[taneq]

> For the most part, it makes no sense to use electricity to power a heat engine.

Exactly. And modern aircraft engines generally just use the turbine to power a high bypass fan, at which point why pretend to be a heat engine when you could just spin the fan directly and save a tremendous amount of power?

[Retric]

Modern high bypass turbines provide a lot of power for the weight and extreme efficiency. Essentially, they are a gas turbine which then gets to extract extra energy from their exhaust gasses without adding a lot of weight or mechanical complexity.

There are a lot of trade offs involved, but for large 500+ MPH aircraft high bypass turbofans are simply the most cost effective option.

[taneq]

Yeah, exactly. Being able to stick the turbine in the middle of the ducted fan and then use its exhaust for extra thrust while directly driving the fan with the generated gases is the cherry on top.

If we were going to replace a high bypass turbofan with anything, though, I'd expect it to be a high power AC motor directly driving a ducted fan. Realistically for long haul air travel, though, my guess is we'll stick with turbofans and just use biodiesel or something.

[jacobush]

An idea I've had is to put a small generator instead of a shaft and gearbox, and put the AC motor powered fan somewhere else. Could improve aerodynamics, stealth properties etc.

[VBprogrammer]

A generator is necessarily on the same order of magnitude in size as the motors it is powering.

[jacobush]

Is your point that they must necessarily be heavy? Because I don't think so. Modern electric motors are very compact and powerful.

Also you'd not need gearbox and shafts. You can completely decouple the fan from the turbine. You could put the fan downwards and the turbine back for instance, or the turbine on the top to hide its heat signature somewhat.

[VBprogrammer]

Doing some back of the envelope calculations to see if we're in the right ball park here:

A Tesla motor weighs in at 70lbs (31kg) on its own, not including the inverter or any gearbox, and generates 362hp. Or 11hp / kg.

Using a turboprop because it's easier to do a straight hp comparison, the PW150 has a dry weight of 716.9kg and produces 5000hp continuous. Or about 7hp / kg.

Of course it's absolutely not a fair comparison for a number of reasons, for a start running a Tesla motor at full power for more than a few minutes at a time is going to result in an overheated motor where as the PT150 can do that all day long. The Tesla motor weight doesn't include the inverter which would be needed (unless it's all designed to run each motor at a synchronous speed with the generator...), nor the cooling system, a gearbox (the low pressure turbine of a high bypass engine is only ~4000rpm, the Tesla motor does 18000rpm). Not to mention the elephant in the room, you still have the weight of the generator plus the prime mover (or battery...).