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How did Little Boy differ from Fat Man?
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atomic bombings of Hiroshima and Nagasaki
A gigantic mushroom cloud rising above Hiroshima, Japan, on August 6, 1945, after a U.S. aircraft dropped an atomic bomb on the city, immediately killing more than 70,000 people.
Little Boy and Fat Man differed primarily in their design, fissile material, and explosive yield.
- Design and Mechanism: Little Boy utilized a "gun-assembly" design, in which an explosive propellant fired one subcritical mass of uranium-235 into another to create a critical mass. In contrast, Fat Man used an "implosion" design, where a spherical core of plutonium-239 was compressed by high explosives to reach a supercritical state.123
- Fissile Material: Little Boy relied on highly enriched uranium, while Fat Man used plutonium. Plutonium was unsuitable for the gun-assembly method because its faster fission rate risked premature detonation.143
- Explosive Yield: Fat Man was significantly more powerful, with an estimated yield of 21,000 tons of TNT, compared to the approximately 15,000-ton yield of Little Boy.124
While Little Boy was used untested on Hiroshima, the Fat Man design was complex enough to require a successful full-scale test, known as the Trinity test, before its deployment over Nagasaki.124