Little Boy ready to be loaded into the Enola Gay Little Boy ready to be lifted into the Enola Gay Manhattan Project scientists and military personnel gather around the bomb pit, ready to watch the Little Boy bomb being loaded into the Enola Gay Little Boy, covered by a protective tarp for security reasons Dropped on the Japanese city of Nagasaki on August 9, 1945, it was the second nuclear weapon used in a war. In the implosion-type device, a core of sub-critical plutonium is surrounded by several thousand pounds of high-explosive designed in such a way that the explosive force of the HE is directed inwards thereby crushing the plutonium core into a super-critical state.

The first was the 'Gadget' detonated at the Trinity site on July 16, 1945. 'Fat Man' was the second plutonium, implosion-type bomb.

Delivery: B-29 Enola Gay piloted by Col.

Use: Dropped on Japanese city of Hiroshima August 6, 1945.

Explosive force: 15,000 tons of TNT equivalent.

1.38% of the uranium fuel actually fissioned

Uranium target component ferried to Tinian via C-54 aircraft of the 509th Composite Group.

Target case, barrel, uranium projectile, and other main parts ferried to Tinian Island via USS Indianapolis.

140 lbs target - 85 lbs and projectile - 55 lbs

Fuel: Highly enriched uranium 'Oralloy'.

Dropped on the Japanese city of Hiroshima on August 6, 1945, it was the first nuclear weapon used in a war. The resulting uranium mass comprised of both projectile and target becomes critical and the chain reaction begins. 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.

For more information, see The Science Behind the Atom Bomb. 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. This is the minimum amount of fissionable material needed to start a chain reaction. 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.

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