During the design of the TX-15 in 1953 it became evident to designers that massive reductions in size and weight of thermonuclear weapons were possible.[4] In November 1954 the TX-Theta Committee proposed the development of the WX-27 and the WX-28. The larger diameter TX-27 was intended as a missile warhead or for internal carriage in aircraft while the smaller diameter TX-28 would be internally or externally carried on high-performance bombers. In a meeting in December the possibility of a small warhead allowing for smaller missiles was considered so the use of the TX-28 as a warhead was also included.[5]
Design of the XW-28 was given to Los Alamos National Laboratory with Sandia National Laboratories working on the non-nuclear components.[4] In February 1955 Sandia proposed that a basic warhead assembly be designed and that different noses, afterbodies, fins and fuzes could be attached to it. Emphasis was placed on obtaining the optimum yield possible within the space and weight limitations assigned which necessitated having a much thinner case design than in previous weapons.[6] Sandia presented a design outline of the weapon to the Division of Military Application in May 1955.[7]
It was decided that if the weapon could not be fitted to all of the required aircraft, first priority would be given to developing a bomb designed for subsonic internal or external carriage.[6] Designers hoped though that they could produce a weapon considerably smaller than the 2,800 pounds (1,300kg) and 25 inches (640mm) diameter specified.[8]
In April 1955 Sandia were working on the fuze design. Barometric fuzing was rejected due to the wide variety of delivery systems the weapon would use. A timer was also considered having benefits in low-level delivery missions, but it was felt that these gains were slight and did not justify the complexity of the system. Eventually, a combination of radar and contact fuzing was chosen.[9]
Two fuze designs were under development; the first used existing components while the latter (called TX-28 Prime) required more development and used pyrotechnic-activated components. A contact backup was not initially included for the airburst design due to the concern that anti-aircraft fire might activate the contact fuze. Additionally, there was a desire to prevent fallout from a contact burst in the tactical mission.[10]
Los Alamos informed the TX-Theta Committee in the same month that the nuclear system could enter production by January 1958, a date that matched Sandia's availability date for the non-nuclear components.[11]
By May 1955 the TX-28 design firmed-up. The warhead itself would be 20 inches (510mm) in diameter and 49 inches (1,200mm) long, with each end capped in a hemisphere. The TX-28 Prime fuze design was also rapidly advancing. The design used pyrotechnically actuated switches to control preheat current, timer selection, thermal battery arming, ground- and air-burst selection, contact fuze arming, timer motor starting, trajectory-arm-switch pressure-port sealing, gas boosting and even thermal battery monitoring. These switches were small, lightweight and resistant to shock.[12]
A baroswitch was included to improve operational safety of the weapon. It was a two-chamber design in which one chamber would close at weapon release and then the baroswitch would measure the pressure difference between the sealed and open chamber as the weapon fell. At some point in the design the fuzing system was changed to allow for contact preclusion to be selected on the ground. With contact preclusion selected the contact fuze would be disabled in the airburst option.[13]
The initial Mod 0 used internal initiation, but in October 1955 Sandia described progress on external initiation of the weapon which eventually became the Mod 1 design. The external initiator system consisted of a power supply, precision timer and a neutron source called an S-unit. The S-unit was a tube filled with tritium gas while one end was titanium-coated and loaded with deuterium. During function the tritium ions were accelerated into the deuterium target which fused, releasing 14 megaelectronvolts (2.2pJ) neutrons.[14]
The Mk-28EX Mod 0 (external) and Mk-28IN Mod 0 (internal) were design-released in June 1957 and early production was achieved in August 1958. The weapon had a diameter of 20 inches (510mm).[15] The external configuration weapon had a length of 170 inches (4,300mm) and weighed approximately 2,040 pounds (930kg) with the nose section containing the fuze.[16] In the internal configuration, the warhead section was turned around with the nose substituted for four wedge fins and the tail replaced with a blunt nose containing radar antennas and contact crystals.[14] In this configuration the weapon had a length of 93.25 inches (2,369mm) and weighed approximately 1,975 pounds (896kg).[16]
The design met almost all of the specified military requirements with some exceptions. One of these exceptions is still classified while the others were that the weapon did not have a visual indication of arming and that the weapon could not be stored for 18 months at the ready condition. Instead of a visual indication, the weapon relied on electrical signals to confirm that the weapon was not armed. The storage requirement was not met, as the weapon initially required pressure-testing at 30-day intervals.[17]
RE and RI weapons
B28RE
