The G7a was a gas/steam propelled system. Inside the torpedo was an internal combustion engine that used a fuel called DEKALIN ( decahydrophtalene ) as fuel and compressed air as an oxidant. The fuel and oxidant mixed which created steam to drive the engine which had a power output of 350bhp at 1,470 rpm. This torpedo had a 3 speed setting which had a direct effect on range,the faster you wanted the torpedo to go the less range you got. The 3 speeds were : 1) 44knts to 5,000meters, 2) 40knts at7,500 meters and 3) 30 knts at12,500 meters. At first this torpedo took 3,730 man hours to make and cost 24,000 RM , but as the torp. was redesigned as the war went along the time to make went down to 1,707 man hours and cost 13.500 RM. Yes this torpedo was called ATOS . So, yes this torp. did have compressed air but it did not really turn the propeller but steam from the break down of the fuel with this oxidizer did and it is this that you see exiting the rear of the torp..