One important aspect of modern jet engines that the article only mentions on the periphery are the materials engineering problems in the hot section. There are many metals (not to mention ceramics) that can survive 1000C temperatures, but there are not many that can permanently resist creep at these temperatures under high tensile loads. The only really viable class of materials at the moment are Nickel-based single-crystal superalloys that contain rare metals like Rhenium and Ruthenium. This comes with serious supply limitations and rather complex manufacturing, where the molten metal is solidified directly in the shape of a turbine blade from a single seed crystal. Fun stuff, in other words :)
I used to work in this industry. One thing that might be interesting for people is the metals do not actually withstand the temperatures directly. Instead cooling vanes are needed throughout various parts of the engine. This is why shutting a gas turbine (aka jet engine) down from full power will destroy it. It is necessary to take the engine down to a lower power setting first and then continue to spin the engine (calling motoring the engine) for quite a while even after it is turned off.
Another interesting thing is some engines cannot withstand certain RPM ranges as the compressor and power turbine can get into a catastrophic resonance. A good example is the T700 (used in the Blackhawk).
Why do turbines have a static duct and micron tolerances for the blades (and creep requirements) instead of a rotating (attached to the blades) duct that can be tensioned separately, and (presumably) no creep/micron tolerances?
Not an expert here, but afaik a turbine section consists of alternating spinning blades attached to the shaft and stationary vanes attached to the duct, which de-spin the air coming off the blades and prepare it for the next set. I'm not sure why the vanes are often hidden in cutaway views.
If you had a spinning duct, you'd presumably need a stationary shaft in the middle for mounting the vanes, and would have similar tolerance issues between the tips of the stationary vanes and the rotating duct. There's reasons that it might be easier to solve (the duct can be lower temperature) and reasons it's harder (bearings for a giant spinning duct). Not sure if anyone has tried such a design.
Many rocket engines, especially the reusable sort, require active cooling of the throttle and combustion chamber. A portion of the fuel is split into channels which run through the combustion chamber, throat, and the nozzle. Generally it is a close loop system, so the fuel makes back to be injected into the combustion chamber.
To get max performance modern engines run hot, aka ox rich, and the regen cooling is generally not enough. So in addition to that, critical surfaces such as nozzle also get protected by injecting a thin layer of fuel. This biases combustion to be fuel heavy in localized areas which is less hot. Of course all of this happens in an extremely dynamic environment where gasses are moving at 2km/s+.
Was actually going to post a similar comment re: NASA and the SSME engines for the Space Shuttle. This graphic shows the coolant system circulation that pumps cold fuel through the outer casing to warm it up to proper temperatures before use. [1]
The blades are hollow and have air injected from where they attach to the outside edge and fin of the blade, so when it’s spinning the blade doesn’t contact the exhaust stream because it’s coated with a layer of relatively cold air. Same thing happens with your car pistons but using an inertial layer.
Image search for a turbine blade and you’ll understand as soon as you see it.
The reason you can’t shut the engine down or power off suddenly is because the blades and housing cool at different speeds, the clearance between the blade tips and housing is as close as possible.
To help with this, hot air from the turbine is sprayed onto the outside of the casing via a hot bleed air bypass when the ecm determines its necessary.
If you shut down suddenly the tips of the blades can contact the housing and best case rub, worst case break.
There’s another problem along these lines which really exemplifies how tight these tolerances are, on the a320, you need to do a bowed rotor procedure if you’ve been sitting with the engines off for 45 minutes before you restart. This involves turning the engine over with the apu to equalize the cooling throughout the engine because the core of the engine cools slower but there’s two shafts running through the middle. These shafts “bend” because the outside is cold but the middle is hot, they can then rub against each other ruining bearings etc.
Your china charger doesn't have clearances that tight.
Turbo timers are a legacy from the days when turbos were primarily oil cooled and synthetic oil wasn't common and shutting down a glowing hot turbo would tend to create sludge if done habitually.
This is amazing yet again that they can ingest rain and snow so the inside can be, what, close to 3000F yet you can come into land in Minneapolis when it's -30F and everything Just Works. Imagine how different aviation would be if in an alternate universe we had modern jet engines but under no circumstances could they ingest water?
Note that at cruising altitude it would be more like -80F. The engine would be more efficient at sea level at -30F as the mass flow rate would be higher. Ingesting water vapour actually improves things for the same reason. The downside is it can cause corrosion over time.
> What do you mean by "metals don't actually withstand temperature"? As in the raw metal would melt were it not for the cooling vanes?
A small addition to the sibling comments: Combustion temperatures in modern turbines are around 1400C, if I recall correctly, but the best nickel superalloys go up to 1050C or thereabouts (for long-term operation). To close this gap, the use of high-temperature alloys is supplemented with active cooling and ceramic coatings, as stated by GP.
> What do you mean by "metals don't actually withstand temperature"? As in the raw metal would melt were it not for the cooling vanes?
Metals don't need to melt to fail. Increasing the temperature leads to gradual reduction of yields limits. For example, the yield stress of steel drops to 50% if it reaches around 500 degrees.
but also yes, the metal would melt if it somehow managed to not fail. Often the turbine blades are operating in an environment above their melting point and only don't melt because of the internal cooling.
> What do you mean by "metals don't actually withstand temperature"? As in the raw metal would melt were it not for the cooling vanes?
They creep. Have you seen, for instance, Blu-tac or glue fail? It doesn't go at once, but slowly, over a period of time. At high temperatures most metals (others on this thread have mentioned single-crystal blades) behave a bit like that.
Although steel is also weaker at temperatures far below its melting point, yes. A simple observation of a blacksmith at work should tell you that. And a think some new jets may be running hotter than Tm for steel now?
> The lower power setting on shutdown does what? Spin it at a low RPM so it doesn't decrease in temp too quickly?
Yup, or more relevantly evenly, although those tend to be related. Given almost all materials expand as they get hotter and contract as they cool, different cooling rates between parts -> different contraction rates -> different relative shape -> Very Bad in precision machinery.
So basically metal gets rubbery when hot, and stopping something all off a sudden could have inertial forces(moving blades, gears etc) wreck the structure?
You have to shut things down step by step, so that rigidity is supplied to the metals as the inertial forces are reduced.
All correct. To add, the main problem with ceramics is their fragility under tensile stresses. Spinning at high speeds puts the blades into tensile stress, which tends to "open up" microscopic defects in the crystal structure and cause complete failure.
Some researchers from the academic lab where I work have been working on a turbine configuration in which ceramic turbine blades undergo compressive, instead of tensile, stresses in rotation: https://www.exonetik.com/turbo Interesting stuff, but it's a huge challenge to bring entirely new jet engines, as TFA mentions, to certification and market.
I think eutectic is referring to the ceramic matrix composites (CMC) used in the General Electric's engine LEAP. Here's some quotes from [1]:
> The engine has one CMC component, a turbine shroud lining its hottest zone, so it can operate at up to 2400 F. The CMC needs less cooling air than nickel-based super-alloys and is part of a suite of technologies that contribute to 15 percent fuel savings for LEAP over its predecessor, the CFM 56 engine.
> GE’s CMC is made of silicon carbide (SiC) ceramic fibers (containing silicon and carbon in equal amounts) coated with a proprietary material containing boron nitride. The coated fibers are shaped into a “preform” that is embedded in SiC containing 10–15 percent silicon.
From what I understand, shroud linings don't rotate, though. They are fixed to the engine casing. So they are not subject to the high centrifugal force that would make creep really problematic.
While you are right about the limited applications for this material, the reason cannot be creep, which should be negligible in this kind of ceramic even at the working temperature. Certainly it must be better regarding creep than the alternative metallic alloys.
In a rotating part, subject to high centrifugal forces and vibrations and shocks, I think that the risk of unpredictable fractures may be too high for a ceramic, even a composite one.
Silicon carbide ceramic has low toughness. A composite should be better, but still far from metallic alloys.
I have seen mentions of research about the feasibility of using silicon carbide composite ceramics for rotating parts, with the goal of reducing their mass and increasing their working temperature, in comparison with metallic parts, but it is unlikely that this has reached the stage of being used in production engines.
Ceramics, e.g. derivatives of zirconia, are frequently used for turbine blades, but only as ceramic thermal barrier coatings on metallic blades, not for the body of the blades.
At GE I kept a few used replacement vanes from a (F414/F110) compressor on my desk. Brand new they run about $4000 a piece. The part is about 1.5x2.0 inches. They don't last long in the desert. Most of the parts we had floating around were from the Saudis' F16s, which had been worn down by the sand.
I've always been fascinated by the power density potential of the gas turbine. Especially the micro turbine class.
> The MT power-to-weight ratio is better than a heavy gas turbine because the reduction of turbine diameters causes an increase in shaft rotational speed. [0]
> A similar microturbine built by the Belgian Katholieke Universiteit Leuven has a rotor diameter of 20 mm and is expected to produce about 1,000 W (1.3 hp). [0]
Efficiency is not fantastic at these scales. But, imagine trying to get that amount of power from a different kind of thermodynamic engine with the same mass-volume budget. For certain scenarios, this tradeoff would be amazing. EV charging is something that comes to mind. If the generator is only 50lbs and fits within a lunch box, you could keep it in your car just like a spare tire. I think the efficiency can be compensated for when considering the benefits of distributed generation, cost & form factor.
One of the other advantages of the smaller engines is that you can use techniques that are wildly infeasible in larger engines. For example, Capstone uses a zero-friction air bearing in their solutions:
> Key to the Capstone design is its use of air bearings, which provides maintenance and fluid-free operation for the lifetime of the turbine and reduces the system to a single moving part. This also eliminates the need for any cooling or other secondary systems. [1]
The reason why microturbines are not taking off is, as you mentioned, low efficiency. "Not fantastic" is a bit of an understatement. Especially if you want the turbine to be reasonably cheap (no superalloys, etc) and if it runs below maximum capacity, you'd probably be happy to get 15-20% out of it, not even half of what is achievable with ICEs of the same size. There are not many applications where power-to-weight-ratio is important enough to overcome that limitation.
I just calculated it for 100 ml of methanol. 4.4 kWh/l / 10 * 0.15 = 66 Wh. Enough to charge a laptop once. Yeah, I expected more from chemical fuel somehow. Gasoline and diesel have twice the energy density, but do you really want to carry that smelly, messy stuff with you?
Ethanol, canola oil, or baby oil might be reasonable things to carry with you if you want to lighten your backpack or just reduce your risk of blindness.
Well, obviously you are not supposed to drink it! For reasons that I don't know, methanol is more commonly used as fuel than ethanol. A nice thing about methanol and ethanol is that they evaporate without a trace if there is a minor spill. That is not true for most any distilled petroleum product or any vegetable oils.
Lighter weights of petroleum oils (from petrol through natural gas) are highly volatile and will typically evaporate with minimal (though probably nonzero) residue. That's what makes them attractive as fuels generally as they require little persuasion to vapourise. OTOH, they're so lightweight that they cannot sustain high compressions (hence anti-knock formulations, most notoriously with leaded fuels).
Vegetable oils are nonvolatile, but also generally nontoxic and hence mostly environmentally benign. (You can choke a river or foul ground-dwelling creatures given sufficient quantities, but a few 100 ml won't cause major problems.)
> OTOH, they're so lightweight that they cannot sustain high compressions (hence anti-knock formulations, most notoriously with leaded fuels).
Anti-knock capability of a fuel has very little to do with how "lightweight" they are. Methane, the lightest hydrocarbon and gaseous at any kind of condition you'll find in an engine, has an octane rating of 120. And diesel fuel, substantially heavier than gasoline, as a much lower octane rating than gasoline.
What I was aware of was that early automobiles typically ran on what we'd now call "distillate", which were lighter fractions of petroleum, some just barely liquid (I don't know specific components), with a result that air-fuel mixes ignited readily at low compression ratios (say, 6:1, as opposed to current petrol engines which are in the range generally of 8:1 to 12:1, with some high-performance engines going as hihg as 16:1).
Anti-knock additives (initially ethanol or methanol, later tetraethyl lead, now ... other stuff, including again alcohol) brought up compression ratios and engine efficiency / power. This information I'm remembering from Yergin's The Prize, FWIW.
Diesel operates at generally higher compression ratios, 14:1 to 23:1 per Wikipedia, which I thought translated to higher octane equivalent, but whatever's impeding ignition point isn't that. I know some (most?) diesel engines are fuel-injected, which permits timing of fuel introduction at maximum compression, but not all as I understand.
I'm doing some online sleuthing about this as I'm curious. Volatility itself may play a role, where petrol vapourises whilst diesel aerosolises. The latter is still a fuel-air suspension but with much lower equivalent surface area (and hence, ignition rate) than a vapour would be.
> What I was aware of was that early automobiles typically ran on what we'd now call "distillate", which were lighter fractions of petroleum, some just barely liquid (I don't know specific components), with a result that air-fuel mixes ignited readily at low compression ratios (say, 6:1, as opposed to current petrol engines which are in the range generally of 8:1 to 12:1, with some high-performance engines going as hihg as 16:1).
Early gasoline was more or less output straight from the refinery distillation tower, yes. Octane rating varied a lot depending on the quality of the crude oil, but usually something in the range of 50-70. Thus necessitating the low compression ratios on those early gasoline engines. But the volatility of that gasoline was approximately similar to modern day gasoline.
What was then developed were various further processing steps to improve the octane rating of gasoline (and as the demand for gasoline increased, to increase the fraction of gasoline that you could get from a given amount of crude oil), like dehydrogenation, catalytic cracking, alkylation etc. First these were used for producing high octane aviation gasoline, but after WWII these processes were also put into use to produce automotive gasoline, enabling higher compression ratios in cars. Anti-knock additives helped a bit as well.
> This information I'm remembering from Yergin's The Prize, FWIW.
A pretty good book, I hear. I should read it.
> Diesel operates at generally higher compression ratios, 14:1 to 23:1 per Wikipedia, which I thought translated to higher octane equivalent, but whatever's impeding ignition point isn't that. I know some (most?) diesel engines are fuel-injected, which permits timing of fuel introduction at maximum compression, but not all as I understand.
Diesels inject ALL of the fuel during the combustion stroke. During the compression stroke, they only compress air. Which is why they can have so high compression ratios, there's no fuel vapor mixed with the air that may ignite and cause knock or detonation. Due to the high temperature and pressure in the air caused by the compression, the fuel ignites by itself more or less immediately as it's injected. No spark plug needed.
If you think about it, diesels want something which is sort-of the opposite of an anti-knock (octane) rating. You want the fuel to ignite by itself as soon as it's injected, not resist ignition. For diesel fuel this scale is called the 'cetane' rating, FWIW.
