Could a hybrid electric steam plane be more efficient at altitude than at sea-level, and how to optimise it to frigid air? In the thirties a steam powered plane was built.
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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.


The steam engine can be down sized to cruise power levels.
The radiator is being cooled by frigid stratospheric air &lt; -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.'
Forget the electric stage - when using steam, you will most likely already have mechanical energy and when ducting that steam through a turbine, even rotational energy to turn your propellers. What advantage do you expect from converting that rotational to electrical and directly again to rotational energy?

That design could be simplified even more: Instead of heating water, the fuel could heat compressed air and let that run through a turbine. That is how any modern jet engine works.

In order to keep mass down, the steam would need to be condensed in a cooling circuit. While the wing surface could be used for this, the required piping would add a substantial weight penalty. In an open circuit where the relaxed steam is simply dumped overboard (like on the Besler biplane or most steam engines), range would be rather limited.

There was to my knowledge a single attempt at building a steam-powered airplane with steam condensation. This was a version of the Messerschmitt 264 bomber which was supposed to run on a mixture of coal powder and heavy oil instead of gasoline. It was never completed.