Mark 4 nuclear bomb

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A Mark 4 bomb casing.

The Mark 4 nuclear bomb was an American nuclear bomb design produced starting in 1949 and in use until 1953.

The Mark 4 was based on the earlier Mark 3 Fat Man design, used in the Trinity test and bombing of Nagasaki. The Mark 3 design was essentially handmade and designed as an emergency wartime expedient design; the Mark 4 utilized essentially the same basic design (materials, dimensions of the nuclear core and explosive components) but reengineered the whole design to be safer and easier to produce.

The Mark 4 was 60 inches in diameter and 128 inches long, the same basic dimensions as Mark 3. It weighed slightly more at 10,800 to 10,900 pounds depending on the specific Mark 4 version (Mark 3 was 10,200 pounds).

In addition to being easier to manufacture, the Mark 4 introduced the concept of in flight insertion or 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. To arm the bomb, the fissile nuclear materials are inserted into the bomb core through a removable segment of the explosive lens assembly, which is then replaced and the weapon closed and armed.

Mark 4 models used composite Uranium and Plutonium fissile pits. The exact pit assemblies were common with several other US nuclear weapons, the Type C and Type D pit assemblies.

Various versions of the Mark 4 had explosive yields of 1, 3.5, 8, 14, 21, 22, and 31 kilotons.

A total of 550 Mark 4 nuclear weapons were produced.

The Mark 4 was succeeded by the Mark 6 nuclear bomb, which was generally similar but much improved.