Initial discussion about a ruggedized laydown weapon was discussed in August 1955 by the TX-Theta Committee. It was noted that Soviet radar capabilities were improving and that high-altitude attacks were becoming less certain. A low-altitude approach would help overcome this but would require a weapon that could survive impact with the ground before detonating once the aircraft was a safe distance away. Sandia had been investigating the problem and believed that designing a bomb to survive an impact shock of 200 to 300 g (2,000 to 2,900m/s2) was possible.[18]
In October 1955 the Special Weapons Development Board met. Sandia stated that they had examined parachutes, rotochutes and retrorockets. Rotochutes could not handle the weapon's weight, while retrorockets placed special operating restrictions on the weapon. Parachutes showed promise, but existing designs were unsuitable, with Sandia working on developing an improved parachute. Sandia had also worked on developing shock-absorbing honeycomb materials, which included drop tests from a 300 feet (91m) tower to simulate the 135 feet per second (41m/s) impact expected in a parachute-retarded weapon.[19]
In early 1956, Sandia concluded that a non-impact-resistant warhead could be used to produce an interim retarded weapon and this weapon system would meet weapon objectives 2 years before the development of a true laydown weapon. The weapon would use a pilot-parachute to deploy a larger drogue-parachute. This required a new bomb tail to be designed, which might aggravate ground and aircraft-clearance problems.[19]
Production authorization for the RE weapon was issued in January 1957 and design release made in April 1958.[20] The changes included a new altitude-sensing arming system to replace the baroswitches based on velocity sensing. This removed certain delivery restrictions on the Mk-28 Mod 0. The design also included an acceleration-integration system to detect if the parachute did not deploy and prevent bomb-arming.[21]
The Mk-28RE (retarded external) was 166 inches (4,200mm) long and weighed 2,140 pounds (970kg).[22] The design consisted of the Mk-28 Mod 1 Fuze and a set of Mk-28 Mod 0 RESC (Retarded External Shape Components). It was only available with the Mod 1 warhead. The Mod 1 warhead had the same yield options as the Mod 0, but not all yield options were stockpiled.[23]
The Mk-28RI (Retarded Internal) weapon was designed released in April 1959 and achieved production in June 1960. The design weighed 2,265 pounds (1,027kg) and was 132 inches (3,400mm) long. The design consisted of the Mk-28 Mod 2 Fuze and the same Mk-28 Mod 0 RESC as the RE weapon. It also used the Mod 1 warhead.[24]
FI weapon
B28FI as used on a B52 bomberB28FI being unloaded from a Boeing B-52H in 1984. The 3 ground crew show the size of this weaponBDU-16/E trainers for the B28FI.
The Mk-28FI was the true laydown weapon desired during the development of the RE and RI weapons. The weapon was based on promising results from the TX-28-X2 warhead (which became the Mod 1 warhead) and would have a full-fuzing option capability. Previous weapons required a B-52 bomber to fly at at least 1,500 feet (460m) for the weapon to survive laydown. It was hoped that a true laydown weapon would reduce this height to under 500 feet (150m).[25]
The proposal included requirements for delayed ground burst (laydown), retarded air burst, free-fall air burst and free-fall contact burst fuzing. The selection of air or contact burst would be selected via aircraft control equipment while the laydown option would automatically engage if the weapon was dropped below a certain pressure altitude.[26] Most components were sourced from other programs meaning the main task of the program was developing a shock-mitigating structure and conducting fuze tests.[25]
In August 1960 operational requirements for the TX-28-X3 warhead were issued. These requirements included the capability to survive release from 500 feet (150m) and to be carried internally by the B-47 and B-52 bombers. The bomb nose was given 8 inches (200mm) of crushable honeycomb and another parachute was added, raising the total number to four.[27]
The Mk-28FI weapon was design released in October 1961. The weapon was 22 inches (560mm) in diameter, 145 inches (3,700mm) long and weighed 2,350 pounds (1,070kg). The design consisted of a Mk-28 Mod 3 fuze and Mk-28 Mod 0 FISC (Full-Fuzing Internal Shape Components). The weapon could not use previous Mk-28 warheads, only being suitable for the TX-28-X3 (now called the Mod 2) and later warheads.[28]
The laydown and retarded airburst time for the weapon was 79 seconds. In laydown mode, the weapon had to be dropped between 500 and 2,400 feet (150 and 730m) to allow the weapon to reach the ground before the 79-second interval. Fall time at 500 feet (150m) was approximately 10 seconds. Burst height in retarded airburst depended on release height. The weapon was unpredictable at releases between 12,000 and 17,000 feet (3,700 and 5,200m) as either the freefall or retarded fuzing would be randomly selected.[29]
Variants
Twenty different versions of the B28 were made, distinguished by their yield and safety features. The B28 used the "building block" principle, allowing various combinations of components for different aircraft and roles.[2]
B28EX– (EXternal), streamlined external-carriage for free-fall delivery. Design specifications called for carriage by the Air Force's B-47, B-52, B-57, B-66, F-84F, F-100, F-101 and F-105; and Navy's AJ-1, AJ-2, A2U-1, A3D-1, A4D, F2H, F3H-2, F7U-3 and P6M.
