A propulsion motor for an underwater vehicle such as an anti-submarine weapon. The motor includes a propulsion chamber into which water is admitted and then rapidly expelled through an exhaust nozzle, developing thrust to propel the vehicle. Gas generators are used to develop the successive hydropulses to expel the water following each filling of the motor chamber with water. In one particular embodiment of an anti-submarine weapon which is directed through the air to the vicinity of a submarine by a rocket motor, the hydropulse underwater propulsion system can use the same chamber as the rocket motor.