Little Boy
Little Boy was a type of atomic bomb created by the Manhattan Project during World War II. The name is also often used to describe the specific bomb used in the bombing of the Japanese city of Hiroshima by the Boeing B-29 Superfortress Enola Gay on 6 August 1945, making it the first nuclear weapon used in warfare, and the second nuclear explosion in history, after the Trinity nuclear test. It exploded with an energy of approximately and had an explosion radius of approximately which caused widespread death across the city. It was a gun-type fission weapon which used uranium that had been enriched in the isotope uranium-235 to power its explosive reaction.
Little Boy was developed by Lieutenant Commander Francis Birch's group at the Los Alamos Laboratory. It was the successor to a plutonium-fueled gun-type fission design, Thin Man, which was abandoned in 1944 after technical difficulties were discovered. Little Boy used a charge of cordite to fire a hollow cylinder of highly enriched uranium through an artillery gun barrel into a solid cylinder of the same material. The design was highly inefficient: the weapon used on Hiroshima contained of uranium, but less than a kilogram underwent nuclear fission. Unlike the implosion design developed for the Trinity test and the Fat Man bomb design that was used against Nagasaki, which required sophisticated coordination of shaped explosive charges, the simpler but inefficient gun-type design was considered almost certain to work, and was never tested prior to its use at Hiroshima.
After the war, numerous components for additional Little Boy bombs were built. By 1950, at least five weapons were completed; all were retired by November 1950.
Naming the bomb
There are two primary accounts of how the first atomic bombs got their names. Los Alamos Laboratory and Project Alberta physicist Robert Serber stated, many decades after the fact, that he had 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. It was also sometimes referred to as the "Mark I" nuclear bomb design, with "Mark II" referring to the abandoned Thin Man, and "Mark III" to the "Fat Man."In September 1945, another Project Alberta physicist, Norman F. Ramsey, stated 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 "Air Force representatives" for "security reasons," so that their communications over telephones sounded "as if they were modifying a plane to carry Roosevelt and Churchill," as opposed to modifying the B-29s to carry the two atomic bomb shapes as part of Project Silverplate in late 1943.
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: that the terms "Thin Man" and "Fat Man" were first developed by someone at or from Los Alamos, but were consciously adopted by the officers in Silverplate when they were adopting their own codenames for their own project. 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. Little Boy was known variously as Thin Man, the Skinny One and a number of similar names.
Development
Early gun-type design work
Because of its perceived simplicity, the gun-type nuclear weapon design was the first approach pursued by the scientists working on bomb design during the Manhattan Project. In 1942, it was not yet known which of the two fissile materials pathways being simultaneously pursued—uranium-235 or plutonium-239—would be successful, or if there were significant differences between the two fuels that would impact the design work. Coordination with British scientists in May 1942 convinced the American scientists, led by J. Robert Oppenheimer, that the atomic bomb would not be difficult to design and that the difficulty would lie only in the production of fuel. Calculations in mid-1942 by theoretical physicists working on the project reinforced the idea that an ordinary artillery gun barrel would be able to impart sufficient velocity to the fissile material projectile.Several different weapon designs, including autocatalytic assembly, a nascent version of implosion, and alternative gun designs were pursued in the early years of the project, while the facilities to manufacture fissile material were being constructed. The belief that the gun design would be an easy engineering task once fuel was available led to a sense of optimism at Los Alamos, although Oppenheimer established a small research group to study implosion as a fallback in early 1943. A full ordnance program for gun-design development was established by March 1943, with expertise provided by E.L. Rose of Jones and Lamson, an experienced gun designer and engineer.
Rose pointed out that the gun durability was immaterial as it would be destroyed when used. So drastic reductions could be made in Thin Man’s size and weight because it would be destroyed in the explosion.
Work was begun to study the properties of barrels, internal and external ballistics, and tampers of gun weapons. Oppenheimer led aspects of the effort, telling Rose that "at the present time our estimates are so ill founded that I think it better for me to take responsibility for putting them forward." He soon delegated the work to Naval Captain William Sterling Parsons, who, along with Ed McMillan, Charles Critchfield, and Joseph Hirschfelder would be responsible for rendering the theory into practice.
File:Thin Man plutonium gun bomb casings.jpg|right|thumb|"Thin Man" plutonium gun test casings at Wendover Army Air Field, as part of Project Alberta in the Manhattan Project, illustrating their relative length and size
Concern that impurities in reactor-bred plutonium would make predetonation more likely meant that much of the gun-design work was focused on the plutonium gun. To achieve high projectile velocities, the plutonium gun was long with a narrow diameter which created considerable difficulty in its ballistics dropping from aircraft and fitting it into the bomb bay of a B-29.
