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The W78 thermonuclear warhead is the warhead used on most of the United States LGM-30G Minuteman III intercontinental ballistic missiles (ICBMs), along with the MK-12A reentry vehicle which carried the warhead. Minuteman III's initially deployed with the older W62 warhead; the W78 was deployed starting in December 1979 onto 300 missiles, three warheads per missile. Declassified records indicate a total of 1,083 W78s were produced.
The W78 was designed at Los Alamos National Laboratory starting in 1974. The design is thought to combine the secondary (fusion) stage design of older ICBM warheads such as the W50 with a more modern primary stage (see Teller-Ulam design for more details).
Dimensions of the W78 are unknown, but it fits within the MK-12A reentry vehicle, which is conically shaped, 21.3 inches in diameter at its base and 71.3 inches long. The W78 is estimated to weigh 700-800 pounds.
The W78 does not use insensitive high explosives, such as TATB, which are used in many nuclear weapons because of their high resistance to accidental detonation due to shock or fire or impact. As a result, the W78 is considered a somewhat dangerous warhead, and cannot be transported by air unless there is no other safe option.