Sandia National Laboratories B61

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B61 in weapon storage vault with F-16 in the background

Contents

Introduction

The B61 series was developed during the 1960s to replace older, tactical Air Force nuclear bombs such as the B28, B43, and B67. Numerically the most important bomb in the US nuclear stockpile, the B61 was optimised for strategic and non-strategic roles. To date there have been 11 different B61 variants, five of which remain in service. The planned life extension programme (LEP) will cover the B61-3, B61-4, B61-7, and B61-10 versions but does not include the B61-11.

Development

The B61 bomb was developed by Los Alamos Scientific Laboratory (now Los Alamos National Laboratory) starting in 1960. The first B61 production unit began in October 1966. Problems stalled the program, and in January 1967 the bomb was withdrawn and changed slightly. Full-scale production started in January 1968. The bomb has been manufactured in six basic modifications, Mods 0 through 5. Three of these versions, Mods 1, 3, and 4, were upgraded with improved characteristics and safety features. Mods 0, 2, and 5 have been retired and dismantled. Programs planned for three other upgrades (Mods 6, 8, and 9) were cancelled. The B61-10 is a converted Pershing II missile warhead. Some sources estimate that the total stockpile of intact B61 bombs is approximately 1,925, of which 1,265 are considered operational. All B61 models are scheduled to undergo life extension and retrofit programs over the next decade, and approximately 400 bombs are scheduled to be “consumed” in quality and reliability testing through 2025. In US service, it is supplemented by the lager B83 bomb.

Description

Dimensions

The intent was to develop an aircraft bomb which was high yield (up to over 100 kilotons) and yet was small enough and had low enough drag to carry under the wing of a fighter or fighter-bomber type aircraft. One major feature was Full Fuzing Options (allowing various air and ground burst usage options; free fall air burst, parachute retarded air burst, free fall ground burst, parachute retarded ground burst, and laydown or parachute retarded time delay after impact ground burst). The overall B61 bomb was 13.3 inches (340 mm) diameter by 141 inches (3,600 mm) long, and weighed 695-715 pounds depending on version. This includes the outer aerodynamic shell, a crushable nose cone, parachute section in the tail, tail fins, etc. (Weight includes tail fins; diameter is of the bomb body itself, without fins). The 141.6-inch long, 13.3-inch diameter bomb averaged approximately 750 pounds, but actual weight varied with each modification. The DBU-388 is the (unarmed) practice round used to simulate the B61.

Physics package

The physics package of the B61 has been adapted to yield several other warheads - the W-80, W-81 (now retired and dismantled), W-84 (now retired and in the inactive stockpile), and the W-85 Pershing II warhead (which was retired, and then readapted to yield the B61 mod 10 variant).

Radar Nose

Once launched, a radar-fuse tells the firing system when the bomb is at the correct altitude for detonation. Production B61 bombs had vacuum tube technology in their radar circuits. In fact, during the 1990s, Sandia National Laboratories scientists developed the MC4033 common radar, which uses solid-state electronics, for planned refurbishments of the B61 and B83 gravity bombs. All B83 bombs now use the common radar, though similar plans to fit a new radar on all B61s have been repeatedly deferred.

Storage

The B61 nuclear bombs in Europe are stored in what is known as the Weapon Storage and Security System (WS3), a nuclear weapons storage capability unique to the European theater. This system enables the weapons to be stored underground in Weapons Storage Vaults (WSV) inside the individual Protective Aircraft Shelters (PAS)9 on each base rather than in igloos in a centralized Weapons Storage Area (WSA). Each vault can store up to four weapons. Usually, the C-130 is used to airlift nuclear weapons within the European theatre, and the C-17 Globemaster is used for transports to/from the United States.

Munition support

Each airbase with B61 weapons has a Munition Support Squadron (MUNSS). Each MUNSS includes approximately 110 personnel that are responsible for the physical security of the weapons, maintenance and logistics of the weapons and the Weapons Storage and Security System (WS3), and handing over the nuclear bombs to the national air forces if ordered to do so by the U.S. National Command Authority. It is estimated that approximately 150 B61s are deployed with US Air Force units in Britain, Germany, and Turkey, and held in US custody for use by NATO allied air force wings and squadrons in Belgium, Germany, Italy, the Netherlands, and Turkey.

