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 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, with the Mod-4 variant weighing 680 pounds. 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 4.96kt 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.