Dr. Thiel's development of the 25 ton rocket motor relied on pump feeding, rather than on the earlier pressure feeding. The motor used centrifugal injection, while using both regenerative cooling and film cooling. Film cooling admitted alcohol into the combustion chamber and exhaust nozzle under slight pressure through four rings of small perforations. The mushroom-shaped injection head was removed from the combustion chamber to a mixing chamber, the combustion chamber was made more spherical while being shortened from 6 to 1 foot in length, and the connection to the nozzle was made cone shaped. The resultant 1.5 ton chamber operated at a combustion pressure of 220 pounds per square inch. Thiel's 1.5 ton chamber was then scaled up to a 4.5 ton motor by arranging three injection heads above the combustion chamber. By 1939, eighteen injection heads in two concentric circles at the head of the 0.12-inch thick sheet-steel chamber, were used to make the 25 ton motor.