Los Alamos Laboratory
|Mass||10,300 pounds (4,670 kg)|
|Length||128 inches (3.3 m)|
|Diameter||60 inches (1.5 m)|
|Filling weight||6.4 kg|
|Blast yield||21 kt (88 TJ)|
"Fat Man" (also known as Mark III) was the codename for the type of
The name Fat Man refers to the early design of the bomb because it had a wide, round shape. Fat Man was an
The feasibility of a plutonium bomb was questioned in 1942.
Oppenheimer reviewed his options in early 1943 and gave priority to the gun-type weapon,
The gun-type and implosion-type designs were codenamed "
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.
The implosion project remained a backup until April 1944, when experiments by
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
The task of the
The size of the bomb was constrained by the available aircraft, which were investigated for suitability by
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 1,857 feet (566 m), 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
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).
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
The explosion symmetrically compressed the plutonium to twice its normal density before the "Urchin" added
- Ansoccer balls).
- The detonation wave (arrows) is initially convex in the...
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. The detonation released the energy equivalent to the detonation of 21 kilotons of TNT or 88 terajoules. About 30% of the yield came from fission of the uranium tamper.
Bombing of Nagasaki
On 7 August, the day after the
Fat Man F31 was assembled on Tinian by Project Alberta personnel,
Bombing of 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. 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.
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.
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
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. Others died later from related blast and burn injuries, and hundreds more from radiation illnesses from exposure to the bomb's initial radiation. Most of the direct deaths and injuries were among munitions or industrial workers.
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.
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.
The Los Alamos Laboratory and the Army Air Forces had already commenced work on improving the design. The
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. Some 120 Mark III Fat Man units were added to the stockpile between 1947 and 1949, when it was superseded by the Mark 4 nuclear bomb. The Mark III Fat Man was retired in 1950.
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
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. 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.
In June 1948, General Omar Bradley, Major General Alfred Gruenther and Brigadier General Anthony McAuliffe visited Sandia and Los Alamos to show them the "special requirements" of atomic weapons. Gruenther asked Brigadier General Kenneth Nichols: "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?" 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.
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".
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- on YouTube
- Fat Man Model in QuickTime VR format
- Samuels, David (23 January 2009) [15 December 2008]. "Atomic John: A truck driver uncovers secrets about the first nuclear bombs". A Reporter at Large (column). The New Yorker. Essay and interview with John Coster-Mullen, the author of Atom Bombs: The Top Secret Inside Story of Little Boy and Fat Man, 2003 (first printed in 1996, self-published), considered a definitive text about Fat Man; illustrations from which are used in the Physics Package section above.
- The Half-Life of Genius Physicist Raemer Schreiber (2017) at IMDb– Biographical film about the life and times of physicist Raemer Schreiber