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Snark (rocket)

Snark (rocket)

__NOTOC__ The Northrop SM-62 Snark was a specialised intercontinental missile with a nuclear warhead briefly operated by the US Strategic Air Command from 1958 until 1961. Basically an unmanned aircraft, the jet engined 20.5 m long device had a top speed of 650 mph (1,046 km/h) and a maximum range of 5,500 nautical miles (10,200 km). The complex stellar navigation based guidance system gave a claimed CEP of 8,000 ft (2.4 km). The Snark was an air-breathing design, launched from a light platform by two rocket booster engines. It would switch to an internal jet engine for the remainder of its flight. The jet was a Pratt and Whitney J57, the first 10,000 lbf (44 kN) thrust design, also used in the early B-52 and the F-100. Lacking a horizontal tail surface and using elevons as the main control surfaces the missile flew an unusual nose high aspect during level flight. During the final phase of flight the nuclear warhead would detach and follow a ballistic trajectory to the target. The missile itself would become unflyable and crash as soon as the warhead detached. It was developed to offer a nuclear threat to the Soviet Union at a time when ICBMs were still in development. Work on the project had begun in 1946. Initially there were two missiles - a subsonic design (the MX775A Snark) and a supersonic design (the MX775B Boojum). Budget reductions threatened the project in its first year but the personal intervention of Jack Northrop with Carl Spaatz saved the project. Despite this funding was low and the program was dogged by requirement changes. The expected due date of 1953 passed with the design still in testing and SAC was becoming less enthusiastic. In 1955 Eisenhower ordered top priority to the ICBM and associated missile programs. Despite considerable difficulties with the missile and military reservations work continued. In 1957 tests the missile had an average CEP of 17 nautical miles (31.5 km) and the most accurate test of 1958 fell 4 nautical miles (7.4 km) short. The design was also notoriously unreliable with the majority of test missiles suffering mechanical failure thousands of miles before reaching the target area. The reduced operating altitude, from 150,000 to 55,000 ft (46 to 17 km), and the inability of the missile to undertake evasive manoeuvres were also cited against it. One of the more advanced features of the Snark was its ability to fly missions of up to 11 hours and return for a landing. If the Snark did not detach its warhead, the missile itself could be flown repeatedly. Lacking landing gear, it was necessary for the Snark to skid to a landing on a flat, level surface. In January 1958 SAC began accepting delivery of operational missiles to Patrick AFB in Florida for training and in 1959 the 702nd Strategic Missile Wing was formed. On 27 May 1959, Presque Isle AFB in Maine, the only Snark base, received its first Snark Missile. Ten months later, on March 18, 1960, a Snark missile officially went on alert status. The 702nd was not declared fully operational until February 1961. In March, 1961 President Kennedy declared the Snark to be "obsolete and of marginal military value" and on 25 June 1961 the 702nd was inactivated.[https://www.patrick.af.mil/heritage/6555th/6555ch2/6555c2-3.htm] The missile name is from the Lewis Carroll poem The Hunting of the Snark. rather than -->

Specifications

General characteristics

- Length: 67 ft 2 in (20.47 m)
- Wingspan: 42 ft 3 in (12.88 m)
- Wing area: ft² ( m²)
- Weight: 48,147 lb (21,839 kg)
- Powerplant: 1× Pratt & Whitney J57 jet engine; 10,500 lbf (46.7 kN) thrust. 2 Aerojet-General solid-propellant rocket boosters; 130,000 lbf (580 kN) thrust

Performance

- Maximum speed: 565 knots (1,050 km/h)
- Combat range: 5,497 nmiles (10,180 km)
- Service ceiling: 50,250 ft (15,320 m)
- Rate of climb: ft/min ( m/min)
- Wing loading: lb/ft² ( kg/m²)
- Thrust/weight: 0.218:1

Armament

- Nuclear warhead

Missile

A missile (CE pronunciation: ; AmE: ) is, in general, a projectile—that is, something thrown or otherwise propelled. Missiles can range from a rock thrown from a slingshot, through a crossbow or ballista bolt, to a Minuteman III intercontinental ballistic missile (ICBM) with multiple nuclear warheads. Modern ICBMs, the largest missiles currently deployed, represent the most destructive weapons ever made in human history.

Etymology

The word missile comes from the Latin verb mittere, literally meaning "to send".

Introduction

Rocket-powered missiles are known as rockets if they lack post-launch guidance or missiles or guided missiles if they are able to continue tracking a target after launch. Cruise missiles typically use some form of jet engine for propulsion. Missiles are often used in warfare as a means of delivering destructive force (usually in the form of an explosive warhead) upon a target. Aside from explosives, other possible types of destructive missile payloads are various forms of chemical or biological agents, nuclear warheads, or simple kinetic energy (where the missile destroys the target by the force of striking it at high speed). Sometimes missiles are used to deliver payloads designed to break infrastructure without harming people. For example, in the Gulf War cruise missiles were used to deliver reels of carbon filament to electricity stations and switches, effectively disabling them by forming short circuits. Missiles which spend most of their trajectory in unpowered flight, and which don't use aerodynamics to alter their course, are known as ballistic missiles (because their motion is largely governed by the laws of ballistics). These are in contrast to cruise missiles, which spend most of their trajectory in powered flight.

Guided missiles

Missiles that have the ability to maneuver through the air can be guided, and are known as guided missiles. These have three key system components:
- tracking
- guidance
- flight A tracking system locates the missile's target. This can be either a human gunner aiming a sight on the target (remotely from the missile) or an automatic tracker. Automatic trackers use radiation emanating from the target or emitted from the launch platform and reflecting back to it from the target. Passive automatic trackers use the target's inherent radiation, usually heat or light, but missiles designed to attack Command & Control posts, aircraft or guided missiles may look for radio waves. Active automatic trackers rely on the target being illuminated by radiation. The target can be "painted" with light (sometimes infrared and/or LASER) or radio waves (radar) which can be detected by the missile. The radiation for the painting can originate in the missile itself or may come from a remote station (for example, a hilltop gunner can illuminate a target with a LASER device and this can be used to direct an air launched guided missile). A guidance system takes data from the missile's tracking system and flight system and computes a flight path for the missile designed to intercept the target. It produces commands for the flight system. The flight system causes the missile to maneuver. There are two main systems: vectored thrust (for missiles that are powered throughout the guidance phase of their flight) and aerodynamic maneuvering (wings, fins, canards, etc). There are some similarities between guided missiles and guided bombs. A guided bomb, dropped from an aircraft, is unpowered and uses aerodynamic fins for forward horizontal maneuvering while falling vertically.

Nuclear weapon

, 1945, rose some 18 km (11 mi) above the hypocenter.]]
- A nuclear weapon is a weapon which derives its destructive force from the nuclear reactions of nuclear fission and/or fusion. As a result, even a nuclear weapon with a small yield is significantly more powerful than the largest conventional explosives, and a single weapon can be capable of destroying or seriously disabling an entire city. In the history of warfare, nuclear weapons have been used on two occasions, both during the closing days of World War II. The first event occurred on the morning of 6 August 1945, when the United States dropped a uranium gun-type device code-named "Little Boy" on the Japanese city of Hiroshima. The second event occurred three days later when a plutonium implosion-type device code-named "Fat Man" was dropped on the city of Nagasaki. The use of the weapons, which resulted in the immediate deaths of at least 120,000 individuals (mostly civilians) and about twice that number over time, was and remains controversial — critics charged that they were unnecessary acts of mass killing, while others claimed that they ultimately reduced casualties on both sides by hastening the end of the war. (See Atomic bombings of Hiroshima and Nagasaki for a full discussion.) Since that time, nuclear weapons have been detonated on over two thousand occasions, mostly for testing purposes, chiefly by the following seven countries: the United States, Soviet Union, France, United Kingdom, People's Republic of China, India and Pakistan. These countries are the declared nuclear powers (with Russia inheriting the weapons of the Soviet Union after its collapse). Various other countries may hold nuclear weapons, but they have never publicly admitted possession, or their claims to possession have not been verified. For example, Israel has modern airbourne delivery systems and appears to have an extensive nuclear program (see Israel and weapons of mass destruction); North Korea has recently stated that it has nuclear capabilities (although it has now stated that it will abandon all of its nuclear weapons programs); Ukraine may possess an obsolete Soviet-era nuclear stockpile due to a post-Soviet administrative error; and Iran is believed to be attempting to develop nuclear capabilities (for more information see List of countries with nuclear weapons). Nuclear weapons in modern times have been used primarily as a method of creating a strategic threat. For example, the worry that North Korea will use nuclear weapons has dominated the relations between the United States and North Korea. Apart from their use as weapons, nuclear explosives have been proposed for various non-military uses.

