B41 nuclear bomb

The B-41 (also known as Mk-41) 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 of TNT (100 petajoules). The B-41 was the only three-stage thermonuclear weapon fielded by the U.S.[1]

The casing of a B-41 thermonuclear bomb.
Mark 41 thermonuclear bomb casing at the National Museum of the United States Air Force.


The development of the B-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 while still in the design stage.


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 (natural uranium), 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 Mt (100 PJ), and weight of 4,850 kg (10,690 lb). It remains the highest yield-to-weight ratio of any weapon created. The US claimed in 1963 that it could produce a 35 Mt (150 PJ) fusion bomb, and put it on a Titan II (3,700 kg [8,200 lb] payload), 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 kt (170–420 TJ) implosion type nuclear fission primary (reportedly based on the Smokey TX-41 shot of Operation Plumbbob)[2] fueled by HEU to trigger the lithium-6 deuteride fusion fuel. The Y1 version, the third ("tertiary") stage was enclosed in a uranium tamper.[1]

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.

Physical characteristicsEdit

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 (4,840 kg). It was carried only by the B-52 Stratofortress and B-47 Stratojet. It could be deployed in free-fall or aerial (parachute) configuration, and could be set for airburst, groundburst, or laydown delivery.

Service lifeEdit

The B-41 (designated Mk-41 until 1968) 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.


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 megatons of TNT per tonne (22 petajoules per tonne) ratio (based on a 25 Mt (100 PJ) yield). Its blast yield was 25% to 50% that of the AN602 Tsar Bomba, which delivered a blast of 50 or 100 Mt (210 or 420 PJ), 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 Mt/t (420 PJ/t), it would still only achieve a yield to weight ratio of ~ 3.7 megatons of TNT per tonne (15 petajoules per tonne), thus the B-41 is the most efficient, highest yield to weight ratio, weapon ever created.[3][2] 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.

See alsoEdit


  1. ^ a b "The B-41 (Mk-41) Bomb" Nuclear Weapon Archive. (accessed April 8, 2015).
  2. ^ a b Carey Sublette, "Operation Plumbbob," Nuclear Weapon Archive, http://nuclearweaponarchive.org/Usa/Tests/Plumbob.html. (accessed December 27, 2006).
  3. ^ The B-41 was ...the most efficient bomb or warhead actually deployed by any country during the Cold War and afterwards. http://www.ieri.be/fr/publications/ierinews/2011/juillet/fission-fusion-and-staging.