> I'm doing some online sleuthing about this as I'm curious. Volatility itself may play a role, where petrol vapourises whilst diesel aerosolises. The latter is still a fuel-air suspension but with much lower equivalent surface area (and hence, ignition rate) than a vapour would be.
I believe you're sort-of right here. Diesel fuel is injected under high pressure, modern common-rail injection systems reach injection pressures of up to 2000 bar FWIW, which causes the fuel to be atomized into small droplets. The actual burn process AFAIU is sort-of a liquid burn process where fuel vaporizes from the droplets and immediately ignites.
On The Prize, it's really phenomenal, and that's from someone who disagrees pretty strongly with Yergin on his general cozyness to the petroleum industry and enthusiasm for its future prospects. As a history the book is a brilliant work, there's an accompanying PBS/BBC miniseries, and the wealth of information contained (and number of head-turning new-to-me revelations) can't be briefly described. If you're into that sort of thing, I'd also recommend as much of Vaclav Smil as you can stand, though would suggest starting with Energy and Civilization, a look at human history through the lens of energy.
The octane ratings you give are about what I recall from Yergin's description (if that's where I first heard it, again, somewhat vague decade-plus recollection).
My understanding of diesel ignition is somewhat informed by WWII-era triple-expansion steamships, which burned bunker fuel, that requiring a lot of heating (utilising spent steam) just to get it flowing toward the boiler, then again getting toasted immediately before going into the burners. External combustion, obviously, but the challenge of getting a very nonvolatile fuel to burn left an impression. That engine room visit left an impression as well....
Otherwise, appreciate the additional knowledge, it fits pretty well with my own weaker understanding. Interesting especially about cetane. Looking that up, the name comes from Hexadecade, a/k/a C16H34, or a sixteen-chain hydrocarbon (double the carbon-atom count of octane, a/k/a C8H18).
> My understanding of diesel ignition is somewhat informed by WWII-era triple-expansion steamships, which burned bunker fuel, that requiring a lot of heating (utilising spent steam) just to get it flowing toward the boiler, then again getting toasted immediately before going into the burners. External combustion, obviously, but the challenge of getting a very nonvolatile fuel to burn left an impression. That engine room visit left an impression as well....
Yes, bunker fuel is very much non-volatile stuff. As an aside, they did eventually figure out that you could run slow-running big diesel engines on that stuff too. Perhaps you've seen pictures of such massive engines big as houses, if not e.g. https://www.youtube.com/watch?v=K30_jf-aA_U
These big diesels have some things in common with the old triple-expansion steam engines, e.g. they are directly connected to the propshaft (and thus they turn slowly, about 100 rpm max or thereabouts), and the engines themselves are reversible, so there's no need for any reduction gearing or gearboxes.
Similarly to steam ships, they need steam lines in the fuel tanks to heat the fuel so it can be pumped, and then further heated to 130C or thereabouts in order to be injected. So they need a small auxiliary steam boiler just for producing the steam to heat the fuel; modern ships often have an 'exhaust heat recovery boiler', which as the name implies utilizes the hot exhaust from the main engine to produce the steam, so that the auxiliary boiler isn't needed when the main engine is running.
Many years ago, I worked for what would now be called a startup building small gas turbines. The turbine was impressive, 400hp in something the size of 2 shoe boxes. However, it spun at 120,000rpm, which meant either a very heavy gearbox or electrical generator had to be connected to it.
High rpms, noise and the difficulty in adjusting the power output quickly, killed the project.
Now I'm thinking of the Koenigsegg Dark Matter, "an 800 hp, 1250 Nm patent-pending Raxial Flux e-motor". What if it was used as a generator? It's 39Kg, although 6 phase.
Well, I'm dumb, it says max motor RPM 8500, so I don't think you'd get close to what's needed as a generator :D
Here's the PowerMEMS Project page from the link you're referring to. Unfortunately, seems like the last update was from 2010. Haven't heard much since. [1][2][3]
I guess nobody cares about efficiency in their model car engine, so it doesn't matter if you need to refuel every 5-10 minutes. But that would be a problem for pretty much any other use case.
Does anyone know how the efficiency per liter of engine volume compares to these small turbine engines?
Suggesting a turbine could go in a gas car on size/weight alone isn’t a great idea.
I’m saying this as someone in the aviation industry. Turbines are amazing pieces of machinery and incredibly reliable, BUT incredibly expensive to operate.
They require all kinds of specialized maintenance and what I would call “exotic” oils that won’t break down in the harsh environment.
It’d make a really great generator for a vehicle, but I don’t think the economics will work out for a family car anytime soon.
There's millions of radial turbines in cars around the world today. They use an internal-combustion engine for their combustor, and they're called turbo-chargers.
While true, they are not sustaining combustion within the turbo. I believe this is what makes the problem more difficult, and it sounded like what the OC was suggesting.
Turbos float on a layer of motor oil, and have a crude design compared to combustion-sustaining turbines.
What about for “microgrids”? If it was possible for a household (or neighborhood) to install one and run completely on corn based ethanol… that might be something better than the IC generators we have today (I understand that corn ethanol isn’t completely green).
This is an idea that needs to go away. We should not burn food for fuel, and there are a lot of externalities in growing corn and then turning it into ethanol that people are not considering.
Corn-based ethanol is just a very inefficient form of solar energy. Use solar panels instead and skip the middleman.
> Food is a stupid argument invented by the oil companies.
It's not that stupid, we still have many millions of people in utter food insecurity, and not just in the "third world" but also across our Western nation.
IMHO, people should come before cars when it comes to distribution of food, and we should electrify automotive to get rid of the entire issue anyway. What few renewable fuels we have, we will sooner or later need to power air flight and ocean-crossing ships, as we do not have any alternative to some sort of combustible fuel for either purpose.
The GP was talking about microgrids in neihgborhoods (presumably fixed, permanent installations), so there isn't really a reason to worry too much about weight.
Reliability, efficiency/cost of operation, and noise is probably the priorities that come way ahead of weight.
Small and micro gas turbines are already used in the sort of Combined Heat and Power plants used to power commercial / retail buildings, refrigerated warehouses, light industrial units and the like. There's no reason why they shouldn't be used for residential neighbourhoods, especially in colder areas where district heating would be a major selling point.
Compared to reciprocating engine-powered CHP, they tend to produce a slightly higher proportion of their energy output in the form of heat than electricity, and the plant is about about 60% of the volume and half the weight - so they make most sense in constrained spaces or for rooftop installations.
Maybe for a remote cabin? One thing I think might be a problem when grid-connecting them is their lower rotational inertia might make it harder to match/keep frequency. Unless it has very good speed regulation.
Their idea was cogeneration, but I’m not sure if the math works out if you have a low efficiency turbine. We just usually don’t need that many BTUs to run a water heater and furnace versus electricity to run everything else. And with heat pumps becoming more of a thing that’s just becoming more apparent.
A Merlin’s lifetime run-time, even with 25+ reuse launches, is just a hair over two hours (162 first stage time times 25 times two for the static burn). That’s assuming the high reuse stages keep all the engines even.
There are likely some compromises engineers can make when the engine is only running for that amount of time with refurbishment in between each 6 minute runtime.
General aviation is still running on pistons. Not because small jet engines can't be built, but because they don't get cheaper as they get smaller. 6-passenger bizjet sized engines seem to be the lower economic limit.
Williams tried and tried. They built good small jet engines, all the way down to jetpack size, but those never got cheap.[1] There are "very light jets", but the smallest in production, the Cirrus Vision Jet, is around US$2 million.
Interestingly, the turbotech engines at least are recuperated engines, which is kind of unusual. But they claim it's necessary to get decent efficiency of such a small engine.
Curious why the Wankel engine never took off in general aviation? A lot of the advantages of the Wankel engine seem like they'd be even more important in aviation: high power-to-weight ratio, can fit in small spaces, higher RPMs, no vibration, can use lower-octane fuel, reacts quickly to increased power demand, etc. The disadvantages - poor efficiency, poor emissions, and maintenance issues with the seals - are pretty big, but it seems like they'd be less of a problem with general aviation, where planes are used a fraction of the time that a family car would be and already have significant maintenance expectations.
They didn’t catch on for all the reasons you state:
* High RPMs are bad in a aero engine. There are very few (no?) propellers on GA airplanes which operate above 3k RPM, so you need a reduction gearbox. That cuts into your weight savings and also reduces reliability.
* Vibration isn’t a significant concern for GA airplanes.
* Throttle response is not a significant concern in GA-sized reciprocating engines.
* Poor efficiency is a major problem because 1 lb of extra fuel is 1 lb less payload. All airplanes are limited by takeoff weight. (The 3000 HP-class radials built by Wright and P&W at the end of WWII are some of the most efficient reciprocating engines ever built)
* Maintenance is a huge concern for GA owners because labor costs $200/hr. Airplanes with 1200 TBO engines sell for a noticeable discount to airplanes with 2000 TBO engines.
I wonder, as batteries and electric (BLDC) motors get better and better, if we will find applications where electric ducted fans outperform (electric driven) propellers, since electric motors are the same complexity regardless of application.
Ducted fans are by nature less efficient than propellers. This is one reason that the next big leap in engine efficiency may come from what are essentially unducted fans.
One of the latest designs appears to do away with the duct/shroud, for the thrust creation part of the engine. It also hopes to achieve a 20$ decrease int fuel usage.
Doesn't this vary for different cruising speed targets? I thought jets & ducted fans were more efficient above 400 kts or so, while (non ducted) propellers were more efficient below maybe 300 kts. But I'm mostly thinking in terms of turboprop vs. turbofan designs - not 100% sure if it applies the same way for electric types, although I assume it probably would.
The physics of gas turbine engines is one reason I am really excited about electric aviation. People don't realize that you are temp limited at altitude. They think the air is cold, but it is about getting mass through that engine so compressing that air to the density needed brings its temp way up. Electric doesn't have that issue so electric engines could go much higher which means those aircraft could become much more efficient. People focus on the problem of putting enough energy into an electric airframe, but they don't realie the potential massive efficiency gains that it can bring because of the physics of flight.
They are not temperature constrained at altitude. It's much colder up there.
They are air, oxygen really, constrained.
You are right that the electric motors themselves won't suffer from the same oxygen starvation, but as the other commenter noted, the props or impeller blades will. They need something to push, there isn't much up there.
I think he's talking about aerodynamic heating. Turbines compress air, and exhaust generates more thrust than resistance, so it's sort of obvious that compressor stages can be temperature limited where the airframe that hosts it is temperature constrained or something.
I'm not sure how it has to do with electric propulsion, though - I'd think systems like NERVA is a more exciting solution in this kind of domain(jk).
Think of it this way, if I took 1lb of air on the ground and put it into a box that box would have sides of x. As I go up x gets bigger because pressure is dropping with altitude so to get the same mass of air I need a bigger box. When you burn fuel you need a ratio of fuel to air that is determined by mass, not volume so I need to take that really big box at altitude and squeeze it down a lot to get the same density as at sea-level (and then squeeze it even more to get the right mixture in the combustion section). The thing is though, 'hot air rises' so just squeezing down to 1 atmosphere of pressure air at altitude is way hotter than the air on the ground and then you squeeze it even more to get it to the density you need for the engine and it is -really- hot. Engines are generally torque limited on the ground and TIT limited at altitude because as they go up you are power limited by TIT (turbine inlet temp, or some other temp limit related to the engine) because of this compression. Designing engines that can handle that massive heat and that massive force is really hard, but electric has the huge benefit of just needing to produce torque so it is way easier to build and can keep producing power at much higher altitudes. There are definitely challenges there, but they are likely much easier than solving both the heat and torque problems that jet engines have.
Duuuuuude, TIT is the temperature after combustion, not compression. Adiabatic compression isn't even close to the main contributor to TIT--heat input from burning fuel is. Also you may be confusing turbofans and turboshafts--helicopters have torque limits (not a helo guy, but my understanding is this is a gearbox or masthead structural limit rather than an engine limit), but if your turbofan can't hit RPM limits on the ground on a cool day you should seriously consider bringing it back for maintenance instead of going flying.
There are a lot of variations. I am most familiar with turbo props so shaft hp is the limiter on the ground and TIT is the limiter at altitude. TIT, of course, gets most of the heat from burning and I did mess up my explanation a bit. Sorry about that! You may be surprised how much of that TIT comes from compression though. The main point still stands, check the temp of that compressed air at sea level vs at altitude and for the same PSI out of the compressor you get a lot more heat at altitude. Either way though, the original point remains, electric has no TIT limit and you don't have to deal with 1000c materials spinning at 100k RPM so way simpler and easier to build something that keeps delivering thrust at extreme altitudes.
The thinner the air, the more efficient your flight can be, but I never saw this as a temperature problem. My understanding is that there just isn't enough oxygen. Maybe there's an issue with the amount of heating that occurs when you try to compress enough air to get enough oxygen to run your engine?
Theory != Practice. If that were the only variable, then yes. Electric would be great. But it's not. It's far from the only thing in play. Lift also suffers from thinner air. Pure electric (as-in battery/solid state energy storage) could have 100% efficiency (specifically in converting prop/turbine torque to thrust of moving air), and it'd still have a terrible efficiency problem with current day tech.
Electric's primary efficiency and efficacy issue is regarding the total operating weight of the aircraft compounded by how that weight does not meaningfully decrease as the battery banks are depleted as compared to consumable fuels. Weight is your biggest enemy in flight, not power nor mechanical efficiency.
Hybrid electric (be it consumable fuel through a generator or fuel cells) is much more promising, but rarely what people mean when discussing "electric propulsion" (without the hybrid qualifier), and still has issues of it's own.
if you stay subsonic. I hear the U-2 has like 1-2kt of leeway when it is at its max altitude because if it went faster it would be supersonic but any slower and it would fall out of the sky.
Electric has the virtually insurmountable problem that they have to haul the entire weight of the batteries around even if they are drained. This is a MASSIVE loss as itliners can burn off over half their weight during the flight.
You need the electric equivalent of a glider tug plane to get you up to altitude. It can then return to base taking its drained batteries with it while you continue to your destination with fully charged batteries.
Given a range of options for a similar problem, glider pilots generally opt for tugs, which suggests that complexity is within range of a general-aviation pilot class, let alone a commercially-certified one. But let's take your point as given.
Fighter aircraft are generally built for speed and have (even relative to commercial aviation) often fairly low range. The equivalent to "tugging" would be external jettisonable fuel tanks, and we have seen those in military use since at least WWII. Given the general lack of electric propulsion in military use, that seems reasonable. The other model has been JATO packs applied to both fighter and cargo aircraft (Fat Albert, a C-130 Hercules, is often fitted with these for air-show demos). Not electrical power, but an external boost assist.
For drone craft, there are deployment scenarios in which a large cargo plane drops (electrically-powered) drone swarms. I don't know the extent to which this has been deployed, but again it's similar.
