A
jet engine is a
reaction engineA reaction engine is an engine which provides propulsion by expelling reaction mass, in accordance with Newton's third law of motion. This law of motion is most commonly paraphrased as: "For every action force there is an equal, but opposite, reaction force"....
that discharges a fast moving
jetA jet is a coherent stream of fluid that is projected into a surrounding medium, usually from some kind of a nozzle or aperture. Jets can travel long distances without dissipating. In the Earth's atmosphere there exist jet streams that travel thousands of miles.Some animals, notably cephalopods use...
of
fluidA fluid is a substance that continually deforms under an applied shear stress. All gases are fluids, but not all liquids are fluids. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
to generate thrust in accordance with
Newton'sSir Isaac Newton FRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian who is perceived and considered by a substantial number of scholars and the general public as one of the most influential men in history...
laws of motionNewton's laws of motion are three physical laws that form the basis for classical mechanics. They are:# In the absence of force, a body either is at rest or moves in a straight line with constant speed....
. This broad definition of jet engines includes
turbojetTurbojets are the oldest kind of general purpose jet engines. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s, although credit for the first turbojet is given to Whittle who submitted...
s,
turbofanA turbofan is a type of aircraft gas turbine engine that provides propulsion using a combination of a ducted fan and a jet exhaust nozzle. Part of the airstream from the ducted fan passes through the core, providing oxygen to burn fuel to create power. However, the rest of the air flow bypasses...
s,
rocketA rocket engine or simply "rocket" is a jet engine[Rocket Propulsion Elements; 7th edition- chapter 1] that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
s,
ramjetA ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of jet engine using the engine's forward motion to compress incoming air, without a rotary compressor...
s,
pulse jetsA pulse jet engine is a very simple type of jet engine in which combustion occurs in pulses. Pulsejet engines can be made with few or no moving parts, and are capable of running statically....
and
pump-jetA pump-jet or water jet is a marine system that creates a jet of water for propulsion. The mechanical arrangement may be a ducted propeller with nozzle, or a centrifugal pump and nozzle...
s. In general, most jet engines are internal combustion engines but non-combusting forms also exist.
In some common parlance, the term 'jet engine' is loosely referred to as an
internal combustionThe internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine the expansion of the high temperature and pressure gases, which are produced by the combustion, directly applies force to a movable...
duct engine, which typically consists of an engine with a rotary (rotating) air compressor powered by a
turbineA turbine is a rotary engine that extracts energy from a fluid or air flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum, with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they...
("
Brayton cycleThe Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the jet engine and others. It is named after George Brayton , the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber in 1791. It is...
"), with the leftover power providing thrust via a
propelling nozzleA propelling nozzle is the component of a jet engine that operates to form an exhaust jet and maximise the thrust from the engine.On non-afterburning engines the nozzle is a fixed size because the differing atmospheric pressure over the operating altitudes makes little difference to the engine...
. These types of jet engines are primarily used by
jet aircraftA jet aircraft is an aircraft propelled by jet engines. Jet aircraft generally fly much faster than propeller-powered aircraft and at higher altitudes — as high as 10,000 to 15,000 meters . At these altitudes, jet engines achieve maximum efficiency over long distances...
for long distance travel. Early jet aircraft used
turbojetTurbojets are the oldest kind of general purpose jet engines. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s, although credit for the first turbojet is given to Whittle who submitted...
engines which were relatively inefficient for subsonic
flightFlight is the process by which an object moves either through the air, or movement beyond earth's atmosphere , by aerodynamically generating lift, propulsive thrust or aerostatically using buoyancy, or by simple ballistic movement.-Buoyant flight:Humans, although not apparently other animals, have...
. Modern subsonic jet aircraft usually use high-bypass turbofan engines which give high speeds, as well as (over long distances) better fuel efficiency than many other forms of transport.
About 7.2% of the oil used in 2004 was consumed by jet engines. 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.
History
Jet engines can be dated back to the invention of the
aeolipileAn aeolipile , also known as a Hero engine, is a rocket style jet engine which spins when heated. In the first century AD, Hero of Alexandria described the device, and many sources give him the credit for its invention.The aeolipile he described is considered to be the first recorded steam engine...
before the first century AD. This device used steam power directed through two nozzles so as to cause a sphere to spin rapidly on its axis. So far as is known, it was not used for supplying mechanical power, and the potential practical applications of this invention were not recognized. It was simply considered a curiosity.
Jet propulsion only literally and figuratively took off with the invention of the
rocketA rocket or rocket vehicle is a missile, spacecraft, aircraft or other vehicle which obtains thrust by the reaction of the rocket to the ejection of a jet of fast moving fluid exhaust from a rocket engine. Chemical rockets create their exhaust by the combustion of rocket propellant...
by the Chinese in the 13th century. Rocket exhaust was initially used in a modest way for
fireworksA firework is a low explosive pyrotechnic device used primarily for aesthetic and entertainment purposes. The most common use of a firework is as part of a fireworks display. A fireworks event is a display of the effects produced by firework devices...
but gradually progressed to propel formidable weaponry; and there the technology stalled for hundreds of years.
