World War 2:The Manhattan Project

The design of Little Boy and Fat Man

Gun-type machines

Early work focused on "gun-type" which fired one piece of uranium into another to create a nuclear chain reaction. Little boy was a gun-type nuclear weapon. It relied on one mass of uranium-235 hitting another to create a nuclear reaction.

Little Boy

The core component was a smoothbore gun barrel though which the uranium projectile would be fired. The final design used 64 kilograms of uranium-235. Sixty percent was formed into the projectile and the remaining forty percent consisted of the target (solid spike measuring seven inches long). Little Boy employed a three-stage fuse system which ensured that the bomber could escape. The three stages were: a employed timer, barometric stage, and a set of doubly-redundant radar altimeters.
Due to lack of uranium-235, no full scale test of the design occurred before the the bomb destruction. Little Boy was considered relatively unsafe by modern. It could have a crash or electrical short circuit that could lead to accidental detonation.


Uranium-235 is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile (it can sustain a fission chain reaction).

Fat Man

The basic structure of the Fat Man consisted of six spheres. The outermost sphere was the explosive lens system. Followed by the absorber shell, the uranium reflector shell, the plutonium pit, and lastly the neutron initiator. The whole assembly was held together by a shell made of strong aluminum called dural (or duralumin)

The absorber shell was an 11.5 cm thick aluminum sphere also weighing 120 kg.
The plutonium pit contained 6.2 kg of plutonium alloy contained in a 9.0 cm shell.

The neutron initiator was a sphere consisting of a hollow beryllium shell, with a solid beryllium pellet inside