In the thirties a steam powered plane was built.
[video][1]
 
Could a hybrid electric steam plane be viable?

Perhaps there are three 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.
 3. The efficiency may be improved if a kalina or lower temperature condensing fluid is used that can exploit the frigid heat sink.

The electric engine is used to reach altitude, but the steam (or other rankine cycle) is used at altitude.

[edit]

[There is a replica of the Besler engine at the Smithsonian. It has more specs than the video. It had a condenser][2]

[Steam Car Developments
and Steam Aviation, VOL. III. JUNE, 1934 NO. 28., The Besler Steam-Driven Aeroplane][3]

'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.'




  [1]: https://youtu.be/nw6NFmcnW-8
  [2]: https://airandspace.si.edu/collection-objects/besler-steam-engine-reproduction
  [3]: http://www.flyingkettle.com/besler6.htm