United States Air Force
The United States Air Force is the aerial and space warfare service branch of the United States Armed Forces. It is one of the five branches of the United States Armed Forces, one of the seven American uniformed services. Formed as a part of the United States Army on 1 August 1907, the USAF was established as a separate branch of the U. S. Armed Forces on 18 September 1947 with the passing of the National Security Act of 1947, it is the youngest branch of the U. S. Armed Forces, the fourth in order of precedence; the USAF is the largest and most technologically advanced air force in the world. The Air Force articulates its core missions as air and space superiority, global integrated intelligence and reconnaissance, rapid global mobility, global strike, command and control; the U. S. Air Force is a military service branch organized within the Department of the Air Force, one of the three military departments of the Department of Defense; the Air Force, through the Department of the Air Force, is headed by the civilian Secretary of the Air Force, who reports to the Secretary of Defense, is appointed by the President with Senate confirmation.
The highest-ranking military officer in the Air Force is the Chief of Staff of the Air Force, who exercises supervision over Air Force units and serves as one of the Joint Chiefs of Staff. Air Force components are assigned, as directed by the Secretary of Defense, to the combatant commands, neither the Secretary of the Air Force nor the Chief of Staff of the Air Force have operational command authority over them. Along with conducting independent air and space operations, the U. S. Air Force provides air support for land and naval forces and aids in the recovery of troops in the field; as of 2017, the service operates more than 5,369 military aircraft, 406 ICBMs and 170 military satellites. It has a $161 billion budget and is the second largest service branch, with 318,415 active duty airmen, 140,169 civilian personnel, 69,200 reserve airmen, 105,700 Air National Guard airmen. According to the National Security Act of 1947, which created the USAF: In general, the United States Air Force shall include aviation forces both combat and service not otherwise assigned.
It shall be organized and equipped for prompt and sustained offensive and defensive air operations. The Air Force shall be responsible for the preparation of the air forces necessary for the effective prosecution of war except as otherwise assigned and, in accordance with integrated joint mobilization plans, for the expansion of the peacetime components of the Air Force to meet the needs of war. §8062 of Title 10 US Code defines the purpose of the USAF as: to preserve the peace and security, provide for the defense, of the United States, the Territories and possessions, any areas occupied by the United States. The stated mission of the USAF today is to "fly and win...in air and cyberspace". "The United States Air Force will be a trusted and reliable joint partner with our sister services known for integrity in all of our activities, including supporting the joint mission first and foremost. We will provide compelling air and cyber capabilities for use by the combatant commanders. We will excel as stewards of all Air Force resources in service to the American people, while providing precise and reliable Global Vigilance and Power for the nation".
The five core missions of the Air Force have not changed since the Air Force became independent in 1947, but they have evolved, are now articulated as air and space superiority, global integrated intelligence and reconnaissance, rapid global mobility, global strike, command and control. The purpose of all of these core missions is to provide, what the Air Force states as, global vigilance, global reach, global power. Air superiority is "that degree of dominance in the air battle of one force over another which permits the conduct of operations by the former and its related land, sea and special operations forces at a given time and place without prohibitive interference by the opposing force". Offensive Counterair is defined as "offensive operations to destroy, disrupt, or neutralize enemy aircraft, launch platforms, their supporting structures and systems both before and after launch, but as close to their source as possible". OCA is the preferred method of countering air and missile threats since it attempts to defeat the enemy closer to its source and enjoys the initiative.
OCA comprises attack operations, sweep and suppression/destruction of enemy air defense. Defensive Counter air is defined as "all the defensive measures designed to detect, identify and destroy or negate enemy forces attempting to penetrate or attack through friendly airspace". A major goal of DCA operations, in concert with OCA operations, is to provide an area from which forces can operate, secure from air and missile threats; the DCA mission comprises both passive defense measures. Active defense is "the employment of limited offensive action and counterattacks to deny a contested area or position to the enemy", it includes both ballistic missile defense and air-breathing threat defense, encompasses point defense, area defense, high-value airborne asset defense. Passive defense is "measures taken to reduce the probability of and to minimize the effects of damage caused by hostile action without the intention of taking the initiative", it includes warning.
