Nuclear fission the Manhattan Project during World War II (September 1, 1939 – September 2, 1945) carried out most of the early scientific work on fission chain reactions, culminating in the three events involving fission bombs that occurred during the war. The first fission bomb, codenamed "The Gadget", was detonated during the Trinity Test in the desert of New Mexico on July 16, 1945. Two other fission bombs, codenamed "Little Boy" and "Fat Man", were used in combat against the Japanese cities of Hiroshima and Nagasaki in on August 6 and 9, 1945 respectively. Even the first fission bombs were thousands of
Mark 4 nuclear bomb The Mark 4 was in diameter and long, the same basic dimensions as Mark 3. It weighed slightly more at , depending on the specific Mark 4 version. (Mark 3 weighed .) In addition to being easier to manufacture, the Mark 4 introduced the concept of in flight insertion (IFI), a weapons safety concept which was used for a number of years. An IFI bomb has either manual or mechanical assembly, which keeps the nuclear core stored outside the bomb until close to the point that it may be dropped. To arm the bomb, the fissile nuclear materials are inserted