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"Little Boy" was the type of
atomic bomb A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bomb ...
dropped on the Japanese city of
Hiroshima is the capital of Hiroshima Prefecture in Japan. , the city had an estimated population of 1,199,391. The gross domestic product (GDP) in Greater Hiroshima, Hiroshima Urban Employment Area, was US$61.3 billion as of 2010. Kazumi Matsui ...
on 6 August 1945 during
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
, making it the first nuclear weapon used in warfare. The bomb was dropped by the
Boeing B-29 Superfortress The Boeing B-29 Superfortress is an American four-engined propeller-driven heavy bomber, designed by Boeing and flown primarily by the United States during World War II and the Korean War. Named in allusion to its predecessor, the B-17 Fl ...
''
Enola Gay The ''Enola Gay'' () is a Boeing B-29 Superfortress bomber, named after Enola Gay Tibbets, the mother of the pilot, Colonel Paul Tibbets. On 6 August 1945, piloted by Tibbets and Robert A. Lewis during the final stages of World War II, it ...
'' piloted by Colonel Paul W. Tibbets, Jr., commander of the 509th Composite Group of the United States Army Air Forces and Captain Robert A. Lewis. It exploded with an energy of approximately and caused widespread death and destruction throughout the city. The
Hiroshima bombing The United States detonated two atomic bombs over the Japanese cities of Hiroshima and Nagasaki on 6 and 9 August 1945, respectively. The two bombings killed between 129,000 and 226,000 people, most of whom were civilians, and remain the onl ...
was the second man-made nuclear explosion in history, after the Trinity nuclear test. Little Boy was developed by Lieutenant Commander Francis Birch's group at the
Manhattan Project The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project w ...
's
Los Alamos Laboratory The Los Alamos Laboratory, also known as Project Y, was a secret laboratory established by the Manhattan Project and operated by the University of California during World War II. Its mission was to design and build the first atomic bombs. Ro ...
during World War II, a reworking of their unsuccessful Thin Man nuclear bomb. Like Thin Man, it was a gun-type fission weapon, but it derived its explosive power from the
nuclear fission Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radio ...
of
uranium-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exi ...
, whereas Thin Man was based on fission of
plutonium-239 Plutonium-239 (239Pu or Pu-239) is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three mai ...
. Fission was accomplished by shooting a hollow cylinder (the "bullet") onto a solid cylinder of the same material (the "target") by means of a charge of
nitrocellulose Nitrocellulose (also known as cellulose nitrate, flash paper, flash cotton, guncotton, pyroxylin and flash string, depending on form) is a highly flammable compound formed by nitrating cellulose through exposure to a mixture of nitric acid and ...
propellant powder. It contained of
highly enriched uranium Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238U ...
, although less than a kilogram underwent nuclear fission. Its components were fabricated at three different plants so that no one would have a copy of the complete design. After 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 began suffering badly from the
Wigner effect The Wigner effect (named for its discoverer, Eugene Wigner), also known as the discomposition effect or Wigner's disease, is the displacement of atoms in a solid caused by neutron radiation. Any solid can display the Wigner effect. The effect ...
, the dislocation of atoms in a solid caused by neutron radiation, and plutonium became scarce, so six Little Boy assemblies were produced at
Sandia Base Sandia Base was the principal nuclear weapons installation of the United States Department of Defense from 1946 to 1971. It was located on the southeastern edge of Albuquerque, New Mexico. For 25 years, the top-secret Sandia Base and its subsidia ...
. The Navy
Bureau of Ordnance The Bureau of Ordnance (BuOrd) was a United States Navy organization, which was responsible for the procurement, storage, and deployment of all naval weapons, between the years 1862 and 1959. History Congress established the Bureau in the Departme ...
built another 25 Little Boy assemblies in 1947 for use by the
Lockheed P2V Neptune The Lockheed P-2 Neptune (designated P2V by the United States Navy prior to September 1962) is a maritime patrol and anti-submarine warfare (ASW) aircraft. It was developed for the US Navy by Lockheed to replace the Lockheed PV-1 Ventura and ...
nuclear strike aircraft which could be launched from the Midway-class aircraft carriers. All the Little Boy units were withdrawn from service by the end of January 1951.


