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Encyclopedia > Jet engine
A Pratt & Whitney F100 turbofan engine for the F-15 Eagle and the F-16 Falcon is tested at Robins Air Force Base, Georgia, USA. The tunnel behind the engine muffles noise and allows exhaust to escape
Simulation of a turbojet's airflow

In common usage, the term 'jet engine' generally refers to a gas turbine driven internal combustion engine, an engine with a rotary compressor powered by a turbine ("Brayton cycle"), with the leftover power providing thrust. These types of jet engines are primarily used by jet aircraft for long distance travel. The early jet aircraft used turbojet engines which were relatively inefficient for subsonic flight. Modern jet aircraft usually use high-bypass turbofan engines which help give high speeds as well as, over long distances, giving better fuel efficiency than many other forms of transport. This machine has a single-stage centrifugal compressor and turbine, a recuperator, and foil bearings. ... A colored automobile engine The internal combustion engine is an engine in which the combustion of fuel and an oxidizer (typically air) occurs in a confined space called a combustion chamber. ... A Siemens steam turbine with the case opened. ... The Brayton cycle is a constant-pressure cycle named after George Brayton (1830â€“1892), the American engineer who developed it. ... Jet aircraft are aircrafts with jet engines. ... For the transportation company in southern China, see TurboJET. Turbojets are the oldest kind of general purpose jet engines. ... Schematic diagram of high-bypass turbofan engine CFM56-3 turbofan, lower half, side view. ...

About 7.2% of the oil used in 2004 was ultimately consumed by jet engines[1] In 2007, the cost of jet fuel, while highly variable from one airline to another, averaged 26.5% of total operating costs, making it the single largest operating expense for most airlines[2]. Year 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ... Year 2007 (MMVII) was a common year starting on Monday of the Gregorian calendar in the 21st century. ...

## History

Jet engines can be dated back to the first century AD, when Hero of Alexandria (a phoenician) invented the aeolipile. This used steam power directed through two jet nozzles so as to cause a sphere to spin rapidly on its axis. So far as is known, it was little used for supplying mechanical power, and the potential practical applications of Hero's invention of the jet engine were not recognized. It was simply considered a curiosity. This article outlines some of the important developments in the early history of the development of the jet engine. ... Hero (or Heron) of Alexandria (Greek: Î‰ÏÏ‰Î½ Î¿ Î‘Î»ÎµÎ¾Î±Î½Î´ÏÎµÏÏ‚) (c. ... Phoenician can mean: The Phoenician ancient civilization The Phoenician alphabet The Phoenician languages This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... An illustration of Herons aeolipile An aeolipile is a device consisting of an air-tight chamber (usually a sphere or cylinder) with bent or curved pipes projecting from it, through which steam is expelled perpendicular to the radius of rotation. ...

Jet propulsion only literally and figuratively took off with the invention of the rocket by the Chinese in the 11th century. Rocket exhaust was initially used in a modest way for fireworks but gradually progressed to propel formidable weaponry; and there the technology stalled for hundreds of years. This article is about vehicles powered by rocket engines. ... For other uses, see Fireworks (disambiguation). ...

In Ottoman Turkey in 1633 Lagari Hasan Çelebi took off with what was described to be a cone shaped rocket and then glided with wings into a successful landing winning a position in the Ottoman army. However, this was essentially a stunt. Lagari Hasan Ã‡elebi is considered the first person to have flown. ... Motto Ø¯ÙˆÙ„Øª Ø§Ø¨Ø¯ Ù…Ø¯Øª Devlet-i Ebed-mÃ¼ddet (The Eternal State) Anthem Ottoman imperial anthem Borders in 1683, see: list of territories Capital SÃ¶ÄŸÃ¼t (1299â€“1326) Bursa (1326â€“1365) Edirne (1365â€“1453) Ä°stanbul (1453â€“1922) Government Monarchy Sultans  - 1281â€“1326 (first) Osman I  - 1918â€“22 (last) Mehmed VI Grand Viziers  - 1320...

The problem was that rockets are simply too inefficient at low speeds to be useful for general aviation. In 1913 René Lorin came up with a form of jet engine, the subsonic ramjet, which would have been somewhat more efficient, but he had no way to achieve high enough speeds for it to operate, and the concept remained theoretical for quite some time. RenÃ© Lorin is the inventor of the ramjet, which he patented in 1908. ... For other uses, see Ramjet (disambiguation). ...

However, engineers were beginning to realize that the piston engine was self-limiting in terms of the maximum performance which could be attained; the limit was essentially one of propeller efficiency.[3] This seemed to peak as blade tips approached the speed of sound. 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 developed. This was the motivation behind the development of the gas turbine engine, commonly called a "jet" engine, which would become almost as revolutionary to aviation as the Wright brothers' first flight. For other uses, see Propeller (disambiguation). ... For other uses, see Speed of sound (disambiguation). ... The Wright brothers, Orville (19 August 1871 â€“ 30 January 1948) and Wilbur (16 April 1867 â€“ 30 May 1912), were two Americans who are generally credited[1][2][3] with inventing and building the worlds first successful airplane and making the first controlled, powered and sustained heavier-than-air human...

The earliest attempts at jet engines were hybrid designs in which an external power source first compressed air, which was then mixed with fuel and burned for jet thrust. In one such system, called a thermojet by Secondo Campini but more commonly, motorjet, the air was compressed by a fan driven by a conventional piston engine. Examples of this type of design were Henri Coandă's Coandă-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. The Campini Caproni CC.2 Motorjet powered aircraft. ... Secondo Campini (born August 28, 1904 in Bologna, died February 7, 1980 in Milan) was an Italian engineer and one of the pioneers of the jet engine. ... The Campini Caproni CC.2 Motorjet powered aircraft. ... Henri Marie CoandÄƒ (June 7, 1886 â€“ November 25, 1972) (IPA: /ÉÊi maÊi kwandÉ™/) was a Romanian inventor, aerodynamics pioneer and the builder of worlds first jet powered aircraft, the Coanda-1910. ... The CoandÄƒ-1910 was the first jet-propelled aircraft ever built. ... The Campini Caproni CC.2 (sometimes referred to as the N-1) was an early thermojet-powered aeroplane. ... The Tsu-11 was a primitive, motorjet-style jet engine produced in small numbers in Japan in the closing stages of World War II. It was principally designed to propel the Japanese Ohka flying bomb, a kamikaze weapon. ... Ohka Model 11 replica at the Yasukuni Shrine The Yokosuka MXY-7 Ohka (æ«»èŠ± cherry blossom) was a purpose-built kamikaze aircraft employed by Japan towards the end of World War II. The United States gave the aircraft the name Baka (Japanese for fool). It was a small flying bomb that... USS Bunker Hill was hit by Ogawa (see picture left) and another kamikaze near KyÅ«shÅ« on May 11, 1945. ... Combatants Allied powers: China France Great Britain Soviet Union United States and others Axis powers: Germany Italy Japan and others Commanders Chiang Kai-shek Charles de Gaulle Winston Churchill Joseph Stalin Franklin Roosevelt Adolf Hitler Benito Mussolini Hideki TÅjÅ Casualties Military dead: 17,000,000 Civilian dead: 33,000...

The key to a practical jet engine was the gas turbine, used to extract energy from the engine itself to drive the compressor. 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. The first gas turbine to successfully run self-sustaining was built in 1903 by Norwegian engineer Ægidius Elling. The first patents for jet propulsion were issued in 1917[citation needed]. Limitations in design and practical engineering and metallurgy prevented such engines reaching manufacture. The main problems were safety, reliability, weight and, especially, sustained operation. A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume. ... This machine has a single-stage centrifugal compressor and turbine, a recuperator, and foil bearings. ... Jens William Ã†gidius Elling (also Aegidus or Aegidius) (born July 26, 1861 - died 1949) was a Norwegian inventor who is considered to be the father of the gas turbine. ...

Albert Fonó In 1915 devised a solution for increasing the range of artillery, comprising a gun-launched projectile which was to be united with a ramjet propulsion unit. This was to make it possible to obtain a long range with low initial muzzle velocities, allowing heavy shells to be fired from relatively lightweight guns. Fonó submitted his invention to the Austro-Hungarian Army but the proposal was rejected. In 1928 he applied for a German patent on supersonic ramjets, and this was awarded in 1932.[4] Albert FonÃ³ (1881-1972) was a mechanical engineer born in Budapest, Hungary. ...

In 1923, Edgar Buckingham of the US National Bureau of Standard published a report[5] expressing scepticism that jet engines would be economically competitive with prop driven aircraft at the low altitudes and airspeeds of the period: "there does not appear to be, at present, any prospect whatever that jet propulsion of the sort here considered will ever be of practical value, even for military purposes." Edgar Buckingham (1867â€“1940) was educated at Harvard and Leipzig, and worked at the (US) National Bureau of Standards (now the National Institute of Standards and Technology, or NIST) 1905--1937. ...

Instead, by the 1930s, the piston engine in its many different forms (rotary and static radial, aircooled and liquid-cooled inline) was the only type of powerplant available to aircraft designers. This was acceptable as long as only low performance aircraft were required, and indeed all that were available. Components of a typical, four stroke cycle, DOHC piston engine. ...

The Whittle W.2/700 engine flew in the Gloster E.28/39, the first British aircraft to fly with a turbojet engine, and the Gloster Meteor

Heinkel He 178, the world's first aircraft to fly purely on turbojet power

Meanwhile, Whittle's engine was starting to look useful, and his 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 specially built for it, and first flew on May 15, 1941 at RAF Cranwell. The Air Ministry was formerly a department of the United Kingdom Government, established in 1918 with the responsibility of managing the affairs of the (then newly formed) Royal Air Force. ... The Gloster E.28/39, (also referred to as the Gloster Whittle, Gloster Pioneer, or Gloster G.40) was the first jet engined aircraft to fly in the United Kingdom. ... Airframe means the mechanical structure of an aircraft[1] and as generally used does not include the engines. ... is the 135th day of the year (136th in leap years) in the Gregorian calendar. ... For other uses, see 1941 (disambiguation). ... RAF Cranwell is a Royal Air Force station in Lincolnshire close to the village of Cranwell, near Sleaford. ...