ArchytasArchytas was an Ancient Greek philosopher, mathematician, astronomer, statesman, and strategist. He was a scientist of the Pythagorean school and famous for being the reputed founder of mathematical mechanics, as well as a good friend of Plato....
, the founder of mathematical mechanics, as described in the writings of Aulus Gellius five centuries after him, was reputed to have designed and built the first artificial, self-propelled flying device. This device was a bird-shaped model propelled by a jet of what was probably steam, said to have actually flown some 200 meters.
In Ottoman, Turkey in 1633
Lagari Hasan ÇelebiLagari Hasan Çelebi was an Ottoman Turk who according to legend was the first person to have made a successful manned rocket flight.-Legendary Flight:...
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
OttomanThe Ottoman Empire or Ottoman State , also known by its contemporaries as the Turkish Empire or Turkey , was an empire that lasted from 1299 to November 1, 1922 The Ottoman Empire or Ottoman State (Ottoman Turkish: دَوْلَتِ عَلِیَّهِ عُثْمَانِیَّه Dawlet-il ʿAliyyat-il ʿOs̠māniyye, Modern Turkish:...
army. However, this was essentially a stunt. The problem was that rockets are simply too inefficient at low speeds to be useful for general aviation.
In 1910
Henri CoandăHenri Marie Coandă was a Romanian inventor, aerodynamics pioneer and the builder of world's first jet powered aircraft, the Coanda-1910. He discovered and gave his name to the Coandă effect.-Life:...
designed, built and piloted the first 'thermojet'-powered aircraft, known as the
Coandă-1910The Coandă-1910 was the first jet-propelled aircraft ever built. It was constructed by Romanian inventor Henri Coandă and exhibited by him at the Second International Aeronautical Exhibition in Paris around October 1910.-Design:...
, which he demonstrated publicly at the second International Aeronautic Salon in Paris. The powerplant used a 4-cylinder piston engine to power a compressor, which fed two burners for thrust, instead of using a propeller. At the airport of
Issy-les-MoulineauxIssy-les-Moulineaux is a commune in the south-western suburban area of Paris, France. It is located from the centre of Paris. On 1 January 2003, Issy-les-Moulineaux became part of the Communauté d'agglomération Arc de Seine along with the other communes of Chaville, Meudon, Vanves and Ville-d'Avray...
near Paris, Coandă lost control of the jet plane, which went off of the runway and caught fire. Fortunately, he escaped with minor injuries to his face and hands. Around that time, Coandă abandoned his experiments due to a lack of interest from the public, scientific and engineering institutions. It would be nearly 30 years until the next thermojet-powered aircraft, the Caproni Campini N.1 (sometimes referred to as C.C.2).
In 1913
René LorinRené Lorin , a graduate of the Ecole Centrale Paris, invented the ramjet, which he patented in 1908; when he patented a subsonic ramjet design, and had not considered supersonic applications....
came up with a form of jet engine, the subsonic pulsejet, 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.
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
propellerA propeller is a type of fan which transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and air or water is accelerated behind the blade...
efficiency. This seemed to peak as blade tips approached the
speed of soundSound is a vibration that travels through an elastic medium as a wave. The speed of sound describes how far this wave travels in a given amount of time. In dry air at , the speed of sound is . This equates to , or about one mile in five seconds...
. 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 brothersThe Wright brothers, Orville and Wilbur , were two Americans who are generally credited with inventing and building the world's first successful airplane and making the first controlled, powered and sustained heavier-than-air human flight, on December 17, 1903...
' first flight.
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 CampiniSecondo Campini was an Italian engineer and one of the pioneers of the jet engine.Campini was born at Bologna, Emilia-Romagna. In 1931 he wrote a proposal for the Italian Air Ministry on the value of jet propulsion and in 1932 demonstrated a jet-powered boat in Venice...
but more commonly,
motorjetA motorjet is a rudimentary type of jet engine which is sometimes referred to as thermojet, a term now commonly used to describe a particular and completely unrelated pulsejet design. At the heart the motorjet is always an ordinary piston engine , but instead of this driving a propeller, it drives...
, the air was compressed by a fan driven by a conventional piston engine. Examples of this type of design were
Henri CoandăHenri Marie Coandă was a Romanian inventor, aerodynamics pioneer and the builder of world's first jet powered aircraft, the Coanda-1910. He discovered and gave his name to the Coandă effect.-Life:...
