W56

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

The W56 was manufactured in a series of models, all the same rough size and 1.2 megaton yield. The Mod-1, Mod-2, and Mod-3 variants weighed 600 pounds, with the Mod-4 variant weighing 680 pounds. All versions were 17.4 in diameter by 47.3 in long.

The W56 demonstrated a yield-to-weight ratio of 4.96kt per kg of device weight, and very close to the predicted 5.1kt/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, 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. One warhead nearly experienced a high-explosive detonation with no nuclear yield in 2005 because an unsafe amount of pressure was applied while it was being disassembled