The W54 (also known as the Mk54 or B54) was a tactical nuclear warhead developed by the United States in the late 1950s. The weapon is notable for being the smallest nuclear weapon in both size and yield to have entered US service. It was a compact implosive device containing plutonium-239 as its fissile material and in its various versions and mods had a yield of 10 to 1,000 tons of TNT (42 to 4,184 gigajoules).
The weapon had two distinct versions; a warhead used in the AIM-4 Falcon air-to-air missile and Davy Crockett recoilless gun, and another used in the Special Atomic Demolition Munition (SADM) system, along with several modifications ("mods") for each version. The two types are distinct in that much of the design between them was different, to the point that during the development of the SADM it was proposed that it be given its own unique mark designation.
Interest in a lightweight, low-yield weapon for the Falcon and Davy Crockett began in 1958. The weapon was initially developed by Lawrence Radiation Laboratory under the XW-51 designation, but in January 1959 the development of the weapon was transferred to Los Alamos National Laboratory and redesignated the XW-54.
For both the Falcon and Davy Crockett, the Department of Defense would supply the weapon systems and adaptation kits for the warheads, while the warheads and firing systems would be the responsibility of the Atomic Energy Commission. First production date for the Falcon warhead was planned for February 1961 while the warhead for the Davy Crockett was given a planned first production date of October 1961. This was because it was felt that the higher acceleration experienced by the warhead in Davy Crockett service would make certification of parts more challenging.
It was hoped during development that the same warhead for both applications could be used, but the requirements for the environmental sensing device - used to detect if the weapon was in its correct military environment and in turn disable weapon safing devices - for each application was quite different. In Falcon use, the warhead experienced 17 g (170 m/s2) of acceleration for 0.8 seconds while in Davy Crockett use the weapon experienced 1,800 to 2,500 g (18,000 to 25,000 m/s2) for 3 milliseconds. The envisioned device was one that would not actuate under less than 10 g (98 m/s2) or under less than 2 g per second (20 m/s2/s).
Sandia reported the problem in May 1959, stating that if Falcon warheads had the greater priority, an interim environmental sensing device could be developed. The Air Force subsequently accelerated the availability date for the Falcon missile and Sandia design released the warhead without an environmental sensing device in October 1959. However, revisions were issued in December before warhead production began with a suitable environmental sensing device for Falcon use.
A single environmental sensing device for both systems was abandoned at this time and development on the XW-54-X1 and XW-54-X2 for Davy Crockett use began. The XW-54-X2 warhead would lack any environmental sensing device for initial deployment, while the XW-54-X1 fitted with a suitable environmental sensing device would replace the XW-54-X2 as the weapon became available. Sandia were against the development of the XW-54-X2 as simply disconnecting the adaptation kit would disable the weapon's safeties.
The Department of Defense cancelled the requirement for the XW-54-X2 (now called the Mk 54 Mod 1) in July 1960 after parts manufacturing slippage delayed production of the warhead. This also caused the accelerated schedule for the Falcon application to slip to January 1961.
Production of both the Falcon warhead (now called the Mk 54 Mod 0) and the XW-54-X1 for the Davy Crockett (now called the Mk 54 Mod 2) was achieved in April 1961. Both the Mod 0 and Mod 2 weapons were interchangeable by changing the environmental sensing device. The final weapon was 10.862 inches (275.9 mm) in diameter, 15.716 inches (399.2 mm) in length and 50.9 pounds (23.1 kg) in weight and was packaged in a fiberglass housing coated in a conductive lacquer to provide an electrical shield.
Special Atomic Demolition Munition
Interest in the Special Atomic Demolition Munition (SADM) began in February 1958 when the Army desired a new munition that could be carried by one man. The project was delayed by the needs of the Falcon and Davy Crockett application until November 1959.
The proposal noted that the existing atomic demolition munition (ADM), the T-4 Atomic Demolition Munition, was based on the Mk-9 gun-type weapon and that transport required 4 men, each carrying a 40-pound (18 kg) section of the weapon. It was felt that the XW-54 SADM proposal could produce a weapon of 11.875 inches (301.6 mm) diameter, 17.5 inches (440 mm) long and a weight of 56 pounds (25 kg) including carrying case. The weapon would also have a waterproof housing, have a pack for carrying in the field and a shock-mitigating container for parachute delivery.
Development began June 1960. Because much of the weapon would be different from the XW-54 warhead, it was proposed that the warhead be giving its own unique mark designation, such as TX-58 (later used for the Polaris A-3 warhead), but the decision was made to retain the existing mark number.
The weapon was delayed until August 1963 due to issues with the timer. This included premature firing of the timer and issues with bearing materials. An interim Mk54 Mod 0 (now called the B54-0) weapon was put into production in April 1963. Drop tests for this weapon were conducted at velocities up to 31 feet per second (9.4 m/s) vertical and 17 feet per second (5.2 m/s) horizontal without damage.
Production of the B54 Mod 1 SADM began in August 1964. The weapon was 12 inches (305 mm) diameter, 18 inches (457 mm) long and weighed 58.5 pounds (26.5 kg), and included the warhead, fuzing and firing system with a mechanical timer, ferroelectric firing set and a sealed housing. The body was constructed with aluminium forgings and moulded fibreglass, and foam-rubber insulation was used between the warhead and case. Dials were illuminated with a tritium-phosphor paint for easy night-reading. A housing for underwater emplacement was provided which included external controls.