's
Coandă-1910The Coandă-1910 was the first jet-propelled aircraft ever built. It was constructed by Romanian inventor Henri Coandă and exhibited by him at the Second International Aeronautical Exhibition in Paris around October 1910.-Design:...
aircraft, and the much later
Campini Caproni CC.2The Caproni Campini N.1 was an early motorjet-powered test aircraft.-Design and development:...
, and the Japanese
Tsu-11The 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....
engine intended to power
OhkaThe Yokosuka MXY-7 Ohka, was a purpose-built, rocket powered human-guided anti-shipping kamikaze attack plane employed by Japan towards the end of World War II...
kamikazeThe were suicide attacks by military aviators from the Empire of Japan against Allied naval vessels in the closing stages of the Pacific campaign of World War II, designed to destroy as many warships as possible....
planes towards the end of
World War IIWorld War II, or the Second World War , was a global military conflict which involved a majority of the world's nations, including all great powers, organized into two opposing military alliances: the Allies and the Axis...
. None were entirely successful and the CC.2 ended up being slower than the same design with a traditional engine and propeller combination.
The key to a practical jet engine was the gas turbine, used to extract energy from the engine itself to drive the
compressorA gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
. The
gas turbineA gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. Energy is added to the gas stream in the combustor, where air is mixed with...
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 EllingJens William Ægidius Elling was a Norwegian inventor who is considered to be the father of the gas turbine. His first gas turbine patent was granted in 1884. In 1903 he completed the first turbine that produced excess power; his original machine used both rotary compressors and turbines to...
. Limitations in design and practical engineering and metallurgy prevented such engines reaching manufacture. The main problems were safety, reliability, weight and, especially, sustained operation.
In
HungaryHungary , in English officially the Republic of Hungary , is a landlocked country in the Carpathian Basin of Central Europe, bordered by Austria, Slovakia, Ukraine, Romania, Serbia, Croatia, and Slovenia. Its capital is Budapest. Hungary is a member of OECD, NATO, EU, V4 and is a Schengen state...
,
Albert FonóAlbert Fonó , was a Hungarian mechanical engineer who was one of the early pioneers of turbojet and ramjet propulsion and was first to patent a ramjet engine in 1928, ....
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 aircraft powered by supersonic ramjets, and this was awarded in 1932.
The first patent for using a gas turbine to power an aircraft was filed in 1921 by Frenchman
Maxime GuillaumeIn aerospace, Maxime Guillaume held a French patent for a turbojet engine in 1921.The first patent for using a gas turbine to power an aircraft was filed in 1921 by Frenchman Maxime Guillaume. His engine was to be an axial-flow turbojet, but was never constructed, as it would have required...
. His engine was an axial-flow turbojet.
In 1923,
Edgar BuckinghamEdgar Buckingham was a physicist.He graduated from Harvard with a bachelor's degree in physics in 1887. He did additional graduate work at the University of Strasbourg and the University of Leipzig, where he studied under chemist Wilhelm Ostwald. Buckingham received a PhD from Leipzig in 1893...
of the US National Bureau of Standard published a report 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."
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.
In 1928, RAF College Cranwell cadet
Frank WhittleAir Commodore Sir Frank Whittle, OM, KBE, CB, FRS, Hon FRAeS was a British Royal Air Force officer. Sharing credit with Germany's Dr. Hans von Ohain for independently inventing the jet engine, he is hailed as a father of jet propulsion.From an early age Whittle demonstrated an aptitude for...
formally submitted his ideas for a turbo-jet to his superiors. In October 1929 he developed his ideas further. . On 16 January 1930 in England, Whittle submitted his first patent (granted in 1932). The patent showed a two-stage
axial compressorAxial compressors are rotating, airfoil based compressors in which the working fluid principally flows parallel to the axis of rotation. This is in contrast with other rotating compressors such as centrifugal, axi-centrifugal and mixed-flow compressors where the air may enter axially but will have...
feeding a single-sided centrifugal compressor. Practical axial compressors were made possible by ideas from
A.A.GriffithAlan Arnold Griffith was an English engineer, who, among many other contributions, is best known for his work on stress and fracture in metals that is now known as metal fatigue, as well as being one of the first to develop a strong theoretical basis for the jet engine.-Early work:Griffith took a...
in a seminal paper in 1926 ("An Aerodynamic Theory of Turbine Design"). Whittle would later concentrate on the simpler centrifugal compressor only, for a variety of practical reasons. Whittle had his first engine running in April 1937. It was liquid-fuelled, and included a self-contained fuel pump. Whittle's team experienced near-panic when the engine would not stop, accelerating even after the fuel was switched off. It turned out that fuel had leaked into the engine and accumulated in pools. So the engine would not stop until all the leaked fuel had burned off. Whittle was unable to interest the government in his invention, and development continued at a slow pace.
