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75 years ago today... J. Robert Oppenheimer sent General Leslie R. Groves a telegram, asking whether, given the success of the implosion design at the Trinity test, they ought to turn the uranium currently designated for the Little Boy bomb into many "composite" bombs.
These "composite cores" would combine uranium and plutonium, allowing them to stretch out the plutonium. As a result they would end up with as many as *sixteen* atomic bombs by November 1945, rather than the five or so they might otherwise plan for by that point.
Groves responded the same day, saying that "factors beyond our control" avoid any change in the plans. He thought it might take four or more atomic bombings against Japan "to conform to planned strategical operations."
These memos, taken together, are pretty interesting. They indicate that the *timing* of the atomic bombing was extremely important (immediately after Potsdam, before the Soviets declared war on Japan).
And they indicate that even Groves didn't think that two atomic bombs would likely lead to Japanese capitulation — he was expecting to drop at least three, maybe four or more.
The Los Alamos scientists did work on "composite" cores after the war ended, but the first ones were not tested until Operation Sandstone in 1948.
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