Types of nuclear weapons

non-military uses The simplest nuclear weapons derive their energy from nuclear fission. A mass of fissile material is rapidly assembled into a critical mass, in which a chain reaction begins and grows exponentially, releasing tremendous amounts of energy. This is accomplished by rapidly creating supercriticality, either by shooting one piece of subcritical material into another, or compressing a subcritical mass. A major challenge in all nuclear weapon designs is ensuring that a significant fraction of the fuel is consumed before the weapon destroys itself. These are colloquially known as atomic bombs. More advanced nuclear weapons also contain a nuclear fission device, but the energy is used to trigger nuclear fusion, releasing even more energy. In such a weapon, the X-ray thermal radiation from a nuclear fission explosion is used to heat and compress a capsule of tritium, deuterium, or lithium, in which fusion occurs. These weapons, colloquially known as hydrogen bombs, can be many hundreds of times more powerful than fission weapons. The so-called "Teller-Ulam design" is thought to be applied for megaton range thermonuclear weapons. More exotic nuclear weapons also exist, designed for special purposes. The detonation of a nuclear weapon is accompanied by a blast of neutron radiation. Surrounding a nuclear weapon with suitable materials (such as cobalt or gold) creates a weapon known as a salted bomb. This device can produce exceptionally large quantities of radioactive contamination. A nuclear weapon may also be designed to permit as many neutrons as possible to escape; such a weapon is called a neutron bomb.

Effects of a nuclear explosion

neutron bomb The energy released from a nuclear weapon comes in four primary categories:
- Blast – 40-60% of total energy
- Thermal radiation – 30-50% of total energy
- Ionizing radiation – 5% of total energy
- Residual radiation (fallout) – 5-10% of total energy The amount of energy released in each form depends on the design of the weapon, and the environment in which it is detonated. The residual radiation of fallout is a delayed release of energy, while the other three forms of energy release occur immediately. The damage from each of the three initial forms of energy release differs with the size (or "yield", see below) of the weapon. Thermal radiation drops off the slowest with distance, so the larger the weapon the more significant the impact of this effect. Ionizing radiation is strongly absorbed by air, so it is only dangerous by itself for smaller weapons. Blast damage falls off more quickly than thermal radiation but more slowly than ionizing radiation. The energy released by a nuclear weapon is generally measured by the explosive power of an equivalent amount of trinitrotoluene, known as the weapon's yield. The yield of nuclear weapons may be rated as equivalent to several kilotons or megatons of TNT. The first fission weapons had yields measurable in the tens of kilotons, while the largest practical hydrogen bombs have yields around 20 megatons. In practice, nuclear weapon yields will vary significantly, from fractional kiloton weapons designed for tactical use on the battlefield (eg. the man-portable Davy Crockett warheads developed by the United States), to the record Tsar Bomba created by the Soviet Union which had a theoretical maximum design yield of around a hundred megatons. Although a nuclear weapon is capable of causing the same destruction as conventional explosives through the effects of blast and thermal radiation, it does so by releasing much larger amounts of energy in a much shorter period of time. Most of the damage caused by a nuclear weapon is not directly related to the nuclear process of energy release, and would be present for any explosion of the same magnitude. In human terms, nuclear weapons are enormously destructive. A weapon with a ten-megaton yield can destroy most of the buildings of a modern city, while a weapon with a hundred-megaton yield (although the deployment of such a weapon would be considered impractical) would set wooden structures and forests alight up to 60-100 miles (100-160 km) from ground zero. A nuclear weapon detonated in the upper atmosphere will also generate an electromagnetic pulse which can disrupt or disable electronic communications and instruments over a wide area, causing more difficulties for those who survive the effects of a detonation. Concerns over the health and environmental effects of nuclear testing led to the passing of the Partial Test Ban Treaty in 1963 which prohibited atmospheric (above-ground), underwater, or outer space nuclear tests (underground testing continued, however). Since most of the effects of nuclear weapons are blast, thermal, or fallout, well-known civil defense efforts could greatly reduce the total loss of life in a nuclear war.

Nuclear strategy

civil defenseed delivery system. Each missile can contain up to ten nuclear warheads (shown in red), each of which can be aimed at a different target. These were developed to make missile defense very difficult for an enemy country.]] Nuclear warfare strategy are ways for either fighting or avoiding a nuclear war. The policy of trying to ward off a potential attack by a nuclear weapon from another country by threatening nuclear retaliation is known as the strategy of nuclear deterrence. The goal in deterrence is to always maintain a second strike status — the ability to respond to a nuclear attack against your country with a nuclear attack of your own. During the Cold War, theorists used game theory to work out models of what sorts of policies could prevent one from ever being attacked by a nuclear weapon. However, many critics have noted that there could be many exceptions to this in practice, and if an attack ever was truly made then many hundreds of thousands if not millions of people would lose their lives as a result. Additionally, the presence of nuclear weapons by one country can spur nuclear proliferation in countries who feel threatened by them and look to deterrence (which requires a nuclear weapon in the first place) as the only solution. Sometimes this theory has been called Mutual Assured Destruction. Weapons which are designed to threaten large populations or to generally deter attacks are known as "strategic" weapons. Weapons which are designed to actually be used on a battlefield in military situations are known as "tactical" weapons. Different forms of nuclear weapons delivery (see below) allow for different types of nuclear strategy, primarily by making it difficult to defend against them and difficult to launch a pre-emptive strike against them. Sometimes this has meant keeping the weapon locations hidden, such as putting them on submarines or train cars whose locations are very hard for an enemy to track, and other times this means burying them in hardened bunkers. Other responses have included attempts to make it seem likely that the country could survive a nuclear attack, by using missile defense (to destroy the missiles before they land) or by means of civil defense (using early warning systems to evacuate citizens to a safe area before an attack).

Weapons delivery

civil defense" weapon dropped on Nagasaki, Japan. These weapons were very large and could only be delivered by larger bomber aircraft.]] Nuclear weapons delivery— the technology and systems used to bring a nuclear weapon to its target—is an important aspect of nuclear weapons relating both to nuclear weapon design and nuclear strategy. Historically the first method of delivery, and the method used in the two nuclear weapons actually used in warfare, is as a gravity bomb, dropped from bomber aircraft. This method is usually the first developed by countries as it does not place many restrictions on the size of the weapon, and weapon miniaturization is something which requires considerable weapons design knowledge. It does, however, limit the range of attack, response time to an impending attack, and number of weapons which can be fielded at any given time. More preferable from a strategic point of view are nuclear weapons mounted onto a missile, which can use a ballistic trajectory to deliver a warhead over the horizon. While even short range missiles allow for a faster and less vulnerable attack, the development of intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBMs) has allowed some nations to plausibly deliver missiles anywhere on the globe with a high likelihood of success. More advanced systems, such as multiple independently targetable reentry vehicles (MIRVs) allow multiple warheads to be launched at a number of targets from any one missile, reducing the chance of any successful missile defense. Today missiles are by far the most common among systems designed for delivery of nuclear weapons. To make a warhead small enough to fit onto a missile, though, can be a difficult task. "Tactical" weapons (see above) have involved the most variety of delivery types, including not only gravity bombs and missiles but also artillery shells, land mines, and nuclear depth charges and torpedoes for anti-submarine warfare. An atomic mortar was also tested at one time by the United States. Small, two-man portable tactical weapons (erroneously referred to as suitcase bombs), such as the Special Atomic Demolition Munition, have been developed, although the difficulty to combine sufficient yield with portability limits their military utility.

History

Special Atomic Demolition Munition The first nuclear weapons were created by the United States, with assistance from the United Kingdom and Canada, during World War II as part of the top-secret Manhattan Project. While the first weapons were developed primarily out of fear that Nazi Germany would first develop them, they were eventually used against the Japanese cities of Hiroshima and Nagasaki in August 1945. The Soviet Union developed and tested their first nuclear weapon in 1949, based partially on information obtained from Soviet espionage in the United States. Both the USA and USSR would go on to develop weapons powered by nuclear fusion (hydrogen bombs) by the mid-1950s. With the invention of reliable rocketry during the 1960s, it became possible for nuclear weapons to be delivered anywhere in the world on a very short notice, and the two Cold War superpowers adopted a strategy of deterrence to maintain a shaky peace. Nuclear weapons were symbols of military and national power, and nuclear testing was often used both to test new designs as well as to send political messages. Other nations also developed nuclear weapons during this time, including the United Kingdom, France, and China. These five members of the "nuclear club" agreed to attempt to limit the spread of nuclear proliferation to other nations, though at least three other countries (India, South Africa, Pakistan, and most likely Israel) developed nuclear arms during this time. At the end of the Cold War in the early 1990s, the Russian Federation inherited the weapons of the former USSR, and along with the USA pledged to reduce their stockpile for increased international safety. Nuclear proliferation has continued, though, with Pakistan testing their first weapons in 1998, and the state of North Korea claiming to have developed nuclear weapons in 2004. Nuclear weapons have been at the heart of many national and international political disputes, and have played a major part in popular culture since their dramatic public debut in the 1940s, and have usually symbolized the ultimate ability of mankind to utilize the strength of nature for destruction. There have been (at least) four major false alarms, the most recent in 1995, that almost resulted in the US or USSR/Russia launching its weapons in retaliation for a supposed attack.[http://www.pbs.org/wgbh/nova/missileers/falsealarms.html] Additionally, during the Cold War the US and USSR came close to nuclear warfare a number of times, most notably during the Cuban Missile Crisis. As of 2005, there are estimated to be at least 29,000 nuclear weapons held by at least seven countries, though 96% of these are in the possession of just two (the United States and the Russian Federation)

