B61 nuclear bomb

The B61 nuclear bomb is the primary thermonuclear gravity bomb in the United States Enduring Stockpile following the end of the Cold War. It is a low to intermediate-yield strategic and tactical nuclear weapon featuring a two-stage radiation implosion design.[5]

B61
B61 training unit intended for ground crew. It accurately replicates the shape and size of a "live" B61 (together with its safety/arming mechanisms) but contains only inert materials
TypeNuclear bomb
Service history
Used byUnited States
Production history
DesignerLos Alamos National Laboratory
Designed1963
ManufacturerPantex Plant
Unit cost$28 million (Mod 12)[1]
Produced1968 (full production)
No. built3,155
Variants12
Specifications
Mass715 pounds (324 kg)[2]
Length141.6 inches (3.60 m)[2]
Diameter13.3 inches (34 cm)[2]
Blast yieldBelieved to be either 0.3–340 kt[3] or 0.3-400 kt[4] in the weapon's various mods.

The B61 is of the variable yield ("dial-a-yield" in informal military jargon) design with a yield of 0.3 to 340 kilotons in its various mods. It is a Full Fuzing Option (FUFO) weapon meaning it is equipped with the full range of fuzing and delivery options including air and ground burst fuzing, and free-fall, retarded free-fall and laydown delivery.[2] It has a streamlined casing capable of withstanding supersonic flight speeds, is 11 ft 8 in (3.56 m) long, with a diameter of about 13 inches (33 cm). Basic weight is about 700 pounds (320 kg), although the weights of individual weapons may vary depending on version and fuze/retardation configuration. As of 2020, it is undergoing a 12th modification. According to the Federation of American Scientists in 2012, the roughly 400 B61-12s will cost $28 million apiece.[1]

Development
A B61 bomb undergoing disassembly.
B61 bomb casing; MAPS Air Museum, North Canton, Ohio.

In 1961 a report was issued indicating Navy and Air Force interest in a lightweight bomb to replace existing weapons, but that both services had considerably different military requirements for such a weapon. However, the report believed that it was possible for a single weapon to fulfil both requirements. This was followed by a report from Sandia in mid-1962, that believed that a lightweight nuclear bomb with full fuzing option was possible and that such a program could be completed in a short time-span such as by 1965 or 1966.[2]

Development of the weapon that would become the B61 was authorised in December 1962. The justification for the program was that the new weapon would modernise the nuclear arsenal, improve the capability of aircraft and simplify the nuclear weapons inventory by replacing lower-yield versions of the B28 and B43 nuclear bombs. The desired production date was June 1965.[2]

The weapon was designated the TX-61 in January 1963. Due to the short time-scales, it was decided to make maximum use of off-the-shelf components such as those developed for the B57 nuclear bomb. Environmental conditions specified included an indefinite temperature range of −60 °F (−51 °C) to 160 °F (71 °C), shocks of up to 40g, and in flight temperatures of up to 275 °F (135 °C) for up to 40 minutes. Parachute deployment was specified to produce a maximum load of 255g.[2]

The weapon was designed and built by the Los Alamos National Laboratory in New Mexico. Production engineering began in 1965, with the first war-reserve B61-0 weapon acepted by the AEC in December 1966. However production was halted in May 1967 for design modifications to be made before resuming in January 1968.[2] Shot Flintlock Halfbeak in June 1966 may have been a full yield test of the weapon with other tests conducted between 1963 and 1968 at the Nevada Test Site.[6]

During testing, spin motors were added to the weapon to produce a spin of 5 revolutions per second. This was to improve stability and decoupling during high speed delivery. Three alternatives to the weapon were also explored for still-classified reasons. The first alternative was halted due to its similarity to the original TX-61 design, and the Department of Defence were unfavourable to the second option which halted its exploration. Though unclear in partially redacted documents, TX-61 design was ordered to "proceed along the lines" of the third alternative in May 1964, though it is unclear what that means.[2]

Total production of all versions was approximately 3,155, of which approximately 540 remain in active service, 415 in inactive service and 520 are awaiting dismantlement as of 2012.[7]

