Engines
Steam is an external combustion engine therefore it requires a pressure vessel to contain the energy (in this case steam). This is bad for at least two reasons:
1. extra weight, and
2. safety hazard - carrying a super heated canister under pressure at altitude creates more bad possibilities than just finding somewhere to stick it.
Wankel rotarys (Mazda, Suzuki, Morton, etc) have reliability issues because all the combustion pressure is kept in check by two apex seals and some flimsy looking side rings. These also tend to twist (the front and back rotor housings) if they are put under stress. Ever wonder why there are not many rotary racing cars?
I can't see CASA approving any hydrogen powered device flying in the near (or any) future because of Hindenburg type events...i_dunno
Just my opinion...
Stanzahero