W56

The W56 was an American thermonuclear warhead produced starting in 1963 which saw service until 1993, on the Minuteman I and II ICBMs.

Technicians work on the W56 Mk-11 warhead of an LGM-30F Minuteman II intercontinental ballistic missile.

The W56 was manufactured in a series of models, all of which were approximately the same size and had a 1.2 megaton yield. The Mod-1, Mod-2, and Mod-3 variants weighed 600 pounds (270 kg), with the Mod-4 variant weighing 680 pounds (310 kg). All versions were 17.4 inches (440 mm) diameter by 47.3 inches (1,200 mm) long.

The W56 demonstrated a yield-to-weight ratio of 4.96 kt per kg of device weight, and very close to the predicted 5.1 kt per kg achievable in the highest yield to weight weapon ever built, the 25 megaton B41. However unlike the B41, which was never tested at full yield, the W56 demonstrated its efficiency in the XW-56X2 Bluestone shot of Operation Dominic in 1962,[1] thus from information available in the public domain, the W56 may hold the distinction of demonstrating the highest efficiency in a nuclear weapon to date.

The early models (Mod-1, Mod-2, and Mod-3) were produced starting in 1963 and retired in September 1966. The Mod-4 model was in production through 1969 and saw service until 1993. 1,000 total were produced, of which 455 were Mod-4.

The early models had to be retrofitted to fix a reliability problem, likely leading to their early retirement.

All retired W56 warheads were stored, to be finally dismantled by the end of June 2006.[2] One warhead which had high-performance but highly sensitive PBX in its explosive lenses is reported to nearly have detonated in 2005 when an unsafe amount of pressure was applied to the explosive while it was being disassembled.[3]