I. Introduction olid rocket motors (SRMs) are being used for a long time in underwater propulsion because of their simplicity, reliability and low price. End-burning grain configurations are common in underwater SRMs because of the long action times needed. The main problem encountered here is that the thrust level is limited by the available burning surface or actually by the motor diameter. Increasing motor diameter is no cure because drag force is increased with the square of the diameter.

To circumvent the problem of low performance of SRMs, much effort has gone to the development of high energy propellants, 1, 2 but there is a certain limit to the performance of any propellant. There have also been extensive studies on drag reduction policies. 3, 4 Nevertheless, finding a way to increase performance independent of propellant characteristics would be of great interest, especially if no considerable extra weight is to be carried with the vehicle to achieve this goal. One way to put this idea to work in an underwater missile is to allow a limited volume of water into combustion chamber, changing it into vapor therein and expelling it through nozzle throat at high velocities together with combustion products. The incoming water could be pressurized naturally by the dynamic pressure of the moving missile.