Fat Man

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Fat Man
Los Alamos Laboratory
No. built120
Mass10,300 pounds (4,670 kg)
Length128 inches (3.3 m)
Diameter60 inches (1.5 m)

Filling weight6.4 kg
Blast yield21 kt (88 TJ)

"Fat Man" (also known as Mark III) was the codename for the type of

Los Alamos Laboratory using plutonium from the Hanford Site, and one was dropped from the Boeing B-29 Superfortress Bockscar piloted by Major Charles Sweeney

The name Fat Man refers to the early design of the bomb because it had a wide, round shape. Fat Man was an

Trinity nuclear test less than a month earlier on 16 July at the Alamogordo Bombing and Gunnery Range in New Mexico. Two more were detonated during the Operation Crossroads nuclear tests at Bikini Atoll in 1946, and some 120 were produced between 1947 and 1949, when it was superseded by the Mark 4 nuclear bomb
. The Fat Man was retired in 1950.

Early decisions

implosion-type nuclear weapon, but the proposal attracted little interest.[2]

The feasibility of a plutonium bomb was questioned in 1942.

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.[4]

Oppenheimer reviewed his options in early 1943 and gave priority to the gun-type weapon,

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.[5]


The gun-type and implosion-type designs were codenamed "

Franklin Roosevelt (Thin Man) and United Kingdom Prime Minister Winston Churchill (Fat Man) on a secret tour of the United States.[7] 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.[8]


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.[9]

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

Earth's core with George Gamow.[11] 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 1943.[10]

The implosion project remained a backup until April 1944, when experiments by

predetonate and blow itself apart during the initial formation of a critical mass.[12] The distance required to accelerate the plutonium to speeds where predetonation 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.[13]

Small-scale slow-motion cut-away of shaped-charge implosion device.

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

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.[16] After Christy's report, the solid-plutonium core weapon was referred to as the "Christy Gadget".[17]

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.[18] 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 300–450 °C (570–840 °F) 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.[19][20]

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 aircraft considered capable of carrying the Fat Man without major modification were the British Avro Lancaster and the American Boeing B-29 Superfortress.[21][22][23] 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 11 feet (3.4 m), width of 5 feet (1.5 m) and weight of 20,000 pounds (9,100 kg). Removing the bomb rails allowed a maximum width of 5.5 feet (1.7 m).[22]

Drop tests began in March 1944, and resulted in modifications to the Silverplate aircraft due to the weight of the bomb.[24] 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.[21] In drop tests in early weeks, the Fat Man missed its target by an average of 1,857 feet (566 m), but this was halved by June as the bombardiers became more proficient with it.[25]

The early Y-1222 model Fat Man was assembled with some 1,500 bolts.[26][27] 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.[27] 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.[28] The final wartime Y-1561 design was assembled with just 90 bolts.[26] On 16 July 1945, a Y-1561 model Fat Man, known as the Gadget, was detonated in a

Trinity" test. It gave a yield of about 25 kilotonnes (100 TJ).[29] Some minor changes were made to the design as a result of the Trinity test.[30] Philip Morrison recalled that "There were some changes of importance... The fundamental thing was, of course, very much the same."[31][32]


The bomb was 128.375 inches (3.2607 m) long and 60.25 inches (153.0 cm) in diameter. It weighed 10,265 pounds (4,656 kg).[33]

  • 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 (0.20-inch [5.1 mm] aluminum sheet)
    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 (0.20-inch [5.1 mm] aluminum sheet)
  • Fat Man internal schematic
    Fat Man internal schematic


Cross section of the Fat Man "physics package". See description and colors in this section for details.
Fat Man's "physics package" nuclear device about to be encased
Fat Man on its transport carriage, with liquid asphalt sealant applied over the casing's seams
Preserved Tinian "bomb pit#2", where Fat Man was loaded aboard Bockscar


tamper was an 8.75-inch-diameter (222 mm) sphere, surrounded by a 0.125-inch-thick (3.2 mm) shell of boron-impregnated plastic. The plastic shell had a 5-inch-diameter (130 mm) 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 18.5-inch-diameter (470 mm) aluminum pusher.[34] The pit was warm to the touch, emitting 2.4 W/kg-Pu, about 15 W for the 6.19-kilogram (13.6 lb) core.[35]

The explosion symmetrically compressed the plutonium to twice its normal density before the "Urchin" added

The result was the fission of about 1 kilogram (2.2 lb) of the 6.19 kilograms (13.6 lb) of plutonium in the pit, i.e. of about 16% of the fissile material present.[43][44] The detonation released the energy equivalent to the detonation of 21 kilotons of TNT or 88 terajoules.[45] About 30% of the yield came from fission of the uranium tamper.[42]