A picture of an early centrifugal engine (DH Goblin II) sectioned to show its internal components

A British aircraft engine designer, Frank Halford, working from Whittle's ideas developed a "straight through" version of the centrifugal jet; his design became the de Havilland Goblin. Image File history File links Download high resolution version (1024x709, 822 KB) Summary A picture of a sectioned de Havilland Goblin II describing the internal components. ... Image File history File links Download high resolution version (1024x709, 822 KB) Summary A picture of a sectioned de Havilland Goblin II describing the internal components. ... Cutaway Goblin II A cutaway diagram of the internal workings of the de Havilland Goblin, as fitted to the Vampire. ... Major Frank Bernard Halford, (1894â€“1955), was an aircraft engine designer. ... Cutaway Goblin II A cutaway diagram of the internal workings of the de Havilland Goblin, as fitted to the Vampire. ...

A cutaway of the Junkers Jumo 004 engine.

In the UK, their first axial-flow engine, the Metrovick F.2, ran in 1941 and was first flown in 1943. Although more powerful than the centrifugal designs at the time, the Ministry considered its complexity and unreliability a drawback in wartime. The work at Metrovick led to the Armstrong Siddeley Sapphire engine which would be built in the US as the J65. The Metrovick F.2 was one of the earliest jet engines, and the first British design to be based on an axial compressor. ... The Sapphire was a jet engine produced by Armstrong Siddeley in the 1950s. ...

Following the end of the war the German jet aircraft and jet engines were extensively studied by the victorious allies and contributed to work on early Soviet and US jet fighters. The legacy of the axial-flow engine is seen in the fact that practically all jet engines on fixed wing aircraft have had some inspiration from this design. Fixed-wing aircraft is a term used to refer to monoplanes, biplanes and triplanes, in fact all conventional aircraft that are neither balloons, airships, autogyros, helicopters or tiltrotors. ...

Centrifugal-flow engines have improved since their introduction. With improvements in bearing technology the shaft speed of the engine was increased, greatly reducing the diameter of the centrifugal compressor. The short engine length remains an advantage of this design, particularly for use in helicopters where overall size is more important than frontal area. Also, its engine components are robust; axial-flow compressors are more liable to foreign object damage. For the song by Green Day, see Dookie FOD damage to the compressor blades of a Honeywell LTS101 turboshaft engine on a Bell 222, caused by a small bolt that passed through the protective inlet screen. ...

Although German designs were more advanced aerodynamically, the combination of simplicity and advanced British metallurgy meant that Whittle-derived designs were far more reliable than their German counterparts. British engines also were licensed widely in the US (see Tizard Mission),and were sold to the USSR who reverse engineered them with the Nene going on to power the famous MiG-15. American and Soviet designs, independent axial-flow types for the most part, would not come fully into their own until the 1960s, although the General Electric J47 provided excellent service in the F-86 Sabre in the 1950s. Sir Henry Tizard, instigator and leader of The Tizard Mission In the late September 1940 during the Battle of Britain in the Second World War, a delegation arrived from the UK in the United States on a mission instigated by Henry Tizard, known as the Tizard Mission. ... The Nene or RB.41, was Rolls-Royces third jet engine to enter production, designed and built in an astonishingly short five month period in 1944, first running on October 27th, 1944. ... The Mikoyan-Gurevich MiG-15 (NATO reporting name Fagot) was a jet fighter developed for the USSR. History Design began under the bureau designation I-310, which first flew in 1947. ... The J47 turbojet was developed from the previous J35 engine and was first flown in May 1948. ... The North American F-86 Sabre (sometimes called the Sabrejet) was a transonic combat aircraft developed for the US Air Force. ...

Relentless improvements in the turboprop pushed the piston engine out of the mainstream entirely, leaving it serving only the smallest general aviation designs, and some use in drone aircraft. The ascension of the jet engine to almost universal use in aircraft took well under twenty years. A schematic diagram showing the operation of a turboprop engine. ... A general aviation scene at Kemble Airfield, England. ... UAVs in a hangar An unmanned aerial vehicle (UAV) is an aircraft with no onboard pilot. ...

However, the story was not quite at an end, for the efficiency of turbojet engines was still rather worse than piston engines, but by the 1970s with the advent of high bypass jet engines, an innovation not foreseen by the early commentators like Edgar Buckingham, at high speeds and high altitudes that seemed absurd to them, only then did the fuel efficiency finally exceeded that of the best piston and propeller engines,[9] and the dream of fast, safe, economical travel around the world finally arrived, and their dour, if well founded for the time, predictions that jet engines would never amount to much, killed forever. CFM56-3 turbofan, lower half, side view. ...

## Types

There are a large number of different types of jet engines, all of which achieve propulsion from a high speed exhaust jet.

Water jet For propelling boats; squirts water out the back through a nozzle Can run in shallow water, high acceleration, no risk of engine overload (unlike propellers), less noise and vibration, highly manoeuvrable at all boat speeds, high speed efficiency, less vulnerable to damage from debris, very reliable, more load flexibility, less harmful to wildlife Can be less efficient than a propeller at low speed, more expensive, higher weight in boat due to entrained water, will not perform well if boat is heavier than the jet is sized for
Motorjet Most primitive airbreathing jet engine. Essentially a supercharged piston engine with a jet exhaust. Higher exhaust velocity than a propeller, offering better thrust at high speed Heavy, inefficient and underpowered
Turbojet Generic term for simple turbine engine Simplicity of design, efficient at supersonic speeds (~M2) A basic design, misses many improvements in efficiency and power for subsonic flight, relatively noisy.
Low-bypass Turbofan One- or two-stage fan added in front bypasses a proportion of the air through a bypass chamber surrounding the core. Compared with its turbojet ancestor, this allows for more efficient operation with somewhat less noise. This is the engine of high-speed military aircraft, some smaller private jets, and older civilian airliners such as the Boeing 707, the McDonnell Douglas DC-8, and their derivatives. As with the turbojet, the design is aerodynamic, with only a modest increase in diameter over the turbojet required to accommodate the bypass fan and chamber. It is capable of supersonic speeds with minimal thrust drop-off at high speeds and altitudes yet still more efficient than the turbojet at subsonic operation. Noisier and less efficient than high-bypass turbofan, with less static (Mach 0) thrust. Added complexity to accommodate dual shaft designs. More inefficient than a turbojet around M2 due to higher cross-sectional area.
High-bypass Turbofan First stage compressor drastically enlarged to provide bypass airflow around engine core, and it provides significant amounts of thrust. Compared to the low-bypass turbofan and no-bypass turbojet, the high-bypass turbfan works on the principle of moving a great deal of air somewhat faster, rather than a small amount extremely fast. This translates into less noise. Most common form of jet engine in civilian use today- used in airliners like the Boeing 747, most 737s, and all Airbus aircraft. Quieter due to greater mass flow and lower total exhaust speed, more efficient for a useful range of subsonic airspeeds for same reason, cooler exhaust temperature. High bypass variants exhibit good fuel economy. Greater complexity (additional ducting, usually multiple shafts) and the need to contain heavy blades. Fan diameter can be extremely large, especially in high bypass turbofans such as the GE90. More subject to FOD and ice damage. Top speed is limited due to the potential for shockwaves to damage engine. Thrust lapse at higher speeds, which necessitates huge diameters and introduces additional drag.
Rocket Carries all propellants and oxidants on-board, 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, no complex air inlet, high compression ratio, very high speed (hypersonic) exhaust, good cost/thrust ratio, fairly easy to test, works in a vacuum-indeed works best exoatmospheric which is kinder on vehicle structure at high speed, fairly small surface area to keep cool, and no turbine in hot exhaust stream. Needs lots of propellant- very low specific impulse — typically 100-450 seconds. Extreme thermal stresses of combustion chamber can make reuse harder. Typically requires carrying oxidiser on-board which increases risks. Extraordinarily noisy.
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 air-breathing jets (thrust/weight ratio up to 30 at optimum speed), cooling much easier than turbojets as no turbine blades to cool. Must have a high initial speed to function, inefficient at slow speeds due to poor compression ratio, difficult to arrange shaft power for accessories, usually limited to a small range of speeds, intake flow must be slowed to subsonic speeds, noisy, fairly difficult to test, finicky to keep lit.
Turboprop (Turboshaft similar) Strictly not a jet at all — a gas turbine engine is used as powerplant to drive propeller shaft (or rotor in the case of a helicopter) High efficiency at lower subsonic airspeeds (300 knots plus), high shaft power to weight Limited top speed (aeroplanes), somewhat noisy, complex transmission
Propfan/Unducted Fan Turboprop engine drives one or more propellers. Similar to a turbofan without the fan cowling. Higher fuel efficiency, potentially 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 is compressed and combusted intermittently instead of continuously. Some designs use valves. Very simple design, commonly used on model aircraft Noisy, inefficient (low compression ratio), works poorly on a large scale, valves on valved designs wear out quickly
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
Air-augmented rocket 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, very noisy, thrust/weight ratio is similar to ramjets.
Scramjet Similar to a ramjet without a diffuser; airflow through the entire engine remains supersonic Few mechanical parts, can operate at very high Mach numbers (Mach 8 to 15) with good efficiencies[10] Still in development stages, must have a very high initial speed to function (Mach >6), cooling difficulties, very poor thrust/weight ratio (~2), extreme aerodynamic complexity, airframe difficulties, testing difficulties/expense
Turborocket A turbojet where an additional oxidizer such as oxygen is added to the airstream to increase maximum 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. Much heavier than simple rockets.
Precooled jets / LACE Intake air is chilled to very low temperatures at inlet in a heat exchanger before passing through a ramjet or turbojet engine. Can be combined with a rocket engine for orbital insertion. Easily tested on ground. Very high thrust/weight ratios are possible (~14) together with good fuel efficiency over a wide range of airspeeds, mach 0-5.5+; this combination of efficiencies may permit launching to orbit, single stage, or very rapid, very long distance intercontinental travel. Exists only at the lab prototyping stage. Examples include RB545, SABRE, ATREX. Requires liquid hydrogen fuel which has very low density and heavily insulated tankage.