Media

References

- p. 54. Bethe, Hans Albrecht. The Road from Los Alamos. Simon and Schuster, New York. (1991 ISBN 0-671-74012-1)
- Glasstone, Samuel and Dolan, Philip J., [http://www.cddc.vt.edu/host/atomic/nukeffct/ The Effects of Nuclear Weapons (third edition)], U.S. Government Printing Office, 1977. [http://www.princeton.edu/~globsec/publications/effects/effects.shtml PDF Version]
- [http://www.fas.org/nuke/guide/usa/doctrine/dod/fm8-9/1toc.htm NATO Handbook on the Medical Aspects of NBC Defensive Operations (Part I - Nuclear)], Departments of the Army, Navy, and Air Force, Washington, D.C., 1996.
- Hansen, Chuck. U.S. Nuclear Weapons: The Secret History, Arlington, TX: Aerofax, 1988.
- Hansen, Chuck. The Swords of Armageddon: U.S. nuclear weapons development since 1945, Sunnyvale, CA: Chukelea Publications, 1995 [http://www.uscoldwar.com/].
- Smyth, Henry DeWolf. [http://nuclearweaponarchive.org/Smyth/ Atomic Energy for Military Purposes], Princeton University Press, 1945. (The first declassified report by the US government on nuclear weapons) (Smyth Report)
- [http://www.fas.org/nuke/intro/nuke/7906/index.html The Effects of Nuclear War], Office of Technology Assessment (May 1979).
- Rhodes, Richard. Dark Sun: The Making of the Hydrogen Bomb. Simon and Schuster, New York, (1995 ISBN 0684824140)
- Rhodes, Richard. The Making of the Atomic Bomb. Simon and Schuster, New York, (1986 ISBN 0684813785)
- Weart, Spencer R. Nuclear Fear: A History of Images. Cambridge, Mass.: Harvard University Press, 1988.

External links

- [http://intergate.cccoe.k12.ca.us/abomb/ "The Race to Build the Atomic Bomb"] educational resource
- [http://nuclearweaponarchive.org Nuclear Weapon Archive from Carey Sublette] is a reliable source of information and has links to other sources and an informative [http://nuclearweaponarchive.org/Nwfaq/Nfaq0.html FAQ].
- [http://www.fas.org/main/content.jsp?formAction=297&contentId=367 Nuclear weapon simulator for several major cities]
- [http://www.fas.org/main/content.jsp?formAction=297&contentId=409 Fallout Calculator for various regions]
- [http://www.neis.org/literature/Brochures/weapcon.htm "Nuclear Power and Nuclear Weapons: Making the Connections"] – an article about the connections between nuclear power and nuclear weapons development by an anti-nuclear group
- The [http://fas.org Federation of American Scientists] provide solid information on weapons of mass destruction, including [http://fas.org/nuke/ nuclear weapons] and their [http://www.fas.org/nuke/intro/nuke/effects.htm effects]
- [http://www.oism.org/nwss/ Nuclear War Survival Skills] is a public domain text about civil defense.
- [http://www.atomicarchive.com/Example/Example1.shtml Step by step scenario of a 150 kiloton bomb exploding in Manhattan] - click on the Next >> button at the bottom of each slide.
- [http://www.ippnw.org IPPNW: International Physicians for the Prevention of Nuclear War] Nobel Peace Prize-winning organization with information about the medical consequences of nuclear weapons, war and militarization.
- [http://www.thebulletin.org Bulletin of the Atomic Scientists] - Magazine founded in 1945 by Manhattan Project scientists. Covers nuclear weapons proliferation and many other global security issues. See [http://www.thebulletin.org/nuclear_weapons_data this page] for comprehensive data on nuclear weapons worldwide.
- [http://alsos.wlu.edu/ Alsos Digital Library for Nuclear Issues] – contains many resources related to nuclear weapons, including a historical and technical overview and searchable bibliography of web and print resources. ko:핵무기 ms:Senjata nuklear ja:核兵器 simple:Nuclear weapon th:อาวุธนิวเคลียร์

Jet engine

is tested at Robins Air Force Base, Georgia, USA. The tunnel behind the engine muffles noise and allows exhaust to escape. The mesh cover at the front of the engine (left of photo) prevents debris—or people—from being pulled into the engine by the huge volume of air rushing into the inlet.]] A jet engine is any engine that accelerates and discharges a fast moving jet of fluid to generate thrust in accordance with Newton's third law of motion. This broad definition of jet engines includes turbojets, turbofans, turboprops, rockets and ramjets, but in common usage, the term generally refers to a gas turbine used to produce a jet of high speed exhaust gases for propulsive purposes.

Turbojet engines

A turbojet engine is a type of internal combustion engine often used to propel aircraft. Air is drawn into the rotating compressor via the intake and is compressed to a higher pressure before entering the combustion chamber. Fuel is mixed with the compressed air and ignited by flame in the eddy of a flame holder. This combustion process significantly raises the temperature of the gas. Hot combustion products leaving the combustor expand through the turbine, where power is extracted to drive the compressor. Although this expansion process reduces both the gas temperature and pressure at exit from the turbine, both parameters are usually still well above ambient conditions. The gas stream exiting the turbine expands to ambient pressure via the propelling nozzle, producing a high velocity jet in the exhaust plume. If the jet velocity exceeds the aircraft flight velocity, there is a net forward thrust upon the airframe. Under normal circumstances, the pumping action of the compressor prevents any backflow, thus facilitating the continuous flow process of the engine. Indeed, the entire process is similar to a four-stroke cycle, but with induction, compression, ignition, expansion and exhaust taking place simultaneously. The efficiency of a jet engine is strongly dependent upon the Overall Pressure Ratio (Combustor Entry Pressure/Intake Delivery Pressure) and the Turbine Inlet Temperature of the cycle. It is also perhaps instructive to compare turbojet engines with propeller engines. Turbojet engines take a relatively small mass of air and accelerate it by a large amount, whereas a propeller takes a large mass of air and accelerates it by a small amount. The high-speed exhaust of a jet engine makes it efficient at high speeds (especially supersonic speeds) and high altitudes. On slower aircraft and those required to fly short stages, a gas turbine-powered propeller engine, commonly known as a turboprop, is more common and much more efficient. Very small aircraft generally use conventional piston engines to drive a propeller but small turboprops are getting smaller as engineering technology improves. The turbojet described above is a single spool design, where a single shaft connects the turbine to the compressor. Higher Overall Pressure Ratio designs often have two concentric shafts, to improve compressor stability during engine throttle movements. The outer (HP) shaft connects the High Pressure (HP) Compressor to the HP turbine. This HP Spool, with the combustor, forms the core or gas generator of the engine. The inner shaft connects the Low Pressure (LP) Compressor to the LP Turbine to create the LP Spool. Both spools are free to operate at their optimum shaft speed.

Turbofan engines

Most modern jet engines are actually turbofans, where the LP Compressor acts as a fan, supplying supercharged air to not only the engine core, but to a bypass duct. The bypass airflow either passes to a separate Cold Nozzle or mixes with LP Turbine exhaust gases, before expanding through a Mixed Flow Nozzle. Forty years ago there was little difference between civil and military jet engines, apart from the use of afterburning in some (supersonic) applications. Turbofans, today, have a low specific thrust (net thrust/airflow) to keep jet noise to a minimum and to improve fuel efficiency. Consequently the bypass ratio (bypass flow/core flow) is relatively high (usually much greater than 3.0). Only a single fan stage is required, because a low specific thrust implies a low fan pressure ratio. Today's military turbofans, however, have a relatively high specific thrust, to maximize the thrust for a given frontal area, jet noise being of little consequence. Multi-stage fans are normally required to achieve the relatively high fan pressure ratio needed for a high specific thrust. Although high Turbine Inlet Temperatures are frequently employed, the bypass ratio tends to be low (usually significantly less than 2.0). An approximate equation for calculating the net thrust of a jet engine is: :F_net = m(v_jfe - v_a ) where: :m = intake mass flow :v_jfe = fully expanded jet velocity (in the exhaust plume) :v_a = aircraft flight velocity While the m·v_jfe term represents the gross thrust of the nozzle, the m·v_a term represents the ram drag of the intake. Most types of jet engine have an air intake, which provides the bulk of the gas exiting the exhaust. There is, however, a penalty for picking this air up and this is known as the ram drag. Conventional rocket motors, however, do not have an air intake, the oxidizer being carried within the airframe. Consequently, rocket motors do not have ram drag; the gross thrust of the nozzle is the net thrust of the engine. Consequently, the thrust characteristics of a rocket motor are completely different from that of an air breathing jet engine; at full throttle, the thrust of a rocket motor improves slightly with increasing altitude (because the back pressure from the atmosphere falls), whereas with a turbojet (or turbofan) the falling density of the air entering the intake causes the net thrust to decrease with increasing altitude.