In 1935
Hans von OhainHans Joachim Pabst von Ohain was one of the inventors of jet propulsion.The engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently during the late 1930s, although credit for the first turbojet is given to Whittle,Ohain designed the first...
started work on a similar design in
GermanyGermany , officially the Federal Republic of Germany , is a country in Central Europe. It is bordered to the north by the North Sea, Denmark, and the Baltic Sea; to the east by Poland and the Czech Republic; to the south by Austria and Switzerland; and to the west by France, Luxembourg, Belgium,...
, apparently unaware of Whittle's work. His first engine was strictly experimental and could only run under external power, but he was able to demonstrate the basic concept. Ohain was then introduced to
Ernst HeinkelErnst Heinkel was a German aircraft designer and manufacturer.-Early life:He was born in Grunbach and as a young man became an apprentice machinist at a foundry. He initially became interested in aviation through a fascination with zeppelins, and in 1909 attended an international airshow in...
, one of the larger aircraft industrialists of the day, who immediately saw the promise of the design. Heinkel had recently purchased the Hirth engine company, and Ohain and his master machinist Max Hahn were set up there as a new division of the Hirth company. They had their first
HeS 1The Heinkel HeS 1 was the first jet engine, which was a stationary test item that ran on hydrogen.-History:In February 1936, Pohl wrote to Ernst Heinkel on behalf of Hans von Ohain, telling him of the design and its possibilities...
centrifugal engine running by September 1937. Unlike Whittle's design, Ohain used
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
as fuel, supplied under external pressure. Their subsequent designs culminated in the gasoline-fuelled
HeS 3The HeS 3 was the world's first operational jet engine to power an aircraft. Designed by Hans von Ohain while working at Heinkel, the engine first flew as the primary power of the Heinkel He 178, piloted by Erich Warsitz on August 27, 1939...
of 1,100 lbf (5 kN), which was fitted to Heinkel's simple and compact
He 178The Heinkel He 178 was the world's first aircraft to fly under turbojet power, and the first practical jet plane, the pioneering example of this type of aircraft. It was a private venture by the German Heinkel company in accordance with director Ernst Heinkel's emphasis on developing technology for...
airframe and flown by
Erich WarsitzErich Warsitz was a German test pilot of the 1930s. He held the rank of Flight-Captain in the Luftwaffe and was selected by the Reich Air Ministry as chief test pilot at Peenemünde West...
in the early morning of August 27, 1939, from
RostockRostock is the largest city in the north German state Mecklenburg-Vorpommern. Rostock is located on the Warnow river; the quarter of Warnemünde 12 km north of the city centre lies directly on the coast of the Baltic Sea.-Geography:Rostock is located nearly centrally on...
-Marienehe aerodrome, an impressively short time for development. The He 178 was the world's first jet plane.
The world's first
turbopropTurboprop engines are a type of aircraft powerplant that use a gas turbine to drive a propeller. The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller...
was the
Jendrassik Cs-1The Jendrassik Cs-1 was the world's first working turboprop engine. It was designed by Hungarian engineer György Jendrassik in 1937, and was intended to power a Hungarian twin-engine heavy fighter, the RMI-1. He patented his "turboprop" invention in 1929...
designed by the
HungarianHungary , in English officially the Republic of Hungary , is a landlocked country in the Carpathian Basin of Central Europe, bordered by Austria, Slovakia, Ukraine, Romania, Serbia, Croatia, and Slovenia. Its capital is Budapest. Hungary is a member of OECD, NATO, EU, V4 and is a Schengen state...
mechanical engineer
György JendrassikGyörgy Jendrassik , Hungarian physicist and mechanical engineer.Jendrassik completed his education at Budapest's József Technical University, then at the University of Berlin attended lectures of the famous physicists Einstein and Planck. In 1922 he obtained his diploma in mechanical engineering in...
. It was produced and tested in the
GanzThe Ganz electric works in Budapest is probably best known for the manufacture of tramcars, but was also a pioneer in the application of three-phase alternating current to electric railways. Ganz also made / makes: ships , bridge steel structures , high voltage equipment...
factory in
BudapestBudapest is the capital of Hungary. As the largest city of Hungary, it serves as the country's principal political, cultural, commercial, industrial, and transportation center and is considered an important hub in Central Europe. In 2009, Budapest had 1,712,210 inhabitants, down from a mid-1980s...
between 1938 and 1942. It was planned to fit to the Varga RMI-1 X/H twin-engined reconnaissance bomber designed by László Varga in 1940, but the program was cancelled. Jendrassik had also designed a small-scale 75 kW turboprop in 1937.
Whittle's engine was starting to look useful, and his
Power JetsPower Jets Ltd was a United Kingdom company set up by Frank Whittle for the purpose of designing and manufacturing jet engines.Founded on January 27, 1936, the company consisted of Whittle, Rolf Dudley-Williams, James Collingwood Tinling, and Lancelot Law Whyte of investment bankers O T Falk &...