## General physical principles

All jet engines are reaction engines that generate thrust by emitting a jet of fluid rearwards at relatively high speed. The forces on the inside of the engine needed to create this jet give a strong thrust on the engine which pushes the craft forwards.

Jet engines make their jet from propellant from tankage that is attached to the engine (as in a 'rocket') or from sucking in external fluid (very typically air) and expelling it at higher speed; or more commonly, a combination of the two sources.

### Thrust

The motion impulse of the engine is equal to the fluid mass multiplied by the speed at which the engine emits this mass:

I = m c

where m is the fluid mass per second and c is the exhaust speed. In other words, a vehicle gets the same thrust if it outputs a lot of exhaust very slowly, or a little exhaust very quickly.

However, when a vehicle moves with certain velocity v, the fluid moves towards it, creating an opposing ram drag at the intake:

m v

Most types of jet engine have an intake, which provides the bulk of the fluid exiting the exhaust. Conventional rocket motors, however, do not have an intake, the oxidizer and fuel both being carried within the vehicle. Therefore, 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.

The jet engine with an intake is only useful if the velocity of the gas from the engine, c, is greater than the vehicle velocity, v, as the net engine thrust is the same as if the gas were emitted with the velocity c-v. So the thrust is actually equal to

S = m (c-v)

### Energy efficiency

Dependence of the energy efficiency (η) upon the vehicle speed/exhaust speed ratio (v/c) for air-breathing jet and rocket engines

Energy efficiency (η) of jet engines has two main components, cycle efficiency (ηc)- how efficiently the engine can accelerate the jet, and propulsive efficiency (ηp)-how much of the kinetic energy of the jet ends up in the vehicle body rather than being dissipated behind the vehicle.

The overall energy efficiency η is simply:

η = ηpηc

For all jet engines the propulsive efficiency is highest when the engine emits an exhaust jet at a speed that is the same as, or nearly the same as, the vehicle velocity as this gives the smallest residual kinetic energy. The exact formula for air-breathing engines moving at speed v with an exhaust velocity c is given in the literature as:[11] is In aircraft design, overall propulsive efficiency is the efficiency, in percent, with which the energy contained in fuel is converted into propulsive energy. ...

$eta_p = frac{2}{1 + frac{c}{v}}$

And for a rocket:

$eta_p= frac {2 frac {v} {c}} {1 + ( frac {v} {c} )^2 }$[12]

In addition to propulsive efficiency, another factor is cycle efficiency; essentially a jet engine is typically a form of heat engine. Heat engine efficiency is determined by the ratio of temperatures that are reached in the engine to that they are exhausted at from the nozzle, which in turn is limited by the overall pressure ratio that can be achieved. Cycle efficiency is highest in rocket engines (~60+%), as they can achieve extremely high combustion temperatures and can have very large, energy efficient nozzles. Cycle efficiency in turbojet and similar is nearer to 30%, the practical combustion temperatures and nozzle efficiencies are much lower. Overall Pressure Ratio is an engine cycle term used in Gas Turbine Engineering and is defined as the ratio of the stagnation pressure at combustor entry, to that at compression entry. ...

Specific impulse as a function of speed for different jet types with kerosene fuel (hydrogen Isp would be about twice as high). Although efficiency plummets with speed, greater distances are covered, it turns out that efficiency per unit distance (per km or mile) is roughly independent of speed for jet engines as a group; however airframes become inefficient at supersonic speeds

Image File history File links Download high resolution version (800x648, 58 KB) Summary From wikibooks JetPropulsion/Specific Impulse Licensing Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1. ... Image File history File links Download high resolution version (800x648, 58 KB) Summary From wikibooks JetPropulsion/Specific Impulse Licensing Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1. ... Specific impulse (usually abbreviated Isp) is a way to describe the efficiency of rocket and jet engines. ...

### Fuel/propellant consumption

A closely related (but different) concept to energy efficiency is the rate of consumption of propellant mass. Propellant consumption in jet engines is measured by Specific Fuel Consumption, Specific impulse or Effective exhaust velocity. They all measure the same thing, specific impulse and effective exhaust velocity are strictly proportional, whereas specific fuel consumption is inversely proportional to the others. Specific fuel consumption, often shortened to SFC, is an engineering term that is used to describe the fuel efficiency of an engine design w/ a mechanical output. ... Specific impulse (usually abbreviated Isp) is a way to describe the efficiency of rocket and jet engines. ...

For airbreathing engines such as turbojets energy efficiency and propellant (fuel) efficiency are much the same thing, since the propellant is a fuel and the source of energy. In rocketry, the propellant is also the exhaust, and this means that a high energy propellant gives better propellant efficiency but lower energy efficiency.

### Comparison of types

Comparative suitability for (left to right) turboshaft, low bypass and turbojet to fly at 10 km altitude in various speeds. Horizontal axis - speed, m/s. Vertical axis displays engine efficiency.

Turboprops obtain little thrust from jet effect, but are useful for comparison. They are gas turbine engines that have a rotating fan that takes and accelerates the large mass of air but by a relatively small change in speed. This low speed limits the speed of any propeller driven airplane. When the plane speed exceeds this limit, propellers no longer provide any thrust (c-v < 0). However, because they accelerate a large mass of air, turboprops are very efficient. Image File history File links JetSuitabilityEn. ... Image File history File links JetSuitabilityEn. ... Schematic diagram showing the operation of a simplified turboshaft engine. ... Schematic diagram of high-bypass turbofan engine CFM56-3 turbofan, lower half, side view. ... For the transportation company in southern China, see TurboJET. Turbojets are the oldest kind of general purpose jet engines. ... A schematic diagram showing the operation of a turboprop engine. ...

turbojets and other similar engines accelerate a much smaller mass of the air and burned fuel, but they emit it at the much higher speeds possible with a de Laval nozzle. This is why they are suitable for supersonic and higher speeds. For the transportation company in southern China, see TurboJET. Turbojets are the oldest kind of general purpose jet engines. ... Diagram of a de Laval nozzle, showing approximate flow velocity increasing from green to red in the direction of flow A de Laval nozzle (or convergent-divergent nozzle, CD nozzle or con-di nozzle) is a tube that is pinched in the middle, making an hourglass-shape. ...

Low bypass turbofans have the mixed exhaust of the two air flows, running at different speeds (c1 and c2). The thrust of such engine is Schematic diagram of high-bypass turbofan engine CFM56-3 turbofan, lower half, side view. ...

S = m1 (c1 - v) + m2 (c2 - v)

where m1 and m2 are the air masses, being blown from the both exhausts. Such engines are effective at lower speeds, than the pure jets, but at higher speeds than the turboshafts and propellers in general. For instance, at the 10 km altitude, turboshafts are most effective at about 0.4 mach, low bypass turbofans become more effective at about 0.75 mach and turbojets become more effective as mixed exhaust engines when the speed approaches 2-3 mach - 2-3x the speed of sound.

Rocket engines have extremely high exhaust velocity and thus are best suited for high speeds (hypersonic) and great altitudes. At any given throttle, the thrust and efficiency of a rocket motor improves slightly with increasing altitude (because the back-pressure falls thus increasing net thrust at the nozzle exit plane), whereas with a turbojet (or turbofan) the falling density of the air entering the intake (and the hot gases leaving the nozzle) causes the net thrust to decrease with increasing altitude. Rocket engines are more efficient than even scramjets above roughly Mach 15.[13] RS-68 being tested at NASAs Stennis Space Center, note the relatively transparent exhaust, this is due to this engines use of hydrogen fuel A rocket engine is a reaction engine that takes all its reaction mass from within tankage and forms it into a high speed jet... Boeing X-43 at Mach 7 In aerodynamics, hypersonic speeds are speeds that are highly supersonic. ...

### Noise

Noise is due to shockwaves that form when the exhaust jet interacts with the external air.

The intensity of the noise is proportional to the thrust as well as proportional to the fourth power of the jet velocity.

Generally then, the lower speed exhaust jets emitted from engines such as high bypass turbofans are the quietest, whereas the fastest jets are the loudest.

Although some variation in jet speed can often be arranged from a jet engine (such as by throttling back and adjusting the nozzle) it is difficult to vary the jet speed from an engine over a very wide range. Therefore since engines for supersonic vehicles such as Concorde, military jets and rockets inherently need to have supersonic exhaust at top speed, so these vehicles are especially noisy even at low speeds.

## Common types

### Turbojet engines

A turbojet engine, in its simplest form is simply a gas turbine with a nozzle attached
Main article: Turbojet

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, but in different sections of the engine. 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. Today Internal combustion engines in cars, trucks, motorcycles, construction machinery and many others, most commonly use a four-stroke cycle. ... In physics, mechanical efficiency is the effectiveness of a machine and is defined as Efficiency is often indicated by a percentage, the efficiency of an ideal machine is 100%. Due to the fact that energy cannot emerge from nothing and the Second law of thermodynamics which states that the quality... Overall Pressure Ratio is an engine cycle term used in Gas Turbine Engineering and is defined as the ratio of the stagnation pressure at combustor entry, to that at compression entry. ...

