Editing Little Boy (section)

==Development==
===Early gun-type design work===
Because of its perceived simplicity, the [[gun-type fission weapon|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 material]]s pathways being simultaneously pursued—[[uranium-235]] or [[plutonium-239]]—would be successful, or if there were significant differences between the two [[Nuclear fuel|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.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|pp=42–44}}

Several different weapon designs, including autocatalytic assembly, a nascent version of implosion, and alternative gun designs (e.g., using high explosives as a propellent, or creating a "double gun" with two projectiles) 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.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|pp=67, 75}} A full ordnance program for gun-design development was established by March 1943, with expertise provided by [[E.L. Rose]], an experienced gun designer and engineer. Work was begun to study the properties of barrels, internal and external [[ballistics]], and [[Tamper (nuclear weapon)|tampers]] of gun weapons. Oppenheimer led aspects of the effort, telling Rose that "at the present time [May 1945] 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 [[Edwin McMillan|Ed McMillan]], [[Charles Critchfield]], and [[Joseph Hirschfelder]] would be responsible for rendering the theory into practice.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|pp=82–84}}

[[File:Thin Man plutonium gun bomb casings.jpg|right|thumb|"Thin Man" plutonium gun test casings at [[Wendover Air Force Base|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 {{convert|17|ft}} long with a narrow diameter (suggesting its codename as the Thin Man) which created considerable difficulty in its ballistics dropping from aircraft and fitting it into the [[bomb bay]] of a B-29.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|pp=87, 114}}

[[File:Little Boy target case and gun tube.jpg|thumb|Little Boy target case (left) attached to a gun tube at Anchor Ranch, Los Alamos, used for testing purposes. Compare with a [[:File:Thin Man testing at Anchor Ranch.jpg|similar image]] of the Thin Man internal components.]]

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]] (Pu-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 [[cyclotron]]s and did not have as much of the contaminating isotope. If reactor-bred plutonium was used in a gun-type design, they concluded, [[Fizzle (nuclear explosion)|it would predetonate]], causing the weapon to destroy itself before achieving the conditions for a large-scale explosion.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|p=228}}

[[File: Atombombe Little Boy.jpg|thumb|As part of [[Project Alberta]], Commander [[Francis Birch (geophysicist)|A. Francis Birch]] (left) assembles the combat unit (L-11) 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 nuclear weapon|implosion-type]] plutonium weapon, and the laboratory was entirely reorganized around the implosion problem. Work on the gun-type weapon continued under Person's Ordnance (O) 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 (United States)|Lieutenant Commander]] [[Francis Birch (geophysicist)|Francis Birch]]'s group.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|pp=245–249}}

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. As 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.{{sfn|Rhodes|1986|p=541}}

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 [[Breechblock|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]].{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|p=257}} The bomb, except for the uranium payload, was ready at the beginning of May 1945.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|p=262}} 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 be a gun type; designing an implosion bomb for enriched uranium was considered, and this would increase the production rate.{{sfn|Nichols|1987|pp=166,175–176}} The enriched uranium projectile was completed on 15 June, and the target was completed on 24 July.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|p=265}} The target and bomb pre-assemblies (partly assembled bombs without the fissile components) left [[Hunters Point Naval Shipyard]], California, on 16 July aboard the [[heavy cruiser]] {{USS|Indianapolis|CA-35|6}}, arriving on 26 July.{{sfn|Coster-Mullen|2012|p=30}} The target inserts followed by air on 30 July.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|p=265}}

Although all of its components had been individually tested,{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|p=265}} no full test of a gun-type nuclear weapon occurred before the Little Boy was dropped over [[Hiroshima]]. The only [[nuclear testing|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. (By comparison, [[B Reactor]] at the [[Hanford Site]] was designed to produce around 20 kilograms of plutonium per month, and each Fat Man bomb used around 6 kilograms of material.){{sfn|Hansen|1995|pp=111–112}} 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 ([[Ernest O. Lawrence]]) 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.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|p=293}}

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 into the "target" cylinder, possibly detonating the bomb from gravity alone (though tests suggested this was unlikely), but easily creating a [[critical mass]] that would release dangerous amounts of radiation.{{sfn|Hansen|1995|p=113}} A crash of the B-29 and subsequent fire could trigger the explosives, causing the weapon to detonate.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|p=333}} 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.{{sfn|Hansen|1995|p=113}} 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.{{sfn|Lewis|Tolzer|1957|p=72}}