Ltd. started receiving
Air MinistryThe Air Ministry was formerly a department of the British Government with the responsibility of managing the affairs of the Royal Air Force, existing from 1918 to 1964...
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
airframeThe term airframe refers to the mechanical structure of an aircraft, and as generally used does not include the propulsion system. Airframe design is a challenging field of engineering, combining aerodynamics, materials technology and manufacturing methods to achieve favorable balances of...
specially built for it, and first flew on May 15, 1941 at
RAF CranwellRAF Cranwell is a Royal Air Force station in Lincolnshire close to the village of Cranwell, near Sleaford. It is currently commanded by Group Captain N Wharmby...
.
A Scottish aircraft engine designer,
Frank HalfordMajor Frank Bernard Halford was an English aircraft engine designer.-Career:Educated at Felsted, In 1913 he left the University of Nottingham before graduating to learn to fly at Brooklands and Bristol Flying School and became a flight instructor using Bristol Boxkites.He served in the First World...
, working from Whittle's ideas developed a "straight through" version of the centrifugal jet; his design became the
de Havilland GoblinThe de Havilland Goblin, originally the Halford H-1, is an early turbojet engine designed by Frank Halford. The Goblin built by de Havilland was the second British jet engine to fly, and the first to pass tests and receive a "Gas Turbine" class type rating.The Goblin was the primary engine of the...
.
One problem with both of these early designs, which are called
centrifugal-flowCentrifugal compressor, are a special class of radial-flow work-absorbing turbomachinery that includes pumps, fans, blowers and compressors....
engines, was that the compressor worked by "throwing" (accelerating) air outward from the central intake to the outer periphery of the engine, where the air was then compressed by a divergent duct setup, converting its velocity into pressure. An advantage of this design was that it was already well understood, having been implemented in centrifugal
superchargerA supercharger is an air compressor used for forced induction of an internal combustion engine. The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally-aspirated engine, which allows more fuel to be provided and more work to be done per cycle,...
s, then in widespread use on piston engines. However, given the early technological limitations on the shaft speed of the engine, the compressor needed to have a very large diameter to produce the power required. This meant that the engines had a large frontal area, which made it less useful as an aircraft powerplant due to drag. A further disadvantage was that the air flow had to be "bent" to flow rearwards through the combustion section and to the turbine and tailpipe, adding complexity and lowering efficiency. Nevertheless, these types of engines had the major advantages of light weight, simplicity and reliability, and development rapidly progressed to practical airworthy designs.
AustriaAustria , officially the Republic of Austria , is a landlocked country of roughly 8.3 million people in Central Europe. It borders both Germany and the Czech Republic to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the west...
n
Anselm FranzAnselm Franz was a pioneering jet engine engineer, known for the development of the Jumo 004 turbojet in Germany during World War II, and his work on turboshaft designs in the U.S. after the war....
of Junkers' engine division (
Junkers Motoren or
Jumo) addressed these problems with the introduction of the axial-flow compressor. Essentially, this is a turbine in reverse. Air coming in the front of the engine is blown towards the rear of the engine by a fan stage (convergent ducts), where it is crushed against a set of non-rotating blades called
stators (divergent ducts). The process is nowhere near as powerful as the centrifugal compressor, so a number of these pairs of fans and stators are placed in series to get the needed compression. Even with all the added complexity, the resulting engine is much smaller in diameter and thus, more aerodynamic. Jumo was assigned the next engine number in the
RLMThe Reich Air Ministry was a government department during the period of Nazi Germany . It is also the original name of a building in Wilhelmstraße in central Berlin, the capital of Germany, which now houses the German Finance Ministry .thumb|300px|The [[Reich Air Ministry Building]], December...
numbering sequence, 4, and the result was the
Jumo 004The Jumo 004 was the world's first turbojet engine in production and operational use, and the first successful axial compressor jet engine ever built. Some 8,000 units were manufactured by Junkers in Germany during late World War II and powered the operational Messerschmitt Me 262 jet fighter,...
engine. After many lesser technical difficulties were solved, mass production of this engine started in 1944 as a powerplant for the world's first jet-fighter aircraft, the
Messerschmitt Me 262The Messerschmitt Me 262 Schwalbe was the world's first operational jet-powered fighter aircraft. It was produced in World War II and saw action starting in 1944 as a multi-role fighter/bomber/reconnaissance/interceptor warplane for the Luftwaffe.The Me 262 claimed a total of 509 Allied kills ...
(and later the world's first jet-bomber aircraft, the
Arado Ar 234The Arado Ar 234 was the world's first operational jet powered bomber, built by the German Arado company in the closing stages of World War II. In the field it was used almost entirely in the reconnaissance role, but in its few uses as a bomber it proved to be nearly impossible to...
). A variety of reasons conspired to delay the engine's availability, this delay caused the fighter to arrive too late to decisively impact Germany's position in
World War IIWorld War II, or the Second World War , was a global military conflict which involved a majority of the world's nations, including all great powers, organized into two opposing military alliances: the Allies and the Axis...