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 turbojet engine makes it efficient at high speeds (especially supersonic speeds) and high altitudes; Concorde used this type for example. 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. For other uses, see Mass (disambiguation). ... For other uses, see Propeller (disambiguation). ... A United States Navy F/A-18E/F Super Hornet in transonic flight. ... For other uses, see Concorde (disambiguation). ... This machine has a single-stage centrifugal compressor and turbine, a recuperator, and foil bearings. ... For other uses, see Propeller (disambiguation). ... A schematic diagram showing the operation of a turboprop engine. ... Components of a typical, four stroke cycle, DOHC piston engine. ...

The turbojet described above is a single-spool design, in which a single shaft connects the turbine to the compressor. Two spool designs have two concentric turbine-compressor systems, that spin independently with the turbine and compressors for each section connected from opposite ends of the engine via concentric shafts. This allows for a higher compression ratio as well as improved compressor stability during engine throttle movements. Three spool designs also exist. Concentric objects share the same center, axis or origin with one inside the other. ...

### Turbofan engines

Main article: Turbofan

Most modern jet engines are actually turbofans, where the low pressure compressor acts as a fan, supplying supercharged air not only to the engine core, but to a bypass duct. The bypass airflow either passes to a separate 'cold nozzle' or mixes with low pressure turbine exhaust gases, before expanding through a 'mixed flow nozzle'. Schematic diagram of high-bypass turbofan engine CFM56-3 turbofan, lower half, side view. ...

Turbofans are used for airliners because they give an exhaust speed that is better matched for subsonic airliners, at airliners flight speed conventional turbojet engines generate an exhaust that ends up travelling very fast backwards, and this wastes energy. By emitting the exhaust so that it ends up travelling more slowly, better fuel consumption is achieved. In addition, the lower exhaust speed gives much lower noise.

In the 1960s there was little difference between civil and military jet engines, apart from the use of afterburning in some (supersonic) applications. Civil turbofans today have a low exhaust speed (low specific thrust -net thrust divided by airflow) to keep jet noise to a minimum and to improve fuel efficiency. Consequently the bypass ratio (bypass flow divided by core flow) is relatively high (ratios from 4:1 up to 8:1 are common). Only a single fan stage is required, because a low specific thrust implies a low fan pressure ratio. For other uses of afterburner, see Afterburner (disambiguation). ... Specific Thrust is a term used in Gas Turbine Engineering to show the relative bulk of a jet engine (e. ... In aeronautical engineering, and jet engine design in particular, bypass ratio is a common measurement that compares the amount of air deliberately blown past the engine to that moving through the core. ...

Today's military turbofans, however, have a relatively high specific thrust, to maximize the thrust for a given frontal area, jet noise being of less concern in military uses relative to civil uses. Multistage fans are normally needed to reach the relatively high fan pressure ratio needed for high specific thrust. Although high turbine inlet temperatures are often 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, be it a turbojet or a mixed turbofan, is:

$F_n = dot{m}(V_{jfe} - V_a),$

where:

$dot{m} = ,$ intake mass flow rate Mass flow is the movement of substances at equal rates or as a single body. ...

$V_{jfe} =,$ fully expanded jet velocity (in the exhaust plume)

$V_a =,$ aircraft flight velocity

While the $dot{m}.V_{jfe},$ term represents the gross thrust of the nozzle, the $dot{m}. V_a,$ term represents the ram drag of the intake.

### Rocket engines

Main article: Rocket engine

The third most common form of jet engine is the rocket engine. RS-68 being tested at NASAs Stennis Space Center, note the relatively transparent exhaust, this is due to this engines use of hydrogen fuel A rocket engine is a reaction engine that takes all its reaction mass from within tankage and forms it into a high speed jet...

This is used for launching satellites, space exploration and manned access, and permitted landing on the moon in 1969. Space exploration is the use of astronomy and space technology to explore outer space. ... The first moon landing by a human was that of American Neil Armstrong, Commander of the Apollo 11 mission. ...

However, the high exhaust speed and the heavier, oxidiser-rich propellant results in more propellant use than turbojets, and their use is largely restricted to very high altitudes, very high speeds, or where very high accelerations are needed as rocket engines themselves have a very high thrust-to-weight ratio. Thrust-to-weight ratio (where weight means weight at the Earths surface) is a dimensionless parameter characteristic of rocket and jet engines, and of vehicles propelled by such engines (typically space launch vehicles and jet aircraft). ...

An approximate equation for the net thrust of a rocket engine is:

$F = dot m g_0 I_{sp-vac} - A_e P ;$

Where F is the thrust, Isp(vac) is the specific impulse, g0 is a standard gravity, Ae is the area of the exhaust bell at the exit and P is the atmospheric pressure. Specific impulse (usually abbreviated Isp) is a way to describe the efficiency of rocket and jet engines. ... g (also gee, g-force or g-load) is a non-SI unit of acceleration defined as exactly 9. ...

## Major components

The major components of a jet engine are similar across the major different types of engines, although not all engine types have all components. The major parts include: Image File history File links Turbofan_operation. ...

• Cold Section:
• Air intake (Inlet) — 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 making more efficient use of the shock wave system.
• Compressor or Fan — The compressor is made up of stages. Each stage consists of vanes which rotate, and stators which remain stationary. As air is drawn deeper through the compressor, its heat and pressure increases. Energy is derived from the turbine (see below), passed along the shaft.
• Bypass ducts much of the thrust of essentially all modern jet engines comes from air from the front compressor that bypasses the combustion chamber and gas turbine section that leads directly to the nozzle or afterburner (where fitted).
• Common:
• Shaft — The shaft connects 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.
• Hot section:
• Combustor or Can or Flameholders or Combustion Chamber — This is a chamber where fuel is continuously burned in the compressed air.
• Turbine — The turbine is a series of bladed discs that act like a windmill, gaining energy from the hot gases leaving the combustor. Some of this energy is used to drive the compressor, and in some turbine engines (ie turboprop, turboshaft or turbofan engines), energy is extracted by additional turbine discs and used to drive devices such as propellers, bypass fans or helicopter rotors. One type, a free turbine, is configured such that the turbine disc driving the compressor rotates independently of the discs that power the external components. Relatively cool air, bled from the compressor, may be used to cool the turbine blades and vanes, to prevent them from melting.
• 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 than the latter.

The various components named above have constraints on how they are put together to generate the most efficiency or performance. The performance and efficiency of an engine can never be taken in isolation; for example fuel/distance efficiency of a supersonic jet engine maximises at about mach 2, whereas the drag for the vehicle carrying it is increasing as a square law and has much extra drag in the transonic region. The highest fuel efficiency for the overall vehicle is thus typically at Mach ~0.85. A frame of reference in physics is a set of axes which enable an observer to measure the aspect, position and motion of all points in a system relative to the reference frame. ... A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume. ... For other uses, see Fan. ... A ring of can type combustors circles the mid section of this gas turbine. ... A flame holder is a component of a jet engine designed to help maintain continual combustion. ... A Siemens steam turbine with the case opened. ... SR-71 in flight with J58 on full afterburner An afterburner is an additional component added to some jet engines, primarily those on military aircraft. ... Rocket Nozzle A nozzle is a mechanical device designed to control the characteristics of a fluid flow as it exits from an enclosed chamber into some medium. ... Diagram of a de Laval nozzle, showing approximate flow velocity increasing from green to red in the direction of flow A de Laval nozzle (or convergent-divergent nozzle, CD nozzle or con-di nozzle) is a tube that is pinched in the middle, making an hourglass-shape. ...

For the engine optimisation for its intended use, 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 // Introduction Inlet cones (sometimes called shock cones) are a component of some supersonic aircraft. ...

#### Subsonic inlets

Pitot intake operating modes

Pitot intakes are the dominant type for subsonic applications. A subsonic pitot inlet is little more than a tube with an aerodynamic fairing around it. Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... A Pitot tube is a measuring instrument used to measure fluid flow. ...

At zero airspeed (i.e., rest), air approaches the intake from a multitude of directions: from directly ahead, radially, or even from behind the plane of the intake lip.

At low airspeeds, the streamtube approaching the lip is larger in cross-section than the lip flow area, whereas at the intake design flight Mach number the two flow areas are equal. At high flight speeds the streamtube is smaller, with excess air spilling over the lip.

Beginning around 0.85 Mach, shock waves can occur as the air accelerates through the intake throat.

Careful radiusing of the lip region is required to optimize intake pressure recovery (and distortion) throughout the flight envelope.

#### Supersonic inlets

Supersonic intakes exploit shock waves to decelerate the airflow to a subsonic condition at compressor entry.

There are basically two forms of shock waves:

1) Normal shock waves lie perpendicular to the direction of the flow. These form sharp fronts and shock the flow to subsonic speeds. Microscopically the air molecules smash into the subsonic crowd of molecules like alpha rays. Normal shock waves tend to cause a large drop in stagnation pressure. Basically, the higher the supersonic entry Mach number to a normal shock wave, the lower the subsonic exit Mach number and the stronger the shock (i.e. the greater the loss in stagnation pressure across the shock wave). An alpha particle is deflected by a magnetic field Alpha particles or alpha rays are a form of particle radiation which are highly ionizing and have low penetration. ... Stagnation pressure is the pressure at a stagnation point in a fluid flow, where the kinetic energy is converted into pressure energy. ...

2) Conical (3-dimensional) and oblique shock waves (2D) are angled rearwards, like the bow wave on a ship or boat, and radiate from a flow disturbance such as a cone or a ramp. For a given inlet Mach number, they are weaker than the equivalent normal shock wave and, although the flow slows down, it remains supersonic throughout. Conical and oblique shock waves turn the flow, which continues in the new direction, until another flow disturbance is encountered downstream.