Bombing of Nagasaki

Bomb assembly

Mushroom cloud after Fat Man exploded over Nagasaki on 9 August 1945

The first

393d Bombardment Squadron, and another from the 216th Army Air Forces Base Unit. The cores were transported to North Field, arriving on 2 August, when F31 was partly disassembled in order to check all its components. F33 was expended near Tinian during a final rehearsal on 8 August. F32 presumably would have been used for a third attack or its rehearsal.[46]

On 7 August, the day after the

Major General Curtis LeMay met on Guam to discuss what should be done next.[47] Since there was no indication of Japan surrendering,[48] they decided to proceed with their orders and drop another bomb. Parsons said that Project Alberta would have it ready by 11 August, but Tibbets pointed to weather reports indicating poor flying conditions on that day due to a storm and asked if the bomb could be made ready by 9 August. Parsons agreed to try to do so.[47][49]

Fat Man F31 was assembled on Tinian by Project Alberta personnel,

Frederick L. Ashworth from Project Alberta as the weaponeer in charge of the bomb.[52]

Bombing of Nagasaki

Detonation of the Mark III 'Fat Man' and ensuing mushroom cloud.
Hypocenter of Fat Man Atomic bomb in Nagasaki

Bockscar lifted off at 03:47 on the morning of 9 August 1945, with Kokura as the primary target and Nagasaki the secondary target. The weapon was already armed, but with the green electrical safety plugs still engaged. Ashworth changed them to red after ten minutes so that Sweeney could climb to 17,000 feet (5,200 m) in order to get above storm clouds.[53] During the pre-flight inspection of Bockscar, the flight engineer notified Sweeney that an inoperative fuel transfer pump made it impossible to use 640 US gallons (2,400 L) of fuel carried in a reserve tank. This fuel would still have to be carried all the way to Japan and back, consuming still more fuel. Replacing the pump would take hours; moving the Fat Man to another aircraft might take just as long and was dangerous as well, as the bomb was live. Colonel Paul Tibbets and Sweeney therefore elected to have Bockscar continue the mission.[54]

Effects of the Fat Man's detonation on Nagasaki

The target for the bomb was the city of Kokura, but it was found to be obscured by clouds and drifting smoke from fires started by a major firebombing raid by 224 B-29s on nearby Yahata the previous day. This covered 70% of the area over Kokura, obscuring the aiming point. Three bomb runs were made over the next 50 minutes, burning fuel and repeatedly exposing the aircraft to the heavy defenses of Yahata, but the bombardier was unable to drop visually. By the time of the third bomb run, Japanese anti-aircraft fire was getting close; Second Lieutenant Jacob Beser was monitoring Japanese communications, and he reported activity on the Japanese fighter direction radio bands.[55]

Sweeney then proceeded to the alternative target of Nagasaki. It was obscured by clouds, as well, and Ashworth ordered Sweeney to make a radar approach. At the last minute, however, bombardier

Kermit K. Beahan[52] found a hole in the clouds. The Fat Man was dropped and exploded at 11:02 local time, following a 43-second free-fall, at an altitude of about 1,650 feet (500 m).[53] There was poor visibility due to cloud cover and the bomb missed its intended detonation point by almost two miles, so the damage was somewhat less extensive than that in Hiroshima

An estimated 35,000–40,000 people were killed outright by the bombing at Nagasaki. A total of 60,000–80,000 fatalities resulted, including from long-term health effects, the strongest of which was leukemia with an attributable risk of 46% for bomb victims.[56] Others died later from related blast and burn injuries, and hundreds more from radiation illnesses from exposure to the bomb's initial radiation.[57] Most of the direct deaths and injuries were among munitions or industrial workers.[58]

Mitsubishi's industrial production in the city was also severed by the attack; the dockyard would have produced at 80 percent of its full capacity within three to four months, the steelworks would have required a year to get back to substantial production, the electric works would have resumed some production within two months and been back at capacity within six months, and the arms plant would have required 15 months to return to 60 to 70 percent of former capacity. The Mitsubishi-Urakami Ordnance Works, which manufactured the Type 91 torpedoes released in the attack on Pearl Harbor, was destroyed in the blast.[58][59]

Post-war development

Crossroads-Baker, 23-kilotons.

