It has been said that a rocket motor is the simplest “ prime mover.” This is correct in theory, since fundamentally a rocket consists of a chamber containing a gas under pressure, having a nozzle at one end through which the gas expands, forming a propulsive jet. Since the pressure ratio of the rocket is always greater than the critical pressure ratio, the nozzle is invariably of the convergent-divergent or de Laval type in which the velocity is subsonic up to the throat, sonic at the throat and supersonic in the divergent portion.

It is a popular fallacy that a rocket or jet engine obtains a forward thrust by allowing the rearward jet of gas to push against the atmosphere in which it is operating. This is not correct and it can be proved that a rocket will operate successfully, and more efficiently, in a vacuum.