'The main object of this invention is to impart to the fluid used as a motor for torpedoes, and the like, the heat needed to maintain it at such a temperature as will cause it to exert the best effect. This is affected by the employment of a liquefiable combustion supporting gas, compressed in a suitable reservoir, and discharged in gaseous form therefrom under pressure into the combustion chamber of the apparatus, so as to have contact with the ignited combustible therein. The gas best adapted for the purpose is nitrous oxide gas, which at 32 deg. Fahr. Liquefies at a pressure of fifty atmospheres, and at about 75 deg. gives a pressure of 1000 lb. to the square inch.