Mark 4 nuclear bomb

The Mark 4 nuclear bomb was an American implosion-type nuclear bomb based on the earlier Mark 3 Fat Man design, used in the Trinity test and the bombing of Nagasaki. 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 issues slowed and delayed the design and production of the Mark 4, the first of which was 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 in 1946 and again for weapons testing at Enewetak for Operation Sandstone 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.