Here is a steam plane from 1934:
Besler Steam Plane - On Youtube
Hybrid Electric Planes are being developed currently. Cruising at altitude requires less power than climbing to altitude. By letting electric motors and batteries power a plane to altitude, combustion engines can be made lighter and optimised to be more efficient . The combustion engine only needs to provide the power for cruise.
This should apply just as much to a steam-engine as a diesel or gas-turbine. However, there is an additional advantage gained by the steam engine. The air in the stratosphere is very cold.
Information on Air temperature at altitude - on Quora
According to the above it is between -50degC and -60degC.
This means the radiator for the steam condenser has a larger temperature gradient across it. This would reduce the size of the condenser.
So, would a modern hybrid-electric steam-plane cruising at altitude on steam power be viable?
