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Little Boy

Little Boy was the name of the type of atomic bomb used in the bombing of the Japanese city of Hiroshima on 6 August 1945 during World War II, making it the first nuclear weapon used in warfare. The bomb was dropped by the Boeing B-29 Superfortress Enola Gay piloted by Colonel Paul W. Tibbets Jr., commander of the 509th Composite Group, and Captain Robert A. Lewis. It exploded with an energy of approximately 15 kilotons of TNT (63 TJ) and had an explosion radius of approximately 1.3 kilometers which caused widespread death across the city. The Hiroshima bombing was the second nuclear explosion in history, after the Trinity nuclear test.

This article is about the type of atomic bomb that was dropped on Hiroshima. For other uses, see Little Boy (disambiguation).

Little Boy

United States

  • Naval Gun Factory,
    Washington, D.C.
  • Naval Ordnance Plant,
    Center Line, Michigan
  • Expert Tool and Die Company,
    Detroit, Michigan

1945–1947

1 wartime + 5 postwar

9,700 pounds (4,400 kg)

10 feet (3.0 m)

28 inches (71 cm)

64 kg

15 kilotons of TNT (63 TJ)

Little Boy was developed by Lieutenant Commander Francis Birch's group at the Manhattan Project's Los Alamos Laboratory during World War II, a reworking of their abandoned Thin Man nuclear bomb. Like Thin Man, it was a gun-type fission weapon. It derived its explosive power from the nuclear fission of uranium-235, whereas Thin Man was based on fission of plutonium-239. Fission was accomplished by shooting a hollow cylinder (the "bullet") onto a solid cylinder of the same material (the "target") by means of a charge of nitrocellulose propellant powder. Little Boy contained 64 kilograms (141 lb) of highly enriched uranium, although less than a kilogram underwent nuclear fission. Its components were fabricated at three different plants so that no one would have a copy of the complete design. Unlike the implosion design, which required sophisticated coordination of shaped explosive charges, the gun-type design was considered almost certain to work so it was never tested before its first use at Hiroshima.


After the war, a number of components for further Little Boy bombs were built. By 1950, only five complete weapons had been created, and these were retired by November 1950.

Naming[edit]

There are two primary accounts of how the first atomic bombs got their names.


Los Alamos and Project Alberta physicist Robert Serber claimed, many decades after the fact, to have named the first two atomic bomb designs during World War II based on their shapes: Thin Man and Fat Man. The "Thin Man" was a long, thin device and its name came from the Dashiell Hammett detective novel and series of movies about The Thin Man. The "Fat Man" was round and fat so it was named after Kasper Gutman, a rotund character in Hammett's 1930 novel The Maltese Falcon, played by Sydney Greenstreet in the 1941 film version. Little Boy was named by others as an allusion to Thin Man since it was based on its design.[1]


In September 1945, another Project Alberta physicist, Norman F. Ramsey, claimed in his brief "History of Project A," that the early bomb ballistic test shapes designs were referred to as "Thin Man" and "Fat Man" by (unspecified) "Air Force representatives" for "security reasons," so that their communications over telephones sounded "as if they were modifying a plane to carry Roosevelt (the Thin Man) and Churchill (the Fat Man)," as opposed to modifying the B-29s to carry the two atomic bomb shapes as part of Project Silverplate in the fall of 1943.[2][3]


Another explanation of the names, from a classified United States Air Force history of Project Silverplate from the 1950s, implies a possible reconciliation of the two versions above: that the terms "Thin Man" and "Fat Man" were first developed by someone at or from Los Alamos (i.e., Serber), but were consciously adopted by the officers in Silverplate when they were adopting their own codenames for their own project (including "Silverplate"). As Silverplate involved modifying B-29s for a secret purpose, deliberately using codenames that would align with modifying vehicles for Roosevelt and Churchill would serve their needs well.[4]

A timer ensured that the bomb would not explode until at least fifteen seconds after release, one-quarter of the predicted fall time, to ensure the safety of the aircraft. The timer was activated when the electrical connecting it to the airplane pulled loose as the bomb fell, switching it to its internal 24-volt battery and starting the timer. At the end of the 15 seconds, the bomb would be 3,600 feet (1,100 m) from the aircraft, and the radar altimeters were powered up and responsibility was passed to the barometric stage.[34]

pull-out plugs

The purpose of the barometric stage was to delay activating the radar altimeter firing command circuit until near detonation altitude. A thin metallic membrane enclosing a vacuum chamber (a similar design is still used today in old-fashioned wall barometers) gradually deformed as ambient air pressure increased during descent. The barometric fuze was not considered accurate enough to detonate the bomb at the precise ignition height, because air pressure varies with local conditions. When the bomb reached the design height for this stage (reportedly 2,000 meters; 6,600 ft), the membrane closed a circuit, activating the radar altimeters. The barometric stage was added because of a worry that external radar signals might detonate the bomb too early.

[34]

Two or more radar altimeters were used to reliably detect final altitude. When the altimeters sensed the correct height, the firing switch closed, igniting the three BuOrd Mk15, Mod 1 Navy gun primers in the breech plug, which set off the charge consisting of four silk powder bags each containing 2 pounds (910 g) of WM slotted-tube cordite. This launched the uranium projectile towards the opposite end of the gun barrel at an eventual muzzle velocity of 300 meters per second (980 ft/s). Approximately 10 milliseconds later the chain reaction occurred, lasting less than 1 microsecond. The radar altimeters used were modified U.S. Army Air Corps APS-13 tail warning radars, nicknamed "Archie", normally used to warn a fighter pilot of another plane approaching from behind.[34]

redundant

at Carey Sublette's NuclearWeaponArchive.org

Little Boy description

Archived 3 March 2007 at the Wayback Machine Definition and explanation of 'Little Boy'

Nuclear Files.org

The Nuclear Weapon Archive

an interactive simulation of "Little Boy"

Simulation of "Little Boy"

Little Boy 3D Model

information about preparation and dropping the Little Boy bomb

Hiroshima & Nagasaki Remembered

Little boy Nuclear Bomb at Imperial War museum London UK (jpg)