In early 1944, Emilio G. Segrè and his P-5 Group at Los Alamos received the first samples of plutonium produced from a nuclear reactor, the X-10 Graphite Reactor at Clinton Engineer Works in Oak Ridge, Tennessee. Analyzing it, they discovered that the presence of the isotope plutonium-240 raised the rate of spontaneous fission of the plutonium to an unacceptable amount. Previous analyses of plutonium had been made from samples created by cyclotrons and did not have as much of the contaminating isotope. If reactor-bred plutonium was used in a gun-type design, they concluded, it would predetonate, causing the weapon to destroy itself before achieving the conditions for a large-scale explosion.
File: Atombombe Little Boy.jpg|thumb|As part of Project Alberta, Commander A. Francis Birch assembles the combat unit while physicist Norman Ramsey watches. This is one of the rare photos where the interior of the bomb can be seen, with the fuzing units and battery boxes visible, attached around the gun tube.
From Thin Man to Little Boy
As a consequence of the discovery of the Pu-240 contamination problem, in July 1944 almost all research at Los Alamos was redirected to the implosion-type plutonium weapon, and the laboratory was entirely reorganized around the implosion problem. Work on the gun-type weapon continued under Parsons's Ordnance Division, for use exclusively with highly enriched uranium as a fuel. All the design, development, and technical work at Los Alamos was consolidated under Lieutenant Commander Francis Birch's group.In contrast to the plutonium implosion-type nuclear weapon and the plutonium gun-type fission weapon, the uranium gun-type weapon was much simpler to design. Because a high-velocity gun was no longer required, the overall length of the gun barrel could be dramatically decreased, and this allowed the weapon to fit into a B-29 bomb bay without difficulty. Though not an optimal use of fissile material compared to the implosion design, it was seen as a nearly guaranteed weapon.
The design specifications were completed in February 1945, and contracts were let to build the components. Three different plants were used so that no one would have a copy of the complete design. The gun and breech were made by the Naval Gun Factory in Washington, D.C.; the target case and some other components by the Naval Ordnance Plant in Center Line, Michigan; and the tail fairing and mounting brackets by the Expert Tool and Die Company in Detroit, Michigan. The bomb, except for the uranium payload, was ready at the beginning of May 1945. Manhattan District Engineer Kenneth Nichols expected on 1 May 1945 to have enriched uranium "for one weapon before August 1 and a second one sometime in December", assuming the second weapon would also be a gun type. Designing an implosion bomb for enriched uranium was considered, and this would increase the production rate. The enriched uranium projectile was completed on 15 June, and the target was completed on 24 July. The target and bomb pre-assemblies left Hunters Point Naval Shipyard, California, on 16 July aboard the heavy cruiser, arriving on 26 July. The target inserts followed by air on 30 July.
Although all of its components had been individually tested, no full test of a gun-type nuclear weapon occurred before the Little Boy was dropped over Hiroshima. The only test explosion of a nuclear weapon concept had been of an implosion-type device employing plutonium as its fissile material, which took place on 16 July 1945 at the Trinity nuclear test. There were several reasons for not testing a Little Boy type of device. Primarily, there was the issue of fissile material availability. K-25 at Clinton Engineer Works was designed to produce around 30 kilograms of enriched uranium per month, and the Little Boy design used over 60 kilograms per bomb. So testing the weapon would incur a considerable delay in use of the weapon. Because of the simplicity of the gun-type design, laboratory testing could establish that its parts worked correctly on their own: for example, dummy projectiles could be shot down the gun barrel to make sure they were "seated" correctly onto a dummy target. Absence of a full-scale test in the implosion-type design made it much more difficult to establish whether the necessary simultaneity of compression had been achieved. While there was at least one prominent scientist who advocated for a full-scale test, by early 1945 Little Boy was regarded as nearly a sure thing and was expected to have a higher yield than the first-generation implosion bombs.
Though Little Boy incorporated various safety mechanisms, an accidental detonation of a fully-assembled weapon was very possible. Should the bomber carrying the device crash, the hollow "bullet" could be driven onto the "target" cylinder, possibly detonating the bomb from gravity alone, but easily creating a critical mass that would release dangerous amounts of radiation. A crash of the B-29 and subsequent fire could trigger the explosives, causing the weapon to detonate. If immersed in water, the uranium components were subject to a neutron moderator effect, which would not cause an explosion but would release radioactive contamination. For this reason, pilots were advised to crash on land rather than at sea. Ultimately, Parsons opted to keep the explosives out of the Little Boy bomb until after the B-29 had taken off, to avoid the risk of a crash that could destroy or damage the military base from which the weapon was launched.