Note: Comments made regarding 3 dimensional conical shock waves, generally also apply to 2D oblique shock waves.

A sharp-lipped version of the pitot intake, described above for subsonic applications, performs quite well at moderate supersonic flight speeds. A detached normal shock wave forms just ahead of the intake lip and 'shocks' the flow down to a subsonic velocity. However, as flight speed increases, the shock wave becomes stronger, causing a larger percentage decrease in stagnation pressure (i.e. poorer pressure recovery). An early US supersonic fighter, the F-100 Super Sabre, used such an intake. F-100A Super Sabre The North American F-100 Super Sabre was a jet fighter aircraft that served with the USAF from 1954 to 1971 and with the ANG until 1979. ...

An unswept lip generate a shock wave, which is reflected multiple times in the inlet. The more reflections before the flow gets subsonic, the better pressure recovery

More advanced supersonic intakes, excluding pitots: Image File history File links Download high resolution version (896x622, 29 KB)Author:Burbank Source:Self Intake pressure recovery, showing impact of more complex shock wave systems File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Download high resolution version (896x622, 29 KB)Author:Burbank Source:Self Intake pressure recovery, showing impact of more complex shock wave systems File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...

a) exploit a combination of conical shock wave/s and a normal shock wave to improve pressure recovery at high supersonic flight speeds. Conical shock wave/s are used to reduce the supersonic Mach number at entry to the normal shock wave, thereby reducing the resultant overall shock losses.

b) have a design shock-on-lip flight Mach number, where the conical/oblique shock wave/s intercept the cowl lip, thus enabling the streamtube capture area to equal the intake lip area. However, below the shock-on-lip flight Mach number, the shock wave angle/s are less oblique, causing the streamline approaching the lip to be deflected by the presence of the cone/ramp. Consequently, the intake capture area is less than the intake lip area, which reduces the intake airflow. Depending on the airflow characteristics of the engine, it may be desirable to lower the ramp angle or move the cone rearwards to refocus the shockwaves onto the cowl lip to maximise intake airflow.

c) are designed to have a normal shock in the ducting downstream of intake lip, so that the flow at compressor/fan entry is always subsonic. However, if the engine is throttled back, there is a reduction in the corrected airflow of the LP compressor/fan, but (at supersonic conditions) the corrected airflow at the intake lip remains constant, because it is determined by the flight Mach number and intake incidence/yaw. This discontinuity is overcome by the normal shock moving to a lower cross-sectional area in the ducting, to decrease the Mach number at entry to the shockwave. This weakens the shockwave, improving the overall intake pressure recovery. So, the absolute airflow stays constant, whilst the corrected airflow at compressor entry falls (because of a higher entry pressure). Excess intake airflow may also be dumped overboard or into the exhaust system, to prevent the conical/oblique shock waves being disturbed by the normal shock being forced too far forward by engine throttling.

Many second generation supersonic fighter aircraft featured an inlet cone, which was used to form the conical shock wave. This type of inlet cone is clearly seen at the very front of the English Electric Lightning and MiG-21 aircraft, for example. // Introduction Inlet cones (sometimes called shock cones) are a component of some supersonic aircraft. ... The English Electric Lightning (later the BAC Lightning) was a supersonic fighter aircraft of the Cold War era, particularly remembered for its great speed and natural metal exterior. ... Mikoyan-Gurevich MiG-21 (NATO reporting name Fishbed) is a fighter aircraft, originally built by the Mikoyan and Gurevich Design Bureau in the Soviet Union. ...

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. The Lockheed F-104 Starfighter was a single-engined, high-performance, supersonic interceptor aircraft that served with the United States Air Force (USAF) from 1958 until 1967. ... The BAC TSR-2 was an ill-fated cold war project developed by the British Aircraft Corporation (BAC) in the early 1960s. ...

Some intakes are biconic; that is they feature two conical surfaces: the first cone is supplemented by a second, less oblique, conical surface, which generates an extra conical shockwave, radiating from the junction between the two cones. A biconic intake is usually more efficient than the equivalent conical intake, because the entry Mach number to the normal shock is reduced by the presence of the second conical shock wave. It has been suggested that this article or section be merged into Shape. ...

A very sophisticated conical intake was featured on the SR-71's Pratt & Whitney J58s that could move a conical spike fore and aft within the engine nacelle, preventing the shockwave formed on the spike from entering the engine and stalling the engine, while keeping it close enough to give good compression. Movable cones are uncommon. The Lockheed SR-71, unofficially known as the Blackbird, is a long-range, advanced, strategic reconnaissance aircraft developed from the Lockheed A-12 and YF-12A aircraft by Lockheeds Skunk works, which was also responsible for the U-2 and many other advanced aircraft. ... The Pratt & Whitney J58 (also known as the JT11D) was the jet engine used on the Lockheed SR-71 Blackbird. The J58 produced 32,000 lbf (142 kN) of thrust. ... // Introduction Inlet cones (sometimes called shock cones) are a component of some supersonic aircraft. ...

A more sophisticated design than cones is to angle the intake so that one of its edges forms a ramp. An oblique shockwave will form at the start of the ramp. The Century Series of US jets featured several variants of this approach, usually with the ramp at the outer vertical edge of the intake, which was then angled back inward towards the fuselage. Typical examples include the Republic F-105 Thunderchief and F-4 Phantom. The Century series aircraft were a series of early US supersonic jet fighters built for the United States Air Force during the 1950s and early 1960s. ... The Republic F-105 Thunderchief, commonly known as the Thud by its crews, was a single-seat supersonic fighter-bomber used by the United States Air Force. ... The F-4 Phantom II (simply F-4 Phantom after 1990) is a two-place (tandem), supersonic, long-range, all-weather fighter-bomber built by McDonnell Douglas Corporation. ...

Concorde intake operating modes

Later this evolved so that the ramp was at the top horizontal edge rather than the outer vertical edge, with a pronounced angle downwards and rearwards. This design simplified the construction of intakes and allowed use of variable ramps to control 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 Concorde. Image File history File links Download high resolution version (938x592, 29 KB)Author:Burbank Source:Self Concorde intake operating modes File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Download high resolution version (938x592, 29 KB)Author:Burbank Source:Self Concorde intake operating modes File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... The Grumman F-14 Tomcat is a supersonic, twin-engine, two-seat, variable geometry wing aircraft. ... The Panavia Tornado is a family of twin-engine combat aircraft, which was jointly developed by the United Kingdom, West Germany and Italy. ... For other uses, see Concorde (disambiguation). ...

From another point of view, like in a supersonic nozzle the corrected (or non-dimensional) flow has to be the same at the intake lip, at the intake throat and at the turbine. One of this three can be fixed. For inlets the throat is made variable and some air is bypassed around the turbine and directly fed into the afterburner. Unlike in a nozzle the inlet is either unstable or inefficient, because a normal shock wave in the throat will suddenly move to the lip, thereby increasing the pressure at the lip, leading to drag and reducing the pressure recovery, leading to turbine surge and the loss of one SR-71. Corrected Flow is the mass flow that would pass through a device (e. ... The Lockheed SR-71, unofficially known as the Blackbird, is a long-range, advanced, strategic reconnaissance aircraft developed from the Lockheed A-12 and YF-12A aircraft by Lockheeds Skunk works, which was also responsible for the U-2 and many other advanced aircraft. ...

### Compressors

Axial compressors
Compressor stage GE J79

Axial compressors rely on spinning blades that have aerofoil sections, similar to aeroplane wings. As with aeroplane wings in some conditions the blades can stall. If this happens, the airflow around the stalled compressor can reverse direction violently. Each design of a compressor has an associated operating map of airflow versus rotational speed for characteristics peculiar to that type (see compressor map). Image File history File links Axial_compressor. ... Image File history File links Axial_compressor. ... Image File history File links Compressor_Stage_GE_J79. ... Image File history File links Compressor_Stage_GE_J79. ... // Introduction Each compressor (or fan) in a gas turbine engine has an operating map. ...

At a given throttle condition, the compressor operates somewhere along the steady state running line. Unfortunately, this operating line is displaced during transients. Many compressors are fitted with anti-stall systems in the form of bleed bands or variable geometry stators to decrease the likelihood of surge. Another method 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 compressors are capable of running at quite high pressure ratios (e.g. 10:1), impeller stress considerations limit the pressure ratio that can be employed in high overall pressure ratio engine cycles.

Increasing overall pressure ratio implies raising the high pressure compressor exit temperature. This implies a higher high pressure shaft speed, to maintain the datum blade tip Mach number on the rear compressor stage. Stress considerations, however, may limit the shaft speed increase, causing the original compressor to throttle-back aerodynamically to a lower pressure ratio than datum.

Combustion chamber GE J79

Image File history File links Combustion_chamber_GE_J79. ... Image File history File links Combustion_chamber_GE_J79. ...

### Combustors

Great 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 (a mixture ratio of around 15:1), some of the compressor air is used to quench the exit temperature of the combustor to an acceptable level (an overall mixture ratio of between 45:1 and 130:1 is used[14]). Air used for combustion is considered to be primary airflow, while excess air used for cooling is called secondary airflow. Combustor configurations include can, annular, and can-annular. Stoichiometry (sometimes called reaction stoichiometry to distinguish it from composition stoichiometry) is the calculation of quantitative (measurable) relationships of the reactants and products in chemical reactions (chemical equations). ...