If a military were to adopt tugs, I'd expect them to be applied to drone or cargo missions, either with a drone tug (similar to the cargo-plane model above, but possibly with remotely-piloted / autonomous tugs), or with some capability for lofting a battery pack that could be detached and flown back to the take-off site after contributing to initial take-off and climb. That is complicated, but might fit certain mission profiles, and for a relatively slow long-haul cargo mission might make the cut.
Worth also noting that most EV aviation concepts are for relatively modest cargoes and distances. The more viable range from 2--12 passengers for perhaps 100--200 km at low speeds. I've seen some more ambitious proposals, but they strike me as not especially viable.
Why? Energy density way beyond anything battery, and power headroom way beyond anything designed for frugal transport.
Short haul passenger flights are not about speed but about getting directly to the stopover, without the unpredictability of ground transport. A "powered glider" with separating assist for the climbout could be a great match for that task.
And the tug would obviously not really be a tug, but a winged battery directly attached to the aircraft it supports, with barely enough wing for a controlled return. An electric drop tank. Not exactly unheard of in military aviation (except for the "electric", obviously)
Very much disagree. Air to air refueling is done in a very stable manner at cruise altitude. Takeoff is a much more dynamic flight regime where things can go very wrong very quickly.
Kinda crazy but might actually work for continental flights over cooperative areas. Parachute the empty batteries down with some minimal steering mechanism to land them at regularly spaced depots, then ship them back to airports fully charged.
Turnaround time for planes is short enough that you’d need to do a battery-swap rather than a battery-charge anyway.
I'd like to see what a typical widebody's fuel drain over time is, but suspect a large share is the takeoff-and-climb portion of flight.
A winged battery which could drop away at ~FL20--30 or so and return to either the origin field or some secondary collection point might be all you need, rather than tossing batteries out the cargo bay throughout the flight.
I also suspect that most EV aviation will be shorter haul such that a large set of drops wouldn't be necessary.
You piqued my interest enough to go hunting - this StackExchange[1] question estimates ~19% of fuel is spent on initial climb-out to 30k feet for a 737-800 on a 5-hour LA->JFK flight.
Without doing hard calculations, it intuitively feels pretty marginal potential flight weight savings for the operational complexity it would add
Worth noting that EV aircraft flight segments are likely far shorter (100--500 km, maybe at a stretch 1,000 km, not the ~5,000 km of JFK->LAX), and cruise much slower (~100--300 knots, say), so climb-out would be a proportionately larger share of the energy budget.
And ditching 20% of your energy storage mass immediately on attaining altitude would still be a considerable savings for the remainder of the flight as that mass doesn't need to be kept aloft.
EV aviation (and aviation itself) is a battle of thin percentages. EV aviation itself has relied strongly on materials advances (advanced fibre composites), and reducing crew (ultimately: autonomous piloting). The need for cabin crew for safety reasons remains, and would be a significant hurdle. The extent to which non-revenue occupants and payload can be minimised likely plays a huge role in any eventual success. A 19% reduction is nothing to be sneezed at, if it can be achieved without significant other compromise.
On the other hand, only some fraction of the energy inherent in jetfuel is converted to work. So fuel based airplanes have to carry a lot of "extra" energy that is then just wasted as heat.
Can you recommend a place to learn more about this? I have been curious about this topic but have struggled to find resources online describing the basic physics of electric flight propulsion.
I think they would basically be just the fan bit of a turbofan (where they replace a turbofan). A turbofan generates some of its thrust from the fast, hot exhaust, which you wouldn't have in an electric fan engine.
Not sure about electrifying engines for slower planes, that currently use turboprops. Would that be an electric prop too?
A very good article, but I was disappointed to see the misunderstanding about the de Havilland Comet failures repeated
> fatigue failures around its rectangular windows caused two crashes, resulting in it being withdrawn from service
While the accident investigation reports refer to "windows", which really doesn't help matters, the failure point was the ADF antenna mounting cutout. The passenger windows had rounded corners and did not fail in service.
The Comet was not withdrawn from service, they re-engineered and launched the Comet 4 (with oval windows, but that choice was to reduce manufacturing costs) in 1958, but the Boeing 707 was introduced that year and the DC-8 in 1959, ending the Comet's status as the only in-service jet airliner it held between 1952 and the grounding of the Comet 1 in 1954. The Comet 4 continued to fly in revenue service until at least the mid 1970s with lower-tier airlines.
The decision to bury the engines in the wings was one of the deciding factors for airlines - engines in nacelles are easier and cheaper to service and swap if required. Re-engining the Comet 4 to new more efficient turbofan engines the DC-8 and Boeing 707 introduced in 1960 and 1961 respectively required a new wing, but a podded engine was much easier to swap on to an existing airframe and this was done for many of the Boeing and Douglas aircraft.
The last Comet-derived aircraft - the Hawker Siddeley Nimrod - flew until 2011 in the RAF. They did look at upgrading them with new wings and avionics, but the plan was scrapped when they discovered that in the grand tradition of British engineering every fuselage was built slightly differently and they couldn't make replacement parts to a standard plan.
As i am sure the OP and GP know pprune has much of this, and concord related stories from a cohort of engineers and pilots who worked on these aircraft.
They did have a "best of" collection at one point, not sure now. Also a lot of flight test stories, ATC stories.
For young aspiring engineers here who may read this and just like the sound of "building jet engine", look into building a pulse jet first.
They're extremely easy to build, having no moving parts, and only requiring some steel tubing, a welder and a large propane tank. I've already done it and can attest to this being true.
The "best" part is that they're incredibly, obnoxiously loud. Like wear earplugs and ear muffs at the same time loud. Efficiency isn't great, you can expect maybe 20-100 lbs thrust from larger models but I suppose that's more than enough for "let's grab an old bicycle and do something really stupid"
(oh and look pulsejets up on youtube for sure, it'll open up a whole world for you in under 20 minutes)
> Developing a new commercial aircraft is another example in this category, as is building a cheap, reusable rocket.
Cheap rockets can be vastly simpler than turbojet engines. Reusability (I'm talking about reusability of an orbital rocket, suborbital reusable rockets can be rather simple, as e.g. Armadillo Aerospace and Masten Space achievements show) adds a lot to the order, but increasing the size the square-cube law improves things to an extent.
The simplest rocket engine doesn't really have moving parts. It's a chamber where the fuel burns and the nozzle which shapes the exhaust. The moving part is the valve somewhere which turns it on.
The more complex rocket engine includes a pump. But today it's feasible not to make a turbopump, but instead use electric pump - batteries get better, and Electron rocket from Rocket Lab uses this approach for years already.
With jet engines you necessarily have to accept incoming air, compress it and burn with fuel - otherwise it's not a jet engine. Batteries are unfortunately still a bit heavy, so electrical aviation is just getting off the ground slowly.
You can't build a liquid-fueled rocket engine without a pump, can you? The liquid will just stay in the tank unless there's something pressurizing it to a higher pressure than you achieve inside the rocket's combustion chamber, won't it?
An electric pump still sounds much more complex than a jet engine, which has, I believe, one moving part to both compress that incoming air and harness the exhaust. Admittedly, it's a moving part subject to high stresses, high temperatures, stringent balancing requirements, and demanding aerodynamics, so the larger number of parts in the electric pump might still be easier to make.
Ultimately I think long-distance aviation will probably get electrified by way of abundant renewable energy powering electrical synthesis of synfuel on the ground which its engines burn.
> You can't build a liquid-fueled rocket engine without a pump, can you?
You most certainly can, and it was done. Why do you think otherwise?
> The liquid will just stay in the tank unless there's something pressurizing it to a higher pressure than you achieve inside the rocket's combustion chamber, won't it?
True, but you can have the pressure in the tank bigger than in the combustion chamber, right?
> An electric pump still sounds much more complex than a jet engine, which has, I believe, one moving part to both compress that incoming air and harness the exhaust. Admittedly, it's a moving part subject to high stresses, high temperatures, stringent balancing requirements, and demanding aerodynamics, so the larger number of parts in the electric pump might still be easier to make.
Yes, the additional materials requirements and others can make single rotating part harder to get right than electrical parts, which can be developed independently from the rest of the system.
> True, but you can have the pressure in the tank bigger than in the combustion chamber, right?
I guess you can if people have done it. I've never built a rocket myself, so I don't know, but I thought the combustion-chamber pressure had to be crazily high to get the high exhaust velocity you need for propulsion.
Surprisingly you don't need to have that large of the pressure in the chamber to get to the sonic speed in the throat - and more than that in the diverging nozzle. E.g. 3 bar pressure in the chamber could be enough for that.
French Diamant rocket, the one used to launch their first satellite, had a pressure-fed first stage. Lunar Expedition Module from Apollo program had a pressure-fed ascent stage.
> There’s no point in designing a new engine if it doesn’t significantly improve on the state of the art
Oh but there is. I would love to see more European alternatives to US designs even at 5% less efficiency and power. Surely it can’t be that expensive to create an engine in 2025 similar to the state of the art 2005, when you have all the hindsight plus unlimited access to the original design?
Events of this week show that this will be very important.
It is. Both GE and P&W newest generation of engines realized on the order of 20% efficiency gains over their previous products. They both cost in the billions / 10's of billions in r&d, which may sound doable, but realize that they were both starting off with organizations (engineers, facilities, decades of experience, etc.) built to do that. China has thrown 10's of billions and 10's of thousands of people at this problem and still hasn't cracked it after 10ish years.
I agree, but it does not seem so bleak. According to Wikipedia[1]:
The manufacturers market share should be led by CFM with 44% followed by Pratt & Whitney with 29% and then Rolls-Royce and General Electric with 10% each.
CFM is a 50/50 American/French joint venture, and Rolls-Royce is British.
Safran can not make a competitive commercial jet engine on their own and Rolls Royce is a generation behind both GE and P&W, and given the state of the UK not likely to catch up. Right now there is really P&W and GE and then everyone else.
There's sort of two tracks when it comes to jet engines: commercial aviation and military. Commercial just focuses on efficiency, while military has other considerations to account for. And in both sectors there's plenty of European competition, US/EU joint ventures, and subcontractors/licensed manufacturing going on.
Europe does have enough aerospace talent to make a jet engine especially at the cutting edge, but there's a significant amount of tech transfer between the US and Europe happening at the same time.
May countries got domestic turbojet running, but most engine projects fail because there wasn't political will eat the billions to keep a competitive program going, especially as complexity increase in turbofans.
One important point is missing from this: building a cheap and good engine is not enough, there are more companies and industries that can do this than it seems. But you also need the maintenance and logistics network, with a ton of professionals trained for your engine type in particular. And for that you need to penetrate the market that is already captured. This is what stopping the most.
I feel, What's more harder are the jet engines on fighter planes. These are usually a decade or two ahead in terms of advancements. The technology here trickles down to commercial jet engines slowly. Things like Metullargy for blades etc are a closely guarded secret. China and India are pouring billions into research just to get theirs close to even the lower end of what GE has to offer.
One of the figures in the article [1] adds something to this point: military engines have much shorter lifetimes. So it seems it's not just "trickle down" technology, there's also some redesign for reliability.
A commercial engine can operate for a cumulative 1 year between overhauls, according to that figure, as of 2010. The military ones last 1/10th as long. I can only imagine how much more challenging it is to iterate on designs when you are dealing with problems that take 10 times longer to manifest.
> Building the understanding required to push jet engine capabilities forward takes time, effort, and expense.
This occurs in a broader cultural context. A society that dreams, enjoys science fiction, rewards hard study of advanced topics and so forth, can produce the work force to staff companies capable of going to the stars.
You're describing Russia and China, but the US still seems to be doing okay at producing spaceships. Maybe that's because many of the dreamers who enjoyed science fiction in India, Ukraine, Russia, South Africa, France, Germany, Mexico, etc., moved there. Will that continue?
Russia lags far behind the US in producing spaceships for some decades. There are other things necessary for the society to build and maintain companies capable of going to the stars.
Starting 14 years ago, Russia had crewed spaceflight capability, and the US didn't; that situation persisted until less than 5 years ago (Crew Dragon Demo-2). There are other things necessary, but Russia wasn't "lagging", except in the sense that they hadn't backslid as quickly as the US. They are now, of course.
I think the US has learned from others without saying a word about it. e.g. the N1 had a lot of smaller engines rather than a few huge ones - so they could be mass-produced. They were also pretty efficient. They lacked the control systems we have now so the N1 was problematic but it was a clever idea.
Those Russian engines were so good that the US has bought a lot of them and used them many years after they were made.
Certain American manufacturers have ..... been making smaller engines that they can mass produce and have gone taken the efficiency approach a step further.
> e.g. the N1 had a lot of smaller engines rather than a few huge ones - so they could be mass-produced.
American Saturn-1 had 8 H-1 engines on the first stage - Wernher von Braun wasn't against putting a bunch of existing engines when he hadn't have a bigger one.
> They were also pretty efficient.
It's pretty impressive SpaceX made full-flow combustion engine to work. Does it improve things enough to justify the complex development? I'm not sure - the Isp isn't that great comparing with even some kerosene engines, and oxygen-rich turbopumps would deliver similar results with less complex development program. On the other hand Raptors are perhaps a good deal in a long term.
Well, exactly. I don't know if the complexity was worth it or not because I'm not an expert.
For comparison there were apparently 30 NK-15 engines on the first stage of the N1. (from Wikipedia of course)
What I did read somewhere was that they had a production line and they filmed it all so they could see what happened to any engine during production and go back to the recordings if something went wrong with it.
I'm not a Russophile at all, but I suspect that there were clever people who solved problems and others quietly took note of it.
Soyuz spacecraft was technologically simpler - yet safer - than Space Shuttle. It can be argued that US had technically superior, but safety-wise inferior access to space capability until 2011, when the last Space Shuttle flight happened, then US had zero crewed spaceflight capability until 2020, and after that US again had technically superior access to space capability.
Russia in contrast didn't develop its crewed spaceflight capability, it uses the technology left from the USSR. Russia maintains that technology, but progress with the improvements is rather slow. So Russia wasn't lagging in a sense of having - and using - a technology, but definitely was and is lagging in a sense of developing a new technology.
As we see, the lagging of US - in a sense of having and using a technology - was for 9 years, and lagging of Russia - in a sense of having and using technology - for now is about 5 years.
In a sense of developing new crewed space technology Russia is lagging roughly since the dissolution of the USSR, so 30+ years. There were quite a few attempts - again, in crewed space technology - but little results.
I disagree that the Space Shuttle was technically superior. It was more complex, more expensive, and less safe; in my book all three of those are forms of technical inferiority. The Space Shuttle program was already a significant regression from the capabilities of Apollo. Yes, it's true that Russia wasn't making much progress on improvements on Soyuz, but neither was the US; instead they were backsliding faster than Russia was.