. Nonetheless, it will be remembered as the first use of jet engines in service.
In the UK, their first axial-flow engine, the
Metrovick F.2The Metropolitan-Vickers F.2 was an early turbojet engine and the first British design to be based on an axial-flow compressor. It was considered too unreliable for use during the war, and never entered production...
, 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.
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.
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 as their engine components are more robust they are less liable to
foreign object damageForeign Object Damage or Foreign Object Debris is a substance, debris or article alien to a vehicle or system that has potential to cause damage. Foreign Object Damage is any damage attributed to a foreign object that can be expressed in physical or economic terms that may or may not degrade the...
than axial-flow compressor engines.
Although German designs were more advanced aerodynamically, the combination of simplicity and the lack of requisite rare metals for the necessary advanced metallurgy (such as
tungstenTungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A steel-gray metal, tungsten is found in several ores, including wolframite and scheelite...
,
chromiumChromium is a chemical element which has the symbol Cr and atomic number 24, first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable...
and
titaniumTitanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the “space age metal”, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color.Titanium can be alloyed with iron, aluminium, vanadium, molybdenum, among other...
) for high-stress components such as turbine blades and
bearingA bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can...
s, etc) meant that the later produced German engines had a short service life and had to be changed after 10–25 hours. British engines were also widely manufactured under license in the US (see
Tizard MissionThe Tizard Mission officially the British Technical and Scientific Mission was a British delegation that visited the United States during the Second World War to get American resources to develop important military technology developed in the UK. It got its popular name from the instigator Henry...
), and were sold to Soviet Russia who reverse engineered them with the
NeneThe Rolls-Royce Nene was a 1940s British centrifugal compressor turbojet engine. It was named after the River Nene in keeping with company tradition for jet engines.-Development:...
going on to power the famous MiG-15. American and Soviet designs, independent axial-flow types for the most part, would strive to attain superior performance until the 1960s, although the
General Electric J47The General Electric J47 turbojet was developed by General Electric from the earlier J35 engine, and first flew in May 1948. The J47 was the first axial-flow turbojet approved for commercial use in the United States. It was used in many types of aircraft and more than 30,000 were manufactured...
provided excellent service in the
F-86 SabreThe North American Aviation F-86 Sabre was a transonic jet fighter aircraft. The Sabre is best known for its Korean War role where it was pitted against the Soviet MiG-15 and obtained UN air superiority...
in the 1950s.
By the 1950s the jet engine was almost universal in combat aircraft, with the exception of cargo, liaison and other specialty types. By this point some of the British designs were already cleared for civilian use, and had appeared on early models like the
de Havilland CometThe de Havilland Comet was the world's first commercial jet airliner to reach production. Developed and manufactured by de Havilland, it first flew in 1949 and was considered a landmark in British aeronautical design...
and Avro Canada Jetliner. By the 1960s all large civilian aircraft were also jet powered, leaving the piston engine in such low-cost niche roles such as
cargoCargo is goods or produce transported, generally for commercial gain, by ship, aircraft, train, van or truck. In modern times, containers are used in most intermodal long-haul cargo transport.- Marine Cargo Types :...
flights.
Relentless improvements in the
turbopropTurboprop engines are a type of aircraft powerplant that use a gas turbine to drive a propeller. The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller...
pushed the piston engine (an internal combustion engine) out of the mainstream entirely, leaving it serving only the smallest
general aviationGeneral aviation is one of two categories of civil aviation. It refers to all flights other than military and scheduled airline flights, both private and commercial. General aviation flights range from gliders and powered parachutes to large, non-scheduled cargo jet flights...
designs and some use in drone aircraft. The ascension of the jet engine to almost universal use in aircraft took well under twenty years.
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 exceed that of the best piston and propeller engines, 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, were killed forever.
Types
There are a large number of different types of jet engines, all of which achieve forward thrust from the principle of
jet propulsion.
| Type |
Description |
Advantages |
Disadvantages |
Water jetA pump-jet or water jet is a marine system that creates a jet of water for propulsion. The mechanical arrangement may be a ducted propeller with nozzle, or a centrifugal pump and nozzle...
|
For propelling water rocketA water rocket is a type of model rocket using water as its reaction mass. The pressure vessel—the engine of the rocket—is usually a used plastic soft drink bottle...
s and jetboatA jetboat is a boat propelled by a jet of water ejected from the back of the craft. Unlike a powerboat or motorboat that uses a propeller in the water below or behind the boat, a jetboat draws the water from under the boat into a pump-jet inside the boat, then expels it through a nozzle at the...