13 versions of the B61 have been designed, known as Mod 0 through Mod 12. Of these, nine have entered production. Each shares the same physics package, with different yield options. The newest variant is the Mod 11, deployed in 1997, which is a ground-penetrating bunker busting weapon. The Russian Continuity of Government facility at Kosvinsky Kamen, finished in early 1996, was designed to resist US earth-penetrating warheads and serves a similar role as the American Cheyenne Mountain Complex.[8][9] The timing of the Kosvinsky completion date is regarded as one explanation for U.S. interest in a new nuclear bunker buster and the declaration of the deployment of the Mod 11 in 1997: Kosvinsky is protected by about 1,000 feet (300 m) of granite.[10]

The B61 unguided bomb should not be confused with the MGM-1 Matador cruise missile, which was originally developed under the bomber designation B-61.

Deployment
B61 bomb components. The nuclear physics package is contained in the silver cylinder center-left

The B61 has been deployed by a variety of U.S. military aircraft. Aircraft cleared for its use have included the FB-111A, B-1 Lancer, B-2 Spirit, B-52 Stratofortress; F-101 Voodoo, F-100 D & F Super Sabre, F-104 Starfighter, F/A-18 Hornet, F-111 Aardvark and F-4 Phantom II fighter bombers; A-4 Skyhawk, A-6 Intruder and A-7 Corsair II attack aircraft; S-3 Viking antisubmarine aircraft, F-15E Strike Eagle and F-16 Falcon; British, German and Italian Panavia Tornado IDS aircraft. USAFE and all NATO dual role aircraft can carry B61s.

B61 tactical variants are deployed with NATO allies in Europe as part of the NATO Nuclear Weapons Sharing Program.[11] About 150 bombs are stored at six bases: Kleine Brogel in Belgium, Büchel Air Base in Germany, Aviano and Ghedi Air Base in Italy, Volkel Air Base in the Netherlands and Incirlik in Turkey.[12] NATO has agreed to vastly improve the capabilities of this force with the increased accuracy of the Mod 12 upgrade and the delivery of the stealthy F-35.[13][14] This will, for the first time, add a modest standoff capability to the B61.[15]

The B61 can fit inside the F-22 Raptor's weapons bays and will also be carried by the Lockheed Martin F-35 Lightning II.[16]

Design
Inert training version of a B61 in an underground Weapons Storage and Security System (WS3) vault at Volkel Air Base, Netherlands. An access panel on the warhead is open, showing the interface for actions such as PAL (safety/arming) and variable yield setting
Internal nuclear components of the B61 bomb. The bomb was assembled at the Burlington AEC Plant and Pantex.

The B61 is a variable yield ("dial-a-yield" colloquially) dual use tactical and strategic bomb equipped with Full Fuzing Option (FUFO)[2] designed for external carriage by high-speed aircraft. It has a streamlined casing capable of withstanding supersonic flight speeds. The original B61-0 weapon was 141.6 inches (3.60 m) long, with a diameter of 13.3 inches (340 mm) and a basic weight of 715 pounds (324 kg)[2] with most later weapons having approximately the same dimensions and weight,[3] except for the Mod 11 version which has a weight of approximately 1,200 pounds (540 kg).[17]

The B61 is armed by ground-based personnel via an access panel located on the side of the bomb, which opens to reveal 9 dials, 2 sockets and a T-handle which manually triggers the "command disable" function. One of the sockets is a MC4142 "strike enable" plug which must be inserted in order to complete critical circuits in the safety/arming and firing mechanisms. The other socket is the PAL connector located in the top right hand corner of the arming panel, which has 23 pins marked with alphabetic letter codes.[18]

The B61 also features a "command disable" mechanism, which functions as follows: after entering the correct 3-digit numeric code it is then possible to turn a dial to "DI" and pull back a T-shaped handle which comes away in the user's hand. This action releases a spring-loaded firing pin which fires the percussion cap on an MC4246A thermal battery, powering it up. Electrical power from the thermal battery is sufficient to "fry" the internal circuitry of the bomb, destroying critical mechanisms without causing detonation. This makes the bomb incapable of being used. Any B61 which has had the command disable facility used must be returned to Pantex for repair.[18]