Naming

Physicist Robert Serber named the first two
atomic bomb A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bomb ...
designs during
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
based on their shapes: Thin Man and
Fat Man "Fat Man" (also known as Mark III) is the codename for the type of nuclear bomb the United States detonated over the Japanese city of Nagasaki on 9 August 1945. It was the second of the only two nuclear weapons ever used in warfare, the fir ...
. The "Thin Man" was a long, thin device and its name came from the
Dashiell Hammett Samuel Dashiell Hammett (; May 27, 1894 – January 10, 1961) was an American writer of hard-boiled detective novels and short stories. He was also a screenwriter and political activist. Among the enduring characters he created are Sam Spade ('' ...
detective novel and series of movies about '' The Thin Man''. The "Fat Man" was round and fat so it was named after Kasper Gutman, a rotund character in Hammett's 1930 novel ''The Maltese Falcon'', played by Sydney Greenstreet in the 1941 film version. Little Boy was named by others as an allusion to Thin Man since it was based on its design.


Development

Because
uranium-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exi ...
was known to be fissionable, it was the first material pursued in the approach to bomb development. As the first design developed (as well as the first deployed for combat), it is sometimes known as the Mark I. The vast majority of the work came in the form of the
isotope enrichment Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" ...
of the uranium necessary for the weapon, since uranium-235 makes up only 1 part in 140 of natural
uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
. Enrichment was performed at
Oak Ridge, Tennessee Oak Ridge is a city in Anderson County, Tennessee, Anderson and Roane County, Tennessee, Roane counties in the East Tennessee, eastern part of the U.S. state of Tennessee, about west of downtown Knoxville, Tennessee, Knoxville. Oak Ridge's popu ...
, where the electromagnetic separation plant, known as Y-12, became fully operational in March 1944. The first shipments of
highly enriched uranium Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238U ...
were sent to the Los Alamos Laboratory in June 1944. Most of the uranium necessary for the production of the bomb came from the Shinkolobwe mine in the
Belgian Congo The Belgian Congo (french: Congo belge, ; nl, Belgisch-Congo) was a Belgian colony in Central Africa from 1908 until independence in 1960. The former colony adopted its present name, the Democratic Republic of the Congo (DRC), in 1964. Colo ...
, and was made available thanks to the foresight of the CEO of the High Katanga Mining Union, Edgar Sengier, who had approximately of uranium ore transported to a warehouse in
Staten Island Staten Island ( ) is a Boroughs of New York City, borough of New York City, coextensive with Richmond County, in the U.S. state of New York (state), New York. Located in the city's southwest portion, the borough is separated from New Jersey b ...
, New York in 1940. At least part of the in addition to the uranium ore and uranium oxide captured by the
Alsos Mission The Alsos Mission was an organized effort by a team of British and United States military, scientific, and intelligence personnel to discover enemy scientific developments during World War II. Its chief focus was on the German nuclear energy pr ...
in 1944 and 1945 went to Oak Ridge for enrichment, as did of uranium oxide captured on the Japan-bound after Germany's surrender in May 1945. Little Boy was a simplification of Thin Man, the previous gun-type fission weapon design. Thin Man, long, was designed to use plutonium, so it was also more than capable of using enriched uranium. The Thin Man design was abandoned after experiments by
Emilio G. Segrè Emilio may refer to: * Emilio Navaira Emilio H. Navaira III (August 23, 1962 – May 16, 2016) was an American singer-songwriter of Tejano and country music. He is the winner of one Grammy Award and one Latin Grammy Award. Known to most by th ...
and his P-5 Group at Los Alamos on the newly reactor-produced
plutonium Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exh ...
from Oak Ridge and the Hanford site showed that it contained impurities in the form of the
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...
plutonium-240. This has a far higher spontaneous fission rate and radioactivity than the
cyclotron A cyclotron is a type of particle accelerator invented by Ernest O. Lawrence in 1929–1930 at the University of California, Berkeley, and patented in 1932. Lawrence, Ernest O. ''Method and apparatus for the acceleration of ions'', filed: Jan ...
-produced plutonium on which the original measurements had been made, and its inclusion in reactor-bred plutonium (needed for bomb-making due to the quantities required) 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. In July 1944, almost all research at Los Alamos was redirected to the implosion-type plutonium weapon. Overall responsibility for the uranium gun-type weapon was assigned to
