[HN Gopher] Why it's so hard to build a jet engine
___________________________________________________________________
Why it's so hard to build a jet engine
Author : mhb
Score : 334 points
Date : 2025-02-28 23:05 UTC (23 hours ago)
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| jmward01 wrote:
| 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.
| sokka_h2otribe wrote:
| 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?
| russdill wrote:
| 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.
| zeusk wrote:
| thinner the air, harder it is to generate lift as well.
|
| Coffin corner is a real thing.
| mhandley wrote:
| Unless you're prepared to go supersonic. Not easy to do
| with electric propulsion though.
| namibj wrote:
| Scimitar props are pretty tame, though, or what part do
| you mean is hard?
| russdill wrote:
| Pretty sure for electric aircraft to do anything useful
| at supersonic speeds you'd need beamed energy. Which
| would be a pretty cool technology
| jmward01 wrote:
| 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.
| tonyarkles wrote:
| To the point where, if you turned too hard, you could
| stall one wing tip while Mach buffeting the other.
| FredPret wrote:
| Obligatory plug for the excellent book Skunk Works by its
| former director, Ben Rich.
| sidnb13 wrote:
| +1, immensely satisfying read for any aviation nut
| 0manrho wrote:
| > 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.
| jmward01 wrote:
| 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.
| scarier wrote:
| 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.
| jmward01 wrote:
| 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.
| 55873445216111 wrote:
| 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.
| iancmceachern wrote:
| 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.
| numpad0 wrote:
| 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).
| TylerE wrote:
| 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.
| mhandley wrote:
| 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.
| nickff wrote:
| If that sort of complexity were viable for commercial
| aviation, we'd be air-to-air refueling airliners.
| namibj wrote:
| Air-to-air is way more difficult than just a tug, though.
| nickff wrote:
| If that's the case, why doesn't the Air Force tug up its
| fighters? It'd be a huge advantage.
| usrusr wrote:
| 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)
| dredmorbius wrote:
| 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.
| TylerE wrote:
| 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.
| almostnormal wrote:
| Drop tanks, well, drop batteries, to get rid of the
| excessive mass.
| jl6 wrote:
| 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.
| dredmorbius wrote:
| 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.
| Galxeagle wrote:
| 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-...
| abdullahkhalids wrote:
| 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.
| bilsbie wrote:
| Would electrics be ducted jet engines but with a motor instead
| of a gas turbine?
| rich_sasha wrote:
| 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?
| tekla wrote:
| You have absolutely no idea what you are talking about.
| Literally made up.
| adiabatichottub wrote:
| For anybody interested in gas turbine engineering, I recommend
| Gas Turbine Theory by Cohen & Rogers.
|
| https://archive.org/details/gasturbinetheory0000sara
| orbital-decay wrote:
| 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.
| avmich wrote:
| > 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.
| kragen wrote:
| Are you talking about cheap liquid-fueled rockets?
| avmich wrote:
| Yes.
| kragen wrote:
| Interesting! How do you make those cheaper than jet
| engines?
| avmich wrote:
| 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.
| kragen wrote:
| 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.
| avmich wrote:
| > 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.
| kragen wrote:
| > _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!
| avmich wrote:
| 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.
| philipwhiuk wrote:
| If by 'rather simple' you mean 'bankrupted two fairly well
| funded aerospace companies' then I'm not sure what your
| definition of complicated is.
| avmich wrote:
| We're comparing with jet engines, and those fairly well
| funded aerospace companies weren't in the league to attempt
| that kind of complexity.
| HeyLaughingBoy wrote:
| As soon as I read your first sentence, I immediately thought of
| Armadillo :-)
| smitty1e wrote:
| > 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.
| kragen wrote:
| 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?
| avmich wrote:
| 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.
| kragen wrote:
| 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.
| avmich wrote:
| 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.
| kragen wrote:
| 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.
| avmich wrote:
| 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.
| t43562 wrote:
| 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.
| avmich wrote:
| > 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.
| bob1029 wrote:
| 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
| nick3443 wrote:
| Tiny nitro RC engines can produce 1+ horsepower in engines that
| weight 1/2 lb.
| esperent wrote:
| 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?
| bob1029 wrote:
| How long can these engines be ran at rated power before you
| have to overhaul or replace?
| jcgrillo wrote:
| 30-50hr before they need to be re-sleeved and given a new
| piston
| mppm wrote:
| 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.
| ahartmetz wrote:
| 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?
| kragen wrote:
| 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.
| ahartmetz wrote:
| 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.
