Fat Man - WikiHQ

"Fat Man" (also known as Mark III) was the design of the nuclear weapon the United States used for seven of the first eight nuclear weapons ever detonated in history. It is also the most powerful design to ever be used in warfare.

A Fat Man device was detonated over the Japanese city of Nagasaki on 9 August 1945. It was the second and larger of the only two nuclear weapons ever used in warfare. It was dropped from the Boeing B-29 Superfortress Bockscar piloted by Major Charles Sweeney. Its detonation marked the third nuclear explosion in history. The name Fat Man refers to the wide, round shape. Fat Man was an implosion-type nuclear weapon with a solid plutonium core, and later with improved cores.

The first Fat Man to be detonated was "The Gadget" in the Trinity nuclear test less than a month earlier on 16 July at the Alamogordo Bombing and Gunnery Range in New Mexico. It was built by scientists and engineers at Los Alamos Laboratory using plutonium manufactured at the Hanford Site. The second nuclear explosion, and the first used in warfare, was Little Boy, a different device based on uranium. Two more Fat Mans were detonated during the Operation Crossroads nuclear tests at Bikini Atoll in 1946. The three tests in the next series, Operation Sandstone in 1948, used Fat Man devices with improved cores. Fat Man was finally superseded by the Mark 4 nuclear bomb in the Operation Ranger tests.

Early decisions

Robert Oppenheimer held conferences in Chicago in June 1942, and in Berkeley, California, in July, at which various engineers and physicists discussed nuclear bomb design issues. They chose a gun-type design in which two sub-critical masses would be brought together by firing a "bullet" into a "target". Richard C. Tolman suggested an implosion-type nuclear weapon, but the proposal attracted little interest.

The feasibility of a plutonium bomb was questioned in 1942. Wallace Akers, the director of the British "Tube Alloys" project, told James Bryant Conant on 14 November that James Chadwick had "concluded that plutonium might not be a practical fissionable material for weapons because of impurities". Conant consulted Ernest Lawrence and Arthur Compton, who acknowledged that their scientists at Berkeley and Chicago, respectively, knew about the problem, but they could offer no ready solution. Conant informed Manhattan Project director Brigadier General Leslie R. Groves Jr., who in turn assembled a special committee consisting of Lawrence, Compton, Oppenheimer, and McMillan to examine the issue. The committee concluded that any problems could be overcome simply by requiring higher purity.

Oppenheimer reviewed his options in early 1943 and gave priority to the gun-type weapon, but he created the E-5 Group at the Los Alamos Laboratory under Seth Neddermeyer to investigate implosion as a hedge against the threat of pre-detonation. Implosion-type bombs were determined to be significantly more efficient in terms of explosive yield per unit mass of fissile material in the bomb, because compressed fissile materials react more rapidly and therefore more completely. Nonetheless, it was decided that the plutonium gun would receive the bulk of the research effort, since it was the project with the least uncertainty involved. It was assumed that the uranium gun-type bomb could be easily adapted from it.

Naming

The gun-type and implosion-type designs were codenamed "Thin Man" and "Fat Man", respectively. These code names were created by Robert Serber, a former student of Oppenheimer's who worked on the Manhattan Project. He chose them based on their design shapes; the Thin Man was a very long device, and the name came from the Dashiell Hammett detective novel The Thin Man and series of movies. The Fat Man was round and fat and was named after Sydney Greenstreet's character in Hammett's The Maltese Falcon. The Little Boy uranium gun-type design came later and was named only to contrast with the Thin Man. Los Alamos's Thin Man and Fat Man code names were adopted by the United States Army Air Forces in their involvement in the Manhattan Project, codenamed Silverplate. A cover story was devised that Silverplate was about modifying a Pullman car for use by President Franklin Roosevelt (Thin Man) and United Kingdom Prime Minister Winston Churchill (Fat Man) on a secret tour of the United States. Air Forces personnel used the code names over the phone to make it sound as though they were modifying a plane for Roosevelt and Churchill. Fat Man was also called the Big Boy, the Round Man, the Big Fellow and the like.