s; squirts water out the back through a nozzle |
In boats, can run in shallow water, high acceleration, no risk of engine overload (unlike propellers), less noise and vibration, highly maneuverable 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 A motorjet is a rudimentary type of jet engine which is sometimes referred to as thermojet, a term now commonly used to describe a particular and completely unrelated pulsejet design. At the heart the motorjet is always an ordinary piston engine , but instead of this driving a propeller, it drives...
|
Works like a turbojet but instead of a turbine driving the compressor a piston engine drives it. |
Higher exhaust velocity than a propeller, offering better thrust at high speed |
Heavy, inefficient and underpowered. Examples include: Coandă-1910 The Coandă-1910 was the first jet-propelled aircraft ever built. It was constructed by Romanian inventor Henri Coandă and exhibited by him at the Second International Aeronautical Exhibition in Paris around October 1910.-Design:...
and Caproni Campini N.1. |
TurbojetTurbojets are the oldest kind of general purpose jet engines. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s, although credit for the first turbojet is given to Whittle who submitted...
|
A tube with a compressor and turbine sharing a common shaft with a burner in between and a propelling nozzleA propelling nozzle is the component of a jet engine that operates to form an exhaust jet and maximise the thrust from the engine.On non-afterburning engines the nozzle is a fixed size because the differing atmospheric pressure over the operating altitudes makes little difference to the engine...
for the exhaust. Uses a high exhaust gas velocity to produce thrust. Has a much higher core flow than bypass type engines |
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 A turbofan is a type of aircraft gas turbine engine that provides propulsion using a combination of a ducted fan and a jet exhaust nozzle. Part of the airstream from the ducted fan passes through the core, providing oxygen to burn fuel to create power. However, the rest of the air flow bypasses...
|
One- or two-stage fan added in front bypasses a proportion of the air through a bypass duct straight to the nozzle/afterburner, avoiding the combustion chamber, with the rest being heated in the combustion chamber and passing through the turbine. 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 707The Boeing 707 is a four-engine commercial passenger jet airliner developed by Boeing in the early 1950s. Its name is most commonly pronounced as "Seven Oh Seven"...
, 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 A turbofan is a type of aircraft gas turbine engine that provides propulsion using a combination of a ducted fan and a jet exhaust nozzle. Part of the airstream from the ducted fan passes through the core, providing oxygen to burn fuel to create power. However, the rest of the air flow bypasses...
|
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 turbofan works on the principle of moving a great deal of air somewhat faster, rather than a small amount extremely fast. Most common form of jet engine in civilian use today- used in airliners like the Boeing 747, most 737s, and all Airbus aircraft. |
Quieter around 10 to 20 percent more than the turbojet engine due to greater mass flow Mass flow rate is the mass of substance which passes through a given surface per unit time. Its unit is mass divided by time, so kilogram per second in SI units, and slug per second or pound per second in US customary units...
and lower total exhaust speed and more efficient for a useful range of subsonic airspeeds for same reason, cooler exhaust temperature. Less noisy and exhibit much better efficiency than low bypass turbofans. |
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. |
RocketA rocket or rocket vehicle is a missile, spacecraft, aircraft or other vehicle which obtains thrust by the reaction of the rocket to the ejection of a jet of fast moving fluid exhaust from a rocket engine. Chemical rockets create their exhaust by the combustion of rocket propellant...
|
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 5.0 or so), thrust/weight ratio over 100, no complex air inlet, high compression ratio, very high speed (hypersonicIn aerodynamics, hypersonic speeds are those that are highly supersonic. Since the 1970s, the term has generally been assumed to refer to speeds of Mach 5 and above...
) 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 impulseSpecific impulse is a way to describe the efficiency of rocket and jet engines. It represents the impulse per unit of propellant. The higher the specific impulse, the less propellant is needed to gain a given amount of momentum...
— 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. |
RamjetA ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of jet engine using the engine's forward motion to compress incoming air, without a rotary compressor...
|
Intake air is compressed entirely by speed of oncoming air and duct shape (divergent), and then it goes through a burner section where it is heated and then passes through a propelling nozzleA propelling nozzle is the component of a jet engine that operates to form an exhaust jet and maximise the thrust from the engine.On non-afterburning engines the nozzle is a fixed size because the differing atmospheric pressure over the operating altitudes makes little difference to the engine...
|
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. |
TurbopropTurboprop engines are a type of aircraft powerplant that use a gas turbine to drive a propeller. The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller...
(TurboshaftA turboshaft engine is a form of gas turbine which is optimized to produce shaft power, rather than jet thrust. In principle, a turboshaft engine is similar to a turbojet, except the former features additional turbine expansion to extract heat energy from the exhaust and convert it into output...
similar) |
Strictly not a jet at all — a gas turbine engine is used as a powerplant to drive a 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 An unducted fan or propfan is a modified turbofan engine, with the fan placed outside of the engine nacelle on the same axis as the compressor blades. Propfans are also known as ultra-high bypass engines and, most recently, open rotor jet engines...
