|Filling weight||64 kg|
|Blast yield||15 kilotons of TNT (63 TJ)|
Little Boy was the name of the type of
Little Boy was developed by
After the war, a number of components for further Little Boy bombs were built. By 1950, only five complete weapons had been created, and these were retired by November 1950.
Because of its perceived simplicity, the gun-type nuclear weapon design was the first approach pursued by the scientists working on bomb design during the Manhattan Project. In 1942, it was not yet known which of the two fissile materials pathways being simultaneously pursued — uranium-235 or plutonium-239 — would be successful, or if there were significant differences between the two fuels that would impact the design work. Coordination with British scientists in May 1942 convinced the American scientists, led by J. Robert Oppenheimer, that the atomic bomb would not be difficult to design, and that the difficulty would lie only in the production of fuel. Early calculations in the summer of 1942 by theoretical physicists working on the project reinforced the idea that an ordinary artillery gun barrel would be able to impact sufficient velocity to the fissile material projectile.
Several different weapon designs, including autocatalytic assembly, a nascent version of implosion, and alternative gun designs (e.g., using high explosives as a propellent, or creating a "double-gun" with two projectiles) were pursued in the early years of the project, while the facilities to manufacture fissile material were being constructed. The belief that the gun design would be an easy engineering task once fuel was available led to a sense of optimism at Los Alamos, although Oppenheimer established a small research group to study implosion as a fallback in early 1943.
Concern that impurities in reactor-bred plutonium would make predetonation more likely meant that much of the gun-design work was focused on the plutonium gun. To achieve high projectile velocities, the plutonium gun was 17 feet (5.2 m) long with a narrow diameter — suggesting its code-name as the Thin Man — which created considerable difficulty in its ballistics dropping from aircraft and fitting it into the bomb bay of a
In the spring of 1944,
Consequently, in July 1944, almost all research at Los Alamos was redirected to the implosion-type plutonium weapon, and the laboratory itself was entirely reorganized around the implosion problem. Work on the gun-type weapon continued under Person's Ordnance (O) Division, for use exclusively with highly-enriched uranium as a fuel. All the design, development, and technical work at Los Alamos was consolidated under
The design specifications were completed in February 1945, and contracts were let to build the components. Three different plants were used so that no one would have a copy of the complete design. The gun and breech were made by the
Although all of its components had been individually tested,
Though Little Boy incorporated various safety mechanisms, an accidental detonation of a fully-assembled weapon was very possible. For example, should the bomber carrying the device crash then the hollow "bullet" could be driven into the "target" cylinder, possibly detonating the bomb from gravity alone (though tests suggested this was unlikely), but easily creating a critical mass that would release dangerous amounts of radiation. And crash of the B-29 and subsequent fire could trigger the explosives, causing the weapon to detonate. If immersed in water, the uranium components were subject to a neutron moderator effect, which would not cause an explosion but would release radioactive contamination. For this reason, pilots were advised to crash on land rather than at sea. Ultimately, Parsons opted to keep the explosives out of the Little Boy bomb until after the B-29 had taken off, to avoid the risk of a crash that could destroy or damage the military base the weapon was being launched from.
The Little Boy was 120 inches (300 cm) in length, 28 inches (71 cm) in diameter and weighed approximately 9,700 pounds (4,400 kg).
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 kilograms [85 lb]). It consisted of a stack of nine uranium rings, each 6.25 inches (159 mm) in diameter with a 4-inch (100 mm) bore in the center, and a total length of 7 inches (180 mm), pressed together into the front end of a thin-walled projectile 16.25 inches (413 mm) 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 42 inches (1,100 mm) along the 72-inch-long (1,800 mm), 6.5-inch-wide (170 mm) smooth-bore gun barrel. The slug "insert" was a 4-inch cylinder, 7 inches in length with a 1-inch (25 mm) axial hole. The slug comprised 40% of the total fissile mass (25.6 kilograms or 56 pounds). The insert was a stack of six washer-like uranium discs somewhat thicker than the projectile rings that were slid over a 1-inch rod. This rod then extended forward through the tungsten carbide tamper plug, impact-absorbing anvil, and nose plug backstop, eventually protruding out of 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 kilograms (5,100 lb).
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, stationary target. However, critical mass considerations dictated that in Little Boy the more extensive, hollow piece would be the projectile. The assembled fissile core had more than two critical masses of uranium-235. This required one of the two pieces to have more than one critical mass, with the larger piece avoiding criticality prior to assembly by means of shape and minimal contact with the neutron-reflecting tungsten carbide tamper.