Captain Captain is a title, an appellative for the commanding officer of a military unit; the supreme leader of a navy ship, merchant ship, aeroplane, spacecraft, or other vessel; or the commander of a port, fire or police department, election precinct, e ...
William S. Parsons's Ordnance (O) Division. All the design, development, and technical work at Los Alamos was consolidated under
Lieutenant Commander Lieutenant commander (also hyphenated lieutenant-commander and abbreviated Lt Cdr, LtCdr. or LCDR) is a commissioned officer rank in many navies. The rank is superior to a lieutenant and subordinate to a commander. The corresponding ran ...
Francis Birch's group. In contrast to the plutonium implosion-type nuclear weapon and the plutonium gun-type fission weapon, the uranium gun-type weapon was straightforward if not trivial to design. The concept was pursued so that in case of a failure to develop a plutonium bomb, it would still be possible to use the gun principle. 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. The simplified weapon was short enough to fit into a B-29 bomb bay. 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
Naval Gun Factory A navy, naval force, or maritime force is the branch of a nation's armed forces principally designated for naval warfare, naval and amphibious warfare; namely, lake-borne, riverine, littoral zone, littoral, or ocean-borne combat operations and ...
in Washington, D.C.; the target case and some other components by the Naval Ordnance Plant in Center Line, Michigan; and the tail fairing and mounting brackets by the Expert Tool and Die Company in
Detroit, Michigan Detroit ( , ; , ) is the largest city in the U.S. state of Michigan. It is also the largest U.S. city on the United States–Canada border, and the seat of government of Wayne County. The City of Detroit had a population of 639,111 at ...
. The bomb, except for the uranium payload, was ready at the beginning of May 1945. Manhattan District Engineer Kenneth Nichols expected on 1 May 1945 to have enriched uranium "for one weapon before August 1 and a second one sometime in December", assuming the second weapon would be a gun-type; designing an implosion bomb for enriched uranium was considered, and this would increase the production rate. The enriched uranium projectile was completed on 15 June, and the target on 24 July. The target and bomb pre-assemblies (partly assembled bombs without the fissile components) left
Hunters Point Naval Shipyard The Hunters Point Naval Shipyard was a United States Navy shipyard in San Francisco, California, located on of waterfront at Hunters Point in the southeast corner of the city. Originally, Hunters Point was a commercial shipyard establishe ...
, California, on 16 July aboard the
heavy cruiser The heavy cruiser was a type of cruiser, a naval warship designed for long range and high speed, armed generally with naval guns of roughly 203 mm (8 inches) in caliber, whose design parameters were dictated by the Washington Naval T ...
, arriving on 26 July. The target inserts followed by air on 30 July. Although all of its components had been tested, no full test of a gun-type nuclear weapon occurred before the Little Boy was dropped over
Hiroshima is the capital of Hiroshima Prefecture in Japan. , the city had an estimated population of 1,199,391. The gross domestic product (GDP) in Greater Hiroshima, Hiroshima Urban Employment Area, was US$61.3 billion as of 2010. Kazumi Matsui ...
. The only test explosion of a nuclear weapon concept had been of an implosion-type device employing plutonium as its fissile material, and took place on 16 July 1945 at the Trinity nuclear test. There were several reasons for not testing a Little Boy type of device. Primarily, there was little enriched uranium as compared with the relatively large amount of plutonium which, it was expected, could be produced by the Hanford Site 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. Though Little Boy incorporated various safety mechanisms, an accidental detonation was nonetheless possible. For example, should the bomber carrying the device crash then the hollow "bullet" could be driven into the "target" cylinder, detonating the bomb or at least releasing massive amounts of radiation; tests showed that this would require a highly unlikely impact of 500 times the force of gravity. Another concern was that a crash and fire could trigger the explosives. If immersed in water, the uranium components were subject to a
neutron moderator In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, ideally without capturing any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy. These thermal neutrons are immensely m ...
effect, which would not cause an explosion but would release
radioactive contamination Radioactive contamination, also called radiological pollution, is the deposition of, or presence of radioactive substances on surfaces or within solids, liquids, or gases (including the human body), where their presence is unintended or undesirab ...
. For this reason, pilots were advised to crash on land rather than at sea.