History

Before the advent of the jet engine, the reciprocating piston engine in its different forms (rotary and static radial, aircooled and liquid-cooled inline) had been the only type of powerplant available to aircraft designers. This was understandable so long as low aircraft performance parameters were considered acceptable, and indeed inevitable. However, by approximately the late 1930s, engineers were beginning to realize that conceptually the piston engine was self-limiting in terms of the maximum performance which could be obtained from it; the limit was essentially one of propeller efficiency, which seemed to peak as blade tips approached supersonic tangential velocity. If engine, and thus aircraft, performance were ever to increase beyond such a barrier, a way would have to be found to radically improve the design of the piston engine, or a wholly new type of powerplant would have to be conceived. The latter would prove to be the case. The gas turbine (turbojet, or simply jet) engine, as subsequently developed, would become almost as revolutionary to aviation as the Wright brothers' first flight. The gas turbine was not an idea developed in the 1930s: the patent for a stationary turbine was granted to John Barber in England in 1791, although Colin Sullivan of Cowplain, England was said to have drawn up identical blueprints 2 years beforehand. The earliest attempts at jet engines were hybrid designs in which an external power source supplied the compression. In this system (called a thermojet by Secondo Campini) the air is first compressed by a fan driven by a conventional piston engine, then it is mixed with fuel and burned for jet thrust. The examples of this type of design were the Henri Coanda's Coanda-1910 aircraft, and the much later Campini Caproni CC.2, and the Japanese Tsu-11 engine intended to power Ohka kamikaze planes towards the end of World War II. None were entirely successful and the CC.2 ended up being slower than the same design with a traditional engine and propeller combination. World War II The key to the useful jet engine was the gas turbine, used to extract energy to drive the compressor from the engine itself. The first gas turbine to successfully run self-sustaining was built in 1903 by Norwegian engineer Aegidius Elling. The first patents for jet propulsion were issued in 1917. Limitations in design and practical engineering and metallurgy prevented such engines reaching manufacture. The main problems were safety, reliability, weight and, especially, sustained operation. On January 16, 1930, in England Frank Whittle submitted patents for his own design for a full-scale aircraft engine (granted in 1932). In 1935 Hans von Ohain started work on a similar design in Germany, seemingly unaware of Whittle's work. Ohain approached Ernst Heinkel, one of the larger aircraft industrialists of the day, who immediately saw the promise of the design. Heinkel had recently purchased the Hirth engine company, and Ohain and his master machinist Max Hahn were set up there as a new division of the Hirth company. They had their first HeS 1 engine running by September 1937. Unlike Whittle's design, Ohain used hydrogen as fuel, which he credits for the early success. Their subsequent designs culminated in the gasoline-fuelled HeS 3 of 1,100 lbf (5 kN), which was fitted to Heinkel's simple and compact He 178 airframe and flown by Erich Warsitz in the early morning of August 27, 1939, from Marienehe aerodrome, an impressively short time for development. The He 178 was the world's first jetplane. The engine was starting to look useful, and Whittle's Power Jets Ltd. started receiving Air Ministry money. In 1941 a flyable version of the engine called the W.1, capable of 1000 lbf (4 kN) of thrust, was fitted to the Gloster E28/39 airframe, and first flew on May 15, 1941 at RAF Cranwell. RAF Cranwell One problem with both of these early designs, which are called centrifugal-flow engines, was that the compressor works by "throwing" (accelerating) air outward from the central intake to the outer periphery of the engine where the air is then compressed by a divergent duct setup—converting velocity into pressure. The advantage was that such compressor designs were well understood in centrifugal superchargers but this leads to a very large cross section for the engine at rotational speeds that were usable at the time. A disadvantage was that the air flow had to be "bent" to flow rearwards through the combustion section and to the turbine and tailpipe. With improvements to bearings, the shaft speed of the engine would increase and the diameter of the centrifugal compressor would reduce greatly. The shortness of this engine is an advantage. The strength of this type of compressor is an advantage over the later axial-flow compressors that are still liable to foreign object damage (FOD in aviation parlance). Austrian Anselm Franz of Junkers' engine division (Junkers Motoren or Jumo) addressed this problem with the introduction of the axial-flow compressor. Essentially, this is a turbine in reverse. Air coming in the front of the engine is blown to the rear of the engine by a fan stage (convergent ducts), where it is crushed against a set of non-rotating blades called stators (divergent ducts). The process is nowhere near as powerful as the centrifugal compressor, so a number of these pairs of fans and stators are placed in series to get the needed compression. Even with all the added complexity, the resulting engine is much smaller in diameter. Jumo was assigned the next engine number, 4, and the result was the Jumo 004 engine. After many lesser technical difficulties were solved, mass production of this engine started in 1944 as a powerplant for the world's first jet-fighter aircraft, the Messerschmitt Me 262. Because Hitler wanted a new bomber the Me 262 came too late to decisively impact Germany's position in World War II, but it will be remembered as the first use of jet engines in service. After the end of the war the German Me 262 aircraft were extensively studied by the victorious allies and contributed to work on early Soviet and US jet fighters. British engines also were licensed widely in the US (see Tizard Mission). Their most famous design, the Nene would also power the USSR's jet aircraft also after a technology exchange. American designs would not come fully into their own until the 1960s.

Types

There are a large number of types of jet engines, which get propulsion from a high speed exhaust jet. Some examples are as follows:

Type	Description	Advantages	Disadvantages
Water jet	Squirts water out the back of a boat	Can run in shallow water, powerful, less harmful to wildlife	Can be less efficient than a propeller, more vulnerable to debris
Thermojet	Most primitive airbreathing jet engine		Very inefficient and underpowered
Turbojet	Generic term for simple turbine engine	Simplicity of design	Basic design, misses many improvements in efficiency and power
Turbofan	Power tapped off exhaust used to drive bypass fan	Quieter due to greater mass flow and lower total exhaust speed, more efficient for a useful range of subsonic airspeeds for same reason	Greater complexity (additional ducting, usually multiple shafts), large diameter engine, need to contain heavy blades. More subject to FOD and ice damage. Different degrees of bypass are possible - this is the design most commonly used on commercial airliners
Rocket	Carries own propellant onboard, emits jet for propulsion	Very few moving parts, Mach 0 to Mach 25+, efficient at very high speed (> Mach 10.0 or so), thrust/weight ratio over 100, relatively simple, no air inlet, doesn't require atmosphere, high compression ratio, very high speed exhaust	very low specific impulse- typically 100-450 seconds. Typically requires carrying oxidiser onboard which increases risks.
Ramjet	Intake air is compressed entirely by speed of oncoming air and duct shape (divergent)	Very few moving parts, Mach 0.8 to Mach 5+, efficient at high speed (> Mach 2.0 or so), lightest of all airbreathing jets (thrust/weight ratio up to 30 at optimum speed)	Must have a high initial speed to function, inherently inefficient at slow speeds due to poor compression ratio, difficult to arrange shaft power for accessories, difficult to engineer to be efficient over a wide range of airspeeds.
Turboprop (Turboshaft similar)	Strictly not a jet at all- a gas turbine engine is used as powerplant to drive (propeller) shaft	High efficiency at lower subsonic airspeeds(300 knots plus), high shaft power to weight	Limited top speed (aeroplanes), somewhat noisy, complexity of propeller drive, very large yaw (aeroplane) if engine fails
Propfan	Turboprop engine drives one or more propellers. much like a turbofan but without ductwork	Higher fuel efficiency, some designs are less noisy than turbofans, could lead to higher-speed commercial aircraft, popular in the 1980s during fuel shortages,	Development of propfan engines has been very limited, typically more noisy than turbofans, complexity
Pulsejet	Air enters a divergent-duct inlet, the front of the combustion area is shut, fuel injected into the air ignites, exhaust vents from other end of engine	Very simple design, commonly used on model aircraft	Noisy, inefficient (low compression ratio), works best at small scale, valves need to be replaced very often
Pulse detonation engine	Similar to a pulsejet, but combustion occurs as a detonation instead of a deflagration, may or may not need valves	Maximum theoretical engine efficiency	Extremely noisy, parts subject to extreme mechanical fatigue, hard to start detonation, not practical for current use
Integral rocket ramjet	Essentially a ramjet where intake air is compressed and burnt with the exhaust from a rocket	Mach 0 to Mach 4.5+ (can also run exoatmospheric), good efficiency at Mach 2 to 4	Similar efficiency to rockets at low speed or exoatmospheric, inlet difficulties, a relatively undeveloped and unexplored type, cooling difficulties
Scramjet	Intake air is compressed but not slowed to below supersonic, intake, combustion and exhaust occur in a single constricted tube	can operate at very high Mach numbers (Mach 8 to 15)[http://www.dod.mil/ddre/downloads/ddre_briefings/Merging_Air_and_Space071603.pdf]	still in development stages, must have a very high initial speed to function (Mach >6), cooling difficulties, inlet difficulties, very poor thrust/weight ratio (~2), airframe difficulties, testing difficulties
Turborocket	An additional oxidizer such as oxygen is added to the airstream to increase max altitude	Very close to existing designs, operates in very high altitude, wide range of altitude and airspeed	Airspeed limited to same range as turbojet engine, carrying oxidizer like LOX can be dangerous
Precooled jets / LACE	Intake air is chilled to very low temperatures at inlet	Very high thrust/weight ratios are possible (~14) together with good fuel efficiency over a wide range of airspeeds, mach 0-5+	Exists only at the lab protoyping stage. Examples include RB545, SABRE, ATREX

