Combustion of O2 and H2 is too voilent. Hard to precisly contro the power required.
Advantages and disadvantages with steam cylinder
1. Easy to control flow rate according to needs
2. Will not heat the cylinder too much. Cooling is easier
3. The power is generated in a two-staged process (fuel to generate steam, then steam to generate thrust), both stages are not highly efficient, the total energy efficiency could be low.
Possible to use kerosene cylinder?
1. Kerosene combustion is a relatively slow process and easy to control the fuel delivery rate to generate the required thrust.
2. Air can be used to provide the oxygen for combusition (in normal or compressed form). No need to store or generate pure oxygen
3. Cylinder cooling might be a little more difficult because of the large amount heat is generated by the combustion. However, if erosion-proof is resolved, sea water is plenty and cools even better than fresh water.