Training

The B61 Mod 4 Type 3A B61 Mod 4 Type 3A are training systems that the US Air Force will use to practice loading and handling operations. The Warhead Simulator Package is a key component of the Trainer, which simulates the electrical functionality of a real War Reserve weapon. The new trainer provides a significant improvement by allowing DoD personnel to realistically practice performing lock/unlock and prelaunch arming/safing operations without exposing a real nuclear weapon to vulnerabilities.

Arming

A system called Permissive Action Link (PAL) is included in or attached to a nuclear weapon system to preclude arming or launching until the insertion of a prescribed discrete code or combination. The earliest versions were 5-digit mechanical combination locks. The most modern electrical versions are the six-digit Cat D and the 12-digit Cat F, both with a “limited try” feature that permits a specific number of attempts to enter the correct code, after which the electrical circuits self-destruct, disabling the weapon. Cat B is an earlier electrical version. The Aircraft Monitoring and Control (AMAC) equipment is installed in an aircraft to permit monitoring and control of the safing, arming, and fuzing functions of a nuclear bomb or missile delivered by the aircraft. It is the datalink for transmission of PALs into the nuclear weapon.

Delivery

The B61 can be dropped at high speeds from altitudes as low as 50 feet. As many as 22 different varieties of aircraft can carry the B61 externally or internally. This weapon can be dropped either by free-fall or as parachute-retarded; it can be detonated either by air burst or ground burst. The B-61 has a 24' kevlar ribbon-type parachute, capable of slowing the ~700 lb weapon from 1000mph to 65mph in about 2 seconds. The retarded ground burst is also called "laydown" because the weapon lies on the ground for a period before detonation. This allows the delivery aircraft to escape.

Live testing

A series of underground tests was conducted from 1963–1968 at the Nevada Test Site to certify the bomb’s yield and confirm its military characteristics. “Shot Halfbeak,” one of six B61-associated tests conducted in 1966, is suspected of being fired on June 30 at full yield—about 350 kilotons. Nuclear testing resumed in the mid-1970s to perfect the Mod 3 and 4 versions, which entered the stockpile in 1979.

Versions

B61 Mod 0

Early strategic model, 500 built. Disassembled between August 1995 and June 1996.

B61 Mod 1

Early model with improved characteristics and safety features, dismantled.

B61 Mod 2

Early model, used by the US Navy, 215 built. After the US Navy terminated the nuclear strike mission from aircraft carriers in the early 1990s, the bombs were retired and disassembled between June 1996 and March 1997.

B61 Mod 3

Tactical model, still in active service. Yield 0.3, 1.5, 60, or 170 kilotons. Cat F PAL certified and uses Insensitive HE (IHE) in primary.

B61 Mod 4

Tactical model, still in active service. 0.3, 1.5, 10, or 45 kilotons yield. Cat F PAL certified and uses Insensitive HE (IHE) in primary.

B51 Mod 5

Early model, used by the US Navy, 236 built. First model to be equipped with the Enhanced Nuclear Detonation System (ENDS), which isolates the electrical elements critical to detonation, to prevent premature arming of nuclear weapons subjected to extreme environments such as extreme heat and radiation (which can be the case on an aircraft carrier). After the US Navy terminated the nuclear strike mission from aircraft carriers in the early 1990s, the bombs were retired and disassembled between March 1997 and August 1997.

B61 Mod 6

A converted Mod 0 with CAT D PAL and IHE, cancelled before production.

B61 Mod 7

Strategic model, produced from 1985–1990 with selectable yield between 100-500 kilotons. Mod 7 is a converted Mod 1 upgraded with Cat F PAL and IHE in primary. About 750 Mod 7 strategic B61's remain in the active inventory and compatible with the B-52 and B-2.

B61 Mod 8

A converted Mod 2 with its CAT D PAL and IHE, cancelled before production.

B61 Mod 9

A converted Mod 0 with new CAT F PAL and IHE, cancelled before production.

B61 Mod 10

The B61 has also served as the basic design for three other warheads: the W80-0 sea-launched cruise missile warhead; the W80-1 warhead for the air-launched cruise missile and the advanced cruise missile; and the W85 warhead for the Martin Marietta MGM-31 Pershing II missile. The Intermediate-Range Nuclear Forces (INF) Treaty, signed on December 8, 1987, marked the Pershing II missile (among others) for elimination. Although the missiles and launchers were destroyed by mid-1991, as the treaty called for, the warheads were retained, converted, and probably returned to European air bases as B61 bombs. The “physics package” (the guts of the nuclear explosive) was removed from the W85 warhead, repackaged in a bomb casing, and re-designated the B61-10.