Configured in the Davy Crockett role, the weapon contained two sets of fuzes: a radar based fuze set for a 40 feet (12 m) airburst and a capacitance based fuze set for 2 feet (0.61 m) airburst. These fuzes represented the high and low airburst modes of the weapon.
Some sources give the yield for the Mod 0 as 250 tons of TNT (1,000 GJ) and the Mod 2 as 10 to 20 tons of TNT (42 to 84 GJ), but declassified warhead development documents indicate the only difference between these two warheads was the environmental sensing devices used and that the warheads were field convertible, suggesting the weapons had the same yield. Official documents give the yield as 20 tonnes of TNT (84 GJ) when configured in the XM388 round for the Davy Crockett.
It has been alleged that the British "Wee Gwen" warhead was a copy of the W54. Though never put into production, Wee Gwen was to contain 1.6 kilograms (3.5 lb) of plutonium and 2.42 kilograms (5.3 lb) uranium.
Special Atomic Demolition Munition
Yield is estimated to be 10 to 1,000 tons of TNT (42 to 4,184 GJ).
The B54 SADM included a Field Wire Remote Control System (FWRCS), a device that enabled the sending of safe/arm and firing signals to the weapon via a wire for safe remote detonation of the weapon by troops. This system was tested for its resistance to electromagnetic radiation in February 1964.
W54 mod numbers overlap between the warhead and SADM weapon. Weapons of the same mod number but in different applications are not the same weapons.
Three mods of the warhead configuration were developed:
W54 Mod 0 — Warhead for AIM-4 Falcon air-to-air missile
W54 Mod 1 — Interim warhead for Davy Crockett. Lacked environmental sensing device. Never entered production.
W54 Mod 2 — Production Davy Crockett warhead with environmental sensing device. Was known as the M-388 when configured into a Davy Crockett round.
Special Atomic Demolition Munition
Three mods of the SADM configuration were developed:
B54 Mod 0 — Interim weapon with timer issues.
B54 Mod 1 — Production weapon. Came with special housing for underwater use.
B54 Mod 2 — Weapon weight increased from 58.5 pounds (26.5 kg) to 70 pounds (32 kg). May be a boosted weapon.
In service, the weapons were known as the XM129 and XM159 Atomic Demolition Charges. Which versions are associated with the XM129 name and which is associated with the XM159 name is not clear.
After the AIM-26 Falcon was retired, 300 units were rebuilt into an improved configuration with a higher yield and redesignated the W72. These warheads were then used to produce a number of nuclear versions of the AGM-62 Walleye television-guided glide bomb system. The W72 variant had a yield of around 600 tons of TNT. The 300 W72 units were produced between 1970 and 1972 and were in service until 1979.
Stockpiled W54 warheads were test-fired at the Nevada Test Site on July 7 and July 17, 1962. In Little Feller II (July 7), the warhead was suspended 3 feet (0.91 m) above the ground. In Little Feller I (July 17), the warhead was launched as a Davy Crockett device from a stationary 155-millimeter launcher and set to detonate low airburst 1.7 miles (2.7 km) from the launch point. This test was the last atmospheric test at Nevada Test Site and was performed in conjunction with Operation Ivy Flats, a simulated military environment. It was observed by Attorney General Robert F. Kennedy and presidential adviser General Maxwell D. Taylor.
- Sublette, Carey (12 June 2020). "Complete List of All U.S. Nuclear Weapons". Retrieved 2021-03-24.
- History of the Mk 54 Weapon (Report). Sandia National Labs. February 1968.
- Safety Rules for Peacetime Operations with the DAVY CROCKETT/MK54 Mod 2 Atomic Weapon System (Report). Headquarters, Department of the Army. 1 November 1961. p. 137-153. MVS-020107-001.
- Brian, Burnell (15 April 2018). "Wee Gwen - British Army Requirement". Retrieved 7 June 2021.
Although there is no hard evidence to directly link Wee Gnat to Wee Gwen, the usual naming convention used by the British for engineered American designs supplied under the 1958 Bilateral Agreement, was to use a name beginning with the same letter of the alphabet, and Gwen and Wee Gwen follows that convention. The US and British warheads also share an identical diameter, an identical length, an identical weight, and identical yields.
- UK Atomic Energy Authority (1964). Weapons Department Atomic Warheads Production Committee, Papers & Minutes (Report). p. 63. TNA AB 16/4675.CS1 maint: uses authors parameter (link)
- Bartlett, J G (11 February 1964). Electromagnetic Radiation Susceptibility of the B54-0 (SADM) and the Field Wire Remote Control System (U) (Report). Sandia National Lab.
- Semiannual Historical Report, Activities for the Period 1 July 1969 - 31 December 1968, Vol I (PDF) (Report). Headquarters Field Command, Defense Atomic Support Agency. p. 260. DASA-69-0492.
Operational Review of the XM-129/XM-159 Atomic Demolition Charge (B54 SADM)
- Ivy Flats Film Report. United States Army. 17 July 1963. Retrieved 2021-03-24.