Our friend AggieSprite mentioned the other day that her latest trip to the thrift store had yielded some silverplate for a future project.
Which, my ears perked up and a memory was triggered.
By the middle of 1943. the scientists and engineers on the Manhattan Project began to have a fair idea what the size and shape of the two types of projected nuclear weapons they were building would be. There were two bomb designs. The first was a plutonium gun type design, known as Thin Man. The second, riskier approach was a plutonium implosion device, known as Fat Man.
With a rough idea of the weights and dimensions of the bombs, they began to consider what airplane should be used to deliver the bombs. The existence of AVRO Lancaster bombers already modified to carry extremely large bombs was a possibility, but range restrictions, and a strong desire to utilize an American design meant the Manhattan Project, in cooperation with the Army Air Forces, settled on using modified B-29 Superfortress bombers as the delivery platform.
But not just any B-29 could drop an atomic weapon. First, no B-29s were equipped with the shackles and release latches needed for extremely large bombs. Second, the Thin Man bomb design was longer than either of the B-29s two bomb bays.
The project to produce and modify a limited number of B-29s for the atomic mission was soon dubbed “Silverplate.”
The Army Air Forces first attempted to design their own shackles and releases, but when that proved problematical, they simply borrowed the existing British designs.
As to the bomb length, the Army Air Force converted the two 12 foot long bomb bays of a standard B-29 to one enormous 33 foot long bomb bay. This was more than enough to carry the projected 17 foot length of the Thin Man bomb.
Soon after modifying the first Silverplate B-29, scientists at Manhattan realized that high spontaneous fission rates would make the plutonium gun type bomb impractical. The bomb design was modified to use Uranium 235. The change in bomb design also meant the bomb would be much shorter, at about 10 feet, meaning the standard B-29 bomb bay length would be sufficient. Accordingly, the first Silverplate had its bomb bays returned to the normal configuration.
Seventeen further Silverplate production aircraft were ordered from the Glenn L. Martin company, with 14 bombers assigned for training to what would eventually become the 509th Composite Group, and three assigned for testing.
These 17 Silverplates, being early production B-29s, suffered the same problems conventional early production B-29s struggled with, including a nasty tendency for the Wright R-3350 engines to burst into flames. By early 1945, a great number of changes to the B-29 had been made on the production line, improving the engines, adding a radar bombing capability, and other myriad improvements. Rather than going through the trouble of modifying the existing Silverplate fleet, another tranche of aircraft were ordered, 28 “next gen” Silverplates were ordered, with 15 of them combat coded for use by the 509th. The remaining 13 were dedicated to the testing program, or held in reserve in Utah as replacements for any losses the 509th might suffer.
A final tranche of 19 Silverplates were ordered just before the atomic missions, and delivered between the end of the war, and 1947. By 1948, the Air Force had an operational fleet of 32 nuclear capable B-29s. They represented the entire US atomic delivery capability.
Soon the B-29 would be supplanted in the nuclear role by the B-50 and B-36 bombers, but for a time, the US nuclear deterrence rested upon an incredibly small fleet of aircraft.