Components

The components of a jet engine are standard across the different types of engines, although not all engine types have all components. The parts include:

Air Induction
The standard reference frame for a jet engine is the aircraft itself. For subsonic aircraft, the air intake to a jet engine presents no special difficulties, and consists essentially of an opening which is designed to minimise drag, as with any other aircraft component. However, the air reaching the compressor of a normal jet engine must be travelling below the speed of sound, even for supersonic aircraft, to sustain the flow mechanics of the compressor and turbine blades. At supersonic flight speeds, shockwaves form in the intake system and reduce the recovered pressure at inlet to the compressor. So some supersonic intakes use devices, such as a cone or ramp, to increase pressure recovery, by more making more efficient use of the shock wave system.
Compressor or Fan
In many cases, the compressor is a series of fans that are spaced very closely together. Each fan compresses the air a little more. Energy is derived from the turbine (see below), passed along the shaft.
Shaft
This carries power from the turbine to the compressor, and runs most of the length of the engine. There may be as many as three concentric shafts, rotating at independent speeds, with as many sets of turbines and compressors. Other services, like a bleed of cool air, may also run down the shaft.
Combustor or Can or Flameholders or Combustion Chamber
This is a chamber where fuel is continuously burned in the compressed air.
Turbine
The turbine acts like a windmill, extracting energy from the hot gases leaving the combustor. This energy is used to drive the compressor through the shaft, or bypass fans, or props, or even (for a gas turbine-powered helicopter) converted entirely to rotational energy for use elsewhere.
Afterburner or reheat (chiefly UK)
(mainly military) Produces extra thrust by burning extra fuel, usually inefficiently, to significantly raise Nozzle Entry Temperature at the exhaust. Owing to a larger volume flow (i.e. lower density) at exit from the afterburner, an increased nozzle flow area is required, to maintain satisfactory engine matching, when the afterburner is alight.
Exhaust or Nozzle
Hot gases leaving the engine exhaust to atmospheric pressure via a nozzle, the objective being to produce a high velocity jet. In most cases, the nozzle is convergent and of fixed flow area.
Supersonic Nozzle
If the Nozzle Pressure Ratio (Nozzle Entry Pressure/Ambient Pressure) is very high, to maximize thrust it may be worthwhile, despite the additional weight, to fit a convergent-divergent (de Laval) nozzle. As the name suggests, initially this type of nozzle is convergent, but beyond the throat (smallest flow area), the flow area starts to increase to form the divergent portion. The expansion to atmospheric pressure and supersonic gas velocity continues downstream of the throat, whereas in a convergent nozzle the expansion beyond sonic velocity occurs externally, in the exhaust plume. The former process is more efficient.

Design considerations

The various components named above have constraints on how they are put together to generate the most efficiency or performance. Important here is air intake design, overall size, number of compressor stages (sets of blades), fuel type, number of exhaust stages, metallurgy of components, amount of bypass air used, where the bypass air is introduced, and many other factors. For instance, let us consider design of the air intake.

Air intakes

See also: Inlet cone
Subsonic inlets
At low speeds a subsonic inlet is little more than a hole, with an aerodynamic fairing around it. However, from around mach 0.85, the air entering the inlet can start to experience shock waves, and then careful radiusing is required for optimum performance at all speeds.
Supersonic inlets
Inlet cone For aircraft travelling at supersonic speeds, a design complexity arises, since the air ingested by the engine must be below supersonic speed, otherwise the engine will "choke" and cease working. This subsonic air speed is achieved by passing the approaching air through a deliberately generated shock wave (since one characteristic of a shock wave is that the air flowing through it is slowed). Therefore, some means is needed to create a shockwave ahead of the intake. The earliest types of supersonic aircraft featured a central shock cone, called an inlet cone, which was used to form the shock wave. This type of shock cone is clearly seen on the English Electric Lightning and MiG-21 aircraft, for example. The same approach can be used for air intakes mounted at the side of the fuselage, where a half cone serves the same purpose with a semicircular air intake, as seen on the F-104 Starfighter and BAC TSR-2. A more sophisticated approach is to angle the intake so that one of its edges forms a leading blade. A shockwave will form at this blade, and the air ingested by the engine will be behind the shockwave and hence subsonic. The Century series of US jets featured a number of variations on this approach, usually with the leading blade at the outer vertical edge of the intake which was then angled back inwards towards the fuselage. Typical examples include the Republic F-105 Thunderchief and F-4 Phantom. Later this evolved so that the leading edge was at the top horizontal edge rather than the outer vertical edge, with a pronounced angle downwards and rearwards. This approach simplified the construction of the intakes and permitted the use of variable ramps to control the airflow into the engine. Most designs since the early 1960s now feature this style of intake, for example the F-14 Tomcat, Panavia Tornado and the Concorde.
SR 71
The SR-71's engines were rather unusual in that a variable air intake design was used to convert the engine from a turbojet to a ramjet, in flight. To get good efficiency over a wide range of speeds the Pratt & Whitney J58 could move a conical spike fore and aft within the engine nacelle, to keep the supersonic shock wave just in front of the inlet. In this manner, the airflow behind the shock wave, and more importantly, through the engine, was kept subsonic at all times. At high mach, the compressor for the J58 was unable to carry the high air flow entering the inlet without stalling its blades, and so the engine directed the excess air through 6 bypass pipes straight to the afterburner. At high speeds the engine actually obtained 80% of its thrust, versus 20% through the turbines itself, in this way. Essentially, this allowed the engine to operate as a ramjet, actually improving specific impulse (fuel efficiency) by 10%–15%.
Heat exchangers
For engines that may need to operate at almost hypersonic speeds (mach 0 to 6), there is strong theoretical and experimental support for using a heat-exchanger to cool the air at the intake. This can increase the density of the air and thus reduce the necessary compression. The lower temperatures also permit lighter alloys to be used hence reducing the engine's weight by several times. This leads to plausible designs like SABRE and ATREX that might permit jet engined vehicles to be used to launch to space. ATREX
Compressors
Each design of compressor has an operating map or characteristic peculiar to that unit. At a given throttle condition, the compressor operates somewhere along the steady state running line. Unfortunately, this operating line is displaced during transients and under extreme conditions can cross the surge or stall line (see compressor map), causing, in some cases, the compressor flow to reverse direction violently. Many compressors are fitted with variable geometry to decrease the likelihood of surge. Another ploy is to split the compressor into two or more units, operating on separate concentric shafts. Another design consideration is the average stage loading. This can be kept at a sensible level either by increasing the number of compression stages (more weight/cost) or the mean blade speed (more blade/disc stress). Although large flow compressors are usually all-axial, the rear stages on smaller units are too small to be robust. Consequently, these stages are often replaced by a single centrifugal unit. Very small flow compressors often employ two centrifugal compressors, connected in series. Although in isolation centrifugal compessors are capable of running at quite high pressure ratios (e.g. 10:1), impeller stress considerations (i.e. T3, NH implications) limit the CF pressure ratio that can be employed in high overall pressure ratio engine cycles. Increasing overall pressure ratio implies a higher (HP) compressor exit temperature (i.e. T3. This implies a higher HP shaft speed, to maintain the datum blade tip Mach number on the rear compressor stages. Stress considerations, however, may limit shaft speed increases, leading to a reduction in the pressure ratio of the rear stages. compressor map
Combustors
Care must be taken to keep the flame burning in a moderately fast moving airstream, at all throttle conditions, as efficiently as possible. Since the turbine cannot withstand stoichiometric temperatures, resulting from the optimum combustion process, some of the compressor air is used to quench the exit temperature of the combustor to an acceptable level.
Turbines
stoichiometric Because a turbine expands from high to low pressure, there is no such thing as turbine surge or stall. Designers must, however, prevent the turbine blades and vanes from melting in a very high temperature and stress environment. Consequently bleed air extracted from the compression system is often used to cool the turbine blades/vanes internally. Other solutions are improved materials and/or special insulating coatings. The discs must be specially shaped to withstand the huge stresses imposed by the rotating blades. Improved materials help to keep disc weight down.
Nozzles
stresses Most jet engines use a simple convergent nozzle, which is relatively easy to design. However, afterburning engines require a variable area nozzle, to maintain sensible engine matching when the afterburner is alight. This is usually accommodated by using a series of interlocking petals (driven by pneumatic or hydraulic rams) to adjust the throat area. Even more complexity is introduced if a convergent-divergent nozzle is fitted, especially if the throat and exit areas are adjusted independently. convergent-divergent nozzle Rocket motors also employ convergent-divergent nozzles, but these are usually of fixed geometry, to minimize weight. Because of the much higher nozzle pressure ratios experienced, rocket motor con-di nozzles have a much greater area ratio (exit/throat) than those fitted to jet engines. At the other extreme, some high bypass ratio civil turbofans use an extremely low area ratio (less than 1.01 area ratio), convergent-divergent, nozzle on the bypass (or mixed exhaust) stream, to control the fan working line. The nozzle acts as if it has variable geometry. At low flight speeds the nozzle is unchoked (less than a Mach number of unity), so the exhaust gas speeds up as it approaches the throat and then slows down slightly as it reaches the divergent section. Consequently, the nozzle exit area controls the fan match and, being larger than the throat, pulls the fan working line slightly away from surge. At higher flight speeds, the ram rise in the intake increases nozzle pressure ratio to the point where the throat becomes choked (M=1.0). Under these circumstances, the throat area dictates the fan match and being smaller than the exit pushes the fan working line slightly towards surge. This is not a problem, since fan surge margin is much better at high flight speeds.
Engine Performance

