The B61, which exists in several mods, is actually a family of weapons based on a single basic weapon and physics package design. 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 (which was retired, and then readapted to yield another B61 variant). See W-80 page for pictures of the actual warhead.

Design Features

Two stage radiation implosion weapon.

Light weight, intermediate yield bomb with variable yield options ("dial-a-yield" or DAY), and flexible fuzing and delivery options. Designed for high-speed external carriage and low altitude delivery. Modular weapon system design.

Contains two neutron generators (supplied by General Electric).

Spin stabilized by rocket. 24 ft. Nylon/Kevlar-29 ribbon parachute ejected by gas generator. Allows rapid deceleration from supersonic velocity (only 2 seconds required to decelerate to 35 mph). Tested at up to Mach 1.2.

The lowest yield option for the B61 (the same for all tactical mods) is 300 tons which probably represents the basic unboosted yield of the fission primary.

The B61 has four major sections:

• Nose Subassembly: contains a dual-channel radar airburst fuze and two piezoelectric crystal impact fuzes, and shock mitigating material for laydown delivery.
• Center Warhead Subassembly: "hard case" containing the actual thermonuclear warhead, sealed and desiccated with polyurethane spacers to support warhead and provide shock isolation. Also contains thermal batteries, safeties, and firing circuits.
• Rear Subassembly: Preflight arming controls, fuzing option switches, safe separation setting dials, and spin rockets for free fall weapon stabilization.
• Tail Subassembly: consists of bomb fins, afterbody structure, parachute with associated deployment and release mechanisms. Complete parachute assembly weighs 115 lb.

Current models are made up of 6000 parts in 1800 subassemblies, manufactured by 570 suppliers and 9 principal contractors.

Features of individual models currently in stockpile:

• Mod 3: Tactical bomb with 4 yield options - 0.3 Kt, 1.5 Kt, 60 Kt, and 170 Kt. This is the highest yield tactical bomb mod. Along with the mod 4, this was the first mod developed using PBX-9502 IHE, and the first to be equipped with a microprocessor based arming and firing system (in 1980).
• Mod 4: Tactical bomb with 4 yield options - 0.3 Kt, 1.5 Kt, 10 Kt, and 45 Kt.
• Mod 7: Strategic bomb with multiple yield options - lowest is 10 Kt, highest is 300 Kt. This is a converted Mod 1 bomb. The conversion replaced the earlier PBX-9404 HE with PBX-9502 IHE. It has the highest yield of any B61 family weapon (including other warhead derivatives, like the W-80).
• Mod 10: Tactical bomb with 4 yield options - 0.3 Kt, 5 Kt, 10 Kt, and 80 Kt. Converted W-85 Pershing II missile warhead.
• Mod 11: Tactical or strategic bomb with multiple yield options presumably ranging from 10 Kt (and possibly lower yields) to 340 Kt. This is a modified Mod 7 with a one-piece case hardened steel center case, and a new nose piece and rear subassembly to provide ground penetration capability for defeating buried targets ("bunker busting"). The parachute assembly has also been removed, and new aerodynamic fins added for high-velocity, accurate delivery. The B61-11 buries itself 3-6 meters underground before detonation, transfering a much higher proportion of the explosion energy to ground shock, compared to surface bursts. The actual warhead itself is identical to the Mod 7. This is the first new model of a U.S. warhead to go into service since warhead production was suspended in 1989. It is being produced by field modification of existing Mod 7s.

Materials

Primary consists of beryllium reflected plutonium (the derivative W80-0 contains supergrade plutonium)
Deuterium-tritium boosted
Primary also contains aluminum and uranium
Contains oralloy (highly enriched uranium), either in primary or as secondary tamper
Lithium-6 (95% enrichment) deuteride fusion fuel
Although early mods used PBX-9404 high explosive, current mods all use PBX-9502 insensitive high explosive (IHE) - a TATB plastic bonded explosive composition.

Delivery Method

Deliverable by any U.S. or NATO nuclear qualified aircraft including:

• B-52
• B-1
• B-2B
• F-15E
• F-16
• F/A-18
• A-6
• AV-8A
• Tornado (NATO)

Currently the B61-11 is deployed for use with the stealthy B-2 bomber, which entered service in a nuclear role on 1 April 1997.

Safeguards and Arming Features

• Common features of current mods
• AMAC (Aircraft Monitoring and Control): Equipment 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 avenue for transmission of PALs.
• Strike enable plug
• Ready/safe switch (8 position motor-driven dual channel switch to isolate batteries from firing circuits)
• Environmental Safety Devices (ESDs):
  • trajectory arming timer
  • safe separation timer
  • inertial switches
  • time delay select switch
• Retracting detonators for thermal safety
• Command disable
• All tactical mods (3, 4 and 10): CAT F PAL, weak link/strong link driven by unique signal generator
• Mod 7 (strategic): CAT D PAL
• Mod 11: CAT D PAL

See Principles of Nuclear Weapons Security and Safety for explanations of these features.

