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Comment by jmward01 | Hacker News Reader

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.

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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).

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.

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)

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.

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)

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?

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.

thinner the air, harder it is to generate lift as well.

Coffin corner is a real thing.

Unless you're prepared to go supersonic. Not easy to do with electric propulsion though.

Scimitar props are pretty tame, though, or what part do you mean is hard?

Pretty sure for electric aircraft to do anything useful at supersonic speeds you'd need beamed energy. Which would be a pretty cool technology

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

> 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.

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.

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.

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