Thin Man (nuclear bomb)

"Thin Man" plutonium gun test casings at Wendover Army Air Field, as part of Project Alberta in the Manhattan Project. In the background, casing designs for "Fat Man" bombs can be seen as well.

The "Thin Man" (formally, Mark 2) nuclear bomb was a proposed plutonium gun-type nuclear bomb which the United States was developing during the Manhattan Project. Its development was aborted when it was discovered that the spontaneous fission rate of nuclear reactor-bred plutonium was too high for use in a gun-type design.^[1]

Specifications

The "Thin Man" design was an early nuclear weapon design proposed before plutonium had been successfully bred in a nuclear reactor from the irradiation of uranium-238. It was assumed that plutonium, like uranium-235, would be able to be assembled into a critical mass by simply shooting a sub-critical piece of it into another sub-critical piece. To avoid pre-detonation, a plutonium "bullet" would need to be accelerated to a speed of at least 3,000 feet per second (910 m/s)—if not, then the plutonium would begin a nuclear fission reaction before fully assembling and blow itself apart prematurely.

Estimated dimensions of the Mark 2 were a diameter of 2 feet (0.61 m) and a length of 18 feet (5.5 m); the long length was necessary in order for the plutonium "bullet" to pick up adequate speed before reaching the "target". Weight was estimated at 7,500 pounds (3,400 kg) for the final weapon model.

History

Early decisions

"Thin Man" plutonium gun test casings.

In 1942, prior to the Army taking over wartime atomic research, Robert Oppenheimer held a conference in Berkeley, California, at which various engineers and physicists discussed nuclear bomb design issues. Three designs were put forth: a uranium-235 gun-type bomb, a plutonium gun-type bomb, and an early form of a plutonium implosion-type bomb. These later became the "Little Boy" (uranium gun), "Thin Man" (plutonium gun), and "Fat Man" (plutonium implosion) code-named design projects.

The names for all three projects were created by Robert Serber, a former student of Oppenheimer's who worked on the project. According to his later memoirs, he chose them based on their design shapes; the "Thin Man" would be a very long device, and the name came from the Dashiell Hammett detective novel The Thin Man and series of movies by the same name; the "Fat Man" bomb would be round and fat and was named after Sidney Greenstreet's character in The Maltese Falcon. "Little Boy" would come last and be named only to contrast to the "Thin Man" bomb.^[2]

At that conference, it was generally agreed that the known physical and nuclear characteristics of uranium-235 and plutonium easily allowed production of gun-type uranium bombs, and with some difficulty plutonium gun-type bombs. Because plutonium had a higher spontaneous fission rate, the plutonium gun bomb would have to have a higher assembly velocity (and a longer, physically much larger gun) than a uranium bomb.

It was estimated that the design of a uranium gun-type bomb, which required a lower velocity, would be much easier than a plutonium gun-type bomb. The early analysis of production methods for plutonium and uranium showed that the Manhattan Project would have problems getting enough uranium for many gun-type bombs before the projected end of the war, but could probably produce enough plutonium.

Implosion-type bombs were determined to be significantly more efficient (in terms of explosive yield per unit mass of fissile material in the bomb), because compressed fissile materials react more rapidly and therefore more completely.

Oppenheimer, reviewing his options in early 1943, determined that two projects should proceed forwards: the "Thin Man" project (plutonium gun) and the "Fat Man" project (plutonium implosion). It was decided that the plutonium gun would receive the bulk of the research effort, since it was the project with the most uncertainty involved. It was assumed that the uranium gun-type bomb could be more easily adapted from it after the fact.

Development

Oppenheimer assembled a team including senior engineer Edwin Rose and senior physicist Charles Critchfield to begin design work on plutonium gun designs. In June 1943, Navy Capt. William Sterling Parsons took over the Ordnance Division and direct management of the "Thin Man" project.^[3]

Design issues

Two major concerns were identified with the "Thin Man" design during its development: first, bomb aerodynamics after being dropped from an aircraft, and second, predetonation.

Aerodynamics

The great length of the "Thin Man" bomb led to aerodynamic instabilities. Subscale models of the bomb were dropped from a TBF Avenger at the US Navy test range at Dahlgren, Virginia starting in August, 1943. The bombs would spin sideways after being dropped, and broke up when they hit the ground.

Extensive aerodynamic testing, particularly of the shape of the bomb's nose and tail and the tail fins, proceeded for the next year. The design ended up with a bulging nose, thin body, and wider tail assembly with the plutonium "target" assembly, plus very long tail fins for stability.

Full scale test units were dropped starting on March 6, 1944, at Muroc Army Air Base (now Edwards Air Force Base) and were successful.

Predetonation

The feasibility of a plutonium bomb had been questioned in 1942, when James Conant heard on 14 November (indirectly via Wallace Akers - then Research Director of ICI) that James Chadwick had recently concluded that plutonium might not be a practical fissionable material for weapons because of impurities. He consulted Lawrence and Compton, who acknowledged that scientists at Chicago and Berkeley had known about the problem since October, but could offer no ready solution. He advised Groves, who assembled a special committee of Lawrence, Compton, Oppenheimer and McMillan. They concluded that any problems could be overcome by requiring higher purity. Du Pont (who were considering taking over all the plutonium project, not just the chemical separation process) were advised, but still had strong doubts about the project.^[4]

In April 1944, experiments by Emilio G. Segrè on the newly reactor-produced plutonium from Hanford showed that it contained impurities in the form of the isotope plutonium-240. Plutonium-240 has a far higher spontaneous fission rates and radioactivity than the cyclotron-produced Pu-239 isotopes on which the original measurements had been made, and its inclusion in reactor-bred plutonium appeared unavoidable. This meant that the background fission rate of the plutonium was so high that it would be highly likely the plutonium would predetonate and blow itself apart in the initial forming of a critical mass. The gun barrel required to speed up the plutonium to levels where predetonation would be less likely would require a gun barrel too long for any existing or planned bomber. The only way to use plutonium in a workable bomb was thus implosion — a far more difficult engineering task.

The impracticability of a gun-type bomb using plutonium was agreed a meeting in Los Alamos on July 17, 1944.^[5] All gun-type work in the Manhattan Project was directed at the enriched uranium gun design (Little Boy), and almost all of the research at Los Alamos was re-oriented around the problems of implosion for the Fat Man bomb.

See also

• List of nuclear weapons
• Nuclear weapon design

Notes

1. ^ Hakim, Joy (1995). A History of Us: War, Peace and all that Jazz. New York: Oxford University Press. ISBN 0-19-509514-6. 
2. ^ Robert Serber, Peace & War: Reminiscences of a Life on the Frontiers of Science (New York: Columbia University Press, 1998): 104.
3. ^ Plutonium Complicates Early Gun Work, Los Alamos National Laboratory.
4. ^ Nichols, Kenneth; The Road to Trinity: A Personal Account of How America’s Nuclear Policies Were Made pp. 62-63 (1987, Morrow, New York) ISBN 068806910X
5. ^ Manhattan Project Chronology at AtomicArchive.com

External links

History links

• Plutonium Complicates Early Gun Work at the Los Alamos National Laboratory history website
• Little Boy and Fat Man at LANL history website
• Nuclear weapons timeline
• The Enola Gay at US Air Force Association
• Silverplate: Aircraft of the Manhattan Project
• Allbombs.html list of all American nuclear bombs at nuclearweaponarchive.org

Physics and design links

• Nuclear Weapon FAQ: Engineering and Design: Predetonation
• Nuclear Weapon FAQ: Engineering and Design: Gun assembly
• Nuclear Weapon FAQ: Engineering and Design: Fissile materials