Neutron capture
Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more than positively charged protons, which are repelled electrostatically. Neutron capture plays an important role in the cosmic nucleosynthesis of heavy elements. In stars it can proceed in two ways: as a slow process. Nuclei of masses greater than 56 cannot be formed by thermonuclear reactions, but can be formed by neutron capture. Neutron capture on protons yields a line at 2.223 MeV predicted and observed in solar flares. At small neutron flux, as in a nuclear reactor, a single neutron is captured by a nucleus. For example, when natural gold is irradiated by neutrons, the isotope 198Au is formed in a excited state, decays to the ground state of 198Au by the emission of γ rays. In this process, the mass number increases by one; this is written in short form 197Au198Au. If thermal neutrons are used, the process is called thermal capture.
The isotope 198Au is a beta emitter. In this process the atomic number rises by one; the r-process happens inside stars if the neutron flux density is so high that the atomic nucleus has no time to decay via beta emission in between neutron captures. The mass number therefore rises by a large amount. Only afterwards, the unstable nuclei decay via many β− decays to stable or unstable nuclei of high atomic number; the absorption neutron cross-section of an isotope of a chemical element is the effective cross sectional area that an atom of that isotope presents to absorption, is a measure of the probability of neutron capture. It is measured in barns. Absorption cross section is highly dependent on neutron energy; as a generality, the likelihood of absorption is proportional to the time the neutron is in the vicinity of the nucleus. The time spent in the vicinity of the nucleus is inversely proportional to the relative velocity between the neutron and nucleus. Other more specific issues modify this general principle.
Two of the most specified measures are the cross-section for thermal neutron absorption, resonance integral which considers the contribution of absorption peaks at certain neutron energies specific to a particular nuclide above the thermal range, but encountered as neutron moderation slows the neutron down from an original high energy. The thermal energy of the nucleus has an effect. In particular, the increase in uranium-238's ability to absorb neutrons at higher temperatures is a negative feedback mechanism that helps keep nuclear reactors under control. Neutron capture is involved in the formation of isotopes of chemical elements; as a consequence of this fact the energy of neutron capture intervenes in the standard enthalpy of formation of isotopes. Neutron activation analysis can be used to remotely detect the chemical composition of materials; this is because different elements release different characteristic radiation when they absorb neutrons. This makes it useful in many fields related to mineral security.
The most important neutron absorber is 10B as 10B4C in control rods, or boric acid as a coolant water additive in PWRs. Other important neutron absorbers that are used in nuclear reactors are xenon, hafnium, cobalt, titanium, erbium, europium and ytterbium; these occur in combinations such as Mo2B5, hafnium diboride, titanium diboride, dysprosium titanate and gadolinium titanate. Hafnium, one of the last stable elements to be discovered, presents an interesting case. Though hafnium is a heavier element, its electron configuration makes it identical with the element zirconium, they are always found in the same ores. However, their nuclear properties are different in a profound way. Hafnium absorbs neutrons avidly, it can be used in reactor control rods, whereas natural zirconium is transparent to neutrons. So, zirconium is a desirable construction material for reactor internal parts, including the metallic cladding of the fuel rods which contain either uranium, plutonium, or mixed oxides of the two elements.
Hence, it is quite important to be able to separate the zirconium from the hafnium in their occurring alloy. This can only be done inexpensively by using modern chemical ion-exchange resins. Similar resins are used in reprocessing nuclear fuel rods, when it is necessary to separate uranium and plutonium, sometimes thorium. Beta decay Induced radioactivity List of particles Neutron emission Radioactive decay Rays: α — β — γ — δ — ε p-process Thermal Neutron Capture Data Thermal Neutron Cross Sections at the International Atomic Energy Agency
Mark 81 bomb
The Mark 81 250 lb general purpose bomb is the smallest of the Mark 80 series of low-drag general-purpose bombs. Developed for United States military forces in the 1950s, it was first used during the Vietnam War; the bomb consists of a cast steel case with 96 lb of Composition Minol or Tritonal explosive. The power of the Mk 81 was found to be inadequate for U. S. military tactical use, it was discontinued, although license-built copies or duplicates of this weapon remain in service with various other nations. Development of a precision guided variant of the Mk 81 bomb was started due to its potential to reduce collateral damage compared to larger bombs, but this program has now been cancelled in favor of the Small Diameter Bomb. Mark 81 Snakeye fitted with a Mark 14 TRD to increase the bomb's drag after release; the bomb's increased air-time, coupled with its forgiving safe drop envelope, allowed for low-level bombing runs at slower speed. Used in the close air support role in Vietnam. Nicknamed "snake", as in the typical Vietnam support loadout of "snake and nape".
