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B53 Nuclear BombThe B53 was one of the most powerful nuclear weapons built by the United States, and one of the last very high-yield thermonuclear bombs in U.S. service. Development of the weapon began in 1955 by Los Alamos Scientific Laboratory, based on the earlier Mk 21 and Mk 46 weapons. In March 1958 the Strategic Air Command issued a request for a new Class C (less than five tons, megaton-range) bomb to replace the earlier Mk 41. A revised version of the Mk 46 became the TX-53 in 1959. The development TX-53 warhead was apparently never tested, although a conceptually similar weapon was detonated 28 June 1958. The Mk 53 entered production in 1962 and was built through June 1965. A total of about 340 bombs were built. It entered service aboard B-47 Stratojet, B-52 Stratofortress, and B-58 Hustler bomber aircraft in the mid-1960s. From 1968 it was redesignated B53. The B53 was 3.81 m (12 ft 6 in) long with a diameter of 50 in (127 cm). It weighed 4,015 kg (8,850 lb), much of which was the parachute system (perhaps 400 kg / 882 lb) and the frangible aluminum nose cone to enable the bomb to survive laydown delivery. It had a total of five parachutes: one 1.52 m (5 ft) pilot chute, one 4.88 m (16 ft) extractor chutes, and three 14.64 m (48 ft) main chutes. Chute deployment depends on delivery mode, with the main chutes used only for laydown delivery (for free-fall delivery, the entire system was jettisoned). The warhead of the B53 uses oralloy (highly enriched uranium), not plutonium, for fission, with a mix of lithium-6 deuteride fuel for fusion. The explosive "lens" is a mixture of RDX and TNT, which is not insensitive. Two variants were made: the B53-Y1, a dirty weapon using a U-238-encased secondary, and the B53-Y2 "clean" version with a non-radioactive (lead or tungsten secondary casing. Explosive yield was a staggering nine megatons. The B53 was intended to be retired in the 1980s, reducing the stockpile to a total of 25 weapons by 1987. On 5 August 1987 SAC decided to halt the retirement and return 25 more weapons to service, for a total of 50. Those weapons are no longer in active service, but are retained as part of the "Hedge" portion of the Enduring Stockpile. The B53's service role has largely been replaced by the earth-penetrating B61 Mod 11. The W-53 warhead of the Titan II ICBM used the same physics package as the B53, albeit without the various air drop-specific components like the parachute system.
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