Design

The Little Boy was in length, in diameter and weighed approximately . The design used the gun method to explosively force a hollow sub-
critical mass In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, its nuclear fi ...
of enriched uranium and a solid target cylinder together into a super-critical mass, initiating a
nuclear chain reaction In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series of these reactions. The specific nu ...
. This was accomplished by shooting one piece of the uranium onto the other by means of four cylindrical silk bags of
cordite Cordite is a family of smokeless propellants developed and produced in the United Kingdom since 1889 to replace black powder as a military propellant. Like modern gunpowder, cordite is classified as a low explosive because of its slow burn ...
powder. This was a widely used smokeless propellant consisting of a mixture of 65 percent
nitrocellulose Nitrocellulose (also known as cellulose nitrate, flash paper, flash cotton, guncotton, pyroxylin and flash string, depending on form) is a highly flammable compound formed by nitrating cellulose through exposure to a mixture of nitric acid and ...
, 30 percent
nitroglycerine Nitroglycerin (NG), (alternative spelling of nitroglycerine) also known as trinitroglycerin (TNG), nitro, glyceryl trinitrate (GTN), or 1,2,3-trinitroxypropane, is a dense, colorless, oily, explosive liquid most commonly produced by nitrating ...
, 3 percent
petroleum jelly Petroleum jelly, petrolatum, white petrolatum, soft paraffin, or multi-hydrocarbon, CAS number 8009-03-8, is a semi-solid mixture of hydrocarbons (with carbon numbers mainly higher than 25), originally promoted as a topical ointment for its h ...
, and 2 percent carbamite that was extruded into tubular granules. This gave it a high surface area and a rapid burning area, and could attain pressures of up to . Cordite for the wartime Little Boy was sourced from Canada; propellant for post-war Little Boys was obtained from the
Picatinny Arsenal The Picatinny Arsenal ( or ) is an American military research and manufacturing facility located on of land in Jefferson and Rockaway Township in Morris County, New Jersey, United States, encompassing Picatinny Lake and Lake Denmark. The Ar ...
. The bomb contained of enriched uranium. Most was enriched to 89% but some was only 50% uranium-235, for an average enrichment of 80%. Less than a kilogram of uranium underwent
nuclear fission Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radio ...
, and of this mass only was transformed into several forms of energy, mostly
kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acce ...
, but also heat and radiation.