GBU-29 Joint Direct Attack Munition, a precision guided version of the Mark 81. Mark 82 bomb Mark 83 bomb Mark 84 bomb Notes Bibliography Mk81 GP Bomb Mk81 General Purpose Bomb DUMB BOMBS, FUZES, AND ASSOCIATED COMPONENTS
BGM-109G Ground Launched Cruise Missile
The Ground Launched Cruise Missile, or GLCM, was a ground-launched cruise missile developed by the United States Air Force in the last decade of the Cold War and destroyed under the INF Treaty. The BGM-109G was developed as a counter to the mobile MRBM and IRBM nuclear missiles deployed by the Soviet Union in Eastern Bloc European countries; the GLCM and the U. S. Army's Pershing II may have been the incentives that fostered Soviet willingness to sign the Intermediate-Range Nuclear Forces Treaty, thus reduced the threat of nuclear wars in Europe. GLCM is a generic term for any ground-launched cruise missile. Since the U. S. deployed only one modern cruise missile in the tactical role, the GLCM name stuck. The GLCM was built by General Dynamics. A conventionally configured cruise missile, the BGM-109 was a small, pilotless flying machine, powered by a turbofan engine. Unlike ballistic missiles, whose aimpoint is determined by gravitic trajectories, a cruise missile is capable of complicated aerial manoeuvres, can fly a range of predetermined flight plans.
It flies at much lower altitudes than a ballistic missile with a terrain-hugging flight plan. The trade-off for this low-observability flight is strike time. GLCM was developed as a ground-launched variant of the Tomahawk missile in use by the U. S. Navy Unlike other variants of the Tomahawk, the GLCM carried; the W84 warhead was a 0.2–150kt variable-yield weapon. This yield contrasts with the yield of the W80 warhead found on other versions of the Tomahawk and on the ALCM from which the W84 was derived, which had a selectable yield of 5 or 150 kt; the Pentagon credited the GLCM with a range of 2000–2500 kilometers. Like other U. S. cruise missiles of this period, accuracy after more than 2000 km of flight was 30 meters. The missile was subsonic, powered by a turbofan engine with a rocket booster assisting at launch. Militarily, the GLCM was targeted against fixed targets—at the outer edge of its range, the missile's flight time with its subsonic turbofan was more than 2½ hours; the missiles were launched from an elevated launcher, with the missile ejected from its canister for about 13 seconds of solid rocket booster flight.
The fins extended at 4 seconds, the air inlet and wings deployed at 10 seconds and the jet engine started at the end of the boost phase. Flying at low level, the missile was guided by TERCOM to the target; this contrasted with Pershing II, which had a flight time of 10–15 minutes. However, the range of the GLCM gave it the ability to strike deep within then-Soviet territory, the missile guidance and low radar cross-section would have made it far more difficult to intercept a GLCM if the launch were detected in time. BGM-109G personnel were trained at Davis-Monthan Air Force Base, Arizona, by the 868th Tactical Missile Training Squadron from 1 July 1981. On 1 October 1985, the squadron became part of the 868th Tactical Missile Training Group; the group and squadron were inactivated on 31 May 1990. An area near Fort Huachuca, Arizona was used for field training for GLCM flights. GLCM testing was conducted at the Dugway Proving Ground in Utah, with many of the people involved in the testing going to operational wings as they were activated.
BGM-109G missiles would be based at six locations throughout Europe. Each location had its own unique problems, but all required extensive construction by the USAF. Initial operating capability occurred in 1983. Normal basing was in blast shelters at military installations; each BGM-109G station was controlled by a Wing that consisted of a Tactical Missile Squadron, responsible for operation and deployment of the missiles. Each TMS consisted of several flights, made up of 22 vehicles; the missile was designed to operate in a flight with sixteen missiles. The flight would be on base, with the missiles and vehicles secured in the hardened storage area called the GAMA. Four transporter erector launchers each carried four BGM-109G missiles in their containers and ready for launch. Two launch control centers, each with two launch officers, were connected to the TELs and interconnected for launch; each TEL and LCC was towed by a large MAN KAT1 8x8 tractor and was capable of traversing rough terrain. There were 16 support vehicles for the flight commander a captain, 19 maintenance technicians, a medical technician and 44 security personnel.
