A steam rocket (also known as a hot water rocket) is a thermal rocket that uses water held in a pressure vessel at a high temperature, such that its saturated vapor pressure is significantly greater than ambient pressure. The water is allowed to escape as steam through a rocket nozzle to produce thrust.
Steam rockets are usually pressure fed, but more complex designs using solar energy or nuclear energy have been proposed. They are probably best known for their use in rocket-powered cars and motorcycles, and they are the type used in aeolipile.
Water, while under pressure, is heated up to a high temperature (approx. 250–500 °C). As the hot water goes through the nozzle (usually a de Laval nozzle) and the pressure reduces, the water flashes to steam pressing on the nozzle, and leaving at high speed. By the recoil the rocket accelerates in the opposite direction to the steam. The nozzle of hot water rockets must be able to withstand high pressure, high temperatures and the particularly corrosive nature of hot water.
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, 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. Although the performance is low, high mass fractions are easy to achieve, and water is expected to be very easy to extract and purify from ice deposits that are found around the solar system.