I think we can date the US's crewed-spaceflight inferiority to Russia to roughly 01972, when Apollo ended; Russia had launched the first space station the year before, and though the US would briefly operate Skylab in 01973–4, but would not catch up to the Russians again in crewed spaceflight until 02020. The Space Shuttle boondoggle made it possible for sufficiently motivated people to deny this until 02011.
Yes, the Shuttle was more complex, expensive and dangerous than Soyuz, and yes, those are forms of technical inferiority.
But the Shuttle was capable of solo flights for couple of weeks without adverse effects of Soyuz - that is, Shuttle was bigger, and that's useful.
Shuttle brought the bigger crew - more than twice bigger, so there could be better specialization and division of labor, and even the amount of tasks done per unit of time.
Shuttle brought significant payload capability - so the crew could make final preparations before the payload would be launched. Similarly Shuttle can "dock" to Hubble to service it. Or crew could work on orbit in SpaceLab which Shuttle carried to orbit and back. Those are advantages.
Shuttle was more gentle in landing - of course, when things went well. Landing on the strip without passing significant acceleration moments before that - that's another advantage.
I don't think SU and Russia had technical superiority over US - except admittedly safety of the Shuttle, and except those periods when US hadn't have the capability at all. Safety is a big item, so Russia can claim superiority for this reason, and also for simplicity and cheapness, but better US solutions - e.g. with Crew Dragon - suggest it's normal that flying to space better - for many reasons, some of which are shown above - may be either more expensive or will require significant changes, like e.g. modern America companies are pushing.
Now Russia doesn't have much of superiority left, and little capabilities to attain it, or at least it seems so. It's arguably better to have the ability to develop to the needed level, than just to carefully preserve achievements of the past.
I was intrigued by an above comment about miniature jet engines - Iran last year announced a jet-powered Shahed drone variant, which uses an engine that has an interesting backstory:
There are many variants of [the French Microturbo TRI 60] engine and it is used in many missiles and UAVs, as listed below. Aside from the known uses listed below, it is widely speculated that Iran illegally purchased many TRI 60 engines from Microturbo to assemble C-802 cruise missiles purchased from China. It is unclear which variant was purchased. Iran also reverse-engineered this engine as the Toloue-4 turbojet engine. Toloue-4 is used in several Iranian military equipment including Iran's copy of C-802, the Noor missile.
It's fascinating how many engineering artifacts turn out to have been invented just once. This is the same engine used in Storm Shadow / SCALP EG, so both sides in the Ukraine war are firing variants of a 1970s miniature French jet engine at each other.
What's beautiful to me is that that combustion turbines have the simplest possible thermodynamic cycle in theory (a steady input flow of X fluid/sec at pressure P, and a steady output flow of Y>X fluid/sec at pressure P), yet it turns out to be one of the most complex cycles to harness in practice!
Is that really the thermodynamic cycle of the turbine? My understanding is that a cycle is something like "adiabatic compression followed by isothermic expansion, followed by ...", i.e. the details of what happens to the working fluid.
In a gas turbine, the phases of the thermodynamic cycle happen simultaneously in time, but in different places inside the turbine.
While a portion of air progresses through the turbine, it passes through the phases of the cycle.
During the first phase, the air passes through the compressor section of the turbine, where it is compressed adiabatically. During the second phase, fuel is added to the air and it burns, heating the air, which expands at an approximately constant pressure. During the third phase, the exhaust gases pass through the expander section of the turbine, being expanded adiabatically.
The last phase of the cycle, which closes the thermodynamic cycle, by reaching the ambient temperature and pressure, happens in the external atmosphere, for the exhaust gases. The meaning of this phase for an open-cycle engine is that its computation provides the value of the energy lost in the exhaust gases, which reduces the achievable efficiency.
This thermodynamic cycle, which approximates what happens in a gas turbine, is named by Americans the Brayton cycle, even if the historically-correct name is the Joule cycle.
(George B. Brayton has patented an engine using this cycle in 1872, without explaining it, but James Prescott Joule had published an article analyzing in great detail this cycle, “On the Air-Engine”, already in 1851, 21 years earlier. Moreover, already in 1859, a textbook by Rankine, “A Manual of the Steam Engine and other Prime Movers”, where all the thermodynamic cycles known at that time were discussed, attributed this cycle to Joule, 13 years before the Brayton patent. Not only the work of Joule happened much earlier than that of Brayton, but the publications of Joule and Rankine have been very important in the development of the industry of thermal engines, unlike the engines produced by Brayton, which had a very limited commercial success and which had a negligible contribution to the education of the engineers working in this domain. Therefore, the use of the term "Brayton cycle" does not appear to be based on any reason, except that Brayton was American and Joule British.)
Seems unlikely. They had their window of opportunity when they had an active Western marketing arm, Russia wasn't a sanctioned nation, COMAC was barely getting started and the early reports of the Superjet were quite positive. Suffice to say the airlines that passed on the opportunity aren't regretting it and the couple that bought them did regret it.
I disagree with you about the effect of sanctions. Their result was that airliners became a strategic priority rather than something Russia was happy to buy overseas forever.
Furthermore, the sanctions demonstrated that there is sovereign risk associated with purchasing Western airliners.
Finally, IIRC the airline's regrets were largely related to the poor early reliability of the French-built parts, specifically combustors, for the Superjet engines. It remains to be seen how the new Russian engines will perform.
Demand for Russian built airlines in Russia /= them being competitive with Boeing and Airbus. The USSR built airliners as a strategic priority for the domestic market for decades: their track record of being terrible was one of the reasons behind scepticism of the Superjet
And airlines in most countries have far more to worry about buying aircraft whose maintenance depends on a faraway pariah state and that are not certified in Europe than they do about US sanctions targeting them. And even if they do, still not necessarily more difficult to circumvent the sanctions (as Mahan Air did with wet leased 747s) and access a worldwide parts supply and MRO market than rely on being able to maintain and sell on your Russian aircraft at reasonable price and timeliness...
It would also be surprising if the new Russian engines were competitive on performance with new Western engines, and likewise with other components they've had to switch to domestic manufacture for.
And most of the companies that'll get parts shipped to you and do your maintenance, especially when you consider getting UAC MRO certifications isn't exactly an exciting opportunity for companies from China, the Middle East or Latin America either. And doing business with Russian aerospace companies was a PITA when you had access to easy international payments and didn't have a risk of becoming a sanctioned company yourself
The sanctions have almost entirely shut down Russian airliner production. They have only managed to deliver a handful of complete aircraft since 2022, and those largely used parts already on hand. Much of their supply chain is just gone and will take years to rebuild. When they eventually do get the complete production system up and running again their engines will still be less fuel efficient: airlines live and die by fuel efficiency.
They are producing MC-21 without engines waiting for the PD-14 to be ready. We will see in a couple of months if the engine's problems have been solved.
Probably the sanctions will be greatly reduced or eliminated this year or next, and the sanctions are great marketing to other countries that fear being sanctioned—which, following Vance's speech in Munich, probably should include Romania, Germany, Sweden, Denmark, and maybe even the UK.
Last year Ukraine wasn't particularly worried the US would cut off military aid, Romania wasn't particularly worried the US would paint it as a poster child of failed democracies, and Denmark wasn't particularly worried the US would annex Greenland. The world is unpredictable.
Nah, Ukraine knew Trump had an excellent chance of winning and was likely to cut off military aid, and the rest of the world was well aware that a Trump return would mean more moronic threats and trash talking.
Trust me, we're not rushing out to buy shitty Russian aircraft as a hedge.
Yeah, waiting to see if the current iteration of PD-14 engine[0] is finally up to the task. Two years ago UAC tested them and found to be in the need of improvement.
The Wikipedia page on Bombardier is ... not especially clear about present ownership, though apparently debt incurred developing the CSeries (Airbus 220) aircraft lead to spin-outs of much of the core business, including large shares (50% and then another acquisition) of CSeries ops by Airbus.
My recollection is that Boeing essentially had insane tariffs applied on US imports of Bombardier commercial aircraft after Delta made a large order & was preparing for delivery.
Shortly thereafter, Airbus came in and acquired a controlling stake of Bombardier Aviation, took over the CS planes, and agreed to manufacture them in the US (Airbus manufacturing is in the EU).
The way it played out seemed to me as if Boeing and Airbus conspired to kill off a viable competitor after they saw how well received the CS100 and CS300 were.
This is all on top of the overall financial troubles the company was facing.
I could be entirely off the mark, so I will let those more knowledgeable chime in from here.
Bombardier makes only private jets now. The C-series was sold to Airbus and is now the 220.. Q-Series turboprops were sold to De Havilland. The CRJ-series regional jets were sold to Mitsubishi.
De Havilland was owned by Bombardier, but Viking Air bought De Havilland's designs and Dash 8, and renamed the holding company De Havilland.
It's hard not because the technology is so special , but because the tolerance for errors is so small . Jet failure can mean loss of many lives and little room to rectify the situation in flight ,whereas an automobile or train engine failure is a more manageable situation.
It's not all that hard to build a jet engine. Nazi Germany built them being constantly bombed, with actively sabotaging slave labor. Starving North Korea builds them. War-torn Ukraine builds them.
What's hard is to build a competitive jet engine. And there, it happens naturally, by itself: the best marketable jet engine is the one where marginal increase of complexity and cost matches marginal fuel savings: buy simpler/cheaper ones and you waste more money on fuel than you save buying the engine, buy a more complex/expensive one and you don't justify the costs with your fuel savings.
Because an engine runs for tens of thousands of hours - some over 100K hours - so 1% of performance improvement is worth ~1000 tons of fuel - there is a lot of complexity that can be pushed into the solution while still being profitable - and competition ensures this is the case.
That's why it is incredibly hard to make a competitive jet engine.
IIRC leading fuel efficient turbo jet cost like ~10m + 1m for maintenance and consumes 70m worth of fuel over 30k hour life time. An engine 15% less fuel efficient would have to cost $0 to be competitive.
One important aspect of modern jet engines that the article only mentions on the periphery are the materials engineering problems in the hot section. There are many metals (not to mention ceramics) that can survive 1000C temperatures, but there are not many that can permanently resist creep at these temperatures under high tensile loads. The only really viable class of materials at the moment are Nickel-based single-crystal superalloys that contain rare metals like Rhenium and Ruthenium. This comes with serious supply limitations and rather complex manufacturing, where the molten metal is solidified directly in the shape of a turbine blade from a single seed crystal. Fun stuff, in other words :)
I used to work in this industry. One thing that might be interesting for people is the metals do not actually withstand the temperatures directly. Instead cooling vanes are needed throughout various parts of the engine. This is why shutting a gas turbine (aka jet engine) down from full power will destroy it. It is necessary to take the engine down to a lower power setting first and then continue to spin the engine (calling motoring the engine) for quite a while even after it is turned off.
Another interesting thing is some engines cannot withstand certain RPM ranges as the compressor and power turbine can get into a catastrophic resonance. A good example is the T700 (used in the Blackhawk).
I've always wanted to ask...
Why do turbines have a static duct and micron tolerances for the blades (and creep requirements) instead of a rotating (attached to the blades) duct that can be tensioned separately, and (presumably) no creep/micron tolerances?
Not an expert here, but afaik a turbine section consists of alternating spinning blades attached to the shaft and stationary vanes attached to the duct, which de-spin the air coming off the blades and prepare it for the next set. I'm not sure why the vanes are often hidden in cutaway views.
If you had a spinning duct, you'd presumably need a stationary shaft in the middle for mounting the vanes, and would have similar tolerance issues between the tips of the stationary vanes and the rotating duct. There's reasons that it might be easier to solve (the duct can be lower temperature) and reasons it's harder (bearings for a giant spinning duct). Not sure if anyone has tried such a design.
a full duct spinning at 10k rpm seems like it would massively increase stress on the blades
Look up "blisks". These are used for ceramic turbines, because those are stronger in compression than tension.
Many rocket engines, especially the reusable sort, require active cooling of the throttle and combustion chamber. A portion of the fuel is split into channels which run through the combustion chamber, throat, and the nozzle. Generally it is a close loop system, so the fuel makes back to be injected into the combustion chamber.
To get max performance modern engines run hot, aka ox rich, and the regen cooling is generally not enough. So in addition to that, critical surfaces such as nozzle also get protected by injecting a thin layer of fuel. This biases combustion to be fuel heavy in localized areas which is less hot. Of course all of this happens in an extremely dynamic environment where gasses are moving at 2km/s+.
Was actually going to post a similar comment re: NASA and the SSME engines for the Space Shuttle. This graphic shows the coolant system circulation that pumps cold fuel through the outer casing to warm it up to proper temperatures before use. [1]
[1] https://en.wikipedia.org/wiki/RS-25#/media/File:Ssme_schemat...
Not just the reusable ones. Almost all of them do. Exception are monoprop ones where the temperatures are just not high enough.
Your comment is really interesting, but I didn't fully understand.
What do you mean by "metals don't actually withstand temperature"? As in the raw metal would melt were it not for the cooling vanes?
'If powered down, the engine would destroy itself' - from what? Overheating?
The lower power setting on shutdown does what? Spin it at a low RPM so it doesn't decrease in temp too quickly?
The blades are hollow and have air injected from where they attach to the outside edge and fin of the blade, so when it’s spinning the blade doesn’t contact the exhaust stream because it’s coated with a layer of relatively cold air. Same thing happens with your car pistons but using an inertial layer.
Image search for a turbine blade and you’ll understand as soon as you see it.
The reason you can’t shut the engine down or power off suddenly is because the blades and housing cool at different speeds, the clearance between the blade tips and housing is as close as possible.
To help with this, hot air from the turbine is sprayed onto the outside of the casing via a hot bleed air bypass when the ecm determines its necessary.
If you shut down suddenly the tips of the blades can contact the housing and best case rub, worst case break.
There’s another problem along these lines which really exemplifies how tight these tolerances are, on the a320, you need to do a bowed rotor procedure if you’ve been sitting with the engines off for 45 minutes before you restart. This involves turning the engine over with the apu to equalize the cooling throughout the engine because the core of the engine cools slower but there’s two shafts running through the middle. These shafts “bend” because the outside is cold but the middle is hot, they can then rub against each other ruining bearings etc.
This also applies to high performance car turbo engines, a “turbo timer” is used so the ignition can’t be shut off until the turbo cools down.
Your china charger doesn't have clearances that tight.
Turbo timers are a legacy from the days when turbos were primarily oil cooled and synthetic oil wasn't common and shutting down a glowing hot turbo would tend to create sludge if done habitually.
This doesn’t seem to be true on modern turbos.