After the war, two Y-1561 Fat Man bombs were used in the Operation "Crossroads" nuclear tests at Bikini Atoll in the Pacific. The first was known as Gilda after Rita Hayworth's character in the 1946 movie Gilda, and it was dropped by the B-29 Dave's Dream; it missed its aim point by 710 yards (650 m). The second bomb was nicknamed Helen of Bikini and was placed without its tail fin assembly in a steel caisson made from a submarine's conning tower; it was detonated 90 feet (27 m) beneath the landing craft USS LSM-60. The two weapons yielded about 23 kilotonnes (96 TJ) each.[60]

The Los Alamos Laboratory and the Army Air Forces had already commenced work on improving the design. The

Project W-47 was continued, and drop tests resumed in January 1946.[61]

Sandstone-Yoke, 49-kilotons; utilized a newly designed 'levitated-pit' to increase yield efficiency.

The Mark III Mod 0 Fat Man was ordered into production in mid-1946. High explosives were manufactured by the Salt Wells Pilot Plant, which had been established by the Manhattan Project as part of Project Camel, and a new plant was established at the Iowa Army Ammunition Plant. Mechanical components were made or procured by the Rock Island Arsenal; electrical and mechanical components for about 50 bombs were stockpiled at Kirtland Army Air Field by August 1946, but only nine plutonium cores were available. Production of the Mod 0 ended in December 1948, by which time there were still only 53 cores available. It was replaced by improved versions known as Mods 1 and 2 which contained a number of minor changes, the most important of which was that they did not charge the X-Unit firing system's capacitors until released from the aircraft. The Mod 0s were withdrawn from service between March and July 1949, and by October they had all been rebuilt as Mods 1 and 2.[62] Some 120 Mark III Fat Man units were added to the stockpile between 1947 and 1949,[63] when it was superseded by the Mark 4 nuclear bomb.[64] The Mark III Fat Man was retired in 1950.[63][65]

A nuclear strike would have been a formidable undertaking in the post-war 1940s due to the limitations of the Mark III Fat Man. The lead-acid batteries which powered the fuzing system remained charged for only 36 hours, after which they needed to be recharged. To do this meant disassembling the bomb, and recharging took 72 hours. The batteries had to be removed in any case after nine days or they corroded. The plutonium core could not be left in for much longer, because its heat damaged the high explosives. Replacing the core also required the bomb to be completely disassembled and reassembled. This required about 40 to 50 men and took between 56 and 72 hours, depending on the skill of the bomb assembly team, and the

RDS–1" (above), which closely resembled Fat Man, even in its external shape.

The only aircraft capable of carrying the bomb were Silverplate B-29s, and the only group equipped with them was the 509th Bombardment Group at Walker Air Force Base in Roswell, New Mexico. They would first have to fly to Sandia Base to collect the bombs, and then to an overseas base from which a strike could be mounted.[66] In March 1948, during the Berlin Blockade, all the assembly teams were in Eniwetok for the Operation Sandstone test, and the military teams were not yet qualified to assemble atomic weapons.[67]

In June 1948, General Omar Bradley, Major General Alfred Gruenther and Brigadier General Anthony McAuliffe visited Sandia and Los Alamos to be shown the "special requirements" of atomic weapons. Gruenther asked Brigadier General Kenneth Nichols (hosting): "When are you going to show us the real thing? Surely this laboratory monstrosity is not the only type of atomic bomb we have in stockpile?"[68] Nichols told him that better weapons would soon become available. After the "astonishingly good" results of Operation Sandstone were available, stockpiling of improved weapons began.[68]

The Soviet Union's first nuclear weapon was based closely on Fat Man's design thanks to spies Klaus Fuchs, Theodore Hall, and David Greenglass, who provided them with secret information concerning the Manhattan Project and Fat Man. It was detonated on 29 August 1949 as part of Operation "First Lightning".[69][70][71]