| dredmorbius wrote:
| 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.)
| jabl wrote:
| > 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.
| kragen wrote:
| 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.
| grapesodaaaaa wrote:
| 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.
| pm90 wrote:
| 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).
| chiph wrote:
| 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.
| salynchnew wrote:
| Why wouldn't this remote cabin be better off with wind or
| solar?
| SoftTalker wrote:
| > 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.
| bluGill wrote:
| All fuel comes from the sun (or other star activity).
| Food is a stupid argument invented by the oil companies.
| dghlsakjg wrote:
| 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.
| adiabatichottub wrote:
| 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.
| pfdietz wrote:
| 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.
| generj wrote:
| 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.
| RachelF wrote:
| 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.
| hinkley wrote:
| 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.
| jabl wrote:
| Well, per wikipedia: "On September 28, 2023, Capstone Green
| Energy declared Chapter 11 bankruptcy"
| gtirloni wrote:
| Aren't these engine designs patented very heavily? How were
| clones popping up less than a decade later?
| sitharus wrote:
| 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 :)
| ggm wrote:
| 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.
| wyager wrote:
| 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!
| eternauta3k wrote:
| 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.
| adrian_b wrote:
| 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.)
| divbzero wrote:
| Related:
|
| _See Thru Jet Engine [video]_ -
| https://news.ycombinator.com/item?id=32145297 - July 2022 (70
| comments)
| I_dream_of_Geni wrote:
| Funny, if you mouse over the graph of transistor costs, they
| become free in 2005! Cool!
| moffkalast wrote:
| Transistor manufacturers are like, "hey free transistors" then
| they bill the entire cost as shipping fees like the average
| aliexpress store.
| drysine wrote:
| >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?
| D_Alex wrote:
| It is part of UAC, along with Ilyushin and Yakovlev.
| drysine wrote:
| Still not in the list)
| D_Alex wrote:
| 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.
| drysine wrote:
| 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...
| D_Alex wrote:
| 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.
| notahacker wrote:
| 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.
| D_Alex wrote:
| 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.
| notahacker wrote:
| 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.
| drysine wrote:
| >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.
| notahacker wrote:
| 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
| nradov wrote:
| 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.
| kragen wrote:
| 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.
| notahacker wrote:
| 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
| kragen wrote:
| 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.
| notahacker wrote:
| 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.
| throwaind29k wrote:
| They have the MC-21 under development as well, though not
| much information seems available.
| drysine wrote:
| 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.
| Cyph0n wrote:
| Bombardier until recently was another, although it was taken
| over (?) by Airbus.
| dredmorbius wrote:
| 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.
| Cyph0n wrote:
| 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.
| dredmorbius wrote:
| I like your take, FWIW.
| ianburrell wrote:
| 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.
| drysine wrote:
| 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.
| mppm wrote:
| 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 :)
| eutectic wrote:
| Silicon carbide fiber reinforced silicon carbide is also being
| increasingly used.
| mppm wrote:
| In production?
| credit_guy wrote:
| 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.
|
| [1] https://www.ornl.gov/news/ceramic-matrix-composites-
| take-fli...
| Invictus0 wrote:
| This is why I love HN
| hnax wrote:
| Me too!
| pfdietz wrote:
| 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.
| adrian_b wrote:
| 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.
| anubiskhan wrote:
| Hell yeah something new to learn about today, thank you.
| osigurdson wrote:
| 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).
| gameshot911 wrote:
| 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?
| motorest wrote:
| > 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.
| aunty_helen wrote:
| 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.
| neuralRiot wrote:
| 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.
| neuralRiot wrote:
| >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.
| evnix wrote:
| 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.
| dguest wrote:
| 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...
| betimsl wrote:
| Material tolerances.
| alkonaut wrote:
| > 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.
| varjag wrote:
| Rolls Royce is British.
| venv wrote:
| And PBS is Czech, to name one.
| 6SixTy wrote:
| 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.
| 1024core wrote:
| How many countries make their own jet engines? US, UK, France ...
| anyone else?
| philwelch wrote:
| Russia and China
| themgt wrote:
| 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
| t43562 wrote:
| The German part of Rolls Royce - that's where the new B-52
| engines are coming from for example.
| paulpauper wrote:
| 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.
| Animats wrote:
| 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
| hnuser123456 wrote:
| 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.
| geocrasher wrote:
| 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.
| Manuel_D wrote:
| I thought adding a shroud to propellers increased
| efficiency? That's why we use turbofan engines instead of
| turboprops.
| jabl wrote:
| 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.
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