Principle of Operation
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 vapourisation 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, still below the up to 465 s a modern hydrogen engines deliver (Vinci).
Read more about this topic: Steam Rocket
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