1. A JET PROPULSION DEVICE FOR OPERATION IN WATER, COMPRISING A DUCT HOUSING HAVING A FIRST WATER INLET OPENING AND A FIRST EXHAUST NOZZLE, A REACTION CHAMBER BETWEEN SAID INLET OPENING AND SAID FIRST EXHAUST OPENING AND ADJOINING SAID FIRST EXHAUST NOZZLE, WATER CONDUIT MEANS CONNECTING SAID WATER INLET OPENING AND SAID REACTION CHAMBER, A SOLID PROPELLANT CHARGE WITHIN SAID HOUSING SURROUNDING SAID CONDUIT MEANS AND ADJOINING SAID REACTION CHAMBER, AND A MEMBER MEANS HAVING A SECOND WATER INLET OPENING, A SECOND EXHAUST NOZZLE BEHIND SAID SECOND WATER INLET OPENING AND A DUCT CONNECTING SAID SECOND WATER INLET OPENING AND SAID SECOND EXHAUST NOZZLE, SAID MEMBER MEANS BEING POSITIONED DOWNSTREAM FROM SAID FIRST-MENTIONED EXHAUST NOZZLE, AND THE FORWARD END OF SAID MEMBER MEANS SURROUNDING THE REAR END OF THE FIRST-MENTIONED EXHAUST NOZZLE, THE REAR END OF THE CONDUIT MEANS ENTERING THE REACTION CHAMBER, A WATER SPRAYING MEANS ATTACHED TO SAID REAR END, AND A FLOW CONTROL MEANS WITHIN SAID CONDUIT MEANS TO CONTROL THE FLOW OF WATER TO A CONSTANT VALUE, WHEREBY WATER ENTERING THE REACTION CHAMBER FROM THE CONDUIT MEANS IS VAPORIZED BY THE HEAT OF REACTION OF THE PROPELLANT CHARGE, THEN PASSES THROUGH THE FIRST EXHAUST NOZZLE, AND THEN CONDENSES IN THE MEMBER MEANS, CAUSING WATER TO FLOW OUT THE SECOND EXHAUST NOZZLE TO PRODUCE THE PROPULSION.