Pumpkin bombs were conventional aerial bombs developed by the Manhattan Project and used by the United States Army Air Forces against Japan during World War II. Its physical characteristics closely replicated those of the Fat Man plutonium bomb, with the same ballistic and handling characteristics, but it used non-nuclear conventional high explosives. While its main purpose was for testing and training in preparation for the 509th Composite Group potentially delivering a Fat Man on a target, post-war analysis deemed the pumpkin bomb to have been an effective weapon when included in combat missions flown by the 509th. The name "pumpkin bomb" was the term used in official documents from the large, fat ellipsoidal shape of the munition casing instead of the more usual cylindrical shape of other bombs, required to enclose the Fat Man's spherical "physics package" (the plutonium implosion nuclear weapon core).
Development Pumpkin bombs were a means of providing realistic training with non-nuclear bombs for the 509th Composite Group's Boeing B-29 Superfortress crews assigned to drop the atomic bomb. The pumpkin bomb had a similar size and weight distribution as a Fat Man plutonium bomb, giving it the same ballistic and handling characteristics. Specifications for the bomb required that it be carried in the forward bomb bay of a Silverplate (nuclear-enabled) B-29 bomber and be fuzed to be effective against actual targets. Pumpkin bombs were produced in both inert and high-explosive variants. The inert versions were filled with a cement-plaster-sand mixture that was combined with water to 1.67 to 1.68 grams per cubic centimetre (0.060 to 0.061 lb/cu in), the density of the Composition B high-explosive versions. The filler of both variants had the same weight and weight distribution as the inner spherical "physics package" of the Fat Man plutonium bomb. The concept for the high-explosive pumpkin bomb was originated in December 1944 by United States Navy Captain William S. Parsons, the head of the Ordnance Division at Manhattan Project's Los Alamos Laboratory, and United States Army Air Forces Lieutenant Colonel Paul W. Tibbets, commander of the 509th Composite Group. Prior testing was carried out with an inert version. The name "pumpkin bomb" originated with Parsons and Dr. Charles C. Lauritsen of the California Institute of Technology, who managed the development team. The name was used for the training bombs in official meetings and documents, and probably derived from its large ellipsoidal shape. On the other hand, anecdotal sources attribute the naming of the bombs to a pumpkin color: While the bombs were painted olive drab or khaki in the field, photographs show that at least the units delivered to Tinian were shipped in the same yellow zinc chromate primer color worn by Fat Man. While many Manhattan scientists expected that the development of the means of delivery of the atomic bomb would be straightforward, Parsons, with his experience of the proximity fuze program, expected that it would involve considerable effort. The test program was initiated on 13 August 1943 at the Naval Proving Ground in Dahlgren, Virginia, where a scale model of the Fat Man plutonium bomb was developed. On 3 March 1944, testing moved to Muroc Army Air Field, California. Initial tests found that the Fat Man assembly was unstable in flight, and that its fuzes did not work properly.
Production The shells of the pumpkin bomb were manufactured by two Los Angeles firms, Consolidated Steel Corporation and Western Pipe and Steel Company, while the 'California Parachute' tail assembly was produced by Centerline Company of Detroit. After initial development, management of the program was turned over to the U.S. Navy Bureau of Ordnance in May 1945. A total of 486 live and inert training bombs were eventually delivered, at a cost of between US$1,000 and $2,000 apiece. All of the inert versions went from the manufacturers directly to Wendover Army Air Field, Utah, by rail, where they were used by the 216th Base Unit in flight testing of the bomb. Some test drop missions were flown by the 509th Composite Group's 393d Bombardment Squadron as training exercises. The bombs intended as live ordnance were shipped to the Naval Ammunition Depot, McAlester, Oklahoma, for filling with explosives. The Composition B was poured as a slurry, solidified in a drying facility for 36 hours, sealed, and shipped by railroad to the Port Chicago Naval Magazine, California, for shipment by sea to Tinian.
Description The pumpkin bombs were externally similar to the Fat Man bomb in size and shape, and both had the same 52-inch (130 cm) California Parachute square tail assembly and single-point attachment lug. The pumpkin bomb had three contact fuzes arranged in an equilateral triangle around the nose of the bomb, while the atomic bomb had four fuse housings. The atomic bomb had its shell sections bolted together and sealed with externally-applied, sprayed-on liquid asphalt, but most if not all of the pumpkin bombs were welded, with a four-inch hole used for filling the shell. The Fat Man also had four external mounting points for radar antennas which the pumpkin bombs did not have. The pumpkin bombs were 10 feet 8 inches (3.25 m) in length and 60 inches (152 cm) in maximum diameter. They weighed 5.26 long tons (5,340 kg), consisting of 3,800 pounds (1,700 kg) for the shell, 425 pounds (193 kg) for the tail assembly, and 6,300 pounds (2,900 kg) of Composition B filler. The shells were made of .375-inch (9.5 mm) steel plate and the tail assemblies from .200-inch (5.1 mm) aluminum plate.
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