previous decision to only use parachute retarded fuzing was reversed and it was asked to reinstate the fuzing selector. Pilot production of the weapon was authorised in April 1959 and full production authorised in September 1959. Early production of the Mark 41 Mod 0 was achieved in September 1960 and production continued until June 1962. Approximately 500 bombs were produced. The weapon was replaced by the more versatile B53 bomb between November 1963 and July 1976. Design The weapon was long, with a body diameter of . It weighed . It was carried only by the B-52 Stratofortress and B-47 Stratojet. It could be deployed in free-fall or retarded free-fall, and had both air burst and ground burst fuzing. The weapon did not have a laydown fuzing capability as the design of the physics package did not make that possible without extensive redesign and further nuclear testing. The B-41 was the only three-stage thermonuclear weapon fielded by the US. Two versions were deployed: The Mk-41Y1, a yield, dirty version with a tertiary stage encased with U-238 (natural uranium); and the Mk-41Y2, a yield, clean version with a lead-encased tertiary. It was the highest-yield nuclear weapon ever fielded by the United States, and had the highest publicly known yield-to-weight ratio of any weapon. Efficiency During its operational lifetime, the B-41 was the most efficient known thermonuclear weapon in terms of yield to actual weight, with a ratio (based on a yield). Its blast yield was to that of the AN602 Tsar Bomba, which delivered a blast of , depending on its own configuration as a clean or dirty bomb. However even at the Tsar Bomb's theoretical maximum yield of , it would still only achieve a yield to weight ratio of ~ , thus the B-41 has the highest yield to weight ratio of any weapon ever created. W41 warhead In November 1956, development of the W41, a warhead version of the B41, began at Lawrence Livermore National Laboratory. Investigated as a possible warhead for the SM-64 Navaho, a cruise missile then in development, work on the warhead continued through July 1957, when the project was canceled. See also List of nuclear weapons Nuclear weapon yield Notes References Cold War aerial bombs of the United States Nuclear bombs of the United States Military equipment introduced in the 1960s