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Comment by spaceheretostay | Hacker News Reader

These are small, low-altitude, low-capacity planes - because this is a hard problem and you have to start small.

I wonder though - does Bye have ambitions in creating larger ones that fly at higher altitudes? I've thought about this ever since Elon speculated that high-altitude electric planes could be insanely efficient: they don't require the intake that normal jet engines do, so they can fly in thinner air and thus require less power to overcome air resistance.

The main problem being battery density, IIRC. I wonder how that has changed since the ideas were floated some years ago.

I want the super-fast, high-flying, supersonic electric planes please! That could do more to alleviate greenhouse gas emissions than even electric cars might!

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Battery energy density hasn't seen any large improvements in a few years. High-flying, long-range commercial passenger aircraft are probably not coming very soon. An aircraft like the Bye eFlyer could help trigger a resurgence in general aviation, which has been on a decline, I suspect in part due to costs. I expect the fuel and maintenance costs for the eFlyer will be substantially lower than those of aircraft typically used for initial training.

The relatively short endurance and range of the first model makes it most suitable as a trainer, but Tesla's first car wasn't what most people would want to use for a long road trip. Refinement of present-day technology might well result in an electric airplane in which four people could reasonably take cross-country trips.

The biggest costs are liability, insurance, and certification of the type. The hardware is the cheapest part of it.

The part about air intakes doesn't make any sense. In order to move the airplane forward it has to suck air in at the front and accelerate it out the back. That's just basic physics, regardless of whether the power comes from batteries or liquid fuel.

(Obviously rocket powered airplanes don't need air intakes but that's not what we're discussing here.)

I presume whatever Musk said was referring to something legitimate, but to put a finer point on your comment: in normal jet engines, only a fraction of air actually goes into the combustion chamber. The rest is just pushed out the back (faster than it came in) by turbine blades that are driven by that combustion. Indeed, modern jet engines are "high by-pass" which I think means that the large majority (>80%) of air isn't part of the combustion.

Performance of internal combustion engines varies greatly depending on altitude due to thinner air. This is even worse on hot days. That's not the case with an electric motor. As you get higher, you need to push more air, but since the air is thinner, it's easier to push. Most likely and electric motor with a variable pitch propeller would be extremely efficient. Electric planes would get all the benefits of high altitude flight (namely thin air means less friction) and none of the downsides.

As someone else mentioned, if the batteries get too cold you run into other issues.

I think the point is that combustion engines require the thicker atmosphere to oxidize the fuel in combustion. The electric planes just need inertial reaction, they don't need oxygen.

No that's not how it works. Turbine engines can and have been designed to run fine at very high altitudes (>100000ft). So electric propulsion holds no particular advantage there.

> I want the super-fast, high-flying, supersonic electric planes please!

Me too, but it's pretty chilly at >35,000 feet up, where most commercial jets fly. Batteries really don't like the cold.

-60°F @ 40k per the in-flight info on a recent flight.

Wind and temperature aloft forecast includes the note: "TEMPS NEG ABOVE 24000" and omit the minus sign.

With normal lapse rates (3.6℉/1000 feet), you're gonna need a heater in any general aviation airplane.

Single engine airplanes tend to use exhaust heat to heat outside air as a heater, so no fuel consumed. For airplanes that use a combustion heater, fuel use needs to be considered in flight planning. Therefore I'd expect the same for electric powered airplanes.

What about solid state batteries? (which will likely start taking over from Li-Ion within a decade)

It's not very difficult to put batteries in a heat controlled chamber. A lot of RC guys are doing it with high altitude long range RC planes. Distances over 500km at 15km are already happening on 10kg planes.

cool! you got a link?

Another problem with aiming for very high altitudes is that you need to have several hour long flights to be able to reach those altitudes and really gain anything.

A typical climb/descent rate for a passenger jet is 1500 to 2000 feet per minute. So for a normal flight, going up and down to 35k will take close to 40 minutes. Going to 55k will take an hour.

So this means you can't start small and focus on cityhoppers with the fancy high-flying electric planes, you have to go straight for competing with the heavy jets.

You also gotta spend more energy going up higher, though. If you take the idea to its conclusion you just end up with the solution of taking a rocket from China to SF.

Yeah, I guess that's where Elon ended up! SpaceX will do it in the extreme then, without batteries and with just rocket engines.

That's what JATO is for. Just strap some rockets under the wings to get you up to altitude ;)

https://youtu.be/97rSobuKBxI?t=27

it's interesting how energy density requirements might above atmosphere flying a feature ..

A propeller works by pushing off of the air. A jet works by exploding fuel. It's not clear how you'll get super-fast in thin air without the exploding fuel part?

That’s not true. You’re describing a rocket, not a turbofan or even a turbojet. Fundamentally, modern high bypass turbofans are just shrouded propellers. Could reach supersonic speeds in the same way using electric.

Lift to drag is about half as good at low supersonic speeds as transonic, so cut your range in half, but fundamentally there’s no reason you can’t achieve supersonic electric flight. Just need good batteries to get that range...

I thought that most commercial jet engines derived most of their thrust from the turbo-fan pushing air through the engine, not direct thrust from the fuel combustion?

Ion thrusters? Not sure if they're at all practical yet, but that seems like a decent solution, especially if you can ionize the air you're flying through.

Air-breathing ion thrusters for aircraft are not at all practical yet. The first demonstration flight of a self-contained unmanned non-payload-carrying aircraft using one took place last year.

Well actually...modern jet engines tend to be high-bypass turbofans, with the PW1000G geared turbofan achieving a bypass ratio of 12.5:1.

In a turbofan, as the name suggests, the (vast) majority of thrust actually comes from the fan in front, and the turbine is really just used to turn the fan. That's what the bypass ratio means.

https://en.wikipedia.org/wiki/Turbofan#High-bypass_turbofan

With one or multiple ductfans in a row you can create enormous pressure differences and thrust. Its less efficient than a regular propellor, at least at low altitudes

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