### Turbines

Turbine Stage GE J79

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. They take the form of impulse, reaction, or combination impulse-reaction shapes. Improved materials help to keep disc weight down. Bleed air in jet turbines is compressed air taken from within the engine, after the compressor stage(s) and before the fuel is injected in the burners. ... A superalloy, or high-performance alloy, is an alloy able to withstand extreme temperatures that would destroy conventional metals like steel and aluminum. ... An abradable coating is a coating made of an abradable material â€“ meaning if it rubs against a more abrasive material in motion, the former will be worn whereas the latter will face no wear. ... Stress is a measure of force per unit area within a body. ...

### Turbopumps

Main article: Turbopump

Turbopumps are centrifugal pumps which are spun by gas turbines and are used to raise the propellant pressure above the pressure in the combustion chamber so that it can be injected and burnt. Turbopumps are very commonly used with rockets, but ramjets and turbojets also have been known to use them. A turbopump can refer to either of two types of pump. ...

### Afterburners (reheat)

Main article: afterburner
Turbofan fitted with afterburner

Due to temperature limitations with the gas turbines, jet engines do not consume all the oxygen in the air ('run stoichiometric'). Afterburners burn the remaining oxygen after exiting the turbines, but usually do so inefficiently due to the low pressures typically found at this part of the jet engine; however this gains significant thrust, which can be useful. Engines intended for extended use with afterburners often have variable nozzles and other details. For other uses of afterburner, see Afterburner (disambiguation). ... In chemistry, stoichiometry is the study of the combination of elements in chemical reactions. ...

Afterburner GE J79

Image File history File links Download high resolution version (678x800, 96 KB) This picture may have usage restrictions - Afterburner GE J79 Source: Own picture File links The following pages link to this file: Jet engine ... Image File history File links Download high resolution version (678x800, 96 KB) This picture may have usage restrictions - Afterburner GE J79 Source: Own picture File links The following pages link to this file: Jet engine ...

### Nozzles

The primary objective of a nozzle is to expand the exhaust stream to atmospheric pressure, and form it into a high speed jet to propel the vehicle. For airbreathing engines, if the fully expanded jet has a higher speed than the aircraft's airspeed, then there is a net rearward momentum gain to the air and there will be a forward thrust on the airframe.

Simple convergent nozzles are used on many jet engines. If the nozzle pressure ratio is above the critical value (about 1.8:1) a convergent nozzle will choke, resulting in some of the expansion to atmospheric pressure taking place downstream of the throat (i.e. smallest flow area), in the jet wake. Although much of the gross thrust produced will still be from the jet momentum, additional (pressure) thrust will come from the imbalance between the throat static pressure and atmospheric pressure.

Many military combat engines incorporate an afterburner (or reheat) in the engine exhaust system. When the system is lit, the nozzle throat area must be increased, to accommodate the extra exhaust volume flow, so that the turbomachinery is unaware that the afterburner is lit. A variable throat area is achieved by moving a series of overlapping petals, which approximate the circular nozzle cross-section.

At high nozzle pressure ratios, the exit pressure is often above ambient and much of the expansion will take place downstream of a convergent nozzle, which is inefficient. Consequently, some jet engines (notably rockets) incorporate a convergent-divergent nozzle, to allow most of the expansion to take place against the inside of a nozzle to maximise thrust. However, unlike the fixed con-di nozzle used on a conventional rocket motor, when such a device is used on a turbojet engine it has to be a complex variable geometry device, to cope with the wide variation in nozzle pressure ratio encountered in flight and engine throttling. This further increases the weight and cost of such an installation. Figure 1: A de Laval nozzle, showing approximate flow velocity increasing from green to red in the direction of flow The main type of rocket engine nozzles used in modern rocket engines is the de Laval nozzle which is used to expand and accelerate the combustion gases, from burning propellants...

Variable Exhaust Nozzle, on the GE F404-400 low-bypass turbofan installed on a Boeing F/A-18 Hornet

The simpler of the two is the ejector nozzle, which creates an effective nozzle through a secondary airflow and spring-loaded petals. At subsonic speeds, the airflow constricts the exhaust to a convergent shape. As the aircraft speeds up, the two nozzles dilate, which allows the exhaust to form a convergent-divergent shape, speeding the exhaust gasses past Mach 1. More complex engines can actually use a tertiary airflow to reduce exit area at very low speeds. Advantages of the ejector nozzle are relative simplicity and reliability. Disadvantages are average performance (compared to the other nozzle type) and relatively high drag due to the secondary airflow. Notable aircraft to have utilized this type of nozzle include the SR-71, Concorde, F-111, and Saab Viggen Image File history File links File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... The Lockheed SR-71, unofficially known as the Blackbird, is a long-range, advanced, strategic reconnaissance aircraft developed from the Lockheed A-12 and YF-12A aircraft by Lockheeds Skunk works, which was also responsible for the U-2 and many other advanced aircraft. ... For other uses, see Concorde (disambiguation). ... A U.S. Air Force F-111 The General Dynamics F-111 Aardvark (the nickname was unofficial for most of its lifespan, but it was officially named Aardvark at its retirement ceremony for the United States Air Force) is a long-range strategic bomber, reconnaissance, and tactical strike aircraft. ... The Saab Viggen (Thunder Bolt or Bolt Â¹) or Saab 37 is a Swedish fighter aircraft manufactured between 1970 and 1990 to replace the Saab Draken (Dragon). ...

For higher performance, it is necessary to use an iris nozzle. This type uses overlapping, hydraulically adjustable "petals". Although more complex than the ejector nozzle, it has significantly higher performance and smoother airflow. As such, it is employed primarily on high-performance fighters such as the F-14, F-15, F-16, though is also used in high-speed bombers such as the B-1B. Some modern iris nozzles additionally have the ability to change the angle of the thrust (see thrust vectoring). Sailors prepare an F-14 Tomcat for flight on the aircraft carrier USS Abraham Lincoln, in support of Operation Iraqi Freedom (2003). ... The McDonnell Douglas F-15 Eagle is an all-weather, extremely maneuverable, tactical fighter designed to permit the U.S. Air Force to gain and maintain air superiority in aerial combat. ... The F-16 Fighting Falcon is a modern multi-role jet fighter aircraft built in the United States and used by dozens of countries all over the world. ... The Boeing IDS (formerly Rockwell) B-1B Lancer is a long-range strategic bomber in service with the USAF. Together with the B-52 Stratofortress, it is the backbone of the United Statess long-range bomber force. ... Thrust vectoring is the ability of an aircraft or other vehicle to direct the thrust from its main engine(s) in a direction other than parallel to the vehicles longitudinal axis. ...

Iris vectored thrust 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. The Convair F-106 Delta Dart has used such a nozzle design, as part of its overall design specification as a aerospace interceptor for high-altitude bomber interception, where conventional nozzle design would prove ineffective. Image File history File links Iris_vectoring_nozzle. ... Image File history File links Iris_vectoring_nozzle. ... A rocket is a vehicle, missile or aircraft which obtains thrust by the reaction to the ejection of fast moving exhaust from within a rocket engine. ... The Convair F-106A Delta Dart was the primary all-weather interceptor aircraft for the US Air Force from the 1960s through the late 1970s. ...

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. In aeronautical engineering, and jet engine design in particular, bypass ratio is a common measurement that compares the amount of air deliberately blown past the engine to that moving through the core. ... Schematic diagram of high-bypass turbofan engine CFM56-3 turbofan, lower half, side view. ... An F/A-18 Hornet at transonic speed and displaying the Prandtl-Glauert singularity just before reaching the speed of sound Mach number (Ma) (generally pronounced , sometimes or ) is the speed of an object moving through air, or any fluid substance, divided by the speed of sound through that substance...

### Thrust reversers

Main article: Thrust reversal

These either consist of cups that swing across the end of the nozzle and deflect the jet thrust forwards (as in the DC-9), or they are two panels behind the cowling that slide backward and reverse only the fan thrust (the fan produces the majority of the thrust). This is the case on many large aircraft such as the 747, C-17, KC-135, etc. KLM Fokker 70 with reverse thrust applied. ...

### Cooling systems

All jet engines require high temperature gas for good efficiency, typically achieved by combusting hydrocarbon or hydrogen fuel. Combustion temperatures can be as high as 3500K (5841F) in rockets, far above the melting point of most materials, but normal airbreathing jet engines use rather lower temperatures.

Cooling systems are employed to keep the temperature of the solid parts below the failure temperature.

#### Air systems

A complex around combustor and is injected into the rim of the rotating turbine disc. The cooling air then passes through complex passages within the turbine blades. After removing heat from the blade material, the air (now fairly hot) is vented, via cooling holes, into the main gas stream. Cooling air for the turbine vanes undergoes a similar process.

Cooling the leading edge of the blade can be difficult, because the pressure of the cooling air just inside the cooling hole may not be much different from that of the oncoming gas stream. One solution is to incorporate a cover plate on the disc. This acts as a centrifugal compressor to pressurize the cooling air before it enters the blade. Another solution is to use an ultra-efficient turbine rim seal to pressurize the area where the cooling air passes across to the rotating disc.

Seals are used to prevent oil leakage, control air for cooling and prevent stray air flows into turbine cavities.

A series of (e.g. labyrinth) seals allow a small flow of bleed air to wash the turbine disc to extract heat and, at the same time, pressurize the turbine rim seal, to prevent hot gases entering the inner part of the engine. Other types of seals are hydraulic, brush, carbon etc.

Small quantities of compressor bleed air are also used to cool the shaft, turbine shrouds, etc. Some air is also used to keep the temperature of the combustion chamber walls below critical. This is done using primary and secondary airholes which allow a thin layer of air to cover the inner walls of the chamber preventing excessive heating.

Exit temperature is dependent on the turbine upper temperature limit depending on the material. Reducing the temperature will also prevent thermal fatigue and hence failure. Accessories may also need their own cooling systems using air from the compressor or outside air.

Air from compressor stages is also used for heating of the fan, airframe anti-icing and for cabin heat. Which stage is bled from depends on the atmospheric conditions at that altitude.

