Little Boy

For other uses, see Little boy (disambiguation).

Little Boy (atomic bomb)
A post-war "Little Boy" model
Type	Nuclear weapon
Place of origin	United States
Production history
Designer	Los Alamos Laboratory
Produced	1945
Number built	1
Specifications
Weight	4,400 kilograms (9,700 lb)
Length	3.0 metres (10 ft)
Diameter	71 centimetres (28 in)
Filling	Uranium-235
Filling weight	64 kg (140 lb)
Blast yield	16 kt (67 TJ)

"Little Boy" was the codename for the atomic bomb dropped on Hiroshima on August 6, 1945 by the Boeing B-29 Superfortress Enola Gay, piloted by Colonel Paul W. Tibbets, Jr., commander of the 509th Composite Group of the United States Army Air Forces. It was the first atomic bomb to be used as a weapon. The second, the "Fat Man", was dropped three days later on Nagasaki.

The weapon was developed by the Manhattan Project during World War II. It derived its explosive power from the nuclear fission of uranium-235. The Hiroshima bombing was the second artificial nuclear explosion in history, after the Trinity test, and the first uranium-based detonation. Approximately 600 to 860 milligrams (9.3 to 13.3 gr) of matter in the bomb was converted into the energy of heat and radiation. It exploded with an energy of 16 kilotons of TNT (67 TJ). It has been estimated that 66,000–166,000 people had died as a result of its use by the end of December 1945. Because of the scarcity of enriched uranium at the time, Little Boy's relatively simple design was not tested in advance, unlike the more complex plutonium-fuelled Fat Man.

Naming

The names for all three atomic bomb design projects during World War II ("Fat Man", "Thin Man", and "Little Boy") were allegedly created by Robert Serber, a former student of Los Alamos Laboratory director Robert Oppenheimer who worked on the Manhattan Project. According to Serber, he chose them based on their design shapes. The "Thin Man" was a long device, and the name came from the Dashiell Hammett detective novel and series of movies by the same name; the "Fat Man" was round and fat, and was named after Sydney Greenstreet's "Kasper Gutman" character in The Maltese Falcon. "Little Boy" would come last and be named only to contrast to the "Thin Man" bomb.

Design

The original "Thin Man" was a gun-type fission weapon 17 feet (5.2 m) long. Like the Fat Man, it would have worked with enriched uranium as well, but the Thin Man design was abandoned after experiments by Emilio G. Segrè and his P-5 Group at Los Alamos on the newly reactor-produced plutonium from Oak Ridge and the Hanford site showed that it contained impurities in the form of the isotope plutonium-240. This has a far higher spontaneous fission rate 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-type design henceforth had to work with enriched uranium only, and this allowed the Thin Man design to be greatly simplified. A high velocity gun was no longer required, and a simpler weapon could be substituted. This greatly shortened the weapon, so that it would fit into a B-29 bomb bay. The design was named "Little Boy".

The "gun" assembly method. When the hollow uranium projectile was driven onto the target cylinder, a nuclear explosion resulted.

The "Little Boy" was 300 centimetres (120 in) in length, 71 centimetres (28 in) in diameter and weighed approximately 4,400 kilograms (9,700 lb). The design used the gun method to explosively force a hollow sub-critical mass of uranium-235 and a solid target cylinder together into a super-critical mass, initiating a nuclear chain reaction. This was accomplished by shooting one piece of the uranium onto the other by means of four cylindrical silk bags of slotted-tube Cordite powder. It contained 64 kg (140 lb) of enriched uranium, of which less than a kilogram underwent nuclear fission, and of this mass only 0.6 g (0.021 oz) was transformed into a different type of energy; initially kinetic energy, then heat and light.

No full test of a gun-type nuclear weapon had occurred before the "Little Boy" device was dropped over [Hiroshima]. The only test explosion of a nuclear weapon concept had been of an implosion-type device employing plutonium as its fissionable material, and took place on July 16, 1945 at the Trinity test. There were several reasons for not testing a "Little Boy" type of device. Primarily, there was little uranium-235 as compared with the relatively large amount of plutonium which, it was expected, could be produced by the Hanford reactors. Additionally, the weapon design was simple enough that it was only deemed necessary to do laboratory tests with the gun-type assembly. Unlike the implosion design, which required sophisticated coordination of shaped explosive charges, the gun-type design was considered almost certain to work.

