Manhattan Project by Bruce Cameron Reed

Author:Bruce Cameron Reed
Language: eng
Format: epub
ISBN: 9783030457341
Publisher: Springer International Publishing

The altitude at which combat bombs would be detonated had to be given careful consideration. In addition to liberating great quantities of electromagnetic radiation and billions of Curies of radioactivity, a nuclear explosion differs from a conventional one of the same energy in that the pressures generated are higher at closer distances. Based on the results of the Trinity test, the detonation heights for the Hiroshima and Nagasaki bombs were set at 1,850 feet. This was chosen to maximize destruction by the shock wave created by the bombs, while minimizing the amount of localized fallout that would be created if they were detonated near ground level and irradiated tons of dirt and debris. The Ordnance Division’s concern with the altitude issue was that most combat bombs detonate near ground level; little thought had been given to mechanisms designed for high-altitude operation. Extreme reliability was the paramount consideration. In a conventional mission where thousands of bombs might be dropped, the failure of a few percent will likely not affect the outcome of the operation. But any type of fuse that failed even one percent of the time would be unacceptable for a single bomb whose development had consumed hundreds of millions of dollars. Fuse specifications called for a less than one in ten-thousand chance of the bomb failing to fire within about 100 feet of the desired altitude.

Two major lines of fuse development were investigated. One was to use barometric switches which would be sensitive to changing air pressure as a function of altitude. The other was to adapt electronic techniques such as proximity fuses or fighter-plane tail-warning radar sets for use with the weapons, presuming that a reliable signal could be obtained with a falling bomb. For both Little Boy and Fat Man, a redundant series-parallel system of clocks, barometers, and four modified tail-warning radars known as “Archies” was adopted. The first stage in the firing process was that when the bombs were released, pullout switches activated timers that counted off a 15-second delay before the arming system became activated; this was to ensure safe separation from the aircraft. Following this, barometric switches activated the radar units at an altitude of 17,000 feet. These were designed to close a relay at a predetermined altitude when any two of them detected the desired firing altitude. To lessen the possibility of failure due to Japanese jamming, each radar operated on a slightly different frequency.

The final Little Boy bomb was ten feet long, 28 inches in diameter, and weighed about 9,700 lb. The gun barrel itself was six feet long and weighed 1,000 lb. The target and projectile pieces were not cast as solid wholes; rather, they each comprised a number of washer-like rings which were cast as uranium became available from Oak Ridge. The projectile was made up of nine rings totaling 7 inches in length, with inside and outside diameters of 4 inches and 6.25 inches. Because the amount of uranium received from Oak Ridge varied from shipment to shipment, none of the individual rings were of the same thickness (nor, likely, of the same enrichment).

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