### Fuel system

Apart from providing fuel to the engine, the fuel system is also used to control propeller speeds, compressor airflow and cool lubrication oil. Fuel is usually introduced by an atomized spray, the amount of which is controlled automatically depending on the rate of airflow.

So the sequence of events for increasing thrust is, the throttle opens and fuel spray pressure is increased, increasing the amount of fuel being burned. This means that exhaust gases are hotter and so are ejected at higher acceleration, which means they exert higher forces and therefore increase the engine thrust directly. It also increases the energy extracted by the turbine which drives the compressor even faster and so there is an increase in air flowing into the engine as well.

Obviously, it is the rate of the mass of the airflow that matters since it is the change in momentum (mass x velocity) that produces the force. However, density varies with altitude and hence inflow of mass will also vary with altitude, temperature etc. which means that throttle values will vary according to all these parameters without changing them manually.

This is why fuel flow is controlled automatically. Usually there are 2 systems, one to control the pressure and the other to control the flow. The inputs are usually from pressure and temperature probes from the intake and at various points through the engine. Also throttle inputs, engine speed etc. are required. These affect the high pressure fuel pump.

#### Fuel control unit (FCU)

This element is something like a mechanical computer. It determines the output of the fuel pump by a system of valves which can change the pressure used to cause the pump stroke, thereby varying the amount of flow.

Take the possibility of increased altitude where there will be reduced air intake pressure. In this case, the chamber within the FCU will expand which causes the spill valve to bleed more fuel. This causes the pump to deliver less fuel until the opposing chamber pressure is equivalent to the air pressure and the spill valve goes back to its position.

When the throttle is opened, it releases i.e. lessens the pressure which lets the throttle valve fall. The pressure is transmitted (because of a back-pressure valve i.e. no air gaps in fuel flow) which closes the FCU spill valves (as they are commonly called) which then increases the pressure and causes a higher flow rate.

The engine speed governor is used to prevent the engine from over-speeding. It has the capability of disregarding the FCU control. It does this by use of a diaphragm which senses the engine speed in terms of the centrifugal pressure caused by the rotating rotor of the pump. At a critical value, this diaphragm causes another spill valve to open and bleed away the fuel flow.

There are other ways of controlling fuel flow for example with the dash-pot throttle lever. The throttle has a gear which meshes with the control valve (like a rack and pinion) causing it to slide along a cylinder which has ports at various positions. Moving the throttle and hence sliding the valve along the cylinder, opens and closes these ports as designed. There are actually 2 valves viz. the throttle and the control valve. The control valve is used to control pressure on one side of the throttle valve such that it gives the right opposition to the throttle control pressure. It does this by controlling the fuel outlet from within the cylinder.

So for example, if the throttle valve is moved up to let more fuel in, it will mean that the throttle valve has moved into a position which allows more fuel to flow through and on the other side, the required pressure ports are opened to keep the pressure balance so that the throttle lever stays where it is.

At initial acceleration, more fuel is required and the unit is adapted to allow more fuel to flow by opening other ports at a particular throttle position. Changes in pressure of outside air i.e. altitude, speed of aircraft etc are sensed by an air capsule.

### Fuel pump

Fuel pumps are used to raise the fuel pressure above the pressure in the combustion chamber so that the fuel can be injected. Fuel pumps are usually driven by the main shaft, via gearing.

Turbopumps are very commonly used with liquid-fuelled rockets and rely on the expansion of an onboard gas through a turbine. A turbopump can refer to either of two types of pump. ...

Ramjet turbopumps use ram air expanding through a turbine.

### Engine starting system

The fuel system as explained above, is one of the 2 systems required for starting the engine. The other is the actual ignition of the air/fuel mixture in the chamber. Usually, an auxiliary power unit is used to start the engines. It has a starter motor which has a high torque transmitted to the compressor unit. When the optimum speed is reached, i.e. the flow of gas through the turbine is sufficient, the turbines take over. There are a number of different starting methods such as electric, hydraulic, pneumatic etc.

The electric starter works with gears and clutch plate linking the motor and the engine. The clutch is used to disengage when optimum speed is achieved. This is usually done automatically. The electric supply is used to start the motor as well as for ignition. The voltage is usually built up slowly as starter gains speed.

Some military aircraft need to be started quicker than the electric method permits and hence they use other methods such as a turbine starter. This is an impulse turbine impacted by burning gases from a cartridge. It is geared to rotate the engine and also connected to an automatic disconnect system. The cartridge is set alight electrically and used to turn the turbine.

Another turbine starter system is almost exactly like a little engine. Again the turbine is connected to the engine via gears. However, the turbine is turned by burning gases - usually the fuel is isopropyl nitrate stored in a tank and sprayed into a combustion chamber. Again, it is ignited with a spark plug. Everything is electrically controlled, such as speed etc. Isopropyl nitrate (IPN, 2-propyl nitrate) is a coloreless liquid monopropellant. ...

Most Commercial aircraft and large Military Transport airplanes usually use what is called an auxiliary power unit or APU. It is normally a small gas turbine. Thus, one could say that using such an APU is using a small gas turbine to start a larger one. High pressure air from the compressor section of the APU is bled off through a system of pipes to the engines where it is directed into the starting system. This "bleed air" is directed into a mechanism to start the engine turning and begin pulling in air. When the rotating speed of the engine is sufficient to pull in enough air to support combustion, fuel is introduced and ignited. Once the engine ignites and reaches idle speed, the bleed air is shut off. The APU exhaust at the tail end of an Airbus A380 An auxiliary power unit (APU) is a device on a vehicle whose purpose is to provide energy for functions other than propulsion. ...

The APUs on aircraft such as the Boeing 737 and Airbus A320 can be seen at the extreme rear of the aircraft. This is the typical location for an APU on most commercial airliners although some may be within the wing root (Boeing 727) or the aft fuselage (DC-9/MD80) as examples and some military transports carry their APU's in one of the main landing gear pods (C-141). The Boeing 737 is a short to medium range, single aisle, narrow body jet airliner. ... The Airbus A320 family of short-to-medium range commercial passenger aircraft are manufactured by Airbus S.A.S.. Family members include the A318, A319, A320, and A321, as well as the ACJ business jet. ... The Boeing 727 is a mid-size, narrow-body, three-engine commercial jet airliner. ... The Douglas DC-9 is a twin-engined jet airliner, first manufactured in 1965 and, in much modified form and under a succession of different names, still in production today as the Boeing 717. ... The Douglas DC-9 is a twin-engined jet airliner, first manufactured in 1965 and, in much modified form and under a succession of different names, still in production today as the Boeing 717. ... The Lockheed C-141 Starlifter is a military strategic airlifter in service with the US Air Force. ...

The APUs also provide enough power to keep the cabin lights, pressure and other systems on while the engines are off. The valves used to control the airflow are usually electrically controlled. They automatically close at a pre-determined speed. As part of the starting sequence on some engines fuel is combined with the supplied air and burned instead of using just air. This usually produces more power per unit weight.

Usually an APU is started by its own electric starter motor which is switched off at the proper speed automatically. When the main engine starts up and reaches the right conditions, this auxiliary unit is then switched off and disengages slowly.

Hydraulic pumps can also be used to start some engines through gears. The pumps are electrically controlled on the ground.

A variation of this is the APU installed in a Boeing F/A-18 Hornet; it is started by a hydraulic motor, which itself receives energy stored in an accumulator. This accumulator is recharged after the right engine is started and develops hydraulic pressure, or by a hand pump in the right hand main landing gear well.

### Ignition

Usually there are 2 igniter plugs in different positions in the combustion system. A high voltage spark is used to ignite the gases. The voltage is stored up from a low voltage supply provided by the starter system. It builds up to the right value and is then released as a high energy spark. Depending on various conditions, the igniter continues to provide sparks to prevent combustion from failing if the flame inside goes out. Of course, in the event that the flame does go out, there must be provision to relight. There is a limit of altitude and air speed at which an engine can obtain a satisfactory relight.

For example, the General Electric F404-400 uses one ignitor for the combustor and one for the afterburner; the ignition system for the A/B incorporates an ultraviolet flame sensor to activate the ignitor.

It should be noted that most modern ignition systems provide enough energy to be a lethal hazard should a person be in contact with the electrical lead when the system is activated, so team communication is vital when working on these systems.

### Lubrication system

A lubrication system serves to ensure lubrication of the bearings and to maintain sufficiently cool temperatures, mostly by eliminating friction.

The lubrication system as a whole should be able to prevent foreign material from entering the plane, and reaching the bearings, gears, and other moving parts. The lubricant must be able to flow easily at relatively low temperatures and not disintegrate or break down at very high temperatures.

Usually the lubrication system has subsystems that deal individually with the pressure of an engine, scavenging, and a breather.

The pressure system components are an oil tank and de-aerator, main oil pump, main oil filter/filter bypass valve, pressure regulating valve (PRV), oil cooler/by pass valve and tubing/jets.
Usually the flow is from the tank to the pump inlet and PRV, pumped to main oil filter or its bypass valve and oil cooler, then through some more filters to jets in the bearings.

Using the PRV method of control, means that the pressure of the feed oil must be below a critical value (usually controlled by other valves which can leak out excess oil back to tank if it exceeds the critical value). The valve opens at a certain pressure and oil is kept moving at a constant rate into the bearing chamber.

If the engine speed increases, the pressure within the bearing chamber also increases, which means the pressure difference between the lubricant feed and the chamber reduces which could reduce slow rate of oil when it is needed even more. As a result, some PRVs can adjust their spring force values using this pressure change in the bearing chamber proportionally to keep the lubricant flow constant.

