The Manhattan Project: Making the Atomic Bomb
Part IV: The Manhattan Engineer District in Operation
Reorganizing for the Final Push
Oppenheimer acted quickly to maximize the laboratory's efforts to master implosion. Only if the implosion method could be perfected would the plutonium produced at Hanford come into play. Without either a plutonium gun bomb or implosion weapon, the burden would fall entirely on uranium and the less efficient gun method. Oppenheimer directed a major reorganization of Los Alamos in July 1944 that prepared the way for the final development of an implosion bomb. Robert Bacher took over G Division (for gadget) to experiment with implosion and design a bomb; George Kistiakowsky led X Division (for explosives) in work on the explosive components; Hans Bethe continued to head up theoretical studies; and "Deke" Parsons now focused on overall bomb construction and delivery.
Field tests performed with uranium-235 prototypes in late 1944 eased doubts about the artillery method to be employed in the uranium bomb. It was clear that the uranium-235 from Oak Ridge would be used in a gun-type nuclear device to meet the August 1 deadline Groves had given General Marshall and the Joint Chiefs of Staff. The plutonium produced at such expense and effort at Hanford would not fit into wartime planning unless a breakthrough in implosion technology occurred.
At the same time, Los Alamos shifted from research to development and production. Time was of the essence, though laboratory research had not yet charted a clear path to the final product. Army Air Force training could wait no longer, and in September at Wendover Field in western Utah, Colonel Paul Tibbets began drilling the 393rd Bombardment Squadron, the heart of the 509th Composite Wing, in test drops with 5,500-pound orange dummy bombs, nicknamed pumpkins. In June 1945, Tibbets and his command moved to Tinian Island in the Marianas, where the Navy SeaBees had built the world's largest airport to accommodate Boeing's new B-29 Superfortresses.