The W54 (also known as the Mark 54 or B54) was a tactical nuclear warhead developed by the United States in the late 1950s. The weapon is notable for being the smallest nuclear weapon in both weight and yield to have entered US service. It was a compact implosion device containing plutonium-239 as its fissile material, and in its various versions and mods it had a yield of . The weapon had two distinct versions: a warhead used in the AIM-26 Falcon air-to-air missile and in the Davy Crockett recoilless gun, and another used in the Special Atomic Demolition Munition (SADM) system, along with several mods (modifications) for each version. The two types are distinct in that much of the design between them was different, to the point that during the development of the SADM it was proposed that it be given its own unique mark designation. A later development was the W72, which was a rebuilt W54 used with the AGM-62 Walleye guided bomb. The W72 was in service until 1979. Development Warhead Interest in a lightweight, low-yield weapon for the Falcon and Davy Crockett began in 1958. The weapon was initially developed by the University of California Radiation Laboratory at Livermore (renamed Lawrence Radiation Laboratory from August 1958 until 1971) under the XW-51 designation, but in January 1959 the development of the weapon was transferred to Los Alamos National Laboratory and redesignated the XW-54. For both the Falcon and Davy Crockett, the Department of Defense would supply the weapon systems and adaptation kits for the warheads, while the warheads and firing systems would be the responsibility of the Atomic Energy Commission. First production date for the Falcon warhead was planned for February 1961 while the warhead for the Davy Crockett was given a planned first production date of October 1961. This was because it was felt that the higher acceleration experienced by the warhead in Davy Crockett service would make certification of parts more challenging. It was hoped during development that the same warhead for both applications could be used, but the requirements for the environmental sensing device - used to detect if the weapon was in its correct military environment and in turn disable weapon safing devices - for each application was quite different. In Falcon use, the warhead experienced of acceleration for 0.8 seconds while in Davy Crockett use the weapon experienced for 3 milliseconds. The