TS Diagram
Mach number °R) = 1 Btu/(lb °F) = 1 kcal/(kg °C) = 4.184 kJ/(kg·K).]] Temperature vs. entropy diagrams (see example, above) are usually used to illustrate the cycle of gas turbine engines. All the reader really needs to know about entropy is that it represents the degree of disorder of the molecules in the fluid and that it tends to increase! Apart from stations 0 and 8s, stagnation pressure and stagnation temperature are used. Station 0 is ambient. The processes depicted are: ;Freestream (stations 0 to 1) :In the example, the aircraft is stationary, so stations 0 and 1 are coincident. Station 1 is not depicted on the diagram. ;Intake (stations 1 to 2) :In the example, a 100% intake pressure recovery is assumed, so stations 1 and 2 are coincident. ;Compression (stations 2 to 3) :The ideal process would appear vertical on a TS diagram. In the real process there is friction, turbulence and, possibly, shock losses, making the exit temperature, for a given pressure ratio, higher than ideal. The shallower the positive slope on the TS diagram, the less efficient the compression process. ;Combustion (stations 3 to 4) :Heat (usually by burning fuel) is added, raising the temperature of the fluid. There is an associated pressure loss, some of which is unavoidable ;Turbine (stations 4 to 5) :The temperature rise in the compressor dictates that there will be an associated temperature drop across the turbine. Ideally the process would be vertical on a TS diagram. However, in the real process, friction and turbulence cause the pressure drop to be greater than ideal. The shallower the negative slope on the TS diagram, the less efficient the expansion process. ;Jetpipe (stations 5 to 8) :In the example the jetpipe is very short, so there is no pressure loss. Consequently, stations 5 and 8 are coincident on the TS diagram. ;Nozzle (stations 8 to 8s) :These two stations are both at the throat of the (convergent) nozzle. Station 8s represents static conditions. Not shown on the TS diagram is the expansion process, external to the nozzle, down to ambient pressure.
Design Point Performance Equations
In theory, any combination of flight condition/throttle setting can be nominated as the engine performance Design Point. Usually, however, the Design Point corresponds to the highest corrected flow at inlet to the compression system (e.g. Take-off Rating, Sea Level Static, ISA) The design point net thrust of any jet engine can be estimated by working through the engine cycle, step by step. Below are the equations for a single spool turbojet. Freestream
$T_1 = t_0 \cdot (1 + (_c-1)\cdot M^2/2)$ $P_1 = p_0 \cdot (T_1/t_0)^$ Intake
$T_2=T_1 \,$ $P_2=P_1 \cdot \mathrm$ Compressor
$T_3 = T_2 \cdot ((P_3/P_2) ^$ $P_3 = P_2 \cdot (P_3/P_2)$ Combustor
$T_4 = \mathrm \,$ $P_4 = P_3 \cdot (P_4/P_3)$ Turbine
Equating the turbine and compressor powers, we have:
$w_4 \cdot C_(T_4-T_5) = w_2 \cdot C_(T_3-T_2)$ A simplyfying assumption sometimes made is for the addition of fuel flow to be exactly offset by an overboard compressor bleed, so mass flow remains constant throughout the cycle. $P4/P5 = (T4/T5)^$ Jetpipe
$T_8 = T_5 \,$ $P_8 = P_5 (P_8/P_5) \,$ Nozzle
$t_ = T_8/((_t+1)/2) \,$ $p_ = P_8/((T_8/t_)^)$ $V_8^2 = 2gJC_(T_8 - t_)$ $_ = p_/(R \cdot t_)$ $A_8 = w_8/(_ \cdot V_8)$ Gross Thrust
$F_g = C_\mathrm((w_8 \cdot V_8/g) + A_8(p_ - p_0))$ Ram Drag
$F_r = w_0 \cdot V_0/g$ Net Thrust
$F_n = F_g - F_r \,$ Note that mass flow is the sizing parameter: doubling the airflow, doubles the thrust. Note:
- A flow area
- C_pc specific heat at constant pressure for air
- C_pt specific heat at constant pressure for combustion products
- C_x thrust coefficient
- g acceleration of gravity
- J mechanical equivalent of heat
- M flight Mach number
- p static pressure
- P total pressure
- prf intake pressure recovery factor
- R gas constant
- RIT (turbine) rotor inlet temperature
- t static temperature
- T total temperature
- V velocity
- w mass flow
- ρ density
- γ_c ratio of specific heats for air
- γ_t ratio of specific heats for combustion products
- η_pc compressor polytropic efficiency
- η_pt turbine polytropic efficiency
Off-design
An engine is said to be running off-design if any of the following apply: :a) change of throttle setting :b) change of altitude :c) change of flight speed :d) change of climate :e) change of installation (e.g. customer bleed or power off-take) Although each off-design point is effectively a design point calculation, the resulting cycle (normally) has the same turbine and nozzle geometry as that at the engine design point. Obviously the final nozzle cannot be over or underfilled with flow. This rule also applies to the turbine nozzle guide vanes, which act like small nozzles. Design point calculations are normally done by a computer program. By the addition of an iterative loop, such a program can also be used to create a crude off-design model. The variables for the single spool turbojet iteration would typically be: RIT (or some other function of fuel flow), w₂,P₃/P₂ Typically, the constraints imposed would be: Engine match (e.g. F_n , fuel flow, etc), A_{8 geometric},w4cor The latter two are the physical constraints that must be met. Corrected flow is the flow that would pass through a device, if the entry pressure and temperature corresponded to ambient conditions at sea level on a Standard Day. A more refined off-design model can be created using compressor maps and turbine maps to predict off-design efficiencies, relative shaft speeds, etc. The nominal net thrust quoted for a jet engine usually refers to the Sea Level Static (SLS) condition, either for the International Standard Atmosphere (ISA) or a hot day condition (e.g. ISA+10 °C). As an example, the GE90-76B has a take-off static thrust of 76,000 lbf (360 kN) at SLS, ISA+15 °C. Naturally, net thrust will decrease with altitude, because of the lower density. There is also, however, a flight speed effect. Initially as the aircraft gains speed down the runway, there will be little increase in nozzle pressure and temperature, because the ram rise in the intake is very small. There will also be little change in mass flow. Consequently, nozzle gross thrust initially only increases marginally with flight speed. However, being an air breathing engine (unlike a conventional rocket) there is a penalty for taking on-board air from the atmosphere. This is known as ram drag. Although the penalty is zero at static conditions, it rapidly increases with flight speed causing the net thrust to be eroded. As flight speed builds up after take-off, the ram rise in the intake starts to have a significant effect upon nozzle pressure/temperature and intake airflow, causing nozzle gross thrust to climb more rapidly. This term now starts to offset the still increasing ram drag, eventually causing net thrust to start to increase. In some engines, the net thrust at say Mach 1.0, sea level can even be slightly greater than the static thrust. Above Mach 1.0, with a subsonic inlet design, shock losses tend to decrease net thrust, however a suitably designed supersonic inlet can give a subsonic airspeed entering the compressor, while giving a useful compression, and thus net thrust and efficiency can continue to climb. The thrust lapse described above depends on the design specific thrust and, to a certain extent, on how the engine is rated with intake temperature.
Rated Performance

Civil
turbine map Nowadays, civil engines are usually flat-rated on net thrust up to a 'break-point' climate. So at a given flight condition, net thrust is held approximately constant over a very wide range of ambient temperature, by increasing (HP) turbine rotor inlet temperature (RIT or SOT). However, beyond the break-point, SOT is held constant and net thrust starts to fall for further increases in ambient temperature. Consequently, aircraft fuel load and/or payload must be decreased. Usually, for a given rating, the kink-point SOT is held constant, regardless of altitude or flight speed. Some engines have a special rating, known as the 'Denver Bump'. This invokes a higher RIT than normal, to enable fully laden aircraft to Take-off safely from Denver, CO in the summer months. Denver Airport is extremely hot in the summer and the runways are over a mile above sea level. Both of these factors affect engine thrust.
Military
turbine map The rating systems used on military engines vary from engine to engine. A typical military rating structure is shown on the left. At low intake temperatures, the engine tends to operate at maximum corrected speed or corrected flow. As intake temperature rises, a limit on (HP) turbine rotor inlet temperature (SOT) takes effect, progressively reducing corrected flow. At even higher intake temperatures, a limit on compressor delivery temperature (T₃) is invoked, which decreases both SOT and corrected flow. corrected flow The impact of design intake temperature is shown on the right hand side. An engine with a low design T₁ combines high corrected flow with high rotor turbine temperature (SOT), maximizing net thrust at low T₁ conditions (e.g. Mach 0.9, 30000 ft, ISA). However, although turbine rotor inlet temperature stays constant as T₁ increases, there is a steady decrease in corrected flow, resulting in poor net thrust at high T₁ conditions (e.g. Mach 0.9, sea level, ISA). Although an engine with a high design T₁ has a high corrected flow at low T₁ conditions, the SOT is low, resulting in a poor net thrust. Only at high T₁ conditions is there the combination of a high corrected flow and a high SOT, to give good thrust characteristics. A compromise between these two extremes would be to design for a medium intake temperature (say 290 K).
See also