This is amazing yet again that they can ingest rain and snow so the inside can be, what, close to 3000F yet you can come into land in Minneapolis when it's -30F and everything Just Works. Imagine how different aviation would be if in an alternate universe we had modern jet engines but under no circumstances could they ingest water?
Note that at cruising altitude it would be more like -80F. The engine would be more efficient at sea level at -30F as the mass flow rate would be higher. Ingesting water vapour actually improves things for the same reason. The downside is it can cause corrosion over time.
> What do you mean by "metals don't actually withstand temperature"? As in the raw metal would melt were it not for the cooling vanes?
A small addition to the sibling comments: Combustion temperatures in modern turbines are around 1400C, if I recall correctly, but the best nickel superalloys go up to 1050C or thereabouts (for long-term operation). To close this gap, the use of high-temperature alloys is supplemented with active cooling and ceramic coatings, as stated by GP.
> What do you mean by "metals don't actually withstand temperature"? As in the raw metal would melt were it not for the cooling vanes?
Metals don't need to melt to fail. Increasing the temperature leads to gradual reduction of yields limits. For example, the yield stress of steel drops to 50% if it reaches around 500 degrees.
> Metals don't need to melt to fail.
Another example: “jet fuel can’t melt steel.”
but also yes, the metal would melt if it somehow managed to not fail. Often the turbine blades are operating in an environment above their melting point and only don't melt because of the internal cooling.
> What do you mean by "metals don't actually withstand temperature"? As in the raw metal would melt were it not for the cooling vanes?
They creep. Have you seen, for instance, Blu-tac or glue fail? It doesn't go at once, but slowly, over a period of time. At high temperatures most metals (others on this thread have mentioned single-crystal blades) behave a bit like that.
Although steel is also weaker at temperatures far below its melting point, yes. A simple observation of a blacksmith at work should tell you that. And a think some new jets may be running hotter than Tm for steel now?
> The lower power setting on shutdown does what? Spin it at a low RPM so it doesn't decrease in temp too quickly?
Yup, or more relevantly evenly, although those tend to be related. Given almost all materials expand as they get hotter and contract as they cool, different cooling rates between parts -> different contraction rates -> different relative shape -> Very Bad in precision machinery.
So basically metal gets rubbery when hot, and stopping something all off a sudden could have inertial forces(moving blades, gears etc) wreck the structure?
You have to shut things down step by step, so that rigidity is supplied to the metals as the inertial forces are reduced.
>What do you mean by "metals don't actually withstand temperature"? As in the raw metal would melt were it not for the cooling vanes?
This is similar to the rocket engines where the thrust nozzle and its extension are cooled by the fuel otherwise they would melt or fail structurally.
All correct. To add, the main problem with ceramics is their fragility under tensile stresses. Spinning at high speeds puts the blades into tensile stress, which tends to "open up" microscopic defects in the crystal structure and cause complete failure.
Some researchers from the academic lab where I work have been working on a turbine configuration in which ceramic turbine blades undergo compressive, instead of tensile, stresses in rotation: https://www.exonetik.com/turbo Interesting stuff, but it's a huge challenge to bring entirely new jet engines, as TFA mentions, to certification and market.
Silicon carbide fiber reinforced silicon carbide is also being increasingly used.
In production?
I think eutectic is referring to the ceramic matrix composites (CMC) used in the General Electric's engine LEAP. Here's some quotes from [1]:
[1] https://www.ornl.gov/news/ceramic-matrix-composites-take-fli...From what I understand, shroud linings don't rotate, though. They are fixed to the engine casing. So they are not subject to the high centrifugal force that would make creep really problematic.
While you are right about the limited applications for this material, the reason cannot be creep, which should be negligible in this kind of ceramic even at the working temperature. Certainly it must be better regarding creep than the alternative metallic alloys.
In a rotating part, subject to high centrifugal forces and vibrations and shocks, I think that the risk of unpredictable fractures may be too high for a ceramic, even a composite one.
Silicon carbide ceramic has low toughness. A composite should be better, but still far from metallic alloys.
I have seen mentions of research about the feasibility of using silicon carbide composite ceramics for rotating parts, with the goal of reducing their mass and increasing their working temperature, in comparison with metallic parts, but it is unlikely that this has reached the stage of being used in production engines.
Ceramics, e.g. derivatives of zirconia, are frequently used for turbine blades, but only as ceramic thermal barrier coatings on metallic blades, not for the body of the blades.
This is why I love HN
Me too!
So we had the chance to get more of these rare materials but trump blew it up?
At GE I kept a few used replacement vanes from a (F414/F110) compressor on my desk. Brand new they run about $4000 a piece. The part is about 1.5x2.0 inches. They don't last long in the desert. Most of the parts we had floating around were from the Saudis' F16s, which had been worn down by the sand.
Hell yeah something new to learn about today, thank you.
I've always been fascinated by the power density potential of the gas turbine. Especially the micro turbine class.
> The MT power-to-weight ratio is better than a heavy gas turbine because the reduction of turbine diameters causes an increase in shaft rotational speed. [0]
> A similar microturbine built by the Belgian Katholieke Universiteit Leuven has a rotor diameter of 20 mm and is expected to produce about 1,000 W (1.3 hp). [0]
Efficiency is not fantastic at these scales. But, imagine trying to get that amount of power from a different kind of thermodynamic engine with the same mass-volume budget. For certain scenarios, this tradeoff would be amazing. EV charging is something that comes to mind. If the generator is only 50lbs and fits within a lunch box, you could keep it in your car just like a spare tire. I think the efficiency can be compensated for when considering the benefits of distributed generation, cost & form factor.
One of the other advantages of the smaller engines is that you can use techniques that are wildly infeasible in larger engines. For example, Capstone uses a zero-friction air bearing in their solutions:
> Key to the Capstone design is its use of air bearings, which provides maintenance and fluid-free operation for the lifetime of the turbine and reduces the system to a single moving part. This also eliminates the need for any cooling or other secondary systems. [1]
[0] https://en.wikipedia.org/wiki/Microturbine
[1] https://en.wikipedia.org/wiki/Capstone_Green_Energy
The reason why microturbines are not taking off is, as you mentioned, low efficiency. "Not fantastic" is a bit of an understatement. Especially if you want the turbine to be reasonably cheap (no superalloys, etc) and if it runs below maximum capacity, you'd probably be happy to get 15-20% out of it, not even half of what is achievable with ICEs of the same size. There are not many applications where power-to-weight-ratio is important enough to overcome that limitation.
I just calculated it for 100 ml of methanol. 4.4 kWh/l / 10 * 0.15 = 66 Wh. Enough to charge a laptop once. Yeah, I expected more from chemical fuel somehow. Gasoline and diesel have twice the energy density, but do you really want to carry that smelly, messy stuff with you?
Ethanol, canola oil, or baby oil might be reasonable things to carry with you if you want to lighten your backpack or just reduce your risk of blindness.
Well, obviously you are not supposed to drink it! For reasons that I don't know, methanol is more commonly used as fuel than ethanol. A nice thing about methanol and ethanol is that they evaporate without a trace if there is a minor spill. That is not true for most any distilled petroleum product or any vegetable oils.
Lighter weights of petroleum oils (from petrol through natural gas) are highly volatile and will typically evaporate with minimal (though probably nonzero) residue. That's what makes them attractive as fuels generally as they require little persuasion to vapourise. OTOH, they're so lightweight that they cannot sustain high compressions (hence anti-knock formulations, most notoriously with leaded fuels).
Vegetable oils are nonvolatile, but also generally nontoxic and hence mostly environmentally benign. (You can choke a river or foul ground-dwelling creatures given sufficient quantities, but a few 100 ml won't cause major problems.)
> OTOH, they're so lightweight that they cannot sustain high compressions (hence anti-knock formulations, most notoriously with leaded fuels).
Anti-knock capability of a fuel has very little to do with how "lightweight" they are. Methane, the lightest hydrocarbon and gaseous at any kind of condition you'll find in an engine, has an octane rating of 120. And diesel fuel, substantially heavier than gasoline, as a much lower octane rating than gasoline.
Huh, I'd not known that about diesel.
What I was aware of was that early automobiles typically ran on what we'd now call "distillate", which were lighter fractions of petroleum, some just barely liquid (I don't know specific components), with a result that air-fuel mixes ignited readily at low compression ratios (say, 6:1, as opposed to current petrol engines which are in the range generally of 8:1 to 12:1, with some high-performance engines going as hihg as 16:1).
Anti-knock additives (initially ethanol or methanol, later tetraethyl lead, now ... other stuff, including again alcohol) brought up compression ratios and engine efficiency / power. This information I'm remembering from Yergin's The Prize, FWIW.
Diesel operates at generally higher compression ratios, 14:1 to 23:1 per Wikipedia, which I thought translated to higher octane equivalent, but whatever's impeding ignition point isn't that. I know some (most?) diesel engines are fuel-injected, which permits timing of fuel introduction at maximum compression, but not all as I understand.
I'm doing some online sleuthing about this as I'm curious. Volatility itself may play a role, where petrol vapourises whilst diesel aerosolises. The latter is still a fuel-air suspension but with much lower equivalent surface area (and hence, ignition rate) than a vapour would be.
> What I was aware of was that early automobiles typically ran on what we'd now call "distillate", which were lighter fractions of petroleum, some just barely liquid (I don't know specific components), with a result that air-fuel mixes ignited readily at low compression ratios (say, 6:1, as opposed to current petrol engines which are in the range generally of 8:1 to 12:1, with some high-performance engines going as hihg as 16:1).
Early gasoline was more or less output straight from the refinery distillation tower, yes. Octane rating varied a lot depending on the quality of the crude oil, but usually something in the range of 50-70. Thus necessitating the low compression ratios on those early gasoline engines. But the volatility of that gasoline was approximately similar to modern day gasoline.
What was then developed were various further processing steps to improve the octane rating of gasoline (and as the demand for gasoline increased, to increase the fraction of gasoline that you could get from a given amount of crude oil), like dehydrogenation, catalytic cracking, alkylation etc. First these were used for producing high octane aviation gasoline, but after WWII these processes were also put into use to produce automotive gasoline, enabling higher compression ratios in cars. Anti-knock additives helped a bit as well.
> This information I'm remembering from Yergin's The Prize, FWIW.
A pretty good book, I hear. I should read it.
> Diesel operates at generally higher compression ratios, 14:1 to 23:1 per Wikipedia, which I thought translated to higher octane equivalent, but whatever's impeding ignition point isn't that. I know some (most?) diesel engines are fuel-injected, which permits timing of fuel introduction at maximum compression, but not all as I understand.
Diesels inject ALL of the fuel during the combustion stroke. During the compression stroke, they only compress air. Which is why they can have so high compression ratios, there's no fuel vapor mixed with the air that may ignite and cause knock or detonation. Due to the high temperature and pressure in the air caused by the compression, the fuel ignites by itself more or less immediately as it's injected. No spark plug needed.
If you think about it, diesels want something which is sort-of the opposite of an anti-knock (octane) rating. You want the fuel to ignite by itself as soon as it's injected, not resist ignition. For diesel fuel this scale is called the 'cetane' rating, FWIW.
> I'm doing some online sleuthing about this as I'm curious. Volatility itself may play a role, where petrol vapourises whilst diesel aerosolises. The latter is still a fuel-air suspension but with much lower equivalent surface area (and hence, ignition rate) than a vapour would be.
I believe you're sort-of right here. Diesel fuel is injected under high pressure, modern common-rail injection systems reach injection pressures of up to 2000 bar FWIW, which causes the fuel to be atomized into small droplets. The actual burn process AFAIU is sort-of a liquid burn process where fuel vaporizes from the droplets and immediately ignites.
Thanks for the info.
On The Prize, it's really phenomenal, and that's from someone who disagrees pretty strongly with Yergin on his general cozyness to the petroleum industry and enthusiasm for its future prospects. As a history the book is a brilliant work, there's an accompanying PBS/BBC miniseries, and the wealth of information contained (and number of head-turning new-to-me revelations) can't be briefly described. If you're into that sort of thing, I'd also recommend as much of Vaclav Smil as you can stand, though would suggest starting with Energy and Civilization, a look at human history through the lens of energy.
The octane ratings you give are about what I recall from Yergin's description (if that's where I first heard it, again, somewhat vague decade-plus recollection).
My understanding of diesel ignition is somewhat informed by WWII-era triple-expansion steamships, which burned bunker fuel, that requiring a lot of heating (utilising spent steam) just to get it flowing toward the boiler, then again getting toasted immediately before going into the burners. External combustion, obviously, but the challenge of getting a very nonvolatile fuel to burn left an impression. That engine room visit left an impression as well....
Otherwise, appreciate the additional knowledge, it fits pretty well with my own weaker understanding. Interesting especially about cetane. Looking that up, the name comes from Hexadecade, a/k/a C16H34, or a sixteen-chain hydrocarbon (double the carbon-atom count of octane, a/k/a C8H18).
<https://en.wikipedia.org/wiki/Hexadecane>
<https://en.wikipedia.org/wiki/Cetane_number>
And for octane: <https://en.wikipedia.org/wiki/Octane_rating>
> My understanding of diesel ignition is somewhat informed by WWII-era triple-expansion steamships, which burned bunker fuel, that requiring a lot of heating (utilising spent steam) just to get it flowing toward the boiler, then again getting toasted immediately before going into the burners. External combustion, obviously, but the challenge of getting a very nonvolatile fuel to burn left an impression. That engine room visit left an impression as well....
Yes, bunker fuel is very much non-volatile stuff. As an aside, they did eventually figure out that you could run slow-running big diesel engines on that stuff too. Perhaps you've seen pictures of such massive engines big as houses, if not e.g. https://www.youtube.com/watch?v=K30_jf-aA_U
These big diesels have some things in common with the old triple-expansion steam engines, e.g. they are directly connected to the propshaft (and thus they turn slowly, about 100 rpm max or thereabouts), and the engines themselves are reversible, so there's no need for any reduction gearing or gearboxes.
Similarly to steam ships, they need steam lines in the fuel tanks to heat the fuel so it can be pumped, and then further heated to 130C or thereabouts in order to be injected. So they need a small auxiliary steam boiler just for producing the steam to heat the fuel; modern ships often have an 'exhaust heat recovery boiler', which as the name implies utilizes the hot exhaust from the main engine to produce the steam, so that the auxiliary boiler isn't needed when the main engine is running.
If you spill it, you might inhale a bunch by accident.
Yeah, soaking your sleeping bag with canola oil would be a pretty bad problem. But a methanol or ethanol spill can also do significant damage.
Xylene or citrus terpenes might be nicer, even if the lethal dose is lower than for ethanol.
