Isn’t the speed of sound lower at higher altitudes?
So, if supersonic tip speed is the efficiency metric then flying higher gets an electric airplane little advantage (well…there is probably a trade-off…higher is better till a point and then it isn’t).
I do not think there is an electric turbine/jet (can’t imagine how that would work). They are all propeller planes.
A high-bypass turbofan engine is really just a turboprop with a specialized (ducted) propeller. You could use a ducted fan with an electric motor–it would even be more efficient due to the smaller cross-section.
I’m not sure what the tradeoff here is, exactly. I believe that unducted props have an efficiency advantage over ducted ones even at supersonic tip speeds, but that would still be dependent on a bunch of factors. A supersonic prop isn’t a complete dealbreaker if you aren’t using it on the ground and you use it in a pusher configuration.
Another possibility is to just increase the amount of prop area, and fold the “spare” props back in high-drag (low-altitude) situations.
There’s a huge unexplored design space here. The lessons learned for turbine engines no longer apply to electric. All of the tradeoffs are different.
IIRC the few planes which had props that had supersonic tips were crazy noisy. So noisy they affected the health of people near them (in a bad way). See Thunderscreech and Tupolev Tu-95.
Like I said, “if you aren’t using it on the ground and you use it in a pusher configuration”. If you can ensure the shockwaves don’t intersect a person, they should be tolerable. But yeah, the Thunderscreech was so loud it gave one of the engineers a seizure.
Sodium as a cooling medium looks great on paper. In real life, it sucks:
" Consider, for example, sodium cooled fast breeder reactors. France, the country most reliant on nuclear power, tried to commercialize this technology after operating pilot-scale and demonstration reactors. This “commercial” version was the Superphénix, which started operating in 1986, experienced a series of accidents, and was shut down in 1997. During this period, it generated less than 8 percent of the electrical energy of what it would have generated running at full power round-the-clock. In the United States, the first and only commercial sodium cooled breeder reactor, Fermi-1, suffered a disastrous meltdown in 1966 as a result of a series of failures that had been dismissed as not credible by reactor engineers.
It doesn’t have to be very good–the point is that it’s better than what you get with solid aluminum or copper, which is nothing (aside from the thermal conductivity). The primary advantage as an electrical conductor is that it’s extremely light, and though the conductivity for a given cross-section is low, it actually comes out ahead per unit mass, which is probably going to be the limiting factor for aircraft.
I dunno, maybe the difficult handling would push the needle back in the other direction, but so far I haven’t seen anyone try it (for aircraft).
Many issues with flying that high. For one thing, a propeller aircraft will necessarily be subsonic. At subsonic speed, you need a huge wing to maintain flight at that altitude. For example the Helios experimental high altitude electric plane made it to 90,000 ft. It did it with a wing with a 31:1 aspect ratio and a span of 247 ft. That’s bigger than a 747. This for an aircraft that only weighed 700 kg or so. The Helios was solar powered, and there was a total of 28 hp in the form of 14 2hp electric motors.
Getting something like a heavy airliner up to that altitude on electric power seems very unlikely. The size of wings you would need would make ground ops very difficult, and structurally it would be a huge challenge.
Oxygen is a big problem. The Concorde flew at 60,000 ft, and even at that altitude it had to be designed with small windows so that in the case of a blowout the cabin woildn’t decompress too quickly. Also, it had a complicated pressurized oxygen system for both the crew and passengers. In addition, it had to demonstrate a very rapid descent capability in the case of a pressurization failure. It could do that because it was a sleek supersonic jet.
At 80,000 ft, it would take a long time to get a slow flying, large winged aircraft down to 40,000 ft. And at those altitudes, full decompression would require pressure suits to survive. The bends would be likely for passengers and crew as well.
You can probably get around the supersonic propeller tip problem by using lots of propellers turning more slowly. But that airplane isn’t going to fly very fast.
The other problem with flying at high altitude is that half the time you are going to be fighting the jet stream. That’s not going to be easy in a slow aircraft.
this is what the Helios looked like before it broke up in flight:
https://commons.wikimedia.org/wiki/File:Helios_in_flight.jpg#/media/File:Helios_in_flight.jpg
I agree that there are… challenges 
.
Some of these things are based on constraints that need to be reevaluated. Does the plane need (passenger) windows at all? Cameras and screens are getting pretty good. Windows are a possible decompression failure point–without them, perhaps you can relax the descent requirements.
Helios had fairly extreme requirements in being totally solar powered. That’s a ways beyond what I’m imagining. These guys have taken a glider up to 76k feet, though:
And I can’t tell where they’re at now, but these people have a reasonably conventional-looking craft that they plan on flying at 80k feet (it has solar panels, but is mostly battery-electric):
In any case, clearly these are very far from commercial craft, but commercial craft wouldn’t be targeting this level of hyper-efficient flight, either. 80k feet also wasn’t meant as some absolute, just a kind of ballpark where it might be worth looking at.
Slow is a bad target for commercial craft, at any rate. The idea isn’t to be as efficient as possible, no matter the tradeoffs, but rather to use the advantages of electric motors to operate in a regime where they can be more efficient and thus have longer range.
This does not imply that an electric aircraft must be subsonic, though. It can achieve supersonic speeds the same way jet aircraft do: by compressing the air first, pumping that, and then letting it expand through a nozzle.
The compressed air is moving much more slowly than the airstream, and is easier to pump due to its density. It can use a multi-stage compressor for better efficiency.
I’m not sure about the losses here. They might be high, but I’m not sure which are “laws of physics” losses vs. engineering difficulties. The compressed air will be hot, but most of that heat should be recoverable once expanded.