B61 Mod 11

Earth Penetration Weapon (EPW) with a reinforced casing and a delayed-action fuze, allowing it 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, a limited number of which had been retained for anti-fortification use. About 50 Mod 11 bombs have been produced, their warheads converted from Mod 7 bombs. At present, the primary carrier for the B61 Mod 11 is the B-2 Spirit.

B61 Mod 12

Modified version with the 50 kT nuclear package of the B61-4, fitted with a tail kit used on the conventional JDAM, increasing the weapon's accuracy and thus requiring a lower yield. Further, the B61 Mod 12 Life Extension Program is transform the B61 “analog controlled” bomb into a “digitally controlled” bomb that mates with the advanced electronics and avionics of the F-35 Lightning II. Total cost is currently estimated at approx. $5 billion. The B61 Mod 12 consolidates four existing B61 types (non-strategic B61-3, B61-4 and B61-10, as well as the strategic B61-7) into one, so the new B61-12 must be able to meet the mission requirements for both the non-strategic and strategic versions. But since the B61-12 will use the nuclear explosive package of the B61-4, which has the lowest yield of the four types (a maximum of 50 kt), increasing the accuracy was added to essentially turn the B61-4 into a B61-7 in terms of targeting capability. The new guided tail kit – the B61 Tail Subassembly (TSA), as it is formally called – will be developed by Lockheed Martin, Raytheon and Boeing for the Air Force and similar to the tail kit used on the conventional JDAM bomb (Boeing has delivered more than 225,000 kits so far). But the B61-12 would be the first time a guided tail kit has been used to increase the accuracy of a deployed nuclear bomb. The B61-12 accuracy is secret, but believed to be similar to the tail kit on the JDAM, which uses a GPS-aided INS. In its most accurate mode it provides the JDAM a CEP of 5 meters or less during free flight when GPS data is available. If GPS data is denied, the JDAM can achieve a 30-meter CEP or less for free flight times up to 100 seconds with a GPS quality handoff from the aircraft. It is yet unclear if the B61-12 will have GPS, which is not hardened against nuclear effects, but many limited regional scenarios probably wouldn’t have sufficient radiation to interfere with GPS.

Alterations

ALT 335

Carried out between October 1998 and September 2003. Installed a Trajectory Sensing Signal Generator (TSSG), a safety improvement that increases the nuclear safety of the bomb in certain normal and abnormal environments. Büchel AB received initial training in May 1996.

ALT 336

Frozen soil proof drop tests conducted in Alaska in March 1998 suggested that the earth-penetration capability of the B61-11 is limited. In 1998 ALT 336 was initiated.

ALT 339

Carried out between October 1998 and September 2003. Installed the MC4519 MCCS Encryption Translator Assembly (MET) in B61-3, -4, and -10 to provide weapons with cryptographic capability to implement end-to-end encryption in the PAL Code Management System (CMS). MC4519 MET coupled with the CMS enables recoding of nuclear weapons in a fully encrypted manner. MET capability improves the positive controls over use of the warhead. Regular monthly shipments started in June 1997. The first CMS became operational on B61s in Europe on November 30, 2001.

ALT 349

During 1999 B61-11 alteration (ALT) 349 units were produced and delivered to the Air Force. Alt 349 is a structural modification to the B61-11 to improve performance under certain impact conditions (believed to be hardened/fortified bunkers or rock). Qualification activities have included three cable pull-down tests, two B-2A airdrops and a horizontal actuator shock test series. Cable pull-down testing consists of suspending units 300 ft in the air from a cable above a concrete target and pulling the unit down with rockets. In addition to testing, extensive finite element modelling has confirmed test results and contributed significantly to the overall design effort.

ALT 350

Alternation to the B61-11, completed in September 2005. Includes integration of the Common Radar, developed for the B83.

ALT 354

Carried out between March 2001 and March 2002. Adjustment of fin cant angle for B61-3, -4, and -10 to improve weapon spin rates when used in conjunction with existing spin motor.

ALT 356

Spin rocket motor for B61-3/-4/-10.

ALT 357

Refurbishment of the canned sub assembly, that contains the secondary — the second stage of the B61-11. Conducted from October 2005 and completed in September 2008.

ALT 358

Spin rocket motor for B61-7. Completed 2009.

ALT 359

Spin rocket motor for B61-11. Completed 2009.

ALT 364

Life Extension Program for the B61. Cancelled.

ALT 365

Life Extension Program for the B61. Cancelled.

ALT 366

Proposal to replace the B61 radar.

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