B28 (nuclear weapon)

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A thermonuclear bomb that had one of the longest service lives, 33 years (1958-1991), in the U.S. inventory, 4,500 of the B28 were built in 20 variants. Weighing approximately 1 ton, its yield ranged from from 70 Kt to 1.45 Mt. It was replaced by the B61 and B83 bombs.

The implosion system used Cyclotol as the fast explosive.


The packaging was extremely modular, with the physics package contained variously in streamlined or parachute-delivered cases. Bomb versions, which could be set for airburst or impact, included:

  • B28EX--Carried externally by F-100 Super Sabre, F-105, and F-4 Phantom II; no parachute.
  • B28RE--Carried externally; equipped with one 4-ft. pilot chute and one 28 ft. ribbon chute.
  • B28IN--Carried internally by B-52 and F-105; no parachute.
  • B28RI--Carried internally; equipped with one 4-ft. pilot parachute, one 16.5-ft. ribbon extraction chute, one 64-ft. solid chute, and one 30-in. stabilization chute.
  • B28FI--Carried internally; equipped with one 4-ft. pilot chute, one 16-ft. and one 24-ft. chute.

In addition to the bomb versions, the W28 variant armed the Warhead for the AGM-28 Hound Dog air-to-surface missile and MGM-13 Mace ground-launched cruise missiles. The British Red Beard was a derivative.

Safety features

Over the B28 life cycle, different bomb versions had Category A, B and D Permissive Action Links (PAL). Based on serious accidents at Thule, Greenland, and Palomares, Spain, with contamination but no nuclear yield, it is apparent no insensitive high explosives were used. Early in development, a 1-point safety problem with the primary stopped initial production, but this was presumably fixed by these 1966 and 1968 incidents. The warhead versions had Category A and B PALs.