Many years ago, I worked for what would now be called a startup building small gas turbines. The turbine was impressive, 400hp in something the size of 2 shoe boxes. However, it spun at 120,000rpm, which meant either a very heavy gearbox or electrical generator had to be connected to it.
High rpms, noise and the difficulty in adjusting the power output quickly, killed the project.
Now I'm thinking of the Koenigsegg Dark Matter, "an 800 hp, 1250 Nm patent-pending Raxial Flux e-motor". What if it was used as a generator? It's 39Kg, although 6 phase.
Well, I'm dumb, it says max motor RPM 8500, so I don't think you'd get close to what's needed as a generator :D
> Well, I'm dumb, it says max motor RPM 8500, so I don't think you'd get close to what's needed as a generator :D
20:1 reduction gear, off you go.
[dead]
Here's the PowerMEMS Project page from the link you're referring to. Unfortunately, seems like the last update was from 2010. Haven't heard much since. [1][2][3]
[1] Turbine Overview: https://www.powermems.be/gasturbine.html
[2] Turboshaft Setup: https://www.powermems.be/Turboshaft.html
[3] 1,200,000 RPM on Aerodynamic Bearings Test Runs: https://www.powermems.be/Pen_setup.html
There's a little bit further from the author (Tobias Waumans) afterward, yet not much publication [4]
[4] https://scholar.google.com/scholar?hl=en&as_sdt=0%2C13&q=T.+...
Mostly a summary pub on the work on the aerodynamic bearing setup in Journal of Micromechanics and Microengineering [5]
[5] Aerodynamic Bearing (pdf): https://lirias.kuleuven.be/retrieve/160403
Tiny nitro RC engines can produce 1+ horsepower in engines that weight 1/2 lb.
I guess nobody cares about efficiency in their model car engine, so it doesn't matter if you need to refuel every 5-10 minutes. But that would be a problem for pretty much any other use case.
Does anyone know how the efficiency per liter of engine volume compares to these small turbine engines?
How long can these engines be ran at rated power before you have to overhaul or replace?
30-50hr before they need to be re-sleeved and given a new piston
Suggesting a turbine could go in a gas car on size/weight alone isn’t a great idea.
I’m saying this as someone in the aviation industry. Turbines are amazing pieces of machinery and incredibly reliable, BUT incredibly expensive to operate.
They require all kinds of specialized maintenance and what I would call “exotic” oils that won’t break down in the harsh environment.
It’d make a really great generator for a vehicle, but I don’t think the economics will work out for a family car anytime soon.
There's millions of radial turbines in cars around the world today. They use an internal-combustion engine for their combustor, and they're called turbo-chargers.
While true, they are not sustaining combustion within the turbo. I believe this is what makes the problem more difficult, and it sounded like what the OC was suggesting.
Turbos float on a layer of motor oil, and have a crude design compared to combustion-sustaining turbines.
What about for “microgrids”? If it was possible for a household (or neighborhood) to install one and run completely on corn based ethanol… that might be something better than the IC generators we have today (I understand that corn ethanol isn’t completely green).
> corn based ethanol
This is an idea that needs to go away. We should not burn food for fuel, and there are a lot of externalities in growing corn and then turning it into ethanol that people are not considering.
Corn-based ethanol is just a very inefficient form of solar energy. Use solar panels instead and skip the middleman.
All fuel comes from the sun (or other star activity). Food is a stupid argument invented by the oil companies.
> Food is a stupid argument invented by the oil companies.
It's not that stupid, we still have many millions of people in utter food insecurity, and not just in the "third world" but also across our Western nation.
IMHO, people should come before cars when it comes to distribution of food, and we should electrify automotive to get rid of the entire issue anyway. What few renewable fuels we have, we will sooner or later need to power air flight and ocean-crossing ships, as we do not have any alternative to some sort of combustible fuel for either purpose.
I think efficiency isn’t great.
A diesel ICE engine can be surprisingly efficient and is not particularly expensive compared to a turbine.
You can also run a diesel engine on green fuels.
Yes, the marine diesels run very slow, are two stroke, and approach 50% efficiency. Turbines have an advantage of weight though.
The GP was talking about microgrids in neihgborhoods (presumably fixed, permanent installations), so there isn't really a reason to worry too much about weight.
Reliability, efficiency/cost of operation, and noise is probably the priorities that come way ahead of weight.
Small and micro gas turbines are already used in the sort of Combined Heat and Power plants used to power commercial / retail buildings, refrigerated warehouses, light industrial units and the like. There's no reason why they shouldn't be used for residential neighbourhoods, especially in colder areas where district heating would be a major selling point.
Compared to reciprocating engine-powered CHP, they tend to produce a slightly higher proportion of their energy output in the form of heat than electricity, and the plant is about about 60% of the volume and half the weight - so they make most sense in constrained spaces or for rooftop installations.
Maybe for a remote cabin? One thing I think might be a problem when grid-connecting them is their lower rotational inertia might make it harder to match/keep frequency. Unless it has very good speed regulation.
Why wouldn't this remote cabin be better off with wind or solar?
Those would be my first choice too but there might be trees that block the sunlight, or are taller than a wind turbine tower.
They'd certainly be quieter than a microturbine and not need fuel brought in.
So it makes sense for Batman, but not for my next car. Got it.
Capstone filed for chapter 11 in 23. I wonder what's the fate of their tech.
I had no idea Capstone was still around.
Their idea was cogeneration, but I’m not sure if the math works out if you have a low efficiency turbine. We just usually don’t need that many BTUs to run a water heater and furnace versus electricity to run everything else. And with heat pumps becoming more of a thing that’s just becoming more apparent.
Well, per wikipedia: "On September 28, 2023, Capstone Green Energy declared Chapter 11 bankruptcy"
Now look at the power density (and power/$) of rocket engines.
A Falcon 9's Merlin 1D engine is reported to cost $400K. Its jet kinetic power in vacuum is 1.5 GW, in an engine with a mass of ~500 kg.
$0.27/kW is insanely cheap for a heat engine.
A Merlin’s lifetime run-time, even with 25+ reuse launches, is just a hair over two hours (162 first stage time times 25 times two for the static burn). That’s assuming the high reuse stages keep all the engines even.
There are likely some compromises engineers can make when the engine is only running for that amount of time with refurbishment in between each 6 minute runtime.
And why they are so expensive.
General aviation is still running on pistons. Not because small jet engines can't be built, but because they don't get cheaper as they get smaller. 6-passenger bizjet sized engines seem to be the lower economic limit.
Williams tried and tried. They built good small jet engines, all the way down to jetpack size, but those never got cheap.[1] There are "very light jets", but the smallest in production, the Cirrus Vision Jet, is around US$2 million.
[1] https://en.wikipedia.org/wiki/Williams_International
[2] https://en.wikipedia.org/wiki/Very_light_jet
There are a couple companies working on 'cheap enough'(?) turbines in the GA size category.
https://turb.aero/ (latest news is from March 2023, not sure the company is still afloat?)
https://www.turbotech-aero.com/
Interestingly, the turbotech engines at least are recuperated engines, which is kind of unusual. But they claim it's necessary to get decent efficiency of such a small engine.
Curious why the Wankel engine never took off in general aviation? A lot of the advantages of the Wankel engine seem like they'd be even more important in aviation: high power-to-weight ratio, can fit in small spaces, higher RPMs, no vibration, can use lower-octane fuel, reacts quickly to increased power demand, etc. The disadvantages - poor efficiency, poor emissions, and maintenance issues with the seals - are pretty big, but it seems like they'd be less of a problem with general aviation, where planes are used a fraction of the time that a family car would be and already have significant maintenance expectations.
They didn’t catch on for all the reasons you state:
* High RPMs are bad in a aero engine. There are very few (no?) propellers on GA airplanes which operate above 3k RPM, so you need a reduction gearbox. That cuts into your weight savings and also reduces reliability.
* Vibration isn’t a significant concern for GA airplanes.
* Throttle response is not a significant concern in GA-sized reciprocating engines.
* Poor efficiency is a major problem because 1 lb of extra fuel is 1 lb less payload. All airplanes are limited by takeoff weight. (The 3000 HP-class radials built by Wright and P&W at the end of WWII are some of the most efficient reciprocating engines ever built)
* Maintenance is a huge concern for GA owners because labor costs $200/hr. Airplanes with 1200 TBO engines sell for a noticeable discount to airplanes with 2000 TBO engines.
>and already have significant maintenance expectations.
Exactly. It's bad enough with conservatively engineered piston engines. Adding apex seals is gonna make it a whole lot worse.
I wonder, as batteries and electric (BLDC) motors get better and better, if we will find applications where electric ducted fans outperform (electric driven) propellers, since electric motors are the same complexity regardless of application.
Ducted fans are by nature less efficient than propellers. This is one reason that the next big leap in engine efficiency may come from what are essentially unducted fans.
I thought adding a shroud to propellers increased efficiency? That's why we use turbofan engines instead of turboprops.
One of the latest designs appears to do away with the duct/shroud, for the thrust creation part of the engine. It also hopes to achieve a 20$ decrease int fuel usage.
https://en.wikipedia.org/wiki/CFM_International_RISE
https://www.cfmaeroengines.com/rise/
Doesn't this vary for different cruising speed targets? I thought jets & ducted fans were more efficient above 400 kts or so, while (non ducted) propellers were more efficient below maybe 300 kts. But I'm mostly thinking in terms of turboprop vs. turbofan designs - not 100% sure if it applies the same way for electric types, although I assume it probably would.
The physics of gas turbine engines is one reason I am really excited about electric aviation. People don't realize that you are temp limited at altitude. They think the air is cold, but it is about getting mass through that engine so compressing that air to the density needed brings its temp way up. Electric doesn't have that issue so electric engines could go much higher which means those aircraft could become much more efficient. People focus on the problem of putting enough energy into an electric airframe, but they don't realie the potential massive efficiency gains that it can bring because of the physics of flight.
They are not temperature constrained at altitude. It's much colder up there.
They are air, oxygen really, constrained.
You are right that the electric motors themselves won't suffer from the same oxygen starvation, but as the other commenter noted, the props or impeller blades will. They need something to push, there isn't much up there.
I think he's talking about aerodynamic heating. Turbines compress air, and exhaust generates more thrust than resistance, so it's sort of obvious that compressor stages can be temperature limited where the airframe that hosts it is temperature constrained or something.
I'm not sure how it has to do with electric propulsion, though - I'd think systems like NERVA is a more exciting solution in this kind of domain(jk).
I am not clear about your description.
Electric propellor planes have similar problems at high altitude that you're pushing thin air.
What are the efficiency gains you're thinking about?
Think of it this way, if I took 1lb of air on the ground and put it into a box that box would have sides of x. As I go up x gets bigger because pressure is dropping with altitude so to get the same mass of air I need a bigger box. When you burn fuel you need a ratio of fuel to air that is determined by mass, not volume so I need to take that really big box at altitude and squeeze it down a lot to get the same density as at sea-level (and then squeeze it even more to get the right mixture in the combustion section). The thing is though, 'hot air rises' so just squeezing down to 1 atmosphere of pressure air at altitude is way hotter than the air on the ground and then you squeeze it even more to get it to the density you need for the engine and it is -really- hot. Engines are generally torque limited on the ground and TIT limited at altitude because as they go up you are power limited by TIT (turbine inlet temp, or some other temp limit related to the engine) because of this compression. Designing engines that can handle that massive heat and that massive force is really hard, but electric has the huge benefit of just needing to produce torque so it is way easier to build and can keep producing power at much higher altitudes. There are definitely challenges there, but they are likely much easier than solving both the heat and torque problems that jet engines have.
Duuuuuude, TIT is the temperature after combustion, not compression. Adiabatic compression isn't even close to the main contributor to TIT--heat input from burning fuel is. Also you may be confusing turbofans and turboshafts--helicopters have torque limits (not a helo guy, but my understanding is this is a gearbox or masthead structural limit rather than an engine limit), but if your turbofan can't hit RPM limits on the ground on a cool day you should seriously consider bringing it back for maintenance instead of going flying.
There are a lot of variations. I am most familiar with turbo props so shaft hp is the limiter on the ground and TIT is the limiter at altitude. TIT, of course, gets most of the heat from burning and I did mess up my explanation a bit. Sorry about that! You may be surprised how much of that TIT comes from compression though. The main point still stands, check the temp of that compressed air at sea level vs at altitude and for the same PSI out of the compressor you get a lot more heat at altitude. Either way though, the original point remains, electric has no TIT limit and you don't have to deal with 1000c materials spinning at 100k RPM so way simpler and easier to build something that keeps delivering thrust at extreme altitudes.
The thinner the air, the more efficient your flight can be, but I never saw this as a temperature problem. My understanding is that there just isn't enough oxygen. Maybe there's an issue with the amount of heating that occurs when you try to compress enough air to get enough oxygen to run your engine?
In any case, electric engines don't need oxygen.
> can be
Theory != Practice. If that were the only variable, then yes. Electric would be great. But it's not. It's far from the only thing in play. Lift also suffers from thinner air. Pure electric (as-in battery/solid state energy storage) could have 100% efficiency (specifically in converting prop/turbine torque to thrust of moving air), and it'd still have a terrible efficiency problem with current day tech.
Electric's primary efficiency and efficacy issue is regarding the total operating weight of the aircraft compounded by how that weight does not meaningfully decrease as the battery banks are depleted as compared to consumable fuels. Weight is your biggest enemy in flight, not power nor mechanical efficiency.
Hybrid electric (be it consumable fuel through a generator or fuel cells) is much more promising, but rarely what people mean when discussing "electric propulsion" (without the hybrid qualifier), and still has issues of it's own.
thinner the air, harder it is to generate lift as well.
Coffin corner is a real thing.
if you stay subsonic. I hear the U-2 has like 1-2kt of leeway when it is at its max altitude because if it went faster it would be supersonic but any slower and it would fall out of the sky.
To the point where, if you turned too hard, you could stall one wing tip while Mach buffeting the other.
Obligatory plug for the excellent book Skunk Works by its former director, Ben Rich.
+1, immensely satisfying read for any aviation nut
Unless you're prepared to go supersonic. Not easy to do with electric propulsion though.
Pretty sure for electric aircraft to do anything useful at supersonic speeds you'd need beamed energy. Which would be a pretty cool technology
Scimitar props are pretty tame, though, or what part do you mean is hard?
Electric has the virtually insurmountable problem that they have to haul the entire weight of the batteries around even if they are drained. This is a MASSIVE loss as itliners can burn off over half their weight during the flight.
You need the electric equivalent of a glider tug plane to get you up to altitude. It can then return to base taking its drained batteries with it while you continue to your destination with fully charged batteries.
If that sort of complexity were viable for commercial aviation, we’d be air-to-air refueling airliners.
Air-to-air is way more difficult than just a tug, though.
