Hot water or steam rocket development and production cost is estimated to be about 95% less than the cost of developing chemical propellant rockets. Hot water rockets can be reused a great many times, making them even more economical. Operating temperatures are low and no insulation is needed for engine walls. Albeit their modest specific impulse, which is in the order of 40-50 sec, hot water rockets have many practical uses, particularly as booster rockets for large payloads. A dimensionless method is described by which hot water rockets of any size can be designed in a few hours.