### J-58 combined ramjet/turbojet

The SR-71's Pratt & Whitney J58 engines were rather unusual. They could convert in flight from being largely a turbojet to being largely a compressor-assisted ramjet. At high speeds (above Mach 2.4), the engine used variable geometry vanes to direct excess air through 6 bypass pipes from downstream of the fourth compressor stage into the afterburner.[15] 80% of the SR-71's thrust at high speed was generated in this way, giving much higher thrust, improving specific impulse by 10-15%, and permitting continuous operation at Mach 3.2. The name coined for this setup is turbo-ramjet. The Lockheed SR-71, unofficially known as the Blackbird, is a long-range, advanced, strategic reconnaissance aircraft developed from the Lockheed A-12 and YF-12A aircraft by Lockheeds Skunk works, which was also responsible for the U-2 and many other advanced aircraft. ... The Pratt & Whitney J58 (also known as the JT11D) was the jet engine used on the Lockheed SR-71 Blackbird. The J58 produced 32,000 lbf (142 kN) of thrust. ... Specific impulse (usually abbreviated Isp) is a way to describe the efficiency of rocket and jet engines. ...

### Hydrogen fuelled jet engines

Jet engines can be run on almost any fuel. Hydrogen is a highly desirable fuel, as, although the energy per mole is not unusually high, the molecule is very much lighter than other molecules. It turns out that the energy per kg of hydrogen is twice that of more common fuels and this gives twice the specific impulse. In addition jet engines running on hydrogen are quite easy to build- the first ever turbojet was run on hydrogen. The mole (symbol: mol) is the SI base unit that measures an amount of substance. ...

However, in almost every other way, hydrogen is problematic. The downside of hydrogen is its density, in gaseous form the tanks are impractical for flight, but even in liquid form it has a density one fourteenth that of water. It is also deeply cryogenic and requires very significant insulation that precludes it being stored in wings. The overall vehicle ends up very large, and they would be difficult for most airports to accommodate. Finally, pure hydrogen is not found in nature, and must be manufactured either via steam reforming or expensive electrolysis. Both are relatively inefficient processes.

### Precooled jet engines

Main article: Precooled jet engine

An idea originated by Robert P. Carmichael in 1955 [16] is that hydrogen fuelled engines could theoretically have much higher performance than hydrocarbon fuelled engines if a heat exchanger were used to cool the incoming air. The low temperature allows lighter materials to be used, a higher mass-flow through the engines, and permits combustors to inject more fuel without overheating the engine.

This idea leads to plausible designs like SABRE, that might permit single-stage-to-orbit,[17] and ATREX, that might permit jet engines to be used up to hypersonic speeds and high altitudes for boosters for launch vehicles. The idea is also being researched by the EU for a concept to achieve non-stop antipodal supersonic passenger travel at Mach 5 (Reaction Engines A2). French naval officers sabre of the 19th Century From left to right: two bayonets, a short curved infantry or artillery briquet, a straight infantry officers sabre, and a carbine. ... The ATREX engine developed in Japan is an experimental precooled jet engine that works as a turbojet at low speeds and a ramjet up to mach 6. ...

### Nuclear-powered ramjet

Project Pluto was a nuclear-powered ramjet, intended for use in a cruise missile. Rather than combusting fuel as in regular jet engines, air was heated using a high-temperature, unshielded nuclear reactor. This dramatically increased the engine burn time, and the ramjet was predicted to be able to cover any required distance at supersonic speeds (Mach 3 at tree-top height). On January 1, 1957, the U.S. Air Force and the U.S. Atomic Energy Commission selected the Lawrence Livermore National Laboratorys (LLNL) predecessor, the Lawrence Radiation Laboratory, to study the feasibility of applying heat from nuclear reactors to ramjet engines. ... A Taurus KEPD 350 cruise missile of the German Luftwaffe A cruise missile is a guided missile which carries an explosive payload and uses a lifting wing and a propulsion system, usually a jet engine, to allow sustained flight; it is essentially a flying bomb. ...

However, there was no obvious way to stop it once it had taken off, which would be a great disadvantage in any non-disposable application. Also, because the reactor was unshielded, it was dangerous to be in or around the flight path of the vehicle (although the exhaust itself wasn't radioactive). These disadvantages limit the application to warhead delivery system for all-out nuclear war, which it was being designed for.

### Scramjets

Main article: Scramjet

Scramjets are an evolution of ramjets that are able to operate at much higher speeds than any other kind of airbreathing engine. They share a similar structure with ramjets, being a specially-shaped tube that compresses air with no moving parts through ram-air compression. Scramjets, however, operate with supersonic airflow through the entire engine. Thus, scramjets do not have the diffuser required by ramjets to slow the incoming airflow to subsonic speeds. X-43A with scramjet attached to the underside A scramjet (supersonic combustion ramjet) is a variation of a ramjet with the key difference being that the flow in the combustor is supersonic. ...

Scramjets start working at speeds of at least Mach 4, and have a maximum useful speed of approximately Mach 17.[18] Due to aerodynamic heating at these high speeds, cooling poses a challenge to engineers. Aerodynamic heating is the heating of a solid body produced by the passage of fluid (such as air) over the body. ...

## Environmental considerations

Jet engines are usually run on fossil fuel propellant, and in that case, are a net source of carbon to the atmosphere.

Some scientists believe that jet engines are also a source of global dimming due to the water vapour in the exhaust causing cloud formations. Global dimming is the gradual reduction in the amount of global direct irradiance at the Earths surface that was observed for several decades after the start of systematic measurements in 1950s. ...

Nitrogen compounds are also formed from the combustion process from atmospheric nitrogen. At low altitudes this is not thought to be especially harmful, but for supersonic aircraft that fly in the stratosphere some destruction of ozone may occur.

Sulphates are also emitted if the fuel contains sulphur.

## Safety and reliability

Main article: Air safety

Jet engines are usually very reliable and have a very good safety record. However failures do sometimes occur. Air safety is a broad term encompassing the theory, investigation and categorization of flight failures, and the prevention of such failures through appropriate regulation, as well as through education and training. ...

One class of failures that has caused accidents in particular is uncontained failures, where rotary parts of the engine break off and exit through the case. These can cut fuel or control lines, and can penetrate the cabin. Although fuel and control lines are usually duplicated for reliability the United Airlines Flight 232 was caused when all control lines were simultaneously severed. United Airlines Flight 232 was a scheduled flight operated by United Airlines between Denver and Philadelphia via Chicago. ...

The most likely failure is compressor blade failure, and modern jet engines are designed with structures that can catch these blades and keep them contained them within the engine casing. Verification of a jet engine design involves testing that this system works correctly.

### Bird strike

Modern jet engines have the capability of surviving an ingestion of a bird. Small fast planes, such as military jet fighters, are at higher risk than big heavy multi-engine ones. This is due to the fact that the fan of a high-bypass turbofan engine, typical on transport aircraft, acts as a centrifugal separator to force ingested materials (birds, ice, etc.) to the outside of the fan's disc. As a result, such materials go through the relatively unobstructed bypass duct, rather than through the core of the engine, which contains the smaller and more delicate compressor blades. Military aircraft designed for high-speed flight typically have pure turbojet, or low-bypass turbofan engines, increasing the risk that ingested materials will get into the core of the engine to cause damage. An A-10 Thunderbolt II, F-86 Sabre, P-38 Lightning and P-51 Mustang fly in formation during an air show at Langley Air Force Base, Virginia. ... Schematic diagram of high-bypass turbofan engine CFM56-3 turbofan, lower half, side view. ... Typical bypass duct, in a high bypass ratio turbofan A bypass duct is an annular passage that allows some of a turbofans airflow to bypass the engine core, or gas generator. ... Military aircraft are airplanes used in warfare. ... For the transportation company in southern China, see TurboJET. Turbojets are the oldest kind of general purpose jet engines. ...

The highest risk of the bird strike is during the takeoff and landing, in low altitudes, which is in the vicinity of the airports. MyTravel Airways Airbus A320 landing Landing is the last part of a flight, where a flying animal or aircraft returns to the ground. ... Altitude is the elevation of an object from a known level or datum. ...

## References

1. ^ How many air-miles are left in the world’s fuel tank?.
2. ^ U.S. Airlines: Operating in an Era of High Jet Fuel Prices
3. ^ propeller efficiency
4. ^ Patent number 554,906
5. ^ sod1280.tmp
6. ^ PBS - Chasing the Sun - Frank Whittle
7. ^ BBC - History - Frank Whittle (1907 - 1996)
8. ^ The History of the Jet Engine - Sir Frank Whittle - Hans Von OhainOhain said that he had not read Whittle's patent and Whittle believed him (Frank Whittle 1907-1996) however the Whittle patent was in German libraries and Whittle's son had suspicions that Ohain had read or heard of it (The History of the Jet Engine - Sir Frank Whittle a genius betrayed - )
9. ^ ch10-3
10. ^ Merging Air and Space
11. ^ K.Honicke, R.Lindner, P.Anders, M.Krahl, H.Hadrich, K.Rohricht. Beschreibung der Konstruktion der Triebwerksanlagen. Interflug, Berlin, 1968
12. ^ Rocket Propulsion elements- seventh edition, pg 37-38
13. ^ High Speed Propulsion
14. ^ The Combustion Chamber
15. ^ J58
16. ^ NASA history Other Interests in Hydrogen
17. ^ The Skylon Spaceplane
18. ^ Astronautix X30
19. ^ Transport Canada - Sharing the Skies
• John Golley (1997). Genesis of the Jet: Frank Whittle and the Invention of the Jet Engine. Crowood Press. ISBN 1-85310-860-X.
• David S Brooks (1997). Vikings at Waterloo: Wartime Work on the Whittle Jet Engine by the Rover Company. Rolls-Royce Heritage Trust. ISBN 1-872922-08-2

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