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 MtonTNT. A top secret document (DCI Briefing to the JCS, 30 July 1963), states “The US has stockpiled bombs of 9 MT and 23 MT...” which would likely be referring to the B-41's actual yield(s). The B-41 was the only three-stage thermonuclear weapon fielded by the U.S.

History
In June 1955, the US Department of Defense requested a feasibility study for a Class B (over 10000 lb weight) bomb and warhead. By summer of 1956, US Air Force Strategic Air Command produced a requirement for a 62 inch Class B bomb, while the DoD produced a requirement for a 60 inch Class B warhead.

The University of California's Radiation Laboratory (now Lawrence Livermore National Laboratory) proposed the use of the existing Bassoon device that was test fired in the Zuni and Tewa shots of Operation Redwing. Stockpiling of this new weapon was planned for January 1959. Dirty and clean (low fission fraction) versions of the device were proposed, with the clean version being dependent on a nuclear test in Operation Hardtack I.

The nomenclature of TX-41 and XW-41 was assigned to the weapon in November 1956, and in December plans were made to conduct drop tests of the weapon from the B-47 bomber. A laydown version of the bomb was requested, however development of such a weapon would add 1 to 2 more years to its development. A non-laydown weapon was subsequently requested. The weapon's military characteristics were approved in February 1957.

The weapon was to be carried by the B-47, B-52, B-66 and systems 110A and 125A. The bomb would be able to withstand without damage the various flight stresses. Fuzing would include contact and air-burst modes, and would be selectable in flight. Contact fuzing would act as a backup for airburst fuzing and a parachute to slow the rate of fall would be developed. Compatibility with the Navaho missile and B-58 bomb pod was also requested.

By March 1957, it was decided to place equal emphasis on the clean and dirty versions of the weapon. By this point the weapon was to be 50 inch in diameter, with a warhead length of 120 inch and weight of 9300 lb, while the bomb was to have a length of 145 inch and weigh 10000 lb. Compatibility with the B-58 was canceled in May 1957, and the warhead version of the weapon was canceled in July 1957.

In July 1957, tests of the contact fuze were made by firing 75 mm shells through the nose of the bomb. These tests showed that there was sufficient time between contact and the firing signal being sent for the fuze design to work correctly. In August 1957, the primary of the device in a device mockup was tested during shot Smoky of Operation Plumbbob, yielding 44 ktTNT. The device was subsequently tested in shots Sycamore, Poplar and Pine of Hardtack I in 1958. Sycamore, a clean test, was a fizzle, producing only 92 ktTNT instead of the predicted 5 MtTNT. Poplar was a retest of Sycamore with a predicted yield of 5 to 10 MtTNT and only 200 ktTNT fission yield. The actual yield was 9.3 MtTNT. Pine was a three-stage variant of the clean device, with a predicted yield of 4 to 6 MtTNT, but the test only yielded 2 MtTNT.

In June 1958, the requirement to be able to select air and ground burst fuzing modes from the cockpit was canceled. This change meant that fuzing selection had to be made on the ground before takeoff. In August 1958, the first production date for the weapon slipped to May 1960. Issues with the weapon now meant that compatibility was limited to the B-47 and B-52 bombers. Compatibility with the B-70 would require significant changes to the aircraft. Further, due to the new emphasis on low level releases to avoid radar detection and due to the fact that the TX-41 could only be dropped from high altitude, the question of continuing the program was raised.

One item raised in support of continuing the program was that the 10000 lb bomb could replace the 17500 lb Mark 36 bomb. Another proposal was to delay the program and include a full-fuzing (FUFO) capability into the weapon that would allow for laydown delivery. However, in September 1958, the Radiation Laboratory and Sandia informed Field Command that to add FUFO to the weapon would require a completely new weapon, including nuclear testing.

In November 1958 it was decided that the weapon would always be deployed in the parachute retarded condition, and thus an option selector switch was no longer needed. In December, Sandia raised issues with the safety of the weapon and proposed additional safing devices. This was provided with additional switches in the aircraft monitor station. In January 1959, the 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 12 ft long, with a body diameter of 4 ft. It weighed 10,670 lb. 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 25 MtTNT yield, dirty version with a tertiary stage encased with U-238 (natural uranium); and the Mk-41Y2, a 10 MtTNT 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 5.2 MtonTNT/t ratio (based on a 25 MtonTNT yield). Its blast yield was 25% to 50% that of the AN602 Tsar Bomba, which delivered a blast of 50 or, depending on its own configuration as a clean or dirty bomb. However even at the Tsar Bomb's theoretical maximum yield of 100 MtonTNT, it would still only achieve a yield to weight ratio of ~ 3.7 MtonTNT/t, 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.