Silverplate - Testing of Bomb Shapes

Testing of Bomb Shapes

The first test drop at Muroc on March 6 involved a Thin Man, followed on March 14 by two drops of an implosion device shape (codenamed Fat Man) fitted with a circular tail fin stabilizer designed by engineers at the National Bureau of Standards. The Thin Man performed without major problems but the Fat Man shapes exhibited significant wobble characteristics, apparently due to poor workmanship and misalignment of the tail fins.

All three bombs had also failed to release immediately, frustrating calibration tests. A fourth testing flight resulted in the premature release of a Thin Man shape while the B-29 was still en route to the test range and severely damaged the aircraft. The modified glider mechanisms had apparently caused all four malfunctions, because of the weight of the bombs, and were replaced with British Type G single-point attachments and Type F releases as used on the Lancaster to carry the 12,000 lb Tallboy bomb.

After repair of the Pullman B-29 at Wright Field, testing resumed with three Thin Man and nine Fat Man shapes dropped in the last two weeks of June 1944. 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", a cubical tail box with fins angled at 45° to the line of fall, was approved.

The Thin Man gun-type design was based on the fissibility of the very pure 239Pu isotope produced in microgram quantities by the Berkeley cyclotron. When the Hanford production reactors came on-line in the spring of 1944, the mix of 239Pu and 240Pu obtained was found to have a high rate of spontaneous fission. To avoid pre-detonation, the muzzle velocity would need to be greatly raised, making the bomb impractically long. The weapon was re-designed to use 235U. The muzzle velocity required for a 235U fission reaction is much lower, reducing the barrel length of the resulting bomb (now code-named Little Boy) to less than 10 ft (3.0 m). This allowed the device to fit into a standard B-29 bomb bay.

The Pullman was modified to its original configuration with the rear bomb bay a standard B-29 design. All subsequent Silverplates were also configured in this manner. The Pullman B-29 was flown to Wendover and assigned to further drop testing in September 1944 with the 216th Base Unit until it was damaged in a landing accident in December.