During periods of increased tension, the flights would be deployed to pre-surveyed, classified locations in the countryside away from the base. The members of the flight would dig in, erect camouflage netting to hide the vehicles, prepare for launch. Flight commanders were tasked to survey and select multiple possible deployment sites, with all details held, the commander selected the location preferred when the flight deployed from the base; when deployed, the flight was self-sustaining, secured with special intrusion detection radar. The launchers were sent out on a number of simulated scrambles. Although deployed in the face of a range of Soviet IRBMs, including the brand-new and capable SS-20 Saber, the
Thin Man (nuclear bomb)
"Thin Man" was the code name for a proposed plutonium gun-type nuclear bomb using plutonium-239 which the United States was developing during the Manhattan Project. They aborted its development when they discovered that the spontaneous fission rate of their nuclear reactor-bred plutonium was too high for use in a gun-type design, due to the high concentration of the isotope plutonium-240. In 1942, prior to the Army taking over wartime atomic research, Robert Oppenheimer held conferences in Chicago in June and Berkeley, California, in July at which various engineers and physicists discussed nuclear bomb design issues. A gun-type design was chosen, in which two sub-critical masses would be brought together by firing a "bullet" into a "target"; the idea of an implosion-type nuclear weapon was suggested by Richard Tolman but attracted scant consideration. Oppenheimer, reviewing his options in early 1943, gave priority to the gun-type weapon, but as a hedge against the threat of pre-detonation, he created the E-5 Group at the Los Alamos Laboratory under Seth Neddermeyer to investigate implosion.
Implosion-type bombs were determined to be more efficient in terms of explosive yield per unit mass of fissile material in the bomb, because compressed fissile materials react more and therefore more completely. It was decided that the plutonium gun would receive the bulk of the research effort, since it was the project with the least amount of uncertainty involved, it was assumed that the uranium gun-type bomb could be more adapted from it. The gun-type and implosion-type designs were codenamed "Thin Man" and "Fat Man" projects respectively; these code names were created by Robert Serber, a former student of Oppenheimer's who worked on the Manhattan Project. He chose them based on their design shapes; the "Fat Man" bomb would be round and fat and was named after Sydney Greenstreet's character in The Maltese Falcon. "Little Boy" would be named only to contrast to the "Thin Man" bomb. Oppenheimer assembled a team at the Los Alamos Laboratory to work on plutonium gun design that included senior engineer Edwin McMillan and senior physicists Charles Critchfield and Joseph Hirschfelder.
Critchfield had been working with sabots, which Oppenheimer believed would be required by the Thin Man to achieve the high muzzle velocities that critical assembly would require. Hirschfelder had been working on internal ballistics. Oppenheimer led the design effort himself until June 1943, when Navy Captain William Sterling Parsons arrived and took over the Ordnance and Engineering Division and direct management of the "Thin Man" project; these four created and tested all the elements of the Thin Man design between April 1943 and August 1944. Parsons, who had developed the proximity fuze for the Navy, ran the division, handled liaison with other agencies; as the head of the E-6 Projectile and Source Group, Critchfield calculated critical masses, instituted a system of live testing with scale models using 20 mm cannon and 3-inch guns. While full-scale Thin Man tubes took months to produce, these were and obtained, it was not possible to conduct tests with plutonium. Indeed, the actual physical characteristics of the metal were little more than educated guesses at this time.
Hirschfelder headed the E-8 Interior Ballistics Group. His group performed mathematical calculations, but he had to identify a suitable powder and primer, his group conducted full-scale tests with their selections. Fixing the physical size of the bomb proved important when it came to selecting a suitable aircraft to carry it; the E-8 group estimated the muzzle velocity of the gun at around 3,000 feet per second, close to the maximum achievable in 1944, calculated that the pressure in the barrel would be up to 75,000 pounds per square inch. Although the weapon's designers thought that bringing a critical mass together would be sufficient, Serber suggested that the design should include an initiator. A polonium-210-beryllium initiator was chosen because polonium 210 has a 140-day half life, which allowed it to be stockpiled, it could be obtained from occurring ores from Port Hope, Ontario. Oppenheimer requested that it be manufactured in the X-10 Graphite Reactor at Oak Ridge, Tennessee or, when they became available, the reactors at the Hanford Site.