Although occasionally used in later experimental devices, the gun design was only used once as a weapon because of the danger of accidental detonation. Little Boy's design is considered unsafe when compared to modern nuclear weapons, which incorporate safety features to endure various accident scenarios. The objective of Little Boy was to create a weapon that was absolutely guaranteed to work. Consequently, Little Boy incorporated only basic safety mechanisms, thus an accidental detonation could easily occur during one or more of the following scenarios:

• A crash could drive the hollow "bullet" onto the "target" cylinder resulting in a massive release of radiation, or possibly nuclear detonation.
• An electrical short circuit of some sort.
• The danger of a misfire was greater over water. If the force of a crash did not trigger the bomb, water leakage into the system could short it out, possibly leading to detonation. The British Red Beard nuclear weapon also suffered from this design flaw.
• Furthermore if immersed in water, the uranium halves were subject to a moderator-effect of the liquid.
• Fire.
• Lightning strike.

Assembly details

The exact specifications of the "Little Boy" bomb remain classified because they could be used today to create a viable nuclear weapon. Even so, many sources have speculated as to the design, relying on limited photographic evidence, interviews with former Manhattan Project personnel, and have pieced together information from declassified sources to reconstruct its internal dimensions.

Inside the weapon, the uranium-235 material was divided into two parts, following the gun principle: the "projectile" and the "target". The projectile was a hollow cylinder with 60% of the total mass (38.5 kg or 85 lb). It consisted of a stack of 9 uranium rings, each 159-millimetre (6.25 in) with a 100-millimetre (4 in) bore of a total length of 180 millimetres (7 in), pressed together into the front end of a thin-walled projectile 413 millimetres (16.25 in) long. Filling in the remainder of the space behind these rings in the projectile was a tungsten carbide disc with a steel back. At ignition, the projectile slug was pushed 1,100 millimetres (42 in) along the 1,800 millimetres (72 in) long, 170-millimetre (6.5 in) smooth-bore gun barrel. The slug "insert" was a 100 millimetres (4 in) cylinder, 180 millimetres (7 in) in length with a 25 millimetres (1 in) axial hole. The slug comprised 40% of the total fissile mass (25.6 kg or 56 lb). The insert was a stack of 6 washer-like uranium discs somewhat thicker than the projectile rings that were slid over a 25 millimetres (1 in) rod. This rod then extended forward through the tungsten carbide tamper plug, impact-absorbing anvil, and nose plug backstop eventually protruding out the front of the bomb casing. This entire target assembly was secured at both ends with locknuts.

When the hollow-front projectile reached the target and slid over the target insert, the assembled super-critical mass of uranium would be completely surrounded by a tamper and neutron reflector of tungsten carbide and steel, both materials having a combined mass of 2,300 kg (5,100 lb). Neutron generators at the base of the projectile would be activated by the impact.

Counter-intuitive design

For the first fifty years after 1945, every published description and drawing of the Little Boy mechanism assumed that a small, solid projectile was fired into the center of a larger target. However, critical mass considerations dictated that in Little Boy the larger, hollow piece would be the projectile. The assembled fissile core had more than two critical masses of U-235. This required one of the two pieces to have more than one critical mass. A hole in the center of the larger mass dispersed the mass and increased the surface area, allowing more fission neutrons to escape, thus preventing a premature chain reaction.

It was also important for the larger piece to have minimal contact with the neutron-reflecting tungsten carbide tamper until detonation, thus only the projectile's back end was in contact with tungsten carbide (see drawing above). The rest of the tungsten carbide surrounded the sub-critical mass target cylinder (called the "insert" by designers) with air space between it and the insert. This arrangement packs the maximum amount of fissile material into a gun-assembly design.

Development of the bomb

Main article: Manhattan Project

The "Little Boy" bomb was constructed through the Manhattan Project during World War II. Because enriched uranium was known to be fissionable, it was the first approach to bomb development pursued. The vast majority of the work in constructing "Little Boy" came in the form of the isotope enrichment of the uranium necessary for the weapon. Enrichment at Oak Ridge, Tennessee began in February 1943, after many years of research.

The development of the first prototypes and the experimental work started in early 1943, at the time when the Los Alamos Design Laboratory became operational in the framework of the Manhattan Project. Originally gun-type designs were pursued for both a uranium and plutonium weapon (the "Thin Man" design), but in April 1944 it was discovered that the spontaneous fission rate for plutonium was too great to use in a gun-type weapon. In July 1944, almost all research at Los Alamos was redirected to the implosion plutonium weapon. In contrast, the uranium bomb was straightforward if not trivial to design.

As part of Project Alberta, Commander A. Francis Birch (left) numbers the bomb while physicist Norman Ramsey watches. This is one of the rare photos where the inside of the bomb can be seen.

With plutonium found unsuitable for the gun-type design, the team working on the gun weapon (led by A. Francis Birch), faced another problem: the bomb was simple, but they lacked the quantity of uranium-235 necessary for its production. Enough fissile material was not going to be available before mid-1945. Despite this, Birch managed to convince others that this concept was worth pursuing, so that in case of a failure of the plutonium bomb, it would still be possible to use the gun principle. In February 1945, the specifications were completed (model 1850). The bomb, except for the uranium payload, was ready at the beginning of May 1945.

