The Mark 4 nuclear bomb was an American implosion-type

nuclear bomb A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bom ...

based on the earlier Mark 3 Fat Man design, used in the

Trinity test Trinity was the code name of the first detonation of a nuclear weapon. It was conducted by the United States Army at 5:29 a.m. on July 16, 1945, as part of the Manhattan Project. The test was conducted in the Jornada del Muerto desert abo ...

and the

bombing of Nagasaki The United States detonated two atomic bombs over the Japanese cities of Hiroshima and Nagasaki on 6 and 9 August 1945, respectively. The two bombings killed between 129,000 and 226,000 people, most of whom were civilians, and remain the o ...

. With the Mark 3 needing each individual component to be hand-assembled by only highly trained technicians under closely controlled conditions, the purpose of the Mark 4 was to produce an atomic weapon as a practical piece of ordnance. The Mark 4 Mod 0 entered the stockpile starting March 19, 1949 and was in use until 1953. With over 500 units procured, the Mark 4 was the first mass-produced nuclear weapon.

Design

The Mark 4 utilized a near-identical nuclear explosive package to the 1561 assembly of the Mark 3 Fat Man, but the newly-formed Z Division of Los Alamos focused on re-engineering the electronics and conventional explosives packages to be more modular for easier assembly in the field. The priority objectives of the Mark 4 bomb program were to have the ability to monitor the weapon battery and electrical systems while being carried within the bomb bay of an aircraft to target. Many things slowed and delayed the design and production of the Mark 4, the first of which is the formation and physical placement of Z Division facilities. The early days of Z Division highlighted the lack of infrastructure and logistics of the nuclear weapon enterprise when production of weapon components had to slow due to lack of basic electrical supplies like soldering rosin and hook-up wire. Personnel from Z Division were also used to assist with the Mark 3 tests in

Operation Crossroads Operation Crossroads was a pair of nuclear weapon tests conducted by the United States at Bikini Atoll in mid-1946. They were the first nuclear weapon tests since Trinity in July 1945, and the first detonations of nuclear devices since the ...

in 1946 and again for weapons testing at Eniwetok for

Operation Sandstone Operation Sandstone was a series of nuclear weapon tests in 1948. It was the third series of American tests, following Trinity in 1945 and Crossroads in 1946, and preceding Ranger. Like the Crossroads tests, the Sandstone tests were carried ou ...

in 1948. Though, between tests following a request from Congress on a report on the status of nuclear ordnance, Los Alamos Scientific Laboratory was able to report positive accomplishments such as developing a 'cartridge-type' assembly to house fuzing components and the decision to implement the 'Archie' radar. Along with other developments for the radar, the main component of the system was the APS/13 Tail Warning device which eventually became the 'Archie'. This radar device would close a relay at a predetermined altitude, using four units in each fuze, requiring at least two to fire, allowing a firing signal to proceed. Despite the delays of the early weapons tests, development and stockpile production of the weapon were solidified and streamlined with the rising tensions in Berlin. The Mark 4 was in diameter and long, the same basic dimensions as Mark 3. It weighed slightly more at , depending on the specific Mark 4 version. (Mark 3 weighed .) In addition to being easier to manufacture, the Mark 4 introduced the concept of in-flight insertion (IFI), a weapons safety concept which was used for a number of years. An IFI bomb has either manual or mechanical assembly, which keeps the nuclear core stored outside the bomb until close to the point that it may be dropped. Arming the Mark 4 required opening the casing's front hatch, removing the forward polar cap, two outer pentagonal lenses with their detonators, and two inner explosive blocks, and exposing the pit (the lenses and blocks weighed an aggregate 156 kilograms). The pit's aluminum pusher had a removable 12 centimeter diameter, 1 kilogram trap door, and its uranium tamper had a removable 12 centimeter diameter, 3 kilogram trap door. The weaponeer could then insert or remove the core with the use of a special vacuum tool. Mark 4 models used composite

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

and

plutonium Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exh ...

fissile pits, as well as uranium and plutonium only pits. The exact pit assemblies were common with several other U.S. nuclear weapons, the Type C and Type D pit assemblies. Along with being composite cores, the device was the first weapon to rely upon levitated-pit implosion. These early weapons with a levitated pit had a removable pit, called an ''open-pit''. It was stored separately, in a special capsule called a ''birdcage''. Various versions of the Mark 4 had explosive yields of 1, 3.5, 8, 14, 21, 22, and 31

kilotons TNT equivalent is a convention for expressing energy, typically used to describe the energy released in an explosion. The is a unit of energy defined by that convention to be , which is the approximate energy released in the detonation of a ...

(4 to 130 TJ).

Operational history

About 550 Mark 4 nuclear weapons were produced. The Mark 4 was succeeded by the Mk6, which was generally similar but much improved.

W4 missile warhead

A variant called the W4 (Warhead 4), intended for use on the

SM-62 Snark The Northrop SM-62 Snark is an early-model intercontinental range ground-launched cruise missile that could carry a W39 thermonuclear warhead. The Snark was deployed by the United States Air Force's Strategic Air Command from 1958 through 1961 ...

cruise missile, was designed but never built. The W4 design was canceled in 1951.

Notes

References

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External links