B28IN– (INternal) unstreamlined internal-carriage version for free-fall delivery, primarily for the Republic F-105 Thunderchief and B-52.
B28RE– (Retarded External) streamlined external-carriage version with a parachute retarder for low-level delivery.
B28RI– (Retarded Internal) unstreamlined internal-carriage laydown weapon. Weapon specifications call for carriage by the B-47, B52 and B-66.
B28FI– (Full-fuzing Internal) unstreamlined internal-carriage version for laydown delivery and full-fuzing option (i.e. with parachute retarded laydown, retarded air burst, free fall air burst and free fall ground burst fuzing). Specifications called for carriage by the B-47 and B52. The B28FI did not use the Mod 0 and Mod 1 warheads, as they were not strengthened for laydown delivery.
Mod 0– Used on TM-76 Mace surface-to-surface missile and the GAM-77 Hound Dog cruise missile. Mod used an internal initiator while subsequent mods used external neutron initiators.
Mod 1– TX-28-X2 design. Also carried by Mace and Hound Dog missiles.
Mod 2– TX-28-X3 design. Mod was hardened for use with the B28FI configuration.
Mod 3– Mod had a Cat A PAL.
Mod 4– Mod had a Cat B PAL. Mod was hardened for use with the B28FI configuration.[30]
Y4– Fission weapon, may have been the W34 warhead.
Y5–1.45 megatonnes of TNT (6,100TJ)
A total of approximately 4,500 B28s were produced. The last weapons in use were retired in 1991.[2]
Related designs
The W49 warhead for the Thor, Atlas, Jupiter, and Titan I ballistic missiles was a W28 Y1 warhead with internal power systems removed. It came in two yield options; the Y1 with a yield of 1.1 megatonnes of TNT (4.6PJ) and Y2 with a yield of 1.45 megatonnes of TNT (6.1PJ). Mods 0 to 2 were internally initiated while Mods 3 to 6 were externally initiated. The initial Mod 0 warhead lacked an environmental sensing device until concerns about accidental or deliberate (sabotage) detonation were raised.[31][2]
The Canadian War Museum, in Ottawa, holds a Mark 28RE training variant in its Cold War gallery. The Mark 28 armed CF-104 Starfighters in Germany, 1963–72, under the "Dual Key" protocol (both the US and Canada had to agree to use, with the weapons in US custody on Canadian bases).[34]
The B28FI, built in 1962, is displayed on a dolly at the Caen Memorial Museum in Normandy, France.
At the Wings over the Rockies Air and Space Museum, the B28FI (Y5) for large bombers is on display alongside the B28RE (Y5) for outboard pylons on fighter-bombers.
A nuclear bunker buster, also known as an earth-penetrating weapon (EPW), is the nuclear equivalent of the conventional bunker buster. The non-nuclear component of the weapon is designed to penetrate soil, rock, or concrete to deliver a nuclear warhead to an underground target. These weapons would be used to destroy hardened, underground military bunkers or other below-ground facilities. An underground explosion releases a larger fraction of its energy into the ground, compared to a surface burst or air burst explosion at or above the surface, and so can destroy an underground target using a lower explosive yield. This in turn could lead to a reduced amount of radioactive fallout. However, it is unlikely that the explosion would be completely contained underground. As a result, significant amounts of rock and soil would be rendered radioactive and lofted as dust or vapor into the atmosphere, generating significant fallout.