The "Thin Man" design was an early nuclear weapon design proposed before plutonium had been bred in a nuclear reactor from the irradiation of uranium-238. It was assumed that plutonium, like uranium-235, could be assembled into a critical mass by a gun-type method, which involved shooting one sub-critical piece into another. To avoid pre-detonation or "fizzle", the plutonium "bullet" would need to be accelerated to a speed of at least 3,000 feet per second —or else the fission reaction would begin before the assembly was complete, blowing the device apart prematurely. Thin Man was 17 feet long, with 38-inch wide tail and nose assemblies, a 23-inch midsection; the length was necessary for the plutonium "bullet" to pick up adequate speed before reaching the "target". Weight was around 8,000 pounds for the final weapon model. There were no aircraft in the Allied inventory. However, the American Boeing B-29 Superfortress could be modified to carry it by removing part of the bulkhead under the main wing spar and some oxygen tanks located between its two bomb bays.
The great length of the "Thin Man" bomb led to aerodynamic instabilities. Subscale models of the bomb were dropp
Nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or from a combination of fission and fusion reactions. Both bomb types release large quantities of energy from small amounts of matter; the first test of a fission bomb released an amount of energy equal to 20,000 tons of TNT. The first thermonuclear bomb test released energy equal to 10 million tons of TNT. A thermonuclear weapon weighing little more than 2,400 pounds can release energy equal to more than 1.2 million tons of TNT. A nuclear device no larger than traditional bombs can devastate an entire city by blast and radiation. Since they are weapons of mass destruction, the proliferation of nuclear weapons is a focus of international relations policy. Nuclear weapons have been used twice in war, both times by the United States against Japan near the end of World War II. On August 6, 1945, the U. S. Army Air Forces detonated a uranium gun-type fission bomb nicknamed "Little Boy" over the Japanese city of Hiroshima.
S. Army Air Forces detonated a plutonium implosion-type fission bomb nicknamed "Fat Man" over the Japanese city of Nagasaki; these bombings caused injuries that resulted in the deaths of 200,000 civilians and military personnel. The ethics of these bombings and their role in Japan's surrender are subjects of debate. Since the atomic bombings of Hiroshima and Nagasaki, nuclear weapons have been detonated over two thousand times for testing and demonstration. Only a few nations are suspected of seeking them; the only countries known to have detonated nuclear weapons—and acknowledge possessing them—are the United States, the Soviet Union, the United Kingdom, China, India and North Korea. Israel is believed to possess nuclear weapons, though, in a policy of deliberate ambiguity, it does not acknowledge having them. Germany, Turkey and the Netherlands are nuclear weapons sharing states. South Africa is the only country to have independently developed and renounced and dismantled its nuclear weapons.
The Treaty on the Non-Proliferation of Nuclear Weapons aims to reduce the spread of nuclear weapons, but its effectiveness has been questioned, political tensions remained high in the 1970s and 1980s. Modernisation of weapons continues to this day. There are two basic types of nuclear weapons: those that derive the majority of their energy from nuclear fission reactions alone, those that use fission reactions to begin nuclear fusion reactions that produce a large amount of the total energy output. All existing nuclear weapons derive some of their explosive energy from nuclear fission reactions. Weapons whose explosive output is from fission reactions are referred to as atomic bombs or atom bombs; this has long been noted as something of a misnomer, as their energy comes from the nucleus of the atom, just as it does with fusion weapons. In fission weapons, a mass of fissile material is forced into supercriticality—allowing an exponential growth of nuclear chain reactions—either by shooting one piece of sub-critical material into another or by compression of a sub-critical sphere or cylinder of fissile material using chemically-fueled explosive lenses.