Assembly details

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 (). It consisted of a stack of nine uranium rings, each in diameter with a bore in the center, and a total length of , pressed together into the front end of a thin-walled projectile long. Filling in the remainder of the space behind these rings in the projectile was a
tungsten carbide Tungsten carbide (chemical formula: WC) is a chemical compound (specifically, a carbide) containing equal parts of tungsten and carbon atoms. In its most basic form, tungsten carbide is a fine gray powder, but it can be pressed and formed into ...
disc with a steel back. At ignition, the projectile slug was pushed along the long, smooth-bore gun barrel. The slug "insert" was a cylinder, in length with a axial hole. The slug comprised 40% of the total fissile mass (). The insert was a stack of six washer-like uranium discs somewhat thicker than the projectile rings that were slid over a 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 .
Neutron initiator A modulated neutron initiator is a neutron source capable of producing a burst of neutrons on activation. It is a crucial part of some nuclear weapons, as its role is to "kick-start" the chain reaction at the optimal moment when the configuration i ...
s at the base of the projectile were 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, 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 mass In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, its nuclear fi ...
es 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.


Fuze system

The fuzing system was designed to trigger at the most destructive altitude, which calculations suggested was . It employed 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 the 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 its internal 24–volt battery and starting the timer. At the end of the 15 seconds, the bomb would be from the aircraft, and 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) 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 ), 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 altimeter A radar altimeter (RA), also called a radio altimeter (RALT), electronic altimeter, reflection altimeter, or low-range radio altimeter (LRRA), measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it t ...
s 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 of WM slotted-tube
cordite Cordite is a family of smokeless propellants developed and produced in the United Kingdom since 1889 to replace black powder as a military propellant. Like modern gunpowder, cordite is classified as a low explosive because of its slow burn ...
. This launched the uranium projectile towards the opposite end of the gun barrel at an eventual
muzzle velocity Muzzle velocity is the speed of a projectile ( bullet, pellet, slug, ball/ shots or shell) with respect to the muzzle at the moment it leaves the end of a gun's barrel (i.e. the muzzle). Firearm muzzle velocities range from approximately ...
of . 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.


Rehearsals

The Little Boy pre-assemblies were designated L-1, L-2, L-3, L-4, L-5, L-6, L-7, and L-11. 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 '' Big Stink'', piloted by
Colonel Colonel (abbreviated as Col., Col or COL) is a senior military officer rank used in many countries. It is also used in some police forces and paramilitary organizations. In the 17th, 18th and 19th centuries, a colonel was typically in charge ...
Paul W. Tibbets Paul Warfield Tibbets Jr. (23 February 1915 – 1 November 2007) was a brigadier general in the United States Air Force. He is best known as the aircraft captain who flew the B-29 Superfortress known as the ''Enola Gay'' (named after his moth ...
, the commander of the 509th Composite Group. Two more drop tests over the sea were made on 24 and 25 July, using the L-2 and L-5 units in order to test all components. Tibbets was the pilot for both missions, but this time the bomber used was the one subsequently known as '' Jabit''. L-6 was used as a dress rehearsal on 29 July. The B-29 '' Next Objective'', piloted by
Major Major ( commandant in certain jurisdictions) is a military rank of commissioned officer status, with corresponding ranks existing in many military forces throughout the world. When used unhyphenated and in conjunction with no other indicato ...
Charles W. Sweeney Charles William Sweeney (December 27, 1919 – July 16, 2004) was an officer in the United States Army Air Forces during World War II and the pilot who flew ''Bockscar'' carrying the Fat Man atomic bomb to the Japanese city of Nagasaki on Augu ...
, flew to
Iwo Jima Iwo Jima (, also ), known in Japan as , is one of the Japanese Volcano Islands and lies south of the Bonin Islands. Together with other islands, they form the Ogasawara Archipelago. The highest point of Iwo Jima is Mount Suribachi at high. ...
, where emergency procedures for loading the bomb onto a standby aircraft were practiced. This rehearsal was repeated on 31 July, but this time L-6 was reloaded onto a different B-29, ''
Enola Gay The ''Enola Gay'' () is a Boeing B-29 Superfortress bomber, named after Enola Gay Tibbets, the mother of the pilot, Colonel Paul Tibbets. On 6 August 1945, piloted by Tibbets and Robert A. Lewis during the final stages of World War II, it ...
'', piloted by Tibbets, and the bomb was test dropped near Tinian. L-11 was the assembly used for the Hiroshima bomb.


