Tuesday, August 09, 2005


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
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. [back row (L-R)] Captain Beahan, Captain Van Pelt, Jr., First Lt. Albury, Second Lt. Olivi, Major Sweeney

Staff Sgt. Buckley, Master Sgt. Kuharek, Sgt. Gallagher, Staff Sgt. DeHart, Sgt. Spitzer

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.

On 9 August 1945, the primary target for the second atomic bomb attack was the nearby city of Kokura, but the bomber pilot found it to be covered in cloud. The industrial areas outside Nagasaki were the secondary target and so, despite a far more powerful bomb, the devastation visited upon Nagasaki was less severe than that experienced by Hiroshima. At 11:02 a.m., the bomb exploded directly above the suburb of Urakami, the site of Urakami Cathedral, then the largest cathedral in East Asia. The fierce blast wind, heat rays reaching several thousand degrees, and deadly radiation generated by the explosion crushed, burned and killed everything in sight and reduced this entire area to a barren field of rubble.

About one-third of Nagasaki City was destroyed and 150,000 people killed or injured, and it was said at the time that this area would be devoid of vegetation for 75 years.

For more informations:
  • The Nagasaki Atomic Bomb Museum
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