A hole in the center of the larger piece dispersed the mass and increased the surface area, allowing more fission neutrons to escape, thus preventing a premature chain reaction. But, for this larger, hollow piece to have minimal contact with the tamper, it must be the projectile, since only the projectile's back end was in contact with the tamper prior to detonation. The rest of the tungsten carbide surrounded the sub-critical mass target cylinder (called the "insert" by the designers) with air space between it and the insert. This arrangement packs the maximum amount of fissile material into a gun-assembly design.
- 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 the safety of the aircraft. The timer was activated when the electrical 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) gradually deformed as ambient air pressure increased during descent. The barometric fuze 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; 6,600 ft), 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 2 pounds (910 g) 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 meters per second (980 ft/s). Approximately 10 milliseconds later the chain reaction occurred, lasting less than 1 microsecond. The radar altimeters used were modified U.S. Army Air Corps APS-13 tail warning radars, nicknamed "Archie", normally used to warn a fighter pilot of another plane approaching from behind.
The Little Boy pre-assemblies were designated L-1, L-2, L-3, L-4, L-5, L-6, L-7, and L-11. Of these, L-1, L-2, L-5, and L-6 were expended in test drops. The first drop test was conducted with L-1 on 23 July 1945. It was dropped over the sea near Tinian in order to test the radar altimeter by the B-29 later known as
Bombing of Hiroshima
Parsons, the Enola Gay's weaponeer, was concerned about the possibility of an accidental detonation if the plane crashed on takeoff, so he decided not to load the four cordite powder bags into the gun breech until the aircraft was in flight. After takeoff, Parsons and his assistant,
The bomb was dropped at approximately 08:15 (JST) on 6 August 1945. After falling for 44.4 seconds, the time and barometric triggers started the firing mechanism. The detonation happened at an altitude of 1,968 ± 50 feet (600 ± 15 m). It was less powerful than the Fat Man, which was dropped on Nagasaki, but 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. Later estimates put the deaths as high as 140,000 people. The United States Strategic Bombing Survey estimated that out of 24,158 Imperial Japanese Army soldiers in Hiroshima at the time of the bombing, 6,789 were killed or missing as a result of the bombing.
The exact measurement of the explosive yield of the bomb was problematic since the weapon had never been tested.
After being selected in April 1945, Hiroshima was spared conventional bombing to serve as a pristine target, where the effects of a nuclear bomb on an undamaged city could be observed. While damage could be studied later, the energy yield of the untested Little Boy design could be determined only at the moment of detonation, using instruments dropped by parachute from a plane flying in formation with the one that dropped the bomb. Radio-transmitted data from these instruments indicated a yield of about 15 kilotons.
Comparing this yield to the observed damage produced a rule of thumb called the 5 pounds per square inch (34 kPa) lethal area rule. Approximately all the people inside the area where the shock wave carried such an overpressure or greater would be killed. At Hiroshima, that area was 3.5 kilometers (2.2 mi) in diameter.
The damage came from three main effects: blast, fire, and radiation.
The blast from a nuclear bomb is the result of X-ray-heated air (the fireball) sending a shock wave or pressure wave in all directions, initially at a velocity greater than the speed of sound, analogous to thunder generated by lightning. Knowledge about urban blast destruction is based largely on studies of Little Boy at Hiroshima. Nagasaki buildings suffered similar damage at similar distances, but the Nagasaki bomb detonated 3.2 kilometers (2.0 mi) from the city center over hilly terrain that was partially bare of buildings.
In Hiroshima, almost everything within 1.6 kilometers (1.0 mi) of the point directly under the explosion was completely destroyed, except for about 50 heavily reinforced, earthquake-resistant concrete buildings, only the shells of which remained standing. Most were completely gutted, with their windows, doors, sashes, and frames ripped out. The perimeter of severe blast damage approximately followed the 5 pounds per square inch (34 kPa) contour at 1.8 kilometers (1.1 mi).
Later test explosions of nuclear weapons with houses and other test structures nearby confirmed the 5 psi overpressure threshold. Ordinary urban buildings experiencing it were crushed, toppled, or gutted by the force of air pressure. The picture at right shows the effects of a nuclear bomb-generated 5 psi pressure wave on a test structure in Nevada in 1953.
A major effect of this kind of structural damage was that it created fuel for fires that were started simultaneously throughout the severe destruction region.
