An improvement of the Mk 3, the Mk 4, developed after WWII, was the first US atomic weapon to be produced on an assembly line basis and to be stockpiled in large numbers. To give better ballistic accuracy, the shape of the case was changed. Its outer casing had been streamlined to avoid the aerodynamic wobble that plagued Nagasaki's Fat Man bomb. Provisions were also made so that the nuclear core of active material could be inserted without complete disassembly of the weapon, by the use of a special detachable device. This gave us the capability of in-flight insertion of the active material. The firing system, commonly referred to as the x-unit, and the fuzing system were also improved and mounted on a cartridge to facilitate checking and testing.
The Mark IV atomic bomb (also referred to as Mark 4, Mark-IV, Mk 4 and Mk-4) weighed 10,800 pounds had a yield of from 1-31 kilotons. The Mark 4 was a redesign of the "Fat Man." After the end of World War II, work at Los Alamos proceeded on developing a completely new implosion system, which evolved into the Mark 4, with improved engineering and production and handling characteristics. Successful demonstration of essential features of the new system, in the Sandstone test series at Eniwetok in the spring of 1948, ended the laboratory-style layout of weapons and opened the way for mass production of components and the use of assembly-line techniques. It was the first assembly-line produced nuclear bomb. It could be configured to for various yields - 1, 3.5, 8, 14, 21, 22, and 31 kilotons. By 1949 a much improved Mark 4 Fat Man bomb was in use, but it was bulky and difficult to work with.
The Mark IV was in service from March 1949 until 1953. In April 1951 President Truman assigned nine MARK 4 atomic bombs to General Hoyt Vandenberg's (USAF Chief of Staff) custody, the first instance DoD and not Atomic Energy Commission authorized to hold both nuclear and non-nuclear components. In March [May?] of 1953, the Mark 4 bombs were finally taken out of service.
The Mark IV, was manually "safed" by removal of a capsule containing the fissionable material through a trap door in the sphere reachable from the nose of the case. Ground handling was accomplished with the N-1 dolly originally procured from Boeing Aircraft Company beginning in late 1947. The N-1 was a low bed roadable dolly without a lifting mechanism. The B-29 was literally lifted up (tilted), the Mark IV rolled in place and the B-29 lowered as the bomb was guided into the bomb bay. It was standard operating procedure at the time that when transporting "special weapons" not to transport the fissionable capsule and the main sphere containing the high explosives trigger together.
|Yield:||1, 3.5, 8, 14, 21, 22, and 31 kilotons|
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