The water is allowed to escape as steam through a rocket nozzle to produce thrust.
The simplest design has a pressurised water tank where the water is heated before launch; however, this gives a very low exhaust velocity since the high latent heat of vaporization means that very little actual steam is produced and the exhaust consists mostly of water, or if high temperatures and pressures are used, then the tank is very heavy.
More complex designs can involve passing the water through pumps and heat exchangers and employing nuclear reactors or solar heating; it is estimated that these can give a specific impulse of over 195 s Isp,[2] which is still well below the standards of more complex designs, for example the 465 s of the hydrogen-oxygen Vinci engine.
Solar or nuclear heated steam thermal rockets have been proposed for use in interplanetary travel.
[7] Steam rockets have occasionally appeared in science fiction stories, especially steampunk.