Pumpkin bomb

Pumpkin bombs were conventional high explosive aerial bombs developed by the Manhattan Project and used by the United States Army Air Forces against Japan during World War II. The pumpkin bomb was a close but non-nuclear replication of the Fat Man plutonium bomb with the same ballistic and handling characteristics. It was mainly used for testing and training purposes, but combat missions were also flown with pumpkin bombs by the 509th Composite Group. The name "pumpkin bomb" resulted from the large ellipsoidal shape of the munition and was the actual reference term used in official documents.

Development
The concept for the pumpkin bomb originated in December 1944 by U.S. 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, the commander of the 509th Composite Group, the unit responsible for delivering the atomic bombs.

The pumpkin bombs were a means of providing realistic training for the 509th Composite Group's Boeing B-29 Superfortress crews assigned to drop the atomic bomb after their deployment to the Pacific. The pumpkin bomb was a close but non-nuclear replication of the Fat Man plutonium bomb with the same ballistic and handling characteristics. Specifications for the bomb required that it be carried in the forward bomb bay of a Silverplate B-29 bomber and be fuzed to be effective against actual targets.

The name "pumpkin bomb" was given to the test bombs by Parsons and Dr. Charles C. Lauritsen of the California Institute of Technology, who managed the development team. The name was used in official meetings and documents, and probably referred to its large ellipsoidal shape. Although anecdotal sources attribute the naming of the bombs to painting them a pumpkin color, the bombs were painted olive drab or khaki, and photographs indicate that the units delivered to Tinian came painted in the same 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. The initial tests demonstrated that the Fat Man assembly was unstable in flight, and that its fuzes did not work properly.

Production
The pumpkin bomb shells were manufactured by two Los Angeles, California, firms, Consolidated Steel Corporation and Western Pipe and Steel Company, while the tail assembly was produced by Centerline Company of Detroit, Michigan. 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 bombs were eventually delivered, at a cost of between $1,000 and $2,000 apiece.

Pumpkin bombs were produced in both inert and high explosive variants, with the inert versions filled with a cement-plaster-sand mixture combined with water to the density of the Composition B used in the high explosive versions. The filler of both variants had the same weight and weight distribution as the inner sphere used in the plutonium bomb. All of the inert versions were 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 in square tail assembly and single-point attachment lug. The pumpkin bomb had three contact fuses arranged in an equilateral triangle around the nose of the bomb while the atomic bomb had four fuse housings. The atomic bomb had its sections bolted together but most if not all of the pumpkin bombs were welded with a 4 in hole used for filling the shell. The Fat Man also had four external mounting points for radar antennae which the pumpkin bombs did not have.

The pumpkin bombs were 12 ft in length and 60 in in maximum diameter. They weighed 5.26 lt, consisting of 3800 lb for the shell, 425 lb for the tail assembly, and 6300 lb of Composition B filler. The shells were made of .375 in steel plate and the tail assemblies from .200 in aluminum plate.

Combat missions
Combat missions were flown by the 509th Composite Group on 20, 23, 26 and 29 July and 8 and 14 August 1945, using the bombs against individual targets in Japanese cities. A total of 49 bombs were dropped on 14 targets, one bomb was jettisoned into the ocean, and two were aboard aircraft that aborted their missions.

Mission parameters were similar to those of the actual atomic bomb missions, and all targets were located in the vicinity of the cities designated for atomic attack. The bombs were released at an altitude of 30000 ft and the aircraft then went into the sharp turn required on a nuclear mission. After the war the Strategic Bombing Survey concluded that the pumpkin bombs were "a reasonably effective weapon against Japanese plants when direct hits were scored on vital areas, or when the near miss was sufficiently close to important buildings to cause severe structural damage."