Most of the uranium necessary for the production of the bomb came from the Shinkolobwe mine and was made available thanks to the foresight of the CEO of the High Katanga Mining Union, Edgar Sengier, who had 1000 tons of uranium ore transported to a New York warehouse in 1939. A small amount may have come from a captured German submarine, U-234, after the German surrender in May 1945. Other sources state that at least part of the 1100 tons of uranium ore and uranium oxide captured by US troops in the second half of April 1945 in Stassfurt, Germany, became ²³⁵U for the bomb. The majority of the uranium for Little Boy was enriched in Oak Ridge, Tennessee, primarily by means of electromagnetic separation in calutrons and through gaseous diffusion plants, with a small amount contributed by the cyclotrons at Ernest O. Lawrence's Radiation Laboratory. The core of Little Boy contained 64 kg of uranium, of which 50 kg was enriched to 88%, and the remaining 14 kg at 50%, resulting in an average enrichment of 79.68%.

Construction and delivery

Little Boy in the bomb pit on Tinian island, before being loaded into Enola Gay's bomb bay. A section of the bomb bay door is visible on the top right.

On July 14, 1945 a train left Los Alamos carrying several "bomb units" (the major non-nuclear parts of a gun-type bomb) together with a single completed uranium projectile; the uranium target was still incomplete. The consignment was delivered to the San Francisco Naval Shipyard at Hunters Point in San Francisco, California. There, two hours before the successful test of Little Boy's plutonium-implosion brother at the Trinity test in New Mexico, the bomb units and the projectile were loaded aboard the heavy cruiser USS Indianapolis. Indianapolis steamed at record speed to the airbase at Tinian island in the Mariana Islands, delivering them ten days later on the 26th. (While returning from this mission Indianapolis was sunk by a Japanese submarine, with great loss of life due to delayed rescue.) Also on the 26th the three sections of the uranium target assembly were shipped from Kirtland Air Force Base near Albuquerque, New Mexico in three C-54 Skymaster aircraft operated by the 509th Composite Group's Green Hornet squadron. With all the necessary components delivered to Tinian, bomb unit L11 was chosen, and the final Little Boy weapon was assembled and ready by August 1.

Handling the completed Little Boy was particularly dangerous. Once cordite was loaded in the breech, any firing of the explosive would at best contaminate the explosion zone and at worst cause a nuclear chain reaction. The mere contact of the two uranium masses could have caused an explosion with dire consequences, from a simple "fizzle" explosion to an explosion large enough to destroy Tinian (including the 500 B-29s based there, and their supporting infrastructure and personnel). Water was also a risk, since it could serve as a moderator between the fissile materials and cause a violent dispersal of the nuclear material. The uranium projectile could only be inserted with an apparatus that produced a force of 300,000 newtons (67,000 lb_f). For safety reasons, the weaponeer, Captain William Sterling Parsons, decided to load the bags of cordite only after take-off.

Fuse system

The bomb employed a fusing system that was designed to detonate the bomb at the most destructive altitude. Calculations showed that for the largest destructive effect, the bomb should explode at an altitude of 580 metres (1,900 ft). The resultant fuse design was a three-stage interlock system:

• A timer ensured that the bomb would not explode until at least fifteen seconds after release, one-quarter of the predicted fall time, to ensure safety of the aircraft. The timer was activated when the electrical pull-out plugs connecting it to the airplane pulled loose as the bomb fell, switching it to internal (24V battery) power and starting the timer. At the end of the 15 seconds the radar altimeters were powered up and responsibility was passed to the barometric stage.
• The purpose of the barometric stage was to delay activating the radar altimeter firing command circuit until near detonation altitude. A thin metallic membrane enclosing a vacuum chamber (a similar design is still used today in old-fashioned wall barometers) was gradually deformed as ambient air pressure increased during descent. The barometric fuse was not considered accurate enough to detonate the bomb at the precise ignition height, because air pressure varies with local conditions. When the bomb reached the design height for this stage (reportedly 2,000 meters) the membrane closed a circuit, activating the radar altimeters. The barometric stage was added because of a worry that external radar signals might detonate the bomb too early.
• Two or more redundant radar altimeters were used to reliably detect final altitude. When the altimeters sensed the correct height, the firing switch closed, igniting the three BuOrd Mk15, Mod 1 Navy gun primers in the breech plug, which set off the charge consisting of four silk powder bags each containing two pounds of WM slotted-tube cordite. This launched the uranium projectile towards the opposite end of the gun barrel at an eventual muzzle velocity of 300 meter per second (1000 feet per second). Approximately 10 milliseconds later the chain reaction occurred, lasting less than 1 μs. The radar altimeters used were modified U.S. Army Air Corps APS-13 fighter tail warning radars, nicknamed "Archie", to warn a pilot of another plane approaching from behind.