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,
Second Lieutenant Second lieutenant is a junior commissioned officer military rank in many armed forces, comparable to NATO OF-1 rank. Australia The rank of second lieutenant existed in the military forces of the Australian colonies and Australian Army unt ...
Morris R. Jeppson, made their way into the bomb bay along the narrow catwalk on the port side. Jeppson held a flashlight while Parsons disconnected the primer wires, removed the breech plug, inserted the powder bags, replaced the breech plug, and reconnected the wires. Before climbing to altitude on approach to the target, Jeppson switched the three safety plugs between the electrical connectors of the internal battery and the firing mechanism from green to red. The bomb was then fully armed. Jeppson monitored the bomb's circuits. 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 . It was less powerful than the
Fat Man "Fat Man" (also known as Mark III) is the codename for the type of nuclear bomb the United States detonated over the Japanese city of Nagasaki on 9 August 1945. It was the second of the only two nuclear weapons ever used in warfare, the fir ...
, which was dropped on
Nagasaki is the capital and the largest city of Nagasaki Prefecture on the island of Kyushu in Japan. It became the sole port used for trade with the Portuguese and Dutch during the 16th through 19th centuries. The Hidden Christian Sites in the ...
, but the damage and the number of victims at Hiroshima were much higher, as Hiroshima was on flat terrain, while the
hypocenter In seismology, a hypocenter or hypocentre () is the point of origin of an earthquake or a subsurface nuclear explosion. A synonym is the focus of an earthquake. Earthquakes An earthquake's hypocenter is the position where the strain ener ...
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. Of those deaths, 20,000 were members of the
Imperial Japanese Army The was the official ground-based armed force of the Empire of Japan from 1868 to 1945. It was controlled by the Imperial Japanese Army General Staff Office and the Ministry of the Army, both of which were nominally subordinate to the Emper ...
. The exact measurement of the yield was problematic since the weapon had never been tested.
President President most commonly refers to: *President (corporate title) * President (education), a leader of a college or university * President (government title) President may also refer to: Automobiles * Nissan President, a 1966–2010 Japanese ...
Harry S. Truman officially announced that the yield was . This was based on Parsons's visual assessment that the blast was greater than what he had seen at the Trinity nuclear test. Since that had been estimated at , speech writers rounded up to 20 kilotons. Further discussion was then suppressed, for fear of lessening the impact of the bomb on the Japanese. Data had been collected by Luis Alvarez, Harold Agnew, and
Lawrence H. Johnston Lawrence Harding Johnston (February 11, 1918 – December 4, 2011) was an American physicist, a young contributor to the Manhattan Project. He was the only man to witness all three atomic explosions in 1945: the Trinity nuclear test in New Me ...
on the instrument plane, ''
The Great Artiste ''The Great Artiste'' was a U.S. Army Air Forces Silverplate B-29 bomber (B-29-40-MO 44-27353, Victor number 89), assigned to the 393d Bomb Squadron, 509th Composite Group. The aircraft was named for its bombardier, Captain Kermit Beahan, ...
'', but this was not used to calculate the yield at the time. After hostilities ended, a survey team from the Manhattan Project that included
William Penney William George Penney, Baron Penney, (24 June 19093 March 1991) was an English mathematician and professor of mathematical physics at the Imperial College London and later the rector of Imperial College London. He had a leading role in the ...
, Robert Serber, and
George T. Reynolds George Thomas Reynolds (May 27, 1917 – April 19, 2005) was an American physicist best known for his accomplishments in particle physics, biophysics and environmental science. Reynolds received his PhD in physics from Princeton in 1943, ...
was sent to Hiroshima to evaluate the effects of the blast. From evaluating the effects on objects and structures, Penney concluded that the yield was 12 ± 1 kilotons. Later calculations based on charring pointed to a yield of 13 to 14 kilotons. In 1953,
Frederick Reines Frederick Reines ( ; March 16, 1918 – August 26, 1998) was an American physicist. He was awarded the 1995 Nobel Prize in Physics for his co-detection of the neutrino with Clyde Cowan in the neutrino experiment. He may be the only scientist i ...
calculated the yield as . This figure became the official yield.