Fuzing and Delivery Mode

In flight fuzing and yield selection merely by turning a dial. Full fuzing options (FUFO): high or low speed delivery, high altitude or low altitude (release heights as low as 50 ft.). Current mods have five fuzing options:

• Free fall air burst (high altitude only)
• Parachute retarded airburst (high or medium altitudes)
• Free fall contact burst (high or medium altitudes)
• Parachute retarded contact burst (high or medium altitudes)
• Parachute retarded laydown delayed surface burst (delivery altitudes up to 5000 feet), 31 and 81 sec delays available

The Mod 11 has a special ground impact time delay feature to allow it to penetrate into the earth before detonating.

Delivery accuracy <600 feet.

Development

Designed and developed by Los Alamos Scientific Laboratory (LASL/LANL)

The B61 was developed to provide a light weight weapon of intermediate yield that could be carried by modern high performance combat aircraft. These aircraft do not carry weapons internally like the older or larger bomber designs, but instead carry them externally on the wings or fuselage. This requires weapons of very low drag, and able to withstand the aerodynamic heating of supersonic flight. In addition, the greater maneuverability of modern combat aircraft, combined with the increased lethality of defense systems makes the option of low altitude delivery both possible and necessary. The initial interest in this type of weapon dates back to 1960.

The original B61 bomb and warhead systems have gone through many modifications since the initial development effort, extending up to the present time (a ground penetrator version is in development right now).

The adoption of IHE in the Mod 3 and 4 resulted in stockpile problems that carried over into a number of other mods and related warhead types. IHE proved vulnerable to unexpectedly reduced performance at low temperatures (perhaps due to the unusual properties of TATB). A low-temperature test in the early 80s revealed a yield reduction of 25%, requiring some redesign.

The latest mod of the B61, the B61-11, began entering service in January 1997. It is a modified Mod 7, a version with built-in warhead shock resistance. The Mod-11 has a greatly strengthened bomb case for destroying buried command centers, and will replace the 9 Megaton B-53 in that role. The Mod-11 was jointly developed by LANL and Sandia National Laboratories. In February 1996 the drop test program began, and 13 drop tests of the Mod 11 were conducted in 1996 (3 in Alaska, and 10 at the Tonopah Test Range in Nevada). By January Sandia had shipped 10 training versions of the modified B61, to the U.S. Air Force last month as training devices, plus nine sets of customized bomb-handling gear. Modifications were scheduled to begin in January. The Kansas City Plant and Oak Ridge are currently delivering conversion kits to the Air Force, whose engineers are performing the actual modifications. Kit fabrication at Kansas City is expected to continue until at least 1999.

Although an August 1995 authorization date is given for the development of the B61-11, the project appears to trace back to the W-61 EPW (earth penetrating warhead) project begun in 1989. The EPW was also a derivative of the B61-7, packaged in a high-strength steel case, and had gotten as far as production engineering in 1991.

The B61-11 has a reported capability to destroy targets at depths of several hundred feet at its highest yield setting.

Efforts are currently underway at Sandia to develop a new weapon using the B61 warhead package. This is the BIOS (Bomb Impact Optimization System), a guided glide bomb that would permit release of nuclear weapons at a greater range from the target, and by using the GPS satellite system would permit delivery with 1 meter accuracy.

B61 Development Schedule

4 August 1960	Sandia and LASL authorized to study light weight FUFO bomb
16 October 1961	Program to develop light weight FUFO bomb initiated
18 January 1963	FUFO bomb designated TX-61
20 August 1963	Free fall ballistic tests begun on bomb
May 1965	Production engineering of B61-0 begun
October 1966	First war reserve quality B61-0 built
21 December 1966	First war reserve quality B61-0 accepted into inventory
January 1967	Quantity production of B61-0 begins
January 1969	Quantity production of B61-0 ends
February 1969	Quantity production of B61-1 begins (later modified to Mod 7, still in stockpile)
April 1971	Quantity production of B61-1 ends
April 1972	Development engineering of B61-3 and B61-4 begins
May 1979	Development engineering of B61-7 (conversion of B61-1 begins)
August 1979	Quantity production of B61-4 begins
October 1979	Quantity production of B61-3 begins
May 1983	Quantity production of W-85 begins (later converted to B61-10)
September 1985	Quantity production of B61-7 begins
April 1989	Production engineering of B61-10 begins (conversion of W-85)
June 1990	Quantity production of B61-10 begins
August 1995	B61-11 development authorized
February 1996	Drop test of B61-11 conducted in Alaska
January 1997	B61-11 enters service, quantity production of conversion kits begins

Deployment

Initial manufacture October 1966
Quantity manufacture begins January 1967
Initial deployment 1967
Approximately 3150 B61 bombs of all mods have been manufactured.
Currently in service: 600 tactical bombs (mods 3, 4 and 10) and 750 strategic bombs (mod-7). Mod-7s are currently being converted to Mod-11, which can be used in either a tactical or strategic role.
Of the 12 mods manufactured, 5 remain in service. The oldest bombs are arguably B61-7 bombs (converted B61-1 bombs), manufactured in their original model starting in 2/69, but since rebuilt (starting in 9/85). The oldest mod currently in service is the B61-4 first manufactured in 8/79. The "newest" bombs are the Mod-11, but these are just Mod-7 warheads repackaged in a new body.