The Manhattan Project: Making the Atomic Bomb
Part IV: The Manhattan Engineer District in Operation
Early Implosion Work
Parsons assigned implosion studies a low priority and placed the emphasis on the more familiar artillery method. Consequently, Seth H. Neddermeyer performed his early implosion tests in relative obscurity. Neddemeyer found it difficult to achieve symmetrical implosions at the low velocities he had achieved. When the Princeton mathematician John von Neumann, a Hungarian refugee, visited Los Alamos late in 1943, he suggested that high-speed assembly and high velocities would prevent predetonation and achieve more symmetrical explosions. A relatively small, subcritical mass could be placed under so much pressure by a symmetrical implosion that an efficient detonation would occur. Less critical material would be required, bombs could be ready earlier, and extreme purification of plutonium would be unnecessary. Von Neumann's theories excited Oppenheimer, who assigned Parsons's deputy, George B. Kistiakowsky, the task of perfecting implosion techniques. Because Parsons and Neddemeyer did not get along, it was Kistiakowsky who worked with the scientists on the implosion project. While experiments on implosion and explosion continued, Parsons directed much of his effort toward developing bomb hardware, including arming and wiring mechanisms and fuzing devices. Working with the Army Air Force, Parsons's group developed two bomb models by March 1944 and began testing them with B-29s. Thin Man, named for President Roosevelt, utilized the plutonium gun design, while Fat Man, named after Winston Churchill, was an implosion prototype. (Segre's lighter, smaller uranium gadget became Little Boy, Thin Man's brother).