67A. This submarine, designated SSX, would have one of three propulsion plants under development: a Walther open cycle hydrogen peroxide plant (termed Alton), a liquid oxygen steam plant (Ellis), and an AIP gas turbine (Wolverine). By late 1951 the Navy realized that while the competing nuclear designs were heavier due to shielding, they were more compact than the three AIP plants: the SSX would be longer than the SSN by nearly 40 feet. The SSN would likely be quieter and less complicated than the AIP technology of this time. By 1952 the nuclear reactors were so far along in development it appeared that the SSX submarine would not be needed as a stopgap. The project was cancelled on 26 October 1953. The USSR and the UK, the only other countries known to be experimenting with the technology at that time, also abandoned it when the US developed the nuclear reactor small enough for submarine propulsion. Other nations, including Germany and Sweden, would later recommence AIP development. It was retained for propelling torpedoes by the British and the Soviet Union, although hastily abandoned by the former following the tragedy. Both this and the loss of the were due to accidents involving hydrogen peroxide propelled torpedoes. Closed-cycle diesel engines This technology uses a submarine diesel engine which can be operated conventionally on the surface, but which can also be provided with oxidant, usually stored as liquid oxygen, when submerged. Since the metal of an engine would burn in pure oxygen, the oxygen is usually diluted with recycled exhaust gas. Argon replaces exhaust gas when the engine is started. In the late 1930s the Soviet Union experimented with closed-cycle engines, and a number of small M-class vessels were built using the REDO system, but none were completed before the German invasion in 1941. During World War II the German Kriegsmarine experimented with such a system as an alternative to the Walter peroxide system, designing variants of their Type XVII U-boat and their Type XXVIIB Seehund midget submarine, the Type XVIIK and Type XXVIIK respectively, though neither was completed before the war's end. After the war the USSR developed the small 650-ton submarine, of which thirty were built between 1953 and 1956. These had three diesel engines—two were conventional and one was closed cycle using liquid oxygen. In the Soviet system, called a "single propulsion system", oxygen was added after the exhaust gases had