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Manhattan Project director discusses origin of names for "Fat Man" and "Little Boy"

Leslie Groves

Typed Letter Signed, "Leslie R. Groves," to Sandia National Laboratories historian Ted Alexander,

referring questions about housing on the Hanford Site (nuclear production facility) to Hanford engineer Franklin T. Matthias and Hanford construction head Gilbert Church, expressing certainty that the names for the nuclear devices detonated at Hiroshima and Nagasaki were the result of his phone conversations with J. Robert Oppenheimer or William Sterling Parsons, noting that testing for the fuses of the two weapons were completed at different times, and promising to send a copy of his new book. 1½ pages, 4to, written on recto and verso of single sheet; folds.

Darien, CT, 29 January 1962.

  • Notes: ". . . I am sure that the names for the fat man ["Fat Man": plutonium-fueled nuclear weapon detonated over Nagasaki] and his friend ["Little Boy": weapon fueled by uranium and detonated by projectile rather than implosion, exploded over Hiroshima] came about in the course of telephone conversations, probably between [J. Robert] Oppenheimer or [William Sterling] Parsons with me. . . . I would not trust any Air Force history with reference to the MED [Manhattan Engineer District: headquarters for the Manhattan Project]. The writers were simply not cognizant of the facts. I doubt too whether the plane modifiers were given any such information. I would guess that if the story is well-founded, that it was based on usage within a small group of which [Norman] Ramsey was a member.
    "The final fusing tests for the fat boy [early version of "Fat Man"] were not run until early in August [1944?]. . . . The fuse tests of the thin boy [early version of "Little Boy"] were much further along and for that reason were certainly carried on with the other tests. . . ." 
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