If that’s the case, why doesn’t the Air Force tug up its fighters? It’d be a huge advantage.
Given a range of options for a similar problem, glider pilots generally opt for tugs, which suggests that complexity is within range of a general-aviation pilot class, let alone a commercially-certified one. But let's take your point as given.
Fighter aircraft are generally built for speed and have (even relative to commercial aviation) often fairly low range. The equivalent to "tugging" would be external jettisonable fuel tanks, and we have seen those in military use since at least WWII. Given the general lack of electric propulsion in military use, that seems reasonable. The other model has been JATO packs applied to both fighter and cargo aircraft (Fat Albert, a C-130 Hercules, is often fitted with these for air-show demos). Not electrical power, but an external boost assist.
For drone craft, there are deployment scenarios in which a large cargo plane drops (electrically-powered) drone swarms. I don't know the extent to which this has been deployed, but again it's similar.
If a military were to adopt tugs, I'd expect them to be applied to drone or cargo missions, either with a drone tug (similar to the cargo-plane model above, but possibly with remotely-piloted / autonomous tugs), or with some capability for lofting a battery pack that could be detached and flown back to the take-off site after contributing to initial take-off and climb. That is complicated, but might fit certain mission profiles, and for a relatively slow long-haul cargo mission might make the cut.
Worth also noting that most EV aviation concepts are for relatively modest cargoes and distances. The more viable range from 2--12 passengers for perhaps 100--200 km at low speeds. I've seen some more ambitious proposals, but they strike me as not especially viable.
Why? Energy density way beyond anything battery, and power headroom way beyond anything designed for frugal transport.
Short haul passenger flights are not about speed but about getting directly to the stopover, without the unpredictability of ground transport. A "powered glider" with separating assist for the climbout could be a great match for that task.
And the tug would obviously not really be a tug, but a winged battery directly attached to the aircraft it supports, with barely enough wing for a controlled return. An electric drop tank. Not exactly unheard of in military aviation (except for the "electric", obviously)
Very much disagree. Air to air refueling is done in a very stable manner at cruise altitude. Takeoff is a much more dynamic flight regime where things can go very wrong very quickly.
Drop tanks, well, drop batteries, to get rid of the excessive mass.
Kinda crazy but might actually work for continental flights over cooperative areas. Parachute the empty batteries down with some minimal steering mechanism to land them at regularly spaced depots, then ship them back to airports fully charged.
Turnaround time for planes is short enough that you’d need to do a battery-swap rather than a battery-charge anyway.
I'd like to see what a typical widebody's fuel drain over time is, but suspect a large share is the takeoff-and-climb portion of flight.
A winged battery which could drop away at ~FL20--30 or so and return to either the origin field or some secondary collection point might be all you need, rather than tossing batteries out the cargo bay throughout the flight.
I also suspect that most EV aviation will be shorter haul such that a large set of drops wouldn't be necessary.
You piqued my interest enough to go hunting - this StackExchange[1] question estimates ~19% of fuel is spent on initial climb-out to 30k feet for a 737-800 on a 5-hour LA->JFK flight.
Without doing hard calculations, it intuitively feels pretty marginal potential flight weight savings for the operational complexity it would add
[1] https://aviation.stackexchange.com/questions/47262/how-much-...
Thanks for that!
Worth noting that EV aircraft flight segments are likely far shorter (100--500 km, maybe at a stretch 1,000 km, not the ~5,000 km of JFK->LAX), and cruise much slower (~100--300 knots, say), so climb-out would be a proportionately larger share of the energy budget.
And ditching 20% of your energy storage mass immediately on attaining altitude would still be a considerable savings for the remainder of the flight as that mass doesn't need to be kept aloft.
EV aviation (and aviation itself) is a battle of thin percentages. EV aviation itself has relied strongly on materials advances (advanced fibre composites), and reducing crew (ultimately: autonomous piloting). The need for cabin crew for safety reasons remains, and would be a significant hurdle. The extent to which non-revenue occupants and payload can be minimised likely plays a huge role in any eventual success. A 19% reduction is nothing to be sneezed at, if it can be achieved without significant other compromise.
Thunderbirds are go!
On the other hand, only some fraction of the energy inherent in jetfuel is converted to work. So fuel based airplanes have to carry a lot of "extra" energy that is then just wasted as heat.
Jet engines are pretty good, over 50%, and electric is more lien 75 (not 100)
Can you recommend a place to learn more about this? I have been curious about this topic but have struggled to find resources online describing the basic physics of electric flight propulsion.
Would electrics be ducted jet engines but with a motor instead of a gas turbine?
I think they would basically be just the fan bit of a turbofan (where they replace a turbofan). A turbofan generates some of its thrust from the fast, hot exhaust, which you wouldn't have in an electric fan engine.
Not sure about electrifying engines for slower planes, that currently use turboprops. Would that be an electric prop too?
You have absolutely no idea what you are talking about. Literally made up.
A very good article, but I was disappointed to see the misunderstanding about the de Havilland Comet failures repeated
> fatigue failures around its rectangular windows caused two crashes, resulting in it being withdrawn from service
While the accident investigation reports refer to "windows", which really doesn't help matters, the failure point was the ADF antenna mounting cutout. The passenger windows had rounded corners and did not fail in service.
The Comet was not withdrawn from service, they re-engineered and launched the Comet 4 (with oval windows, but that choice was to reduce manufacturing costs) in 1958, but the Boeing 707 was introduced that year and the DC-8 in 1959, ending the Comet's status as the only in-service jet airliner it held between 1952 and the grounding of the Comet 1 in 1954. The Comet 4 continued to fly in revenue service until at least the mid 1970s with lower-tier airlines.
The decision to bury the engines in the wings was one of the deciding factors for airlines - engines in nacelles are easier and cheaper to service and swap if required. Re-engining the Comet 4 to new more efficient turbofan engines the DC-8 and Boeing 707 introduced in 1960 and 1961 respectively required a new wing, but a podded engine was much easier to swap on to an existing airframe and this was done for many of the Boeing and Douglas aircraft.
The last Comet-derived aircraft - the Hawker Siddeley Nimrod - flew until 2011 in the RAF. They did look at upgrading them with new wings and avionics, but the plan was scrapped when they discovered that in the grand tradition of British engineering every fuselage was built slightly differently and they couldn't make replacement parts to a standard plan.
Anyway that's my rant in to the void today :)
As i am sure the OP and GP know pprune has much of this, and concord related stories from a cohort of engineers and pilots who worked on these aircraft.
They did have a "best of" collection at one point, not sure now. Also a lot of flight test stories, ATC stories.
For young aspiring engineers here who may read this and just like the sound of "building jet engine", look into building a pulse jet first.
They're extremely easy to build, having no moving parts, and only requiring some steel tubing, a welder and a large propane tank. I've already done it and can attest to this being true.
The "best" part is that they're incredibly, obnoxiously loud. Like wear earplugs and ear muffs at the same time loud. Efficiency isn't great, you can expect maybe 20-100 lbs thrust from larger models but I suppose that's more than enough for "let's grab an old bicycle and do something really stupid" (oh and look pulsejets up on youtube for sure, it'll open up a whole world for you in under 20 minutes)
https://youtu.be/xJhazf0apN8?si=fcb8vehZbqUFRhIy
A great DIY from a great YouTuber
https://youtube.com/watch?v=-fDM9Eb16Do&t=176s&pp=ygUcY29saW...
I like this one
For anybody interested in gas turbine engineering, I recommend Gas Turbine Theory by Cohen & Rogers.
https://archive.org/details/gasturbinetheory0000sara
> Developing a new commercial aircraft is another example in this category, as is building a cheap, reusable rocket.
Cheap rockets can be vastly simpler than turbojet engines. Reusability (I'm talking about reusability of an orbital rocket, suborbital reusable rockets can be rather simple, as e.g. Armadillo Aerospace and Masten Space achievements show) adds a lot to the order, but increasing the size the square-cube law improves things to an extent.
If by 'rather simple' you mean 'bankrupted two fairly well funded aerospace companies' then I'm not sure what your definition of complicated is.
We're comparing with jet engines, and those fairly well funded aerospace companies weren't in the league to attempt that kind of complexity.
As soon as I read your first sentence, I immediately thought of Armadillo :-)
Are you talking about cheap liquid-fueled rockets?
Yes.
Interesting! How do you make those cheaper than jet engines?
The simplest rocket engine doesn't really have moving parts. It's a chamber where the fuel burns and the nozzle which shapes the exhaust. The moving part is the valve somewhere which turns it on.
The more complex rocket engine includes a pump. But today it's feasible not to make a turbopump, but instead use electric pump - batteries get better, and Electron rocket from Rocket Lab uses this approach for years already.
With jet engines you necessarily have to accept incoming air, compress it and burn with fuel - otherwise it's not a jet engine. Batteries are unfortunately still a bit heavy, so electrical aviation is just getting off the ground slowly.
You can't build a liquid-fueled rocket engine without a pump, can you? The liquid will just stay in the tank unless there's something pressurizing it to a higher pressure than you achieve inside the rocket's combustion chamber, won't it?
An electric pump still sounds much more complex than a jet engine, which has, I believe, one moving part to both compress that incoming air and harness the exhaust. Admittedly, it's a moving part subject to high stresses, high temperatures, stringent balancing requirements, and demanding aerodynamics, so the larger number of parts in the electric pump might still be easier to make.
Ultimately I think long-distance aviation will probably get electrified by way of abundant renewable energy powering electrical synthesis of synfuel on the ground which its engines burn.
> You can't build a liquid-fueled rocket engine without a pump, can you?
You most certainly can, and it was done. Why do you think otherwise?
> The liquid will just stay in the tank unless there's something pressurizing it to a higher pressure than you achieve inside the rocket's combustion chamber, won't it?
True, but you can have the pressure in the tank bigger than in the combustion chamber, right?
> An electric pump still sounds much more complex than a jet engine, which has, I believe, one moving part to both compress that incoming air and harness the exhaust. Admittedly, it's a moving part subject to high stresses, high temperatures, stringent balancing requirements, and demanding aerodynamics, so the larger number of parts in the electric pump might still be easier to make.
Yes, the additional materials requirements and others can make single rotating part harder to get right than electrical parts, which can be developed independently from the rest of the system.
> True, but you can have the pressure in the tank bigger than in the combustion chamber, right?
I guess you can if people have done it. I've never built a rocket myself, so I don't know, but I thought the combustion-chamber pressure had to be crazily high to get the high exhaust velocity you need for propulsion.
Thank you very much for enlightening me!
Surprisingly you don't need to have that large of the pressure in the chamber to get to the sonic speed in the throat - and more than that in the diverging nozzle. E.g. 3 bar pressure in the chamber could be enough for that.
French Diamant rocket, the one used to launch their first satellite, had a pressure-fed first stage. Lunar Expedition Module from Apollo program had a pressure-fed ascent stage.
> There’s no point in designing a new engine if it doesn’t significantly improve on the state of the art
Oh but there is. I would love to see more European alternatives to US designs even at 5% less efficiency and power. Surely it can’t be that expensive to create an engine in 2025 similar to the state of the art 2005, when you have all the hindsight plus unlimited access to the original design?
Events of this week show that this will be very important.
It is. Both GE and P&W newest generation of engines realized on the order of 20% efficiency gains over their previous products. They both cost in the billions / 10's of billions in r&d, which may sound doable, but realize that they were both starting off with organizations (engineers, facilities, decades of experience, etc.) built to do that. China has thrown 10's of billions and 10's of thousands of people at this problem and still hasn't cracked it after 10ish years.
Rolls Royce is British.
And PBS is Czech, to name one.
I agree, but it does not seem so bleak. According to Wikipedia[1]:
CFM is a 50/50 American/French joint venture, and Rolls-Royce is British.1: https://en.m.wikipedia.org/wiki/List_of_turbofan_manufacture...
Safran can not make a competitive commercial jet engine on their own and Rolls Royce is a generation behind both GE and P&W, and given the state of the UK not likely to catch up. Right now there is really P&W and GE and then everyone else.
SAFRAN (French national aerospace company) make the LEAP turbofans used on some A320s and 737s - https://www.safran-group.com/group/profile/aircraft-propulsi...
There's sort of two tracks when it comes to jet engines: commercial aviation and military. Commercial just focuses on efficiency, while military has other considerations to account for. And in both sectors there's plenty of European competition, US/EU joint ventures, and subcontractors/licensed manufacturing going on.
Europe does have enough aerospace talent to make a jet engine especially at the cutting edge, but there's a significant amount of tech transfer between the US and Europe happening at the same time.
May countries got domestic turbojet running, but most engine projects fail because there wasn't political will eat the billions to keep a competitive program going, especially as complexity increase in turbofans.
One important point is missing from this: building a cheap and good engine is not enough, there are more companies and industries that can do this than it seems. But you also need the maintenance and logistics network, with a ton of professionals trained for your engine type in particular. And for that you need to penetrate the market that is already captured. This is what stopping the most.
Funny, if you mouse over the graph of transistor costs, they become free in 2005! Cool!
Transistor manufacturers are like, "hey free transistors" then they bill the entire cost as shipping fees like the average aliexpress store.
Related:
See Thru Jet Engine [video] - https://news.ycombinator.com/item?id=32145297 - July 2022 (70 comments)
I wonder if there aren't other propulsion principles to pursue in modern aircrafts beside the jet engine.
I feel, What's more harder are the jet engines on fighter planes. These are usually a decade or two ahead in terms of advancements. The technology here trickles down to commercial jet engines slowly. Things like Metullargy for blades etc are a closely guarded secret. China and India are pouring billions into research just to get theirs close to even the lower end of what GE has to offer.
One of the figures in the article [1] adds something to this point: military engines have much shorter lifetimes. So it seems it's not just "trickle down" technology, there's also some redesign for reliability.
A commercial engine can operate for a cumulative 1 year between overhauls, according to that figure, as of 2010. The military ones last 1/10th as long. I can only imagine how much more challenging it is to iterate on designs when you are dealing with problems that take 10 times longer to manifest.
[1]: https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_pr...
> Building the understanding required to push jet engine capabilities forward takes time, effort, and expense.
This occurs in a broader cultural context. A society that dreams, enjoys science fiction, rewards hard study of advanced topics and so forth, can produce the work force to staff companies capable of going to the stars.
Let us encourage that.
You're describing Russia and China, but the US still seems to be doing okay at producing spaceships. Maybe that's because many of the dreamers who enjoyed science fiction in India, Ukraine, Russia, South Africa, France, Germany, Mexico, etc., moved there. Will that continue?
Russia lags far behind the US in producing spaceships for some decades. There are other things necessary for the society to build and maintain companies capable of going to the stars.