As part of its full fuzing option capability, the B61 can be set for airburst and groundburst detonation, and freefall, retarded freefall and laydown delivery, the latter two delivery options being achieved through the use of a parachute to slow the weapon down during release from the delivery aircraft.[2] Only the Mod 0 to 10 versions of the B61 are equipped with a parachute retarder (currently a 24-ft (7.3 m) diameter nylon/Kevlar chute). This offers the aircraft a chance to escape the blast in its retarded delivery modes, or allows the weapon to survive impact with the ground in laydown delivery mode. The weapon can be released at speeds up to Mach 2 and altitudes as low as 50 feet (15 m). In one of the weapon's laydown modes, it detonates 31 seconds after weapon release.[19]

The Mod 11 is a hardened penetration bomb with a reinforced casing and a delayed-action fuze, this allows the weapon to penetrate several metres into the ground before detonating, damaging fortified structures further underground. Developed from 1994, the Mod 11 went into service in 1997 replacing the older megaton-yield B53 bomb. About 50 Mod 11 bombs have been produced, their warheads converted from Mod 7 bombs. At present, the primary carrier for the Mod 11 is the B-2 Spirit.[17]

A W61 earth penetrating warhead variant with a yield of 340 kt was developed for the MGM-134 Midgetman small ICBM which was cancelled at the end of the Cold War.[4]

The B61 has 13 variants, referred to as Mod 0 through Mod 12.

Mod Status Date Number produced Role Yields PAL type Notes
0 Retired 1968 to ?[3] 500[4] Cat B[6] First production weapons
1 Retired 1969 to ?[3] 700[4] Strategic Up to 340 kt None[6]
2 Retired 1975 to ?[6] 235[4] Cat D
3 Active 1979 to present[3] 545[4] Tactical 0.3, 1.5, 60, or 170 kt[3] Cat F
4 Active 1979 to present[3] 695[4] Tactical 0.3, 1.5, 10, or 45 kt[3] Cat F
5 Retired 1977 to ?[6] 265[4] Strategic Up to 340 kt Cat D
6 Never entered production ? ? Cat D Retrofit of Mod 0
7 Active 1985 to present[3] 600[4] Strategic Allegedly 4 yield settings, including 10, or 340 kt.[3] Cat D Retrofit of Mod 1
8 Never entered production ? Cat D Retrofit of Mod 2 and Mod 5
9 Never entered production ? Cat F Retrofit of Mod 0
10 Inactive stockpile 1990 to present[3] 215[4] Tactical 0.3, 5, 10, or 80 kt[3] Cat F Remanufactured from retired Pershing II W85 warheads.
11 Active 1997 to present 50[4] Tactical/Strategic Either 0.3, 340,[3] or 400 kt[4] Cat F A retrofit of the Mod 7, earth penetrating weapon.
12 In development 2019 onwards 400 to 500 weapons planned Tactical/Strategic[20][21][22] 0.3, 1.5, 10, or 50 kt[4][23] Cat F GPS guided, earth-penetrating weapon

Cost

In May 2010 the National Nuclear Security Administration asked Congress for $40 million to redesign the bomb to enable the Lockheed Martin F-35 Lightning II to carry the weapon internally by 2017.[24] This version is designated Mod 12.[25] The four hundred Mod 12 bombs will be used by both tactical aircraft (such as the F-35) and strategic aircraft (such as the B-2) and the Tail Subassembly (TSA) will give them Joint Direct Attack Munition levels of accuracy, allowing the fifty kiloton warhead to have strategic effects from all carrying aircraft.[26] However, refitting the 400 weapons is now expected to cost over $10 billion.[27] The Mod 12 tail assembly contract was awarded to Boeing on November 27, 2012 for $178 million.[28] Boeing will use their experience with the Joint Direct Attack Munition to yield JDAM-equivalent accuracy in a nuclear bomb.[29] This contract is only the first part of the billion dollar expense of producing and applying the tail kits, over and above the $10 billion cost to refurbish the warheads.[30] The Mod 12 uses an internal guidance system and can glide to its target. On 1 July 2015, the National Nuclear Security Administration (NNSA) conducted the first of three flight tests of the Mod 12 tail kit assembly.[31]

