Your quotes seem to very clearly support the idea its a an onboard heat source as claimed by gp. Specifically >>the mirror can reflect those small photons to r

Your quotes seem to very clearly support the idea its a an onboard heat source as claimed by gp. Specifically >>the mirror can reflect those small photons to reheat the thermal source I'm pretty sure holding a mirror up to the sun is not going to change the sun's temperature much. > I am seeing figures for steam turbine efficiency of between 40 and 80% There's more that goes into choosing an engine for a drone than just efficiency. Particularly important would be power/weight. A Carnot engine would get much higher efficiency for sure, but would not put out nearly enough power per weight for a drone to fly. (It's so low power we don't even use it for cars!)