- Jet aircraft
- Jetboat
- Spacecraft propulsion
- Supercharger
- Turbocharger
- Gas turbine
- Kurt Schreckling who built practical jet engines for model aircraft
- Wikibooks: Jet propulsion
External link

- [http://www.rmcybernetics.com/projects/DIY_Devices/homemade_jet_engine.htm RMCybernetics - A simple Homemade Jet Engine]
- [http://www.rolls-royce.com/education/schools/journey02/flash.html Journey through a jet engine(flash)]
- [http://travel.howstuffworks.com/turbine.htm How Stuff Works article on how a Gas Turbine Engine works]
- [http://www.generalatomic.com/jetmakers/chapter15.html Influence of the Jet Engine on the Aerospace Industry]
- [http://www.rand.org/publications/MR/MR1596/MR1596.appb.pdf An Overview of Military Jet Engine History] (Rand Corp., 24 pgs, PDF)
- [http://bikerodnkustom3.homestead.com/danger.html A jet propulsion bicycle] Category:Energy conversion Category:Gas turbines Category:Jet engines ja:ジェットエンジン

Circular error probable

In the military science of ballistics, Circular Error Probability or circular error probable (CEP) is a simple measure of a weapon system's precision. It is defined as the radius of a circle into which a missile, bomb, or projectile will land at least half the time. For example, a Trident II warhead has a CEP of 90 meters; thus, each warhead will impact within 90 meters of the target point with a probability of 50%. For LGM-30 Minuteman III warheads, the CEP is 275 meters for the three 170kT W62 warheads contained in General Electric (GE) Mk 12 RVs, and 220 meters for the three 335kT W78 warheads contained in GE Mk 12A RVs. In its most accurate mode, Joint Direct Attack Munition provides a CEP of 13 meters or less when GPS data is available. The impact of munitions near the target tends to be bivariate normally distributed around the aim point, with most reasonably close, progressively fewer and fewer further away, and very few indeed at long distance. One component of the bivariate normal will represent range errors and the other azimuth errors. Unless the munition is arriving exactly vertically downwards the standard deviation of range errors is usually larger than the standard deviation of azimuth errors, and the resulting confidence regions is elliptical. Generally, the munition will not be exactly on target, i.e. the mean vector will not be (0,0). This is referred to as bias. The mean error squared (MSE) will be the sum of the variance of the range error plus the variance of the azimuth error plus the covariance of the range error with the azimuth error plus the square of the bias. Thus the MSE results from pooling all these sources of error. The square root of the MSE is the circular error probable, commonly abbreviated to CEP. Geometrically, it corresponds to radius of a circle within which 50 percent of rounds will land. It should be noted that the concept of CEP is only strictly meaningful if misses are roughly normally distributed. This is generally not true for precision-guided munitions. Generally, if CEP is n meters, 50 percent of rounds land within n meters of the target, 43 percent between n and twice that distance and 7 percent between two and three times that distance. If misses were exactly normally distributed as in this theory, then the proportion of rounds that land farther than three times the CEP from the target is less than 0.2%. With precision-guided munitions, the number of 'close misses' is higher. ms:Kemungkinan ralat bulatan ja:CEP Category:Artillery Category:Ballistics Category:Air-dropped bombs Category:Weapon guidance

B-52 Stratofortress

The Boeing B-52 Stratofortress is a long-range strategic bomber flown by the United States Air Force since 1954, replacing the Convair B-36 and the Boeing B-47. Although built for the role of Cold War-era nuclear deterrent, its conventional capabilities are these days the more important role in USAF operations, where its long range, heavy weapons load and fearsome reputation have proven valuable.

Mission

Air Combat Command's B-52 is a long-range heavy bomber that can perform a variety of missions. The bomber is capable of flying at high subsonic speeds at altitudes up to 50,000 feet (15 km). It can carry nuclear or precision guided conventional ordnance and has the capability to navigate the world precisely.

Background

For more than 50 years, the B-52 Stratofortress has been the backbone of the manned strategic bomber force for the United States. The B-52 is capable of dropping or launching a wide array of weapons in the U.S. inventory, including gravity bombs, cluster bombs, precision guided missiles and Joint Direct Attack Munitions. When updated with the latest technology, the B-52 will be capable of delivering the full complement of joint developed weapons; allowing it to continue into the 21st century as an important element of U.S. military capabilities. Current engineering analyses show the B-52's life span to extend beyond the year 2045. 2045Two B-52 prototypes were built, and were designated XB-52 and YB-52. In actuality, both aircraft were almost identical, but the YB-52 incorporated enough changes to warrant a different designation. The most notable difference between the prototypes and the B-52A was that the X and Y aircraft used a tandem cockpit for the pilot and co-pilot, very similar to that on the B-47. The cockpit for the B-52A was completely redesigned due to the insistence of General Curtis LeMay, Commander of the Strategic Air Command, who was opposed to the tandem seating arrangement. Although the XB-52 was the first prototype to be completed and rolled out, the YB-52 was the first to fly - on April 15, 1952 - due to damage on the XB-52's wing trailing edges caused by a hydraulic system failure. The XB-52 eventually flew for the first time on October 2, 1952. Unfortunately, both aircraft were scrapped in the mid-1960s, though the YB-52 was available for viewing in the USAF Museum from the late '50s until the time when it was decided to scrap it. The B-52A first flew in August 1954 and the B model entered service in 1955. A total of 744 B-52s were built with the last, a B-52H, delivered in October 1962. Only the H model is still in the Air Force inventory and is assigned to Air Combat Command and the Air Force Reserves. The oldest B-52 still flying was a B-52B that was built in 1955, though it also has the fewest flight hours of any surviving B-52. It was operated by NASA's Dryden Flight Research Center and was used for drop tests of various research aircraft until its retirement on December 17, 2004. On July 30, 2001, Dryden received a B-52H that is expected to fully replace the older B-model aircraft by the end of 2004. The first of 102 B-52H's was delivered to Strategic Air Command in May 1961. The H model can carry up to 20 air launched cruise missiles. In addition, it can carry the conventional cruise missile that was launched in several contingencies during the 1990s, starting with Operation Desert Storm and culminating with Operation Allied Force. The B-52 contributed to the U.S. success in Operation Enduring Freedom in Afghanistan, providing the ability to loiter high over the battlefield and provide Close Air Support (CAS) through the use of precision guided munitions. The long range and endurance of the B-52 provided a U.S. presence unmatched by any other combat aircraft. B-52's also played a key role in the second Gulf War in 2002-2003 (Operation Iraqi Freedom), where they provided close air support and bombing. The Air Force intends to keep the B-52 in service until around 2050, an unprecedented length of service for an aircraft model. This is especially amazing considering that the last plane was built in 1962; the Air Force fully expects to be flying 90-year-old airframes. Boeing has suggested re-engining of the B-52H fleet with the Rolls-Royce RB211 534E-4. This would involve replacing the eight Pratt & Whitney TF33s (total thrust 17,000 lb or 605 kN) with four RB211s (total thrust 37,400 lb.). The RR engines will increase the range/payload of the fleet and reduce fuel consumption. However the cost of the project would be significant. Procurement would cost approximately $2.56 billion ($36 million × 71 aircraft). A General Accounting Office study of the proposal concluded that Boeing's estimated savings of $4.7 billion would not be realized. They found that it would cost the Air Force $1.3 billion over keeping the existing engines. [http://www.globalsecurity.org/wmd/systems/b-52-upgrade.htm] Another recently approved upgrade for the B-52 is the B-52 SOJ (Stand Off Jammer) program which will allow it to assume an airborne communications/jamming role. Approximately a quarter of the fleet will be converted to take on this mission, with the Air Force seeking funding to convert the entire fleet. The B-52 SOJ will retain all of its bomber functions and capabilities, however now after having expended its weapons load it will continue to loiter over the combat area providing electronic warfare cover for follow on strikes. The additional equipment will be carried in 30 ft external pods under the wings. [http://www.isrjournal.com/story.php?F=1166029]