The first effect of the explosion was blinding light, accompanied by radiant heat from the fireball. The Hiroshima fireball was 370 meters (1,200 ft) in diameter, with a surface temperature of 6,000 °C (10,830 °F), about the same temperature as at the surface of the sun. Near ground zero, everything flammable burst into flame. One famous, anonymous Hiroshima victim, sitting on stone steps 260 metres (850 ft) from the hypocenter, left only a shadow, having absorbed the fireball heat that permanently bleached the surrounding stone. Simultaneous fires were started throughout the blast-damaged area by fireball heat and by overturned stoves and furnaces, electrical shorts, etc. Twenty minutes after the detonation, these fires had merged into a firestorm, pulling in surface air from all directions to feed an inferno which consumed everything flammable.
The Hiroshima firestorm was roughly 3.2 kilometers (2.0 mi) in diameter, corresponding closely to the severe blast-damage zone. (See the USSBS map, right.) Blast-damaged buildings provided fuel for the fire. Structural lumber and furniture were splintered and scattered about. Debris-choked roads obstructed firefighters. Broken gas pipes fueled the fire, and broken water pipes rendered hydrants useless. At Nagasaki, the fires failed to merge into a single firestorm, and the fire-damaged area was only one-quarter as great as at Hiroshima, due in part to a southwest wind that pushed the fires away from the city.
As the map shows, the Hiroshima firestorm jumped natural firebreaks (river channels), as well as prepared firebreaks. The spread of fire stopped only when it reached the edge of the blast-damaged area, encountering less available fuel. The Manhattan Project report on Hiroshima estimated that 60% of immediate deaths were caused by fire, but with the caveat that "many persons near the center of explosion suffered fatal injuries from more than one of the bomb effects."
Local fallout is dust and ash from a bomb crater, contaminated with radioactive fission products. It falls to earth downwind of the crater and can produce, with radiation alone, a lethal area much larger than that from blast and fire. With an air burst, the fission products rise into the stratosphere, where they dissipate and become part of the global environment. Because Little Boy was an air burst 580 meters (1,900 ft) above the ground, there was no bomb crater and no local radioactive fallout.
However, a burst of intense
After the surrender of Japan was finalized, Manhattan Project scientists began to immediately survey the city of Hiroshima to better understand the damage, and to communicate with Japanese physicians about radiation effects in particular. The collaboration became the Atomic Bomb Casualty Commission in 1946, a joint U.S.–Japanese project to track radiation injuries among survivors. In 1975 its work was superseded by the Radiation Effects Research Foundation.
In 1962, scientists at Los Alamos created a mockup of Little Boy known as "Project Ichiban" in order to answer some of the unanswered questions about the exact radiation output of the bomb, which would be useful for setting benchmarks for interpreting the relationship between radiation exposure and later health outcomes. But it failed to clear up all the issues. In 1982, Los Alamos created a replica Little Boy from the original drawings and specifications. This was then tested with enriched uranium but in a safe configuration that would not cause a nuclear explosion. A hydraulic lift was used to move the projectile, and experiments were run to assess neutron emission.
Conventional weapon equivalent
After hostilities ended, a survey team from the Manhattan Project that included
Although Little Boy exploded with the energy equivalent of around 15 kilotons of TNT, in 1946 the
When the war ended, it was not expected that the inefficient Little Boy design would ever again be required, and many plans and diagrams were destroyed. However, by mid-1946 the Hanford Site reactors were suffering badly from the
At Sandia Base, three Army officers, Captains Albert Bethel, Richard Meyer, and Bobbie Griffin attempted to re-create the Little Boy. They were supervised by Harlow W. Russ, an expert on Little Boy who served with Project Alberta on Tinian, and was now leader of the Z-11 Group of the Los Alamos Laboratory's Z Division at Sandia. Gradually, they managed to locate the correct drawings and parts, and figured out how they went together. Eventually, they built six Little Boy assemblies. Although the casings, barrels, and components were tested, no enriched uranium was supplied for the bombs. By early 1947, the problem caused by the Wigner effect was on its way to solution, and the three officers were reassigned.
The Smithsonian Institution displayed a Little Boy (complete, except for enriched uranium), until 1986. The Department of Energy took the weapon from the museum to remove its inner components, so the bomb could not be stolen and detonated with fissile material. The government returned the emptied casing to the Smithsonian in 1993. Three other disarmed bombs are on display in the United States; another is at the Imperial War Museum in London.
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- Simulation of "Little Boy" an interactive simulation of "Little Boy"
- Little Boy 3D Model
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- Little boy Nuclear Bomb at Imperial War museum London UK (jpg)