The W38 was an American thermonuclear warhead used in the early to mid-1960s as a warhead for Atlas E and F, and LGM-25 Titan I ICBMs. It was first built in 1961 and was in service from 1961 to 1965. 70 were deployed on Titan I missiles and 110 on Atlas missiles. It used the Avco Mark 4 reentry vehicle. The W38 was 32 inches (81 cm) in diameter and 82.5 inches (2 m) long. It weighed and had a design yield of 3.75 megatons with an airburst or contact fuze. The W38 was the first thermonuclear ICBM warhead developed by the UCRL (University of California Radiation Laboratory), which is now known as the Lawrence Livermore National Laboratory. The W38 was superseded by the Titan II missile family with a W53 warhead and 9 megaton payload. See also List of nuclear weapons References External links List of all US nuclear weapons Nuclear warheads of the United States Military equipment introduced in the 1960s