Starting 14 years ago, Russia had crewed spaceflight capability, and the US didn't; that situation persisted until less than 5 years ago (Crew Dragon Demo-2). There are other things necessary, but Russia wasn't "lagging", except in the sense that they hadn't backslid as quickly as the US. They are now, of course.
I think the US has learned from others without saying a word about it. e.g. the N1 had a lot of smaller engines rather than a few huge ones - so they could be mass-produced. They were also pretty efficient. They lacked the control systems we have now so the N1 was problematic but it was a clever idea.
Those Russian engines were so good that the US has bought a lot of them and used them many years after they were made.
Certain American manufacturers have ..... been making smaller engines that they can mass produce and have gone taken the efficiency approach a step further.
> e.g. the N1 had a lot of smaller engines rather than a few huge ones - so they could be mass-produced.
American Saturn-1 had 8 H-1 engines on the first stage - Wernher von Braun wasn't against putting a bunch of existing engines when he hadn't have a bigger one.
> They were also pretty efficient.
It's pretty impressive SpaceX made full-flow combustion engine to work. Does it improve things enough to justify the complex development? I'm not sure - the Isp isn't that great comparing with even some kerosene engines, and oxygen-rich turbopumps would deliver similar results with less complex development program. On the other hand Raptors are perhaps a good deal in a long term.
Well, exactly. I don't know if the complexity was worth it or not because I'm not an expert.
For comparison there were apparently 30 NK-15 engines on the first stage of the N1. (from Wikipedia of course)
What I did read somewhere was that they had a production line and they filmed it all so they could see what happened to any engine during production and go back to the recordings if something went wrong with it.
I'm not a Russophile at all, but I suspect that there were clever people who solved problems and others quietly took note of it.
Soyuz spacecraft was technologically simpler - yet safer - than Space Shuttle. It can be argued that US had technically superior, but safety-wise inferior access to space capability until 2011, when the last Space Shuttle flight happened, then US had zero crewed spaceflight capability until 2020, and after that US again had technically superior access to space capability.
Russia in contrast didn't develop its crewed spaceflight capability, it uses the technology left from the USSR. Russia maintains that technology, but progress with the improvements is rather slow. So Russia wasn't lagging in a sense of having - and using - a technology, but definitely was and is lagging in a sense of developing a new technology.
As we see, the lagging of US - in a sense of having and using a technology - was for 9 years, and lagging of Russia - in a sense of having and using technology - for now is about 5 years.
In a sense of developing new crewed space technology Russia is lagging roughly since the dissolution of the USSR, so 30+ years. There were quite a few attempts - again, in crewed space technology - but little results.
I disagree that the Space Shuttle was technically superior. It was more complex, more expensive, and less safe; in my book all three of those are forms of technical inferiority. The Space Shuttle program was already a significant regression from the capabilities of Apollo. Yes, it's true that Russia wasn't making much progress on improvements on Soyuz, but neither was the US; instead they were backsliding faster than Russia was.
I think we can date the US's crewed-spaceflight inferiority to Russia to roughly 01972, when Apollo ended; Russia had launched the first space station the year before, and though the US would briefly operate Skylab in 01973–4, but would not catch up to the Russians again in crewed spaceflight until 02020. The Space Shuttle boondoggle made it possible for sufficiently motivated people to deny this until 02011.
Yes, the Shuttle was more complex, expensive and dangerous than Soyuz, and yes, those are forms of technical inferiority.
But the Shuttle was capable of solo flights for couple of weeks without adverse effects of Soyuz - that is, Shuttle was bigger, and that's useful.
Shuttle brought the bigger crew - more than twice bigger, so there could be better specialization and division of labor, and even the amount of tasks done per unit of time.
Shuttle brought significant payload capability - so the crew could make final preparations before the payload would be launched. Similarly Shuttle can "dock" to Hubble to service it. Or crew could work on orbit in SpaceLab which Shuttle carried to orbit and back. Those are advantages.
Shuttle was more gentle in landing - of course, when things went well. Landing on the strip without passing significant acceleration moments before that - that's another advantage.
I don't think SU and Russia had technical superiority over US - except admittedly safety of the Shuttle, and except those periods when US hadn't have the capability at all. Safety is a big item, so Russia can claim superiority for this reason, and also for simplicity and cheapness, but better US solutions - e.g. with Crew Dragon - suggest it's normal that flying to space better - for many reasons, some of which are shown above - may be either more expensive or will require significant changes, like e.g. modern America companies are pushing.
Now Russia doesn't have much of superiority left, and little capabilities to attain it, or at least it seems so. It's arguably better to have the ability to develop to the needed level, than just to carefully preserve achievements of the past.
How many countries make their own jet engines? US, UK, France ... anyone else?
Russia, China, India, both South and North Korea, Iran, Turkey, Israel, and Czech Republic all manufacture jet engines domestically.
The German part of Rolls Royce - that's where the new B-52 engines are coming from for example.
Russia and China
I was intrigued by an above comment about miniature jet engines - Iran last year announced a jet-powered Shahed drone variant, which uses an engine that has an interesting backstory:
There are many variants of [the French Microturbo TRI 60] engine and it is used in many missiles and UAVs, as listed below. Aside from the known uses listed below, it is widely speculated that Iran illegally purchased many TRI 60 engines from Microturbo to assemble C-802 cruise missiles purchased from China. It is unclear which variant was purchased. Iran also reverse-engineered this engine as the Toloue-4 turbojet engine. Toloue-4 is used in several Iranian military equipment including Iran's copy of C-802, the Noor missile.
It's fascinating how many engineering artifacts turn out to have been invented just once. This is the same engine used in Storm Shadow / SCALP EG, so both sides in the Ukraine war are firing variants of a 1970s miniature French jet engine at each other.
https://en.wikipedia.org/wiki/TEM_Toloue-4
https://en.wikipedia.org/wiki/Microturbo_TRI_60
What's beautiful to me is that that combustion turbines have the simplest possible thermodynamic cycle in theory (a steady input flow of X fluid/sec at pressure P, and a steady output flow of Y>X fluid/sec at pressure P), yet it turns out to be one of the most complex cycles to harness in practice!
Is that really the thermodynamic cycle of the turbine? My understanding is that a cycle is something like "adiabatic compression followed by isothermic expansion, followed by ...", i.e. the details of what happens to the working fluid.
In a gas turbine, the phases of the thermodynamic cycle happen simultaneously in time, but in different places inside the turbine.
While a portion of air progresses through the turbine, it passes through the phases of the cycle.
During the first phase, the air passes through the compressor section of the turbine, where it is compressed adiabatically. During the second phase, fuel is added to the air and it burns, heating the air, which expands at an approximately constant pressure. During the third phase, the exhaust gases pass through the expander section of the turbine, being expanded adiabatically.
The last phase of the cycle, which closes the thermodynamic cycle, by reaching the ambient temperature and pressure, happens in the external atmosphere, for the exhaust gases. The meaning of this phase for an open-cycle engine is that its computation provides the value of the energy lost in the exhaust gases, which reduces the achievable efficiency.
This thermodynamic cycle, which approximates what happens in a gas turbine, is named by Americans the Brayton cycle, even if the historically-correct name is the Joule cycle.
(George B. Brayton has patented an engine using this cycle in 1872, without explaining it, but James Prescott Joule had published an article analyzing in great detail this cycle, “On the Air-Engine”, already in 1851, 21 years earlier. Moreover, already in 1859, a textbook by Rankine, “A Manual of the Steam Engine and other Prime Movers”, where all the thermodynamic cycles known at that time were discussed, attributed this cycle to Joule, 13 years before the Brayton patent. Not only the work of Joule happened much earlier than that of Brayton, but the publications of Joule and Rankine have been very important in the development of the industry of thermal engines, unlike the engines produced by Brayton, which had a very limited commercial success and which had a negligible contribution to the education of the engineers working in this domain. Therefore, the use of the term "Brayton cycle" does not appear to be based on any reason, except that Brayton was American and Joule British.)
Aren't these engine designs patented very heavily? How were clones popping up less than a decade later?
>Depending on how you count, there are just two to four builders of large commercial aircraft (Airbus, Boeing, Embraer, and now COMAC).
Where is Russian Sukhoi?
It is part of UAC, along with Ilyushin and Yakovlev.
Still not in the list)
Yes, and they are ahead of COMAC in the number of aircraft produced.
It will be interesting to see if UAC emerges as a serious competitor to Boeing and Airbus (and COMAC) in the near future.
Seems unlikely. They had their window of opportunity when they had an active Western marketing arm, Russia wasn't a sanctioned nation, COMAC was barely getting started and the early reports of the Superjet were quite positive. Suffice to say the airlines that passed on the opportunity aren't regretting it and the couple that bought them did regret it.
I disagree with you about the effect of sanctions. Their result was that airliners became a strategic priority rather than something Russia was happy to buy overseas forever.
Furthermore, the sanctions demonstrated that there is sovereign risk associated with purchasing Western airliners.
Finally, IIRC the airline's regrets were largely related to the poor early reliability of the French-built parts, specifically combustors, for the Superjet engines. It remains to be seen how the new Russian engines will perform.
Demand for Russian built airlines in Russia /= them being competitive with Boeing and Airbus. The USSR built airliners as a strategic priority for the domestic market for decades: their track record of being terrible was one of the reasons behind scepticism of the Superjet
And airlines in most countries have far more to worry about buying aircraft whose maintenance depends on a faraway pariah state and that are not certified in Europe than they do about US sanctions targeting them. And even if they do, still not necessarily more difficult to circumvent the sanctions (as Mahan Air did with wet leased 747s) and access a worldwide parts supply and MRO market than rely on being able to maintain and sell on your Russian aircraft at reasonable price and timeliness...
It would also be surprising if the new Russian engines were competitive on performance with new Western engines, and likewise with other components they've had to switch to domestic manufacture for.
>faraway pariah state
By that you mean a state sanctioned by the US and the EU, which together comprise about 10% of the world's population.
And most of the companies that'll get parts shipped to you and do your maintenance, especially when you consider getting UAC MRO certifications isn't exactly an exciting opportunity for companies from China, the Middle East or Latin America either. And doing business with Russian aerospace companies was a PITA when you had access to easy international payments and didn't have a risk of becoming a sanctioned company yourself
The sanctions have almost entirely shut down Russian airliner production. They have only managed to deliver a handful of complete aircraft since 2022, and those largely used parts already on hand. Much of their supply chain is just gone and will take years to rebuild. When they eventually do get the complete production system up and running again their engines will still be less fuel efficient: airlines live and die by fuel efficiency.
They have the MC-21 under development as well, though not much information seems available.
They are producing MC-21 without engines waiting for the PD-14 to be ready. We will see in a couple of months if the engine's problems have been solved.
Probably the sanctions will be greatly reduced or eliminated this year or next, and the sanctions are great marketing to other countries that fear being sanctioned—which, following Vance's speech in Munich, probably should include Romania, Germany, Sweden, Denmark, and maybe even the UK.
I don't think Swedes and Brits are particularly worried about being unable to obtain parts and maintenance for Boeing aircraft, never mind Airbus...
Even Iran is flying old Western aircraft
Last year Ukraine wasn't particularly worried the US would cut off military aid, Romania wasn't particularly worried the US would paint it as a poster child of failed democracies, and Denmark wasn't particularly worried the US would annex Greenland. The world is unpredictable.
Nah, Ukraine knew Trump had an excellent chance of winning and was likely to cut off military aid, and the rest of the world was well aware that a Trump return would mean more moronic threats and trash talking.
Trust me, we're not rushing out to buy shitty Russian aircraft as a hedge.
Yeah, waiting to see if the current iteration of PD-14 engine[0] is finally up to the task. Two years ago UAC tested them and found to be in the need of improvement.
[0] https://rostec.ru/media/news/rostekh-peredal-partiyu-seriyny...
Yep... it is hard to build a competitive jet engine.
If the Russians manage to do this, it would be another example of the stupidity of the sanctions.
I bet COMAC is cheering them on too.
Bombardier until recently was another, although it was taken over (?) by Airbus.
Huh, I'd not heard that.
The Wikipedia page on Bombardier is ... not especially clear about present ownership, though apparently debt incurred developing the CSeries (Airbus 220) aircraft lead to spin-outs of much of the core business, including large shares (50% and then another acquisition) of CSeries ops by Airbus.
<https://en.wikipedia.org/wiki/Bombardier_Inc.>
The top of the article seems to portray Bombardier as an independent company, other bits not so much.
My recollection is that Boeing essentially had insane tariffs applied on US imports of Bombardier commercial aircraft after Delta made a large order & was preparing for delivery.
Shortly thereafter, Airbus came in and acquired a controlling stake of Bombardier Aviation, took over the CS planes, and agreed to manufacture them in the US (Airbus manufacturing is in the EU).
The way it played out seemed to me as if Boeing and Airbus conspired to kill off a viable competitor after they saw how well received the CS100 and CS300 were.
This is all on top of the overall financial troubles the company was facing.
I could be entirely off the mark, so I will let those more knowledgeable chime in from here.
CS300 are the best aircraft I have ever flown in. A lot of room, very good air inside, very nice design aesthetic.
I like your take, FWIW.
Bombardier makes only private jets now. The C-series was sold to Airbus and is now the 220.. Q-Series turboprops were sold to De Havilland. The CRJ-series regional jets were sold to Mitsubishi.
De Havilland was owned by Bombardier, but Viking Air bought De Havilland's designs and Dash 8, and renamed the holding company De Havilland.
Looks like they only kept the business jet division, selling the rest to different buyers including Airbus. [0]
[0] https://en.wikipedia.org/wiki/Bombardier_Inc.
because it flies away
It's hard not because the technology is so special , but because the tolerance for errors is so small . Jet failure can mean loss of many lives and little room to rectify the situation in flight ,whereas an automobile or train engine failure is a more manageable situation.
It's not all that hard to build a jet engine. Nazi Germany built them being constantly bombed, with actively sabotaging slave labor. Starving North Korea builds them. War-torn Ukraine builds them.
What's hard is to build a competitive jet engine. And there, it happens naturally, by itself: the best marketable jet engine is the one where marginal increase of complexity and cost matches marginal fuel savings: buy simpler/cheaper ones and you waste more money on fuel than you save buying the engine, buy a more complex/expensive one and you don't justify the costs with your fuel savings.
Because an engine runs for tens of thousands of hours - some over 100K hours - so 1% of performance improvement is worth ~1000 tons of fuel - there is a lot of complexity that can be pushed into the solution while still being profitable - and competition ensures this is the case.
That's why it is incredibly hard to make a competitive jet engine.
IIRC leading fuel efficient turbo jet cost like ~10m + 1m for maintenance and consumes 70m worth of fuel over 30k hour life time. An engine 15% less fuel efficient would have to cost $0 to be competitive.
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Material tolerances.