Project Ichiban

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, 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. Based on this and the data from ''The Great Artiste'', the yield was estimated at 16.6 ± 0.3 kilotons. After considering many estimation methods, a 1985 report concluded that the yield was ± 20%.


Physical effects

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 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 in diameter. The damage came from three main effects: blast, fire, and radiation.


Blast

The blast from a nuclear bomb is the result of
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
-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 from the city center over hilly terrain that was partially bare of buildings. In Hiroshima, almost everything within 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 contour at . 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.


Fire

The first effect of the explosion was blinding light, accompanied by radiant heat from the fireball. The Hiroshima fireball was in diameter, with a surface temperature of , 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 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 A firestorm is a conflagration which attains such intensity that it creates and sustains its own wind system. It is most commonly a natural phenomenon, created during some of the largest bushfires and wildfires. Although the term has been used ...
, pulling in surface air from all directions to feed an inferno which consumed everything flammable. The Hiroshima firestorm was roughly 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 fourth 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."


Radiation

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 An air burst or airburst is the detonation of an explosive device such as an anti-personnel artillery shell or a nuclear weapon in the air instead of on contact with the ground or target. The principal military advantage of an air burst over ...
, the fission products rise into the
stratosphere The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air h ...
, where they dissipate and become part of the global environment. Because Little Boy was an air burst above the ground, there was no bomb crater and no local radioactive fallout. However, a burst of intense
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the atomic nucleus, nuclei of atoms. Since protons and ...
and
gamma radiation A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically s ...
came directly from the fission of the uranium. Its lethal radius was approximately , covering about half of the firestorm area. An estimated 30% of immediate fatalities were people who received lethal doses of this direct radiation, but died in the firestorm before their radiation injuries would have become apparent. Over 6,000 people survived the blast and fire, but died of radiation injuries. Among injured survivors, 30% had radiation injuries from which they recovered, but with a lifelong increase in
cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
risk. To date, no radiation-related evidence of heritable diseases has been observed among the survivors' children.


Conventional weapon equivalent

Although Little Boy exploded with the energy equivalent of 16,000 tons of TNT, the
Strategic Bombing Survey The United States Strategic Bombing Survey (USSBS) was a written report created by a board of experts assembled to produce an impartial assessment of the effects of the Anglo-American strategic bombing of Nazi Germany during the European theatre ...
estimated that the same blast and fire effect could have been caused by 2,100 tons of conventional bombs: "220 B-29s carrying 1,200 tons of
incendiary bombs Incendiary weapons, incendiary devices, incendiary munitions, or incendiary bombs are weapons designed to start fires or destroy sensitive equipment using fire (and sometimes used as anti-personnel weaponry), that use materials such as napalm, th ...
, 400 tons of high-explosive bombs, and 500 tons of
anti-personnel An anti-personnel weapon is a weapon primarily used to maim or kill infantry and other personnel not behind armor, as opposed to attacking structures or vehicles, or hunting game. The development of defensive fortification and combat vehicles ...
fragmentation bombs." Since the target was spread across a two-dimensional plane, the vertical component of a single spherical nuclear explosion was largely wasted. A
cluster bomb A cluster munition is a form of air-dropped or ground-launched explosive weapon that releases or ejects smaller submunitions. Commonly, this is a cluster bomb that ejects explosive bomblets that are designed to kill personnel and destroy vehicl ...
pattern of smaller explosions would have been a more energy-efficient match to the target.


