Little Boy


August 20, 2022

Little Boy (Model: L11) was the cover name or nickname of the nuclear bomb dropped on Hiroshima on August 6, 1945 from the Enola Gay. It was 3 m long, 71 cm in diameter and weighed 4,000 kg. Its operation was based on rings of the uranium isotope 235U below the critical mass, which together formed a critical mass to cause a chain reaction. Little Boy's explosive power was equivalent to about 13 kilotons (ie 13 million kg of TNT). The fissile material weighed 61 kg, although some say 64 kg; although it was not quite a perfectly pure 235 isotope of uranium, the enrichment level was very close to 80 percent. The rest was uranium isotope 238U.


Little Boy was a Mk.1 type nuclear weapon. It was modeled, among other things, by dropping a subcritical, i.e. under-fissile, piece of uranium into a donut-shaped larger piece of uranium, so that for a tenth of a second, the critical mass was just barely exceeded.

Development and use

This type of bomb had not been test detonated. One important reason must have been the scarcity of resources (such as the necessary fissile material), funds and time used for both experiments and development work. In addition, there were security risks caused by radiation and possible data leaks. Instead, the nuclear weapon detonated in the Trinity test was the equivalent of the Fat Man bomb that was dropped on Nagasaki. The core of the bomb itself, or the "uranium projectile target" has been described as being "bowl-like" - and was delivered to Tinian by the heavy cruiser USS Indianapolis (CA-35). The caliber of the cannon-type weapon was 5 inches, or about 127 mm. The uranium target was delivered in a 40 liter lead cylinder. The untried nature of the bomb was due to the fact that only a part of the reactant worked in the actual explosion reaction; the rest of the substance decomposed and caused the bulk of the precipitation. It has been estimated that the power of the explosion would have been about two and a half times (2½x so a total of approximately 40 kT) if all the reaction material had worked as planned. The power of the explosion still exceeded expectations. However, various tests were used to model the nuclear weapon. Except for the Trinity nuclear test, they could not be full scale. In addition to the device itself, there was a tungsten carbide coating. Its purpose was to prolong the explosion reaction. Codenamed Bronx Shipments, the operation transported bomb parts to Tinian. The pipe and uranium pieces were delivered by the heavy cruiser USS Indianapolis, which sank on its return trip after leaving Guam. The ship was torpedoed by Imperial Japanese Navy submarine I-58 (I.58). The other parts of the bomb had been supplied by the 509th Special Heavy Bombardment Regiment and its 320th Transport Squadron: using their own C-54 type (Green Hornets) transport aircraft. The parts of the bomb had been manufactured at Los Alamos (LANL). The bomb was assembled by members of the 1st Ordnance Squadron in Tinian, and the bomb was activated, i.e. brought into operational readiness, only on the flight to Japan.

Launching the bomb

Various security systems had been developed for the use of the bomb. One of them was a barometer system that prevented the bomb from going off at an altitude of more than 1,500 meters. In addition to this, there was a timer that prevented the bomb from going off during the first 15 seconds after it was dropped. Finally, there was a proximity fuze, i.e. a miniature radar. This weapon was fired by a traditional powder charge containing cordite. William S. Parsons did not load the bomb until the Enola Gay was in flight. 1. Sub-Lieutenant Morris R. Jeppson of the Separate Ordnance Squadron observed the condition of the bomb, especially its circuits, during the final stages of the bombing flight - Jeppson changed the bomb's "spark plugs". After all, the bomb worked with battery-powered, electric 24V ignition.

Photo Gallery

See also

Fat Man The bombings of Hiroshima and Nagasaki The Manhattan Project


Silvennoinen, Martti: Heartbeats after Hiroshima. Publisher: Kirjapaja, Helsinki, 19