The Special Atomic Demolition Munition (SADM), also known as the XM129 and XM159 Atomic Demolition Charges, and the B54 bomb was a nuclear man-portable atomic demolition munition (ADM) system fielded by the US military from the 1960s to 1980s but never used in combat.
The W25 was a small nuclear warhead that was developed by the Los Alamos Scientific Laboratory for air-defense use. It was a fission device with a nominal yield of 1.7 kt.
The B83 is a variable-yield thermonuclear gravity bomb developed by the United States in the late 1970s that entered service in 1983. With a maximum yield of 1.2 megatonnes of TNT (5.0 PJ), it has been the most powerful nuclear weapon in the United States nuclear arsenal since October 25, 2011 after retirement of the B53. It was designed by Lawrence Livermore National Laboratory.
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.
The B57 nuclear bomb was a tactical nuclear weapon developed by the United States during the Cold War.
The Mk/B53 was a high-yield bunker buster thermonuclear weapon developed by the United States during the Cold War. Deployed on Strategic Air Command bombers, the B53, with a yield of 9 megatons, was the most powerful weapon in the U.S. nuclear arsenal after the last B41 nuclear bombs were retired in 1976.
The B-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 megatons of TNT. A top secret document, 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.
Laydown delivery is a mode of delivery found in some nuclear gravity bombs: the bomb's descent to the target is slowed by parachute so that it lands on the ground without detonating. The bomb then detonates by timer some time later. Laydown delivery requires the weapon to be reinforced so that it can survive the force of impact.
The W54 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 weight and yield to have entered US service. It was a compact implosion device containing plutonium-239 as its fissile material, and in its various versions and mods it had a yield of 10 to 1,000 tons of TNT.
Medium Atomic Demolition Munition (MADM) was a tactical nuclear weapon developed by the United States during the Cold War. It was an atomic demolition munition (ADM), a combat engineering device for demolition of structures and for battlefield shaping. The device contained a W45 warhead with an estimated yield of 0.5 to 15 kilotonnes of TNT. Each MADM weighed 391 pounds (177 kg) in its transportation container. They were deployed between 1962 and 1986.
The W50 was an American thermonuclear warhead deployed on the MGM-31 Pershing theater ballistic missile. Initially developed for the LIM-49 Nike Zeus anti-ballistic missile, this application was cancelled before deployment. The W50 was developed by Los Alamos National Laboratory. The W50 was manufactured from 1963 through 1965, with a total of 280 being produced. They were retired from service starting in 1973 with the last units retired in 1991.
The W49 was an American thermonuclear warhead, used on the Thor, Atlas, Jupiter, and Titan I ballistic missile systems. W49 warheads were manufactured starting in 1958 and were in service until 1965, with a few warheads being retained until 1975.
The W56 was an American thermonuclear warhead produced starting in 1963 which saw service until 1993, on the Minuteman I and II ICBMs.
The W58 was an American thermonuclear warhead used on the Polaris A-3 submarine-launched ballistic missile. Three W58 warheads were fitted as multiple warheads on each Polaris A-3 missile.
The Mark 39 nuclear bomb and W39 nuclear warhead were versions of an American thermonuclear weapon, which were in service from 1957 to 1966.
The W59 was an American thermonuclear warhead used on some Minuteman I ICBM missiles from 1962 to 1969, and planned to be used on the cancelled GAM-87 Skybolt air-launched ballistic missile.
The B61 Family is a series of nuclear weapons based on the B61 nuclear bomb.
The Mark 17 and Mark 24 were the first mass-produced hydrogen bombs deployed by the United States. The two differed in their "primary" stages. They entered service in 1954, and were phased out by 1957.
Contact preclusion is a fuzing feature found in some nuclear weapons in which backup contact fuzes in a nuclear weapon can be disabled when the weapon is set to air burst fuzing.
↑ A Summary of Accidents and Significant Incidents Involving US Nuclear Weapons and Nuclear Weapon Systems (U) (Report). Sandia National Labs. 1990.
↑ MARK 28 THERMONUCLEAR BOMBArchived 2013-04-04 at the Wayback Machine // National Museum of the USAF, 8/16/2012: "The artifacts on exhibit are BDU-16/E training variants of the Mk-28 and are displayed on an MHU-7/M Bomb Lift Trailer... return to the Cold War Gallery."
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