In the thirties a steam powered plane was built. video

Could a hybrid electric steam plane be more efficient at altitude than at sea-level, and how could such a steam engine be optimised to benefit from frigid air at altitude?

Perhaps there are two advantages to going hybrid.

1. The steam engine can be down sized to cruise power levels.
2. The radiator is being cooled by frigid stratospheric air < -40 deg so can be smaller.

This is a parallel hybrid, the electric motor drives the propeller until the plane is at altitude. Once at altitude the electric motor is disengaged and the small steam engine starts up and engages with the propeller.

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There is a replica of the Besler engine at the Smithsonian. It has more specs than the video. It had a condenser

Steam Car Developments and Steam Aviation, VOL. III. JUNE, 1934 NO. 28., The Besler Steam-Driven Aeroplane

'Under the fuselage nose is the condenser, which is simply a section of an ordinary petrol car radiator, and this is said to be sufficient to recover more than ninety per cent. of the water from the exhaust steam.'

'The tests have shown that ten gallons of water is sufficient for a flight of 400 miles.'