Would a hybrid-electric steam-plane work?
Would Hybrid-Electric Steam-Planes be viable?
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?
I would imagine the two biggest hurtles are weight and startup time. Steam engines, especially turbines, with all of their supporting equipment can be heavy. So the power density is lower than ICEs, which is a problem when planes need to be light. I'm not particularly familiar with piston/cylinder steam engines like the one in the video, so maybe it would work.
If you wanted a higher efficiency by using a turbine though, there would also be startup time requirements, the larger the turbine the longer the wait. The reason is that the entire mass of the turbine has to equalize at operating temperature before it can be used. Hot metal expands, and uneven expansion can wreak havoc, because the seals have very tight clearances and imbalance in the blades can cause bad vibrations, or even mechanical failure.