Development

Neddermeyer discarded Serber and Tolman's initial concept of implosion as assembling a series of pieces in favor of one in which a hollow sphere was imploded by an explosive shell. He was assisted in this work by Hugh Bradner, Charles Critchfield, and John Streib. L. T. E. Thompson was brought in as a consultant and discussed the problem with Neddermeyer in June 1943. Thompson was skeptical that an implosion could be made sufficiently symmetric. Oppenheimer arranged for Neddermeyer and Edwin McMillan to visit the National Defense Research Committee's Explosives Research Laboratory near the laboratories of the Bureau of Mines in Bruceton, Pennsylvania (a Pittsburgh suburb), where they spoke to George Kistiakowsky and his team. But Neddermeyer's efforts in July and August at imploding tubes to produce cylinders tended to produce objects that resembled rocks. Neddermeyer was the only person who believed that implosion was practical, and only his enthusiasm kept the project alive.

upright=1.25|thumb|left|alt=Fat Man Replica|Replica mockup of a Fat Man displayed in the [[National Museum of the United States Air Force, beside the Bockscar B-29 that dropped the original device – black liquid asphalt sealant was sprayed over the original bomb casing's seams, simulated on the mockup.]]

Oppenheimer brought John von Neumann to Los Alamos in September to take a fresh look at implosion. After reviewing Neddermeyer's studies, and discussing the matter with Edward Teller, von Neumann suggested the use of high explosives in shaped charges to implode a sphere, which he showed could not only result in a faster assembly of fissile material than was possible with the gun method, but greatly reduce the amount of material required because of the resulting higher density. The idea that, under such pressures, the plutonium metal would be compressed came from Teller, whose knowledge of how dense metals behaved under heavy pressure was influenced by his pre-war theoretical studies of the Earth's core with George Gamow. The prospect of more-efficient nuclear weapons impressed Oppenheimer, Teller, and Hans Bethe, but they decided that an expert on explosives would be required. Kistiakowsky's name was immediately suggested, and Kistiakowsky was brought into the project as a consultant in October.

The implosion project remained a backup until April 1944, when experiments by Emilio G. Segrè and his P-5 Group at Los Alamos on the newly reactor-produced plutonium from the X-10 Graphite Reactor at Oak Ridge and the B Reactor at the Hanford Site showed that it contained impurities in the form of the isotope plutonium-240. This has a far higher spontaneous fission rate and radioactivity than plutonium-239. The cyclotron-produced isotopes, on which the original measurements had been made, held much lower traces of plutonium-240. Its inclusion in reactor-bred plutonium appeared unavoidable. This meant that the spontaneous fission rate of the reactor plutonium was so high that pre-detonation was highly likely and that the bomb would blow itself apart during the initial formation of critical mass, creating a "fizzle." The distance required to accelerate the plutonium to speeds where pre--detonation would be less likely would need a gun barrel too long for any existing or planned bomber. The only way to use plutonium in a workable bomb was therefore implosion.

The impracticability of a gun-type bomb using plutonium was agreed at a meeting in Los Alamos on 17 July 1944. All gun-type work in the Manhattan Project was re-directed towards the Little Boy, enriched uranium gun design, and the Los Alamos Laboratory was reorganized with almost all of the research focused on the problems of implosion for the Fat Man bomb. The idea of using shaped charges as three-dimensional explosive lenses came from James L. Tuck and was developed by von Neumann. The success of the bomb relied on absolute precision in all of the plates moving inward at the same time. To overcome the difficulty of synchronizing multiple detonations, Luis Alvarez and Lawrence Johnston invented exploding-bridgewire detonators to replace the less precise primacord detonation system. Robert Christy is credited with doing the calculations that showed how a solid subcritical sphere of plutonium could be compressed to a critical state, greatly simplifying the task, since earlier efforts had attempted the more-difficult compression of a hollow spherical shell. After Christy's report, the solid-plutonium core weapon was referred to as the "Christy Gadget".

The task of the metallurgists was to determine how to cast plutonium into a sphere. The difficulties became apparent when attempts to measure the density of plutonium gave inconsistent results. At first contamination was believed to be the cause, but it was soon determined that there were multiple allotropes of plutonium. The brittle α phase that exists at room temperature changes to the plastic β phase at higher temperatures. Attention then shifted to the even more malleable δ phase that normally exists in the range. It was found that this was stable at room temperature when alloyed with aluminum, but aluminum emits neutrons when bombarded with alpha particles, which would exacerbate the pre-ignition problem. The metallurgists then hit upon a plutonium–gallium alloy, which stabilized the δ phase and could be hot pressed into the desired shape. They found it easier to cast hemispheres than spheres. The core consisted of two hemispheres with a ring with a triangular cross-section between them to keep them aligned and prevent jets forming. As plutonium was found to corrode readily, the sphere was coated with nickel.

