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The B41 was a thermonuclear weapon deployed by the United States Strategic Air Command in the early 1960s. It was the most powerful nuclear bomb ever developed by the United States, with a maximum yield of 25 megatons. The Mk 41 was the only three-stage thermonuclear weapon fielded by the U.S.
The development of the Mk 41 began in 1955 with a USAF requirement for a Class B (high-yield, over 10,000 lb or 4,500 kg) weapon. It was based on the "Bassoon" test device first fired in the Redwing Zuni test of 27 May 1956. An ICBM warhead version of the weapon was cancelled in 1957.
The Mk 41 was the only three-stage thermonuclear weapon fielded by the U.S. It had a deuterium-tritium boosted primary, probably with lithium-6-enriched deuteride fuel for the fusion reaction in the secondary stage. This was followed by a yet-larger third fusion stage, the tertiary stage, compressed by the secondary stage. Finally, there was a fission jacket.
Two versions were deployed, Y1, a "dirty" version with a tertiary stage encased with U-238 (natural uranium), and Y2, a "clean" version with a lead-encased tertiary. It was the highest-yield nuclear weapon ever deployed by the United States, with a maximum yield of 25 megatons (Mt), and weighing in at 4,850 kg. It remains the highest yield-to-weight ratio of any weapon created. The US claimed in 1963 that it could produce a 35 Mt fusion bomb, and put it on a Titan II (3,700 kg payload), almost doubling the yield-to-weight ratio of the B-41.
The Mk 41 was of the usual long cylindrical shape and weighed 10,670 lb (4,840 kg). The nuclear fusion warhead was of the Teller-Ulam type and used a 40–100 kiloton implosion type nuclear fission primary fueled by HEU to trigger the lithium-6 deuteride fusion fuel. Between 500 and 1,000 kg of lithium deuteride was used and was contained in a cylinder of natural uranium (U-238) with an inner casing of U-235.
The Mk 41 was an example of a fission-fusion-fusion-fission type thermonuclear weapon, or tertiary stage bomb. The additional tertiary fusion stage, compressed by a previous fusion stage, could be used to make a bomb with yields as large as desired (see Tsar Bomba, a Soviet three-stage bomb and the highest-yield nuclear weapon ever built or tested).
The weapon was 12 ft 4 in (3.76 m) long, with a body diameter of 4 ft 4 in (1.32 m). It weighed 10,670 lb (4,850 kg). It was carried only by the B-52 Stratofortress and B-47 Stratojet. It could be deployed in free-fall or retarded (parachute) configuration, and could be set for airburst, groundburst, or laydown delivery.
The Mk 41 (designated B41 from 1968 on) entered service in 1961. About 500 of these weapons were manufactured between September 1960 and June 1962. The Mk 41 was progressively phased out of service from 1963 in favor of the B53 nuclear bomb. The last B41s were retired in July 1976.
During its operation, the Mk-41 was the most efficient known thermonuclear weapon in terms of yield to actual weight, with a 5.2 Megaton/tonne ratio (based on a 25 Mt yield). Its blast yield was 25 to 50% that of the AN602 Tsar Bomba, which delivered a blast of 50 or 100 megatons of TNT, depending on its own configuration as a "clean" (lead encased) or dirty (uranium encased) bomb. However even at the Tsar Bomba's theoretical maximum yield of 100 megatons, it would still only achieve a yield to weight ratio of ~ 3.7 Megaton/tonne, thus the Mk-41 is the most efficient, highest yield to weight ratio, weapon ever created.