  1. ^ Hoddeson et al. 1993, pp. 42–44.
  2. ^ a b Hoddeson et al. 1993, p. 55.
  3. ^ Nichols 1987, p. 64.
  4. ^ Nichols 1987, pp. 64–65.
  5. ^ Hoddeson et al. 1993, p. 87.
  6. ^ Serber & Crease 1998, p. 104.
  7. ^ Bowen 1959, p. 96.
  8. ^ Rhodes 1986, p. 481.
  9. ^ Hoddeson et al. 1993, pp. 86–90.
  10. ^ a b Hoddeson et al. 1993, pp. 130–133.
  11. ^ Teller 2001, pp. 174–176.
  12. ^ Hoddeson et al. 1993, p. 228.
  13. ^ a b Hoddeson et al. 1993, pp. 240–244.
  14. ^ a b Hoddeson et al. 1993, p. 163.
  15. ^ Coster-Mullen 2012, p. 110.
  16. ^ Hoddeson et al. 1993, pp. 270–271.
  17. ^ Hoddeson et al. 1993, pp. 293, 307–308.
  18. ^ Hewlett & Anderson 1962, pp. 244–245.
  19. ^ Baker, Hecker & Harbur 1983, pp. 144–145.
  20. ^ Wellerstein, Alex. "You don't know Fat Man". Restricted Data: The Nuclear Secrecy Blog. Archived from the original on 7 April 2014. Retrieved 4 April 2014.
  21. ^ a b Hoddeson et al. 1993, pp. 380–383.
  22. ^ a b Hansen 1995, pp. 119–120.
  23. ^ Groves 1962, p. 254.
  24. ^ Campbell 2005, pp. 8–10.
  25. ^ Hansen 1995, p. 131.
  26. ^ a b c Coster-Mullen 2012, p. 52.
  27. ^ a b Hansen 1995, p. 121.
  28. ^ Hansen 1995, p. 127.
  29. S2CID 244134027
  30. ^ Hoddeson et al. 1993, p. 377.
  31. ^ Coster-Mullen 2012, p. 53.
  32. ^ The most significant change involved the use of an anti-jet ring within the plutonium pit, described earlier. In the Trinity Gadget, the possibility of a fine jet of neutrons going between the seams of the pit was avoided by adding some crumpled gold foil around the initiator. Additionally, in the Trinity Gadget, the pit was electroplated with silver, whereas with the later Fat Man bombs, nickel was used.
  33. ^ Coster-Mullen 2012, p. 47.
  34. ^ a b Coster-Mullen 2012, p. 186.
  35. ^ Coster-Mullen 2012, p. 49.
  36. ^ a b Coster-Mullen 2012, p. 45.
  37. ^ a b c Coster-Mullen 2012, p. 41.
  38. ^ a b Hansen 1995, pp. 122–123.
  39. ^ Coster-Mullen 2012, p. 48.
  40. ^ Coster-Mullen 2012, p. 57.
  41. ^ Sublette, Carey (3 July 2007). "Section 8.0 The First Nuclear Weapons". Nuclear Weapons FAQ. Retrieved 29 August 2013.
  42. ^ a b Wellerstein, Alex (10 November 2014). "The Fat Man's Uranium". Restricted Data. Retrieved 9 December 2020.
  43. ^ Coster-Mullen 2012, p. 46.
  44. ^ Wellerstein, Alex (23 December 2013). "Kilotons per kilogram". Restricted Data. Retrieved 9 December 2020.
  45. ^ Malik 1985, p. 25.
  46. ^ a b Campbell 2005, pp. 38–40.
  47. ^ a b Russ 1990, pp. 64–65.
  48. ^ Frank 1999, pp. 283–284.
  49. ^ Groves 1962, p. 342.
  50. ^ Coster-Mullen 2012, p. 67.
  51. ^ "Bockscar … The Forgotten Plane That Dropped The Atomic Bomb « A Little Touch Of History". Awesometalks.wordpress.com. 7 August 2008. Retrieved 31 August 2012.
  52. ^ a b Campbell 2005, p. 32.
  53. ^ a b c Rhodes 1986, p. 740.
  54. ^ Sweeney, Antonucci & Antonucci 1997, pp. 204–205.
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  56. ^ Columbia university center for nuclear studies: Hiroshima and Nagasaki: The Long Term Health Effects Archived 23 July 2015 at the Wayback Machine, updated 7/3/2014
  57. ^ Craven & Cate 1953, pp. 723–725.
  58. ^ a b Nuke-Rebuke: Writers & Artists Against Nuclear Energy & Weapons (The Contemporary anthology series). The Spirit That Moves Us Press. 1 May 1984. pp. 22–29.
  59. ^ "United States Strategic Bombing Survey Summary Report (Pacific War) The Effects Of The Atomic Bombs". U.S. Strategic Bombing Survey. p. 24.
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  61. ^ Hansen 1995, pp. 137–142.
  62. ^ Hansen 1995, pp. 142–145.
  63. ^ a b Coster-Mullen 2012, p. 87.
  64. ^ Hansen 1995, p. 143.
  65. ^ Hansen 1995, p. 150.
  66. ^ Hansen 1995, pp. 147–149.
  67. ^ Nichols 1987, pp. 260, 264, 265.
  68. ^ a b Nichols 1987, p. 264.
  69. ^ Holmes, Marian Smith (19 April 2009). "Spies Who Spilled Atomic Bomb Secrets". Smithsonian. Retrieved 5 April 2019.
  70. .
  71. ^ Sublette, Carey (3 July 2007). "Section 8.1.1 The Design of Gadget, Fat Man, and "Joe 1" (RDS-1)". Nuclear Weapons FAQ. Retrieved 12 August 2011.


See also

  • Third Shot, a Fat Man-type weapon intended for a third Japanese target after Nagasaki

External links