According to the Federation of American Scientists in 2012, the roughly 400 B61-12s will cost $28 million apiece.[1]

Mod 12

As of 2013 the Pentagon saw the B83 nuclear bomb believing that deploying a megaton-yield gravity bomb, the highest level nuclear weapon left in the U.S. inventory, to Europe is "inconceivable" at this point. It also can only be carried by the B-2 bomber, and integrating it onto additional aircraft would be costly. The Mod 12 upgrade is being pursued as a forward-deployed tactical/strategic nuclear weapon to protect NATO and Asian allies since it can be used from dual-capable fighter aircraft, as well as planned to arm the F-35 and B-21 Raider, and its lower yield options make it more flexible with less collateral effects. Recapitalizing the B61 is hoped to lead to the retirement of the B83, resulting in the elimination of the last megaton-yield U.S. bomb and leave the B61-series as the only U.S. gravity nuclear bomb.[32]

In 2013, the Pentagon and NNSA stated that if B61 refurbishment did not begin by 2019, components in the existing weapons could begin to fail.[33] In 2013 Tom Collina of the Arms Control Association said that the new development could complicate arms control efforts with Russia.[34]

In 2014, Congress slashed funding for the project and called for alternates to be studied.[35] In January 2014, former Air Force Chief of Staff Norton A. Schwartz stated that the Mod 12 nuclear bomb upgrade would have enhanced accuracy and a lower yield with less fallout compared to previous versions of the weapon. Accuracy has not been a guarantee for air-dropped nuclear weapons, so consequently large warheads were needed to effectively impact a target; the Mod 11 nuclear earth-penetrator is accurate to 110–170 meters from the desired detonation location, so it requires a 400-kiloton warhead. The Mod 12 is accurate to 30 meters from a target and only requires a 50-kiloton warhead. Schwartz believes that greater accuracy would both improve the weapon and create a different target set it can be useful against. An example is the higher-yield Mod 11's role of attacking underground bunkers that need a ground burst to create a crater and destroy it through the shockwave. A 50-kiloton yield detonating on the ground produces a crater with a radius of 30–68 meters, depending on the density of the surface, effectively putting the bunker within the circular error probability.[36]

In 2014 critics said that a more accurate and less destructive nuclear weapon would make leaders less cautious about deploying it, while Schwartz said it would deter adversaries more because the U.S. would be more willing to use it in situations where necessary. The improved accuracy would make it more effective than the previous Mod 3/4/7 currently deployed to the continent. F-16 and Panavia Tornado aircraft cannot interface with the new bomb due to electronic differences, but NATO countries buying the F-35 would be able to utilize it.[36] The first flight test for an inert Mod 12 was conducted in 2015, with a second successful test in August 2017.[37][31]

In November 2015, a test of the Mod 12 was conducted where the bomb penetrated underground, showing its potential as a nuclear earth-penetrator. Although ground penetration was not an objective of the Mod 12 upgrade, this could allow it to take up the penetrating mission of the Mod 11, which has no life-extension planned and will expire in the 2030s. Being able to penetrate underground increases its effectiveness against buried targets, as it more efficiently transmits explosive energy through enhanced ground-shock coupling, allowing its max yield of 50 kilotons underground to have the equivalent surface-burst capability of a 750 kt to 1.25 megaton weapon. The Mod 12's increased accuracy and earth-penetration capability allows a lower strike yield to be selected, reducing radioactive fallout risk, potentially making it more attractive to military planners.[38] However the Mod 12 does not have the reinforced structure of the Mod 11 which will be retained in service for the ground penetrating mission.[39]

The Mod 12 Life Extension Program continued in 2018 and on 29 June 2018 two successful non-nuclear system qualification flight tests at Tonopah Test Range were reported.[40] In October 2018, the Mod 12 guided tail-kit assembly received Milestone C approval to enter the production phase; the TKA went through the traditional test program in under 11 months, achieving a 100% success rate for all 31 bomb drops.[41] The B61-12 nuclear bomb completed its successful flight tests with the US Air Force’s F-15E in June 2020. It was dropped from above 25,000ft and was in the air for approximately 55 seconds before hitting the target.[42]

See also
B61s on a bomb rack

References
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