[rbanffy]

> Do you think todays aircraft are not designed with the idea that the engine can fail?

Of course they are - but the engines are also designed to be extremely reliable, and that's why you get away with two engines on long flights over water, something previously only available for planes with four engines.

> No - in effect building jet-engines (that are commercially viable i.e. fuel and efficeny) is not a easy to disrupt business.

That's true. My point being that building a better jet engine might be the hardest way to disrupt the business - making a better propulsion system, which might or not include a jet engine, is a less difficult approach. If you have an electric plane with two motors, a big APU-like turbine charging a battery and powering the motors, you might get away with a cheaper turbine, running in less extreme regimes, and still have a more fuel efficient plane requiring less maintenance than a pure turboprop would.

You don't win a stacked game by playing it by the rules. You win by changing the game to another one you can actually win. China did that with cars already.

[coderenegade]

Aerospace as a discipline has tried just about every propulsion system under the sun. What you're proposing has already been flight tested on an unmanned vehicle, albeit with many smaller props for a larger effective prop disk than the two you're proposing. This is actually better, because electric motors want to use rpm control, so you need to keep the moment of inertia of the propellers low. The efficiency penalty of smaller props is overcome by having many of them arranged closely together to create a large effective prop disk.

For a hybrid system to be worth it, you need to claw back more efficiency than you lose in going from mechanical energy to electrical energy, and then back again. For cars, this is generally the case, because they're always accelerating and decelerating. Their wide operating band means that the engine will always be a game of compromises, which is why sticking a motor and battery in the loop and decoupling the engine from the wheels is beneficial. But planes aren't like that; they go from setpoint to setpoint, and they stay in a given configuration for long periods of time. They have very narrow, highly optimized operating bands, so hybridization just isn't as effective.

[rbanffy]

> For a hybrid system to be worth it, you need to claw back more efficiency than you lose in going from mechanical energy to electrical energy, and then back again.

For short-haul you spend a lot of time climbing and descending in relation to long flights, so the plane spends less time at the optimum the engines were designed for (cruise), so, if the power unit can be at the peak efficiency throughout the flight, with extra energy being supplied by the onboard battery for take-off and the power unit be shut down for descent, we might get to a point where it's economically viable, depending on battery operating costs and weight.

You are right to point out the aerospace industry has tried everything conceivable to see what sticks, but technology evolution sometimes throws us a curveball.

[coderenegade]

It's unlikely; the energy density just isn't there, even for a hybrid system. Because of the way flight profiles work, you're still talking about enough battery to provide meaningful power for half a flight, which probably isn't feasible in the near term.

And it's not like the industry has missed this possibility. I've seen papers on systems as exotic as combining high temperature fuel cells (SOFCs, PCFCs) with the Brayton cycle to achieve a hybrid powerplant. The bar for novel propulsion systems in aerospace is extraordinarily high, higher (imo) than it is for automotive. The exception is unmanned systems, which are new enough and varied enough that there's been an explosion of activity, which has been exciting to see.

[londons_explore]

> If you have an electric plane with two motors, a big APU-like turbine charging a battery and powering the motors, you might get away with a cheaper turbine, running in less extreme regimes, and still have a more fuel efficient plane r

This works for hybrid cars because the power demand when driving varies widely, as does the output speed. The 'hybrid' bit gets huge gains by always running the engine at optimal rpm+throttle, and using the battery to cover peaks and absorb troughs (regen).

A plane cruises for many hours at a fairly constant speed and throttle, which is designed to hit peak efficiency of all components. The 'hybrid' design therefore falls far behind on weight, efficiency and cost.

[rbanffy]

> A plane cruises for many hours

This is why short-haul flight will be the first to adopt hybrid propulsion.

[londons_explore]

There are already some trials with very short flights - ie. Sub 25 miles.

Used for getting between islands for example

[nradov]

That's correct to an extent. It could never work for long haul flights due to the weight penalty but there is potential for regional flights. Current airliner turbine engines are sized to deliver enough thrust to safely take off and climb even if one engine fails. In theory the necessary excess thrust could be delivered by a hybrid system during critical phases of flight. We might see that for flights like SFO to LAX in a few decades.