Features

In a conventional conflict, the B-52 can perform strategic attack, air interdiction, offensive counter-air and maritime operations. During Operation Desert Storm, B-52s delivered 40 percent of all the weapons dropped by coalition forces. All B-52s are equipped with an electro-optical viewing system that uses platinum silicide forward-looking infrared and high resolution low-light-level television sensors to augment targeting, battle assessment, and flight safety, thus further improving its combat ability and low-level flight capability. Pilots wear night vision goggles (NVGs) to enhance their vision during night operations. Night vision goggles provide greater safety during night operations by increasing the pilot's ability to visually clear terrain, avoid enemy radar and see other aircraft in a covert/lights-out environment. Starting in 1989, on-going modifications incorporates the Global Positioning System, heavy stores adapter beams for carrying 2,000 pound (900 kg) munitions, and a full array of advanced weapons currently under development. The use of aerial refueling gives the B-52 a range limited only by crew endurance, or in the extreme, required maintenance. It has an unrefueled combat range in excess of 8,800 statute miles (14,000 km). It is highly effective when used for ocean surveillance, and can assist the U.S. Navy in anti-ship and mine-laying operations. Two B-52s, in two hours, can monitor 140,000 square miles (364,000 km²) of ocean surface. If on land, this area is about as large as a circle centered at New York City and covered as far as Washington, DC, Syracuse and Boston (radius = 212 statute miles or 340 km). However, the actual shape of coverage would vary. The aircraft's flexibility was evident in Operation Desert Storm and again during Operation Allied Force. B-52s struck wide-area troop concentrations, fixed installations and bunkers, and ruined the morale of Iraq's Republican Guard. The Persian Gulf War involved the longest strike mission in the history of aerial warfare when B-52s took off from Barksdale Air Force Base, Louisiana, launched conventional air launched cruise missiles and returned to Barksdale—a 35 hour, non-stop combat mission. During Operation Allied Force, B-52s opened the conflict with conventional cruise missile attacks and then transitioned to delivering general purpose bombs and cluster bomb units on Serbian army positions and staging areas.

General characteristics

staging area
- Contractor: Boeing Military Airplane Co.
- Speed: 650 mph, 1000 km/h (Mach 0.86)
- Range: Unrefueled 8,800 statute miles (14,200 km)
- Armament: Approximately 70,000 lb (31,500 kg) mixed ordnance—bombs, land mines and missiles. (Modified to carry air-launched cruise missiles, AGM-84 Harpoon anti-ship and AGM-142 Have Nap missiles.)
- The nuclear weapons capacity has previously included B28, B43, B53, B61, and B83 free-fall nuclear bombs, or various combinations of twelve AGM-129 Advanced Cruise Missiles (ACMS), 20 AGM-86A Air Launched Cruise Missiles (ALCM) and eight bombs.
- The B-52A through F carried a tail-mounted armament of four .50 cal (12.7 mm) machine guns with the gunner sitting in the tail, The B-52G retained the quad .50 cals but the gunner moved up front with the rest of the crew and controlled the guns via remote. The B-52H replaced the quad .50's with a single 20 mm M61A1 Vulcan which offered much greater defensive fire power. In the mid-1990s, the tail gun was removed from all of the B-52H aircraft to reduce weight.
- The G and H models are distinguishable from previous models due to their shorter (by 8 feet) vertical tailplane. This configuration had previously been tested on a B-52A.
- The H model is distinguishable from all previous variants by having visually different engine pods. The B-52H uses TF33-3 turbofan engines, which provided 20% greater range, 70% more thrust and are considerably quieter than the J57 engine which had been used on all previous variants
- The B-52 is the only known bomber to have shot down jet-powered fighter aircraft; one unit of the type shot down two MiG-17 fighter planes during the Vietnam War.
- Accommodations: Five (Pilot, Co-Pilot, Navigator, Radar Navigator (AKA Bombardier) & Electronic Warfare Officer with all sitting in ejection seats
- Unit Cost: $74 million
- Date Deployed: February 1955 · Inventory: Active force, 85; ANG, 0; Reserve, 9

Production

- XB-52 - The first B-52 prototype. 1 built
- YB-52 - The second protoype. 1 built
- B-52A - The first production model. 3 built
- B-52B - 50
- RB-52B - 27 B-52Bs converted into reconnaissance aircraft. 2X 20mm Cannon Replaced 4X .50 cal in tail
- B-52C - 35
- B-52D - 170
- B-52E - 100
- B-52F - 89
- B-52G - 193
- B-52H - 102
- Total produced - 744

Specifications (B-52H)

right

Trivia

- Among its crew, the B-52 is affectionately known as the "BUFF", an acronym for "big ugly fat fucker" (or "big ugly fat fellow" in more polite company).
- The B-52's wheels are all steerable, because the enormous wingspan of the plane means it cannot always orient itself properly with the runway when landing in a strong cross-wind. The wheels are made to point down the runway even if the nose of the plane does not.
- A hairstyle known as the "B-52", because of its resemblance to the nose cone of this aircraft, was popular in the 1950s, 60s and 70s.
- The musical band The B-52's were not directly named after this aircraft, but after the B-52 hairstyle members of the band wear.
- There is also a cocktail named for the B-52, the B-52 shooter.
- The B-52 bomber gained notoriety after Stanley Kubrick's Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb Cold War movie satire. The cockpit of the plane is one of only four movie settings. The Air Force refused to allow Stanley Kubrick permission to photograph the cockpit interior; he developed his B-52 cockpit by extrapolating from photos of the interior of a B-50 Superfortress. His guess was so accurate that his production company was later investigated by the Department of Defense.
- In The Living Daylights,a B-52 was parked at a British base in Gibraltar.
- The NASA B-52B Mothership, NASA tail number 008, was retired from active service with NASA on 17 December, 2004, after almost 50 years flying service. This was the B-52 famous for dropping such aerospace research vehicles as the X-15, X-24, HiMAT, Lifting Body vehicles, X-43, and others. It was the oldest active B-52 at the time, having first flown on June 11, 1955, and entering service with NASA in 1959. It was the last B-52B in service.
- The B-52's longevity is marked by the fact that in at least one family of airmen, the grandfather, father, and son have all served as B-52 crew.
- Two of the six ejection seats eject downwards from the bottom of the plane.
- In the early 1980s Boeing submitted an unsolicited proposal for a "Super" B-52. It would have offered upgraded engines, improved electronics and avionics and vastly improved ergonomics for the crew. The plan was considered but dropped in favor of the B-1B that was then being considered to replace the then-20+ year old B-52G/H fleet.
- On the night of December 27, 1972, North Vietnamese pilot and future cosmonaut Pham Tuan became the first person ever to shoot down a US B-52 bomber, during the Vietnam War. The bomber had been circling the Hanoi sky during the US campaign Operation Rolling Thunder.
- As part of the 1991 Strategic Arms Reduction Treaty between the United States and Russia, 365 B-52s were flown to the Aerospace Maintenance and Regeneration Center at Davis-Monthan Air Force Base in Arizona. The bombers were stripped of all usable parts, then unceremoniously chopped into five pieces by a 13,000-pound steel blade dropped from a crane. The modern-day guillotine crashed down four times on each plane, severing the mammoth wings and leaving the fuselage in three pieces. The ruined B-52s remained in place for three months in order for orbiting Russian satellites to confirm the bombers had been destroyed, after which they were sold for scrap at 12 cents a pound.

External links

- http://www.fas.org/nuke/guide/usa/bomber/b-52_hist.htm -- detailed historical overview
- http://www.af.mil/factsheets/factsheet.asp?fsID=83 -- B-52 Stratofortress Fact Sheet
- [http://fmc.dotnet-services.nl/operation_iraqi_freedom.htm USAF B-52 mission flights from Fairford to Iraq (2003) monitored by the Frequency Monitor Centre]
- [http://www.nasa.gov/centers/dryden/news/FactSheets/FS-005-DFRC.html NASA Dryden B-52 fact sheet] ja:B-52 (爆撃機)

Soviet Union

The Union of Soviet Socialist Republics, abbreviated USSR ( (СССР) ; tr.: Soyuz Sovetskikh Sotsialisticheskikh Respublik [SSSR])), more commonly known as the Soviet Union (; tr.: Sovetsky Soyuz) was an officially socialist state founded in 1922, centered on Russia, and dissolved in 1991. From 1945 until its dissolution it was historically notable as one of the world's two superpowers. The formation of the Soviet Union was the culmination of the Russian Revolution of 1917, which overthrew short-lived Provisional Government (established after Tsar Nicholas II abdicated on March 15, 1917), and later the Red Army victory in the violent Russian Civil War from 1918-1920. The geographic boundaries of the Soviet Union varied with time, but by 1945 it approximately corresponded to that of historic Imperial Russia, with the notable exclusions of Poland and Finland. The geographic size of the Soviet Union remained from 1945 until its dissolution. The Soviet Union, founded three decades before the Cold War, became a primary model for future Communist states; the socialist government and the political organization of the country were defined by the only permitted political party, the Communist Party of the Soviet Union.

History

The Soviet Union is traditionally considered to be the successor of the Russian Empire. The last Russian monarch, Tsar Nicholas II, ruled until March 1917 and was eventually executed. The Soviet Union was established in December 1922 as the union of the Russian, Ukrainian, Belarusian, and Transcaucasian Soviet republics ruled by Bolshevik parties. By Soviet historiography, revolutionary activity in Russia began with the Decembrist Revolt of 1825, and although serfdom was abolished in 1861, its abolition was achieved on terms unfavorable to the peasants and served to encourage revolutionaries. A parliament, the State Duma, was established in 1906, after the 1905

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