The latter approach, the "implosion" method, is more sophisticated than the former. A major challenge in all nuclear weapon designs is to ensure that a significant fraction of the fuel is consumed before the weapon destroys itself; the amount of energy released by fission bombs can range from the equivalent of just under a ton to upwards of 500,000 tons of TNT. All fission reactions generate the remains of the split atomic nuclei. Many fission products are either radioactive or moderately radioactive, as such, they are a serious form of radioactive contamination. Fission products are the principal radioactive component of nuclear fallout. Another source of radioactivity is the burst of free neutrons produced by the weapon; when they collide with other nuclei in surrounding material, the neutrons transmute those nuclei into other isotopes, altering their stability and making them radioactive. The most used fissile materials for nuclear weapons applications have been uranium-235 and plutonium-239.
Less used has been uranium-233. Neptunium-237 and some isotopes of americium may be usable for nuclear explosives as well, but it is not clear that this has been implemented, their plausible use in nuclear weapons is a matter of dispute; the other basic type of nuclear weapon produces a large proportion of its energy in nuclear fusion reactions. Such fusion weapons are referred to as thermonuclear weapons or more colloquially as hydrogen bombs, as they rely on fusion reactions between isotopes of hydrogen. All such weapons derive a significant portion of their energy from fission reactions used to "trigger" fusion reactions, fusion reactions can themselves trigger additional fission reactions. Only six countries—United States, United Kingdom, China and India—have conducted thermonuclear weapon tests. North Korea claims to have tested a fusion weapon as of January 2016. Thermonuclear weapons a
Little Boy
"Little Boy" was the code name for the type of atomic bomb dropped on the Japanese city of Hiroshima on 6 August 1945 during World War II. It was the first nuclear weapon used in warfare; the bomb was dropped 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 exploded with an energy of 15 kilotons of TNT and caused widespread death and destruction throughout the city; the Hiroshima bombing was the second nuclear explosion in history, after the Trinity test, the first uranium-based detonation. Little Boy was developed by Lieutenant Commander Francis Birch's group at the Manhattan Project's Los Alamos Laboratory during World War II, a development of the 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 of uranium-235, whereas Thin Man was based on fission of plutonium-239. Fission was accomplished by shooting a hollow cylinder of enriched uranium onto a solid cylinder of the same material by means of a charge of nitrocellulose propellant powder.
It contained 64 kg of enriched uranium. 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 again be required, many plans and diagrams were destroyed. However, by mid-1946, the Hanford Site reactors began suffering badly from the Wigner effect, the dislocation of atoms in a solid caused by neutron radiation, plutonium became scarce, so six Little Boy assemblies were produced at Sandia Base; the Navy Bureau of Ordnance built another 25 Little Boy assemblies in 1947 for use by the Lockheed P2V Neptune 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. Physicist Robert Serber named the first two atomic bomb designs during World War II based on their shapes: Thin Man and Fat Man; the "Thin Man" was a long, thin device and its name came from the Dashiell Hammett 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 novel The Maltese Falcon, played by Sydney Greenstreet in the film version. Little Boy was named by others as an allusion to Thin Man; because uranium-235 was known to be fissionable, it was the first material pursued in the approach to bomb development. As the first design developed, it is sometimes known as the Mark I; the vast majority of the work came in the form of the isotope enrichment of the uranium necessary for the weapon, since uranium-235 makes up only 1 part in 140 of natural uranium. Enrichment was performed at Oak Ridge, where the electromagnetic separation plant, known as Y-12, became operational in March 1944; the first shipments of enriched uranium 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 and was made available thanks to the foresight of the CEO of the High Katanga Mining Union, Edgar Sengier, who had 1,200 short tons of uranium ore transported to a New York warehouse in 1940.
At least part of the 1,200 short tons of uranium ore and uranium oxide captured by the Alsos Mission in 1944 and 1945 went to Oak Ridge for enrichment, as did 1,232 pounds of uranium oxide captured on the Japan-bound German submarine U-234 after Germany's surrender in May 1945. Little Boy was a simplification of the previous gun-type fission weapon design. Thin Man, 17 feet long, was designed to use plutonium, so it was more than capable of using enriched uranium; 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 plutonium on which the original measurements had been made, 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 likely the plutonium would predetonate and blow itself apart in the initial forming of a critical mass.
In July 1944 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 William S. Parsons's Ordnance Division. All the design and technical work at Los Alamos was consolidated under Lieutenant Commander 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, this allowed the Thin Man design to be simplified. A high-velocity gun was no longer required, 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, contracts were let to build the components.
Three different plants were used so that no one would have a copy of the compl