thumb|left|A [[pumpkin bomb (Fat Man test unit) being raised from the pit into the bomb bay of a B-29 for bombing practice during the weeks before the attack on Nagasaki]]

The size of the bomb was constrained by the available aircraft, which were investigated for suitability by Norman Foster Ramsey. The only Allied bombers considered capable of carrying the Fat Man without major modification were the British Avro Lancaster and the American Boeing B-29 Superfortress. British scientist James Chadwick advocated the Lancaster which had a limited range but had a larger single bomb bay; but this was less of a problem when the Fat Man replaced the long ( Thin Man. At the time, the B-29 represented the epitome of bomber technology with significant advantages in maximum takeoff weight, range, speed, flight ceiling, and survivability. Without the availability of the B-29, dropping the bomb would likely have been impossible. However, this still constrained the bomb to a maximum length of , width of and weight of . Removing the bomb rails allowed a maximum width of .

Drop tests began in March 1944 and resulted in modifications to the Silverplate aircraft due to the weight of the bomb. High-speed photographs revealed that the tail fins folded under the pressure, resulting in an erratic descent. Various combinations of stabilizer boxes and fins were tested on the Fat Man shape to eliminate its persistent wobble until an arrangement dubbed a "California Parachute" was approved, a cubical open-rear tail box outer surface with eight radial fins inside of it, four angled at 45 degrees and four perpendicular to the line of fall holding the outer square-fin box to the bomb's rear end. In drop tests in early weeks, the Fat Man missed its target by an average of , but this was halved by June as the bombardiers became more proficient with it.

The early Y-1222 model Fat Man was assembled with some 1,500 bolts. This was superseded by the Y-1291 design in December 1944. This redesign work was substantial, and only the Y-1222 tail design was retained. Later versions included the Y-1560, which had 72 detonators; the Y-1561, which had 32; and the Y-1562, which had 132. There were also the Y-1563 and Y-1564, which were practice bombs with no detonators at all. The final wartime Y-1561 design was assembled with just 90 bolts.

On 16 July 1945, a Y-1561 model Fat Man, known as the Gadget, was detonated in a test explosion at a remote site in New Mexico, known as the "Trinity" test. It gave a yield of about . Some minor changes were made to the design as a result of the Trinity test. Philip Morrison recalled that "There were some changes of importance... The fundamental thing was, of course, very much the same." For certain non-atomic parts the manufacturer had not met the delivery schedule, so many tests had to be done twice over, one with all components except for the missing item, and at a critically late date with the complete assembly minus the nuclear components. So the first live tests of the missing part were conducted only a few days before it was dropped on Nagasaki.

Interior

The bomb was long and in diameter. It weighed .

File:Fat Man External.svg|Fat Man external schematic. 1. One of four AN 219 contact fuzes 2. Archie radar antenna 3. Plate with batteries (to detonate charge surrounding nuclear components) 4. X-Unit, a firing set placed near the charge 5. Hinge fixing the two ellipsoidal parts of the bomb 6. Physics package (see details below) 7. Plate with instruments (radars, baroswitches, and timers) 8. Barotube collector 9. California Parachute tail assembly ( aluminum sheet)

File:Fat Man Internal Components.png|Fat Man internal schematic

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Assembly

thumb|Cross section of the Fat Man "physics package". See description and colors in this section for details.|440x440px

thumb|right|Fat Man's "physics package" nuclear device about to be encased

thumb|right|Fat Man on its transport carriage, with liquid asphalt sealant applied over the casing's seams

thumb|right|Preserved Tinian "bomb pit#2", where Fat Man was loaded aboard Bockscar

The plutonium pit was in diameter and contained an "Urchin" modulated neutron initiator that was in diameter. The depleted uranium tamper was an sphere, surrounded by a shell of boron-impregnated plastic. The plastic shell had a cylindrical hole running through it, like the hole in a cored apple, in order to allow insertion of the pit as late as possible. The missing tamper cylinder containing the pit could be slipped in through a hole in the surrounding aluminum pusher. The pit was warm to the touch, emitting 2.4 W/kg-Pu, about 15 W for the core.

The explosion symmetrically compressed the plutonium to twice its normal density before the "Urchin" added free neutrons to initiate a fission chain reaction.