The W55 was an American thermonuclear warhead used as the warhead for the SUBROC antisubmarine rocket system. The W55 was designed in the early-1960s, and produced from 1964 to 1968. The last units were retired from service in 1990. A total of 285 W55 weapons were produced. The W55 is 35 cm (13.5 inches) in diameter, 1 metre (39.4 inches) long, and weighs 213 kg (470 pounds). Researcher Chuck Hansen claims based on his US nuclear program research that the W55 and W58 warheads shared a common Primary or fission first stage, and that this design was nicknamed the Kinglet primary by Hansen in 2001