The bombing of Hiroshima

The mushroom cloud over Hiroshima after the dropping of "Little Boy"

Main articles: Enola Gay and atomic bombings of Hiroshima and Nagasaki

Because the Enola Gay weaponeer "Deak" Parsons was concerned about the possibility of an accidental detonation he did not load the four cordite powder bags into the gun breech until the aircraft was on its mission flight. Before climbing to altitude on approach to the target, Parsons' assistant, Morris R. Jeppson, removed three safety plugs between the electrical connectors of the internal battery and the firing mechanism.

The bomb was dropped at approximately 08:15  (JST) August 6, 1945. After falling for 44.4 seconds, the time and barometric triggers started the firing mechanism. The detonation happened at an altitude of 600 ± 15 metres (1,968 ± 50 ft). The official yield estimate of "Little Boy" was about 13 kilotons of TNT equivalent in explosive force, i.e. 6.3 × 10¹³ joules = 63 TJ (tera-joules). The actual yield estimated after an analysis of blast damage was closer to 16 kilotons. When one pound (454 g) of U-235 undergoes complete fission, the yield is 8 kilotons. The 16-kiloton yield of the Little Boy bomb was therefore produced by the fission no more than two pounds (907 g) of U-235, out of the 141 pounds (64 kg) in the pit. The remaining 139 pounds (63 kg), 98.5% of the total, contributed nothing to the energy yield.

It was less powerful than "Fat Man", which was dropped on Nagasaki (21–23 kt). However, the damage and the number of victims at Hiroshima were much higher, as Hiroshima was on flat terrain, while the hypocenter of Nagasaki lay in a small valley. According to figures published in 1945, 66,000 people were killed as a direct result of the Hiroshima blast, and 69,000 were injured to varying degrees.

References

• Campbell, Richard H. (2005). The Silverplate Bombers: A History and Registry of the Enola Gay and Other B-29s Configured to Carry Atomic Bombs. Jefferson, North Carolina: McFarland & Company. ISBN 0-7864-2139-8. OCLC 58554961. 
• Coster-Mullen, John (2012). Atom Bombs: The Top Secret Inside Story of Little Boy and Fat Man. United States: J. Coster-Mullen. OCLC 298514167. 
• Groves, Leslie (1962). Now it Can be Told: The Story of the Manhattan Project. New York: Harper & Row. ISBN 978-0-306-70738-4. OCLC 537684. 
• Hansen, Chuck (1995). Volume V: US Nuclear Weapons Histories. Swords of Armageddon: US Nuclear Weapons Development since 1945. Sunnyvale, California: Chukelea Publications. ISBN 978-0-9791915-0-3. OCLC 231585284. 
• Hoddeson, Lillian; Henriksen, Paul W.; Meade, Roger A.; Westfall, Catherine L. (1993). Critical Assembly: A Technical History of Los Alamos During the Oppenheimer Years, 1943–1945. New York: Cambridge University Press. ISBN 0-521-44132-3. OCLC 26764320. 
• Krauss, Robert; Krauss, Amelia, eds. (2005). The 509th Remembered: A History of the 509th Composite Group as Told by the Veterans Themselves, 509th Anniversary Reunion, Wichita, Kansas October 7-10, 2004. Buchanan, Michigan: 509th Press. ISBN 9780923568665. OCLC 59148135. 
• Lawren, William (1988). The General and the Bomb: A Biography of General Leslie R. Groves, Director of the Manhattan Project. New York: Dodd Mead. ISBN 0-396-08761-2. OCLC 16868107. 
• Rhodes, Richard (1986). The Making of the Atomic Bomb. New York: Simon & Schuster. ISBN 0-684-81378-5. OCLC 13793436. 
• Serber, Robert; Crease, Robert P. (1998). Peace & War: Reminiscences of a Life on the Frontiers of Science. New York: Columbia University Press. ISBN 9780231105460. OCLC 37631186. 

External links

• Footage of Hiroshima Atomic Bomb Attack
• Little Boy description at Carey Sublette's NuclearWeaponArchive.org
• Nuclear Files.org Definition and explanation of 'Little Boy'
• The Nuclear Weapon Archive
• Little Boy 3D Model
• Hiroshima & Nagasaki Remembered information about preparation and dropping the Little Boy bomb
• Atomic John: A truck driver uncovers secrets about the first nuclear bombs. Essay and interview with John Coster-Mullen by David Samuels in the New Yorker, December 15, 2008 issue. Coster-Mullen is the author of Atom Bombs: The Top Secret Inside Story of Little Boy and Fat Man, 2003 (first printed in 1996, self-published), considered a definitive text about Little Boy; illustrations from which are used in the Assembly details section above.