The development of the B-41 began in 1955 with a USAF requirement for a Class B 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 while still in the design stage.
Composition
The B-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, and Y2, a "clean" version with a lead-encased tertiary. It was the highest-yield nuclear weapon ever fielded by the United States, with a maximum yield of 25 megatons, and weight of. 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, almost doubling the yield-to-weight ratio of the B-41. The B-41 was of the usual long cylindrical shape. The nuclear fusion warhead was of the Teller-Ulam type and used a 40–100 kiloton implosion typenuclear fission primary fueled by HEU to trigger the lithium-6 deuteride fusion fuel. The Y1 version, the third stage was enclosed in a uranium tamper. The B-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.
The B-41 entered service in 1961. About 500 of these weapons were manufactured between September 1960 and June 1962. The B-41 was progressively phased out of service beginning in 1963, superseded by the B53 nuclear bomb. The last B-41s were retired in July 1976.
Efficiency
During its operational lifetime, the B-41 was the most efficient known thermonuclear weapon in terms of yield to actual weight, with a 5.2 Megaton/tonne ratio. 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" or dirty 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 B-41 is the most efficient, highest yield to weight ratio, weapon ever created. However, since neither full yield versions of the B-41 nor "Tsar Bomba" were ever demonstrably tested, the B-41's high efficiency is merely theoretical; the most efficient tested and proven nuclear physics package is the W56.
