| Themes > Science > Chemistry > Nuclear Chemistry > Nuclear Weapons > The First Nuclear Chain Reaction > The First Nuclear Weapons > Fat Man |
The combat configuration for the implosion bomb (the Model 1561) basically consisted of the Gadget device encapsulated in a steel armor egg. The two steel half-ellipsoids were bolted to the dural equatorial band of the explosive assembly, with the necessary X-Unit, batteries, and fuzing and firing electronics located in the front and aft shell. For use in combat, each Fat Man bomb required assembly almost from scratch - a demanding and time consuming job. Assembly of a Fat Man bomb was (and may still be) the most complex field preparation operation for any weapon ever made. Like Little Boy, Fat Man was fuzed by four radar units called "Archies", the antennas for which were mounted on the tail of the bomb. Developed originally as fighter tail warning systems, these units measured the bomb's height above the ground and were set to detonate at a pre-calculated altitude (set to 1850 ft, +/- 100 ft). A barometric switch acted as a "fail-safe", preventing detonation until the bomb had fallen below 7000'. Fat Man was 60 inches in diameter, was 12 feet long, and weighed 10,300 lb. The Fat Man plutonium core, and its initiator, left Kirtland Air Force Base, for Tinian Island on July 26, 1945 in a C-54 transport plane. It arrived on Tinian on July 28. Also on July 28, three specially-modified B-29s flew from Kirtland Field carrying three Fat Man bomb assemblies, including units F-31 and F-32, each encased in an outer ballistic shell. These arrived at Tinian on August 2, the first Fat Man units to do so. The bombing date was set for August 11 at this time, with Kokura as the target. Assembly of practice (non-nuclear) weapons began shortly afterward, with the first completed bomb (Fat Man unit F33) ready on Aug. 5. On August 7 a forecast of 5 days of bad weather around the 11th moved the bombing date up to August 10, then to August 9. This compressed the bomb assembly schedule so much that many check-out procedures had to be skipped during assembly. On August 8 the assembly of Fat Man unit F31, with the plutonium core, was completed in the early morning. At 2200, Fat Man was loaded on the B-29 "Bock's Car". August 9, 1945 - * 0347, Bock's Car takes off from Tinian, the target of choice is Kokura Arsenal. Charles Sweeney is pilot. Soon after takeoff he discovers that the fuel system will not pump from the 600 gallon reserve tank. * 1044, Bock's Car arrives at Kokura but finds it covered by haze, the aimpoint cannot be seen. Flak and fighters appear, forcing the plane to stop searching. Sweeney turns toward Nagasaki, the only secondary target in range. * Upon arriving at Nagasaki, Bock's Car has enough fuel for only one pass over the city even with an emergency landing at Okinawa. Nagasaki is covered with clouds, but one gap allows a drop several miles from the intended aimpoint. * 11:02 (Nagasaki time) Fat Man explodes at 1650 +/- 33 feet (503 m) near the perimeter of the city with a yield of 22+/-2 kt. Due to the hilly terrain around ground zero, five shock waves were felt in the aircraft (the initial shock, and four reflections). Although Fat Man fell on the border of an uninhabited area, the eventual casualties still exceeded 70,000. Also ground zero turned out to be the Mitsubishi Arms Manufacturing Plant, the major military target in Nagasaki. It was utterly destroyed. The 1987 reassessment of the Japanese bombings placed the yield at 21 kt. At the extreme estimate ranges for Little Boy and Fat Man (low for Little Boy, high for Fat Man), a ratio of nearly 2-to-1 has been implied. The 1987 best estimate figures make Fat Man only about 40% larger than Little Boy (and possibly as little as 15% more). Using the 21 kt figure, the optimal burst height for Fat Man would have been about 3100 feet. The actual burst height was optimal for 15 psi overpressure The burst height was sufficient to prevent any fallout over Japan. |
|
|