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The Smithsonian and the Enola Gay:
The Atomic Bomb
The Manhattan Project
Spurred on by German success in splitting the atom and fearing the
Germans would develop a nuclear bomb first, US scientists had been
working toward an atomic weapon since 1939. They pursued two approaches
to creating fissionable material, one to extract U-235 nuclear fuel from
natural uranium (U-238) and the other to produce plutonium. Both
approaches would be successful.
In 1942, the program was transferred to the Army Corps of Engineers and
designated the “Manhattan Project,” taking its name from the Corps’s
Manhattan Engineer District. Col. Leslie R. Groves—later a major general—was
appointed as director.
Plants at Oak Ridge, Tenn., and Hanford, Wash., produced the U-235 and the
plutonium. At the University of Chicago, Enrico Fermi and his team succeeded
in generating the world’s first controlled nuclear chain reaction. Scientists
and engineers at Los Alamos, N. M., headed by physicist J. Robert Oppenheimer,
worked on designing and building an atomic bomb.
Los Alamos tried two possible designs, a bulbous 10-foot bomb called “Fat Man”
and a long, skinny 17-foot bomb called “Thin Man.” Eventually, Thin Man was
canceled in favor of a shorter design dubbed “Little Boy.”
The program was ready for testing by 1945, but there was only enough U-235 for
one bomb, so the test bomb—known as “the gadget”—was a plutonium device,
similar to “Fat Man,” the bomb that would be dropped on Nagasaki. The “gadget”
was tested successfully at the “Trinity” site in the New Mexico desert, July 16, 1945.
Los Alamos produced two operational bombs: “Little Boy,” the uranium bomb dropped on
Hiroshima, and the “Fat Man” plutonium bomb used at Nagasaki.
Click here
for more from the National Atomic Museum:
www.atomicmuseum.com/tour/manhattanproject.cfm
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Little Boy
(Hiroshima)
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Fat Man
(Nagasaki)
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Weight
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9,700 lbs.
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10,800 lbs.
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Length
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10 ft. |
10 ft. 8 in.
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Diameter
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28 in.
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60 in.
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Fuel
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enriched uranium
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enriched plutonium
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Detonation
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gun type
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implosion
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Explosive Force |
15,000 tons TNT |
21,000 tons TNT |
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Although the explosive power of Fat Man was greater
than that of Little Boy, the damage was less extensive because of the hilly
terrain around Nagasaki.
Sources: The Manhattan Project Heritage
Preservation Association; USAF Museum.
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Little Boy Return To Top
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Little Boy was anything but “little.” It weighed almost 10,000 pounds.
On Tinian, the B-29 was pulled astraddle a loading pit and the bomb was hoisted
into the bomb bay of the aircraft with a hydraulic lift.
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Little Boy was the only nuclear weapon to use uranium as the fissionable
material. It was simpler, but also less efficient, than implosion bombs,
like Fat Man.
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It was detonated by a mechanism that resembled a cannon. At the muzzle was the
“target,” a hollowed-out subcritical mass of uranium. The “cannon ball” was another
subcritical mass of uranium, a perfect fit to plug the hollow of the target. The
plug was propelled down the cannon barrel by several thousand pounds of high
explosive. When it hit, the combination of compression and increased mass pushed
the uranium to the supercritical level and the bomb went off.
The Enola Gay weaponeer, Navy Capt. Deak Parsons, was concerned about taking
off with Little Boy fully assembled and live. Some heavily loaded B-29s had
crashed on takeoff from Tinian. If that happened to the Enola Gay, the bomb might
explode and wipe out half the island. Thus, Parsons, assisted by Lt. Morris Jeppson,
finished the assembly and armed the bomb in the bomb bay after takeoff.
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Fat Man Return To Top
The Nagasaki bomb worked on the basis of implosion, as did the “gadget”
test device at Trinity. Fat Man had a core of plutonium at subcritical mass
in the center of a sphere, surrounded by 64 precisely-timed high explosive
charges. Upon detonation, the high explosive compressed the plutonium core
from the size of a grapefruit to the size of tennis ball, achieving supercritical
mass and inducing the nuclear explosion.
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Fat Man was too complex to arm in flight. Bockscar took off with the bomb fully armed.
Further Reading Return To Top
CHRISTMAN, Albert B. Target Hiroshima: Deak Parsons and the
Creation of the Atomic Bomb. Naval Institute Press, 1998.
RHODES, Richard. The Making of the Atomic Bomb. Simon & Schuster, 1986.
WHEELER, Keith. The Fall of Japan. Time-Life Books, 1983.
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Return To Top
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The Manhattan Project
