Was thinking about this, have a 2 engine regular turboprop with a third electric engine for extra performance (imagine a DC3 like the Conroy Tri-Turbo-Three). The 3rd electric engine would be run from the turboprops' alternator/generator and solar, with minimal batteries for emergency use.

An alternate version has the 3rd engine run by steam, the turbine engines produce the exhaust heat to power it, and cold upper air temps will cool the steam quickly.

In either case the 3rd engine does not have to be as powerful, it's just there for performance/emergency. Does either sound feasible?

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    $\begingroup$ Weight weight weight. $\endgroup$ – user3528438 Sep 15 at 20:24
  • $\begingroup$ upper air is low density air. Cooling your steam is everything but easy. But if the flight Mach number is high enough, those coolers could help to produce thrust. $\endgroup$ – Peter Kämpf Sep 15 at 20:37
  • $\begingroup$ The thermal efficiency of steam power is not great. And if you're going to power an electric motor from a turboshaft powered generator, you have losses from the conversion and you might as well run the prop directly from the turboshaft. Then when you get to batteries, the energy density is only about 1/3rd of fuel, in terms of power per lb of overall weight. In the end, it's all too heavy. $\endgroup$ – John K Sep 15 at 21:29
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    $\begingroup$ @user3528438: And thermodynamics. As in if you use the hot exhaust from the turbines to produce steam, they are not producing thrust. Also, except for special cases (like surveillance aircraft that need a long loiter time) you are running the engines at something like 80% of full power at cruise. Hybrid cars work because cruise is 20% or less of full throttle, so there's lots of room to downsize the engine. $\endgroup$ – jamesqf Sep 16 at 5:23
  • $\begingroup$ @JohnK 1/3rd? I remember hearing something like 1/50 $\endgroup$ – Abdullah yesterday

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