Fat Man and Little Boy Bombs
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Fat Man and Little Boy Bombs

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Fat Man and Little Boy

The United States was fully engaged in war in the Pacific and Atlantic oceans in the summer of 1942. In the Atlantic, German U-boats sank an average of 100 ships a month in 1942, losing only 21 submarines in the process. In the Pacific, U.S. forces first engaged the Japanese in the Solomon Islands at Guadalcanal in August, setting off a bitter six-month campaign. The outcome of both conflicts was uncertain. With the prospect of a long war, a group of theorists under the direction of J. Robert Oppenheimer met at Berkeley during the summer of 1942 to develop preliminary plans for designing and building a nuclear weapon.


Crucial questions remained, however, about the properties of fast neutrons. John Manley, a physicist at the University of Chicago's Metallurgical Laboratory, was assigned to help Oppenheimer find answers to these questions by coordinating several experimental physics groups scattered across the country.

Keep in mind that an explosive nuclear chain reaction occurs when a sufficient quantity of nuclear fuel, such as uranium or plutonium, is brought together to form a critical mass. This is the minimum amount of fissionable material needed to start a chain reaction. The chain reaction starts when neutrons strike the heavy uranium or plutonium nucleus which splits releasing a tremendous amount of energy along with two or more neutrons which, in turn split more nuclei, and so on. In this gun-type device, the critical mass is achieved when a uranium projectile which is sub-critical is fired through a gun barrel at a uranium target which is also sub-critical. The resulting uranium mass comprised of both projectile and target becomes critical and the chain reaction begins.

The Little Boy Bomb:

Dropped on the Japanese city of Hiroshima on August 6, 1945, it was the first nuclear weapon used in a war. Following are some approximate statistics for Little Boy. If you require more extensive information on this weapon, please contact us:

  • Weight: 9,700 lbs
  • Length: 10 ft.; Diameter: 28 in.
  • Fuel: Highly enriched uranium; "Oralloy"
  • Uranium Fuel: approx. 140 lbs; target - 85 lbs and projectile - 55 lbs
  • Target case, barrel, uranium projectile, and other main parts ferried to Tinian Island via USS Indianapolis
  • Uranium target component ferried to Tinian via C-54 aircraft of the 509th Composite Group
  • Efficiency of weapon: poor
  • Approx. 1.38% of the uranium fuel actually fissioned
  • Explosive force: 15,000 tons of TNT equivalent
  • Use: Dropped on Japanese city of Hiroshima; August 6, 1945
  • Delivery: B-29 Enola Gay piloted by Col. Paul Tibbets

The Fat Man Bomb:

Dropped on the Japanese city of Nagasaki on August 9, 1945, it was the second nuclear weapon used in a war. Following are some approximate statistics for Fat Man.

  • Weight: 10,800 lbs
  • Length: 10 ft 8 in.; Diameter: 60 in.
  • Fuel: Highly enriched plutonium 239
  • Plutonium Fuel: approx. 13.6 lbs; approx. size of a softball
  • Plutonium core surrounded by 5,300 lbs of high explosives; plutonium core reduced to size of tennis ball
  • Bomb Initiator: Beryllium - Polonium
  • All components of Fat Man ferried to Tinian Island aboard B-29's of the 509th CG
  • Efficiency of weapon: 10 times that of Little Boy
  • Approx 1.176 grams of plutonium converted to energy
  • Explosive force: 21,000 tons of TNT equivalent
  • Use: Dropped on Japanese city of Nagasaki; August 9, 1945
  • Nuclear Weaponeer: Cdr. Frederick Ashworth
  • Delivery: B-29 Bockscar piloted by Maj. Charles Sweeney


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