Post-war

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
Wigner effect The Wigner effect (named for its discoverer, Eugene Wigner), also known as the discomposition effect or Wigner's disease, is the displacement of atoms in a solid caused by neutron radiation. Any solid can display the Wigner effect. The effect ...
. Faced with the prospect of no more plutonium for new cores and no more
polonium Polonium is a chemical element with the symbol Po and atomic number 84. Polonium is a chalcogen. A rare and highly radioactive metal with no stable isotopes, polonium is chemically similar to selenium and tellurium, though its metallic character ...
for the initiators for the cores that had already been produced, the Director of the Manhattan Project,
Major General Major general (abbreviated MG, maj. gen. and similar) is a military rank used in many countries. It is derived from the older rank of sergeant major general. The disappearance of the "sergeant" in the title explains the apparent confusion of ...
Leslie R. Groves, ordered that some Little Boys be prepared as an interim measure until a solution could be found. No Little Boy assemblies were available, and no comprehensive set of diagrams of the Little Boy could be found, although there were drawings of the various components, and stocks of spare parts. At
Sandia Base Sandia Base was the principal nuclear weapons installation of the United States Department of Defense from 1946 to 1971. It was located on the southeastern edge of Albuquerque, New Mexico. For 25 years, the top-secret Sandia Base and its subsidia ...
, three Army officers,
Captains Captain is a title, an appellative for the commanding officer of a military unit; the supreme leader of a navy ship, merchant ship, aeroplane, spacecraft, or other vessel; or the commander of a port, fire or police department, election precinct, e ...
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 Project Alberta, also known as Project A, was a section of the Manhattan Project which assisted in delivering the first nuclear weapons in the atomic bombing of Hiroshima and Nagasaki during World War II. Project Alberta was formed in Marc ...
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 Navy
Bureau of Ordnance The Bureau of Ordnance (BuOrd) was a United States Navy organization, which was responsible for the procurement, storage, and deployment of all naval weapons, between the years 1862 and 1959. History Congress established the Bureau in the Departme ...
built 25 Little Boy assemblies in 1947 for use by the nuclear-capable
Lockheed P2V Neptune The Lockheed P-2 Neptune (designated P2V by the United States Navy prior to September 1962) is a maritime patrol and anti-submarine warfare (ASW) aircraft. It was developed for the US Navy by Lockheed to replace the Lockheed PV-1 Ventura and ...
aircraft carrier An aircraft carrier is a warship that serves as a seagoing airbase, equipped with a full-length flight deck and facilities for carrying, arming, deploying, and recovering aircraft. Typically, it is the capital ship of a fleet, as it allows a ...
aircraft (which could be launched from but not land on the Midway-class aircraft carriers). Components were produced by the Naval Ordnance Plants in
Pocatello, Idaho Pocatello () is the county seat of and largest city in Bannock County, with a small portion on the Fort Hall Indian Reservation in neighboring Power County, in the southeastern part of the U.S. state of Idaho. It is the principal city of the ...
, and
Louisville, Kentucky Louisville ( , , ) is the largest city in the Commonwealth of Kentucky and the 28th most-populous city in the United States. Louisville is the historical seat and, since 2003, the nominal seat of Jefferson County, on the Indiana border ...
. Enough fissionable material was available by 1948 to build ten projectiles and targets, although there were only enough initiators for six. All the Little Boy units were withdrawn from service by the end of January 1951. The
Smithsonian Institution The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums and education and research centers, the largest such complex in the world, created by the U.S. government "for the increase and diffusion of knowledge". Found ...
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 bombs 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 Imperial War Museums (IWM) is a British national museum organisation with branches at five locations in England, three of which are in London. Founded as the Imperial War Museum in 1917, the museum was intended to record the civil and military ...
in London.


Notes


References

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External links



at Carey Sublette's NuclearWeaponArchive.org

Definition and explanation of 'Little Boy'


Simulation of "Little Boy"
an interactive simulation of "Little Boy"
Little Boy 3D Model


information about preparation and dropping the Little Boy bomb
Little boy Nuclear Bomb at Imperial War museum London UK (jpg)
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