/Unducted Fan |
Turbojet engine that also 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 A pulse-detonation engine, or "PDE", is a type of propulsion system that can operate from subsonic up to hypersonic speeds. In theory the PDE design can produce an engine with a burn efficiency higher than other designs, with considerably fewer moving parts. The burn efficiency of a PDE can exceed...
|
Similar to a pulsejet, but combustion occurs as a detonation Detonation is a process of combustion in which a supersonic shock wave is propagated through a fluid due to an energy release in a reaction zone. It is the more powerful of the two general classes of combustion, the other one being deflagration. In a detonation, the shock compresses the material...
instead of a deflagrationDeflagration is a technical term describing subsonic combustion that usually propagates through thermal conductivity . Most "fire" found in daily life, from flames to explosions, is technically 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 rocketAir-augmented rockets use the supersonic exhaust of some kind of rocket engine to further compress air collected by ram effect during flight to use as additional working mass, leading to greater effective thrust for any given amount of fuel than either the rocket or a ramjet...
|
Essentially a ramjet where intake air is compressed and burnt with the exhaust from a rocket |
Mach 0 to Mach 4.5+ (can also run exoatmospheric), good efficiency at Mach 2 to 4 |
Similar efficiency to rockets at low speed or exoatmospheric, inlet difficulties, a relatively undeveloped and unexplored type, cooling difficulties, very noisy, thrust/weight ratio is similar to ramjets. |
| Scramjet A scramjet is a variation of a ramjet distinguished by supersonic combustion. A scramjet, like a ramjet, essentially consists of a constricted tube through which inlet air is compressed by the high speed of the vehicle, a combustion chamber where fuel is combusted, and a nozzle through which the...
|
Similar to a ramjet without a diffuser; airflow through the entire engine remains supersonic |
Few mechanical parts, can operate at very high Mach numberMach number is the speed of an object moving through air, or any fluid substance, divided by the speed of sound as it is in that substance...
s (Mach 8 to 15) with good efficiencies |
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 turborocket is a type of aircraft engine combining elements of a jet engine and a rocket. It typically comprises a multi-stage fan driven by a turbine, which is driven by the hot gases exhausting from a series of small rocket-like motors mounted around the turbine inlet...
|
A turbojet where an additional oxidizer such as oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
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 LOXLox is salmon fillet that has been cured. In its most popular form, it is thinly sliced—less than in thickness—and, typically, served on a bagel, often with cream cheese and capers....
can be dangerous. Much heavier than simple rockets. |
| Precooled jets / LACE A liquid air cycle engine is a spacecraft propulsion engine that attempts to gain efficiency by gathering part of its oxidizer from the atmosphere...
|
Intake air is chilled to very low temperatures at inlet in a heat exchanger before passing through a ramjet and/or turbojet and/or rocket engine. |
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 The RB545 was an air-breathing rocket engine that was proposed to propel a British space shuttle to orbit using a single stage.-Design:...
, Reaction Engines SABRE, ATREXThe 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 6.0....
. Requires liquid hydrogen fuel which has very low density and heavily insulated tankage. |
Uses
Jet engines are usually used as
aircraft engineAn aircraft engine is a propulsion system for an aircraft. Aircraft engines are almost always either lightweight piston engines or gas turbines. This article is an overview of the basic types of aircraft engines and the design concepts employed in engine development for aircraft.- Engine design...
s for
jet aircraftA jet aircraft is an aircraft propelled by jet engines. Jet aircraft generally fly much faster than propeller-powered aircraft and at higher altitudes — as high as 10,000 to 15,000 meters . At these altitudes, jet engines achieve maximum efficiency over long distances...
. They are also used for
cruise missileA cruise missile is a guided missile that 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. Cruise missiles are generally designed to carry a large conventional or nuclear warhead many...
s and
unmanned aerial vehicleAn unmanned aerial vehicle is an aircraft that flies without a human crew. Their largest uses are in military applications. To distinguish UAVs from missiles, a UAV is defined as a reusable, uncrewed vehicle capable of controlled, sustained, level flight and powered by a jet or reciprocating...
s.
In the form of rocket engines they are used for
fireworksA firework is a low explosive pyrotechnic device used primarily for aesthetic and entertainment purposes. The most common use of a firework is as part of a fireworks display. A fireworks event is a display of the effects produced by firework devices...
, model rocketry,
spaceflightSpaceflight is the use of space technology to achieve the flight of spacecraft into and through outer space.Spaceflight is used in space exploration, and also in commercial activities like space tourism and satellite telecommunications...
, and military
missileA missile is a self-propelled projectile used as a weapon. Missiles are typically propelled by rockets or jet engines. Missiles generally have one or more explosive warheads, although other weapon types may also be used...
s.
Jet engines have also been used to propel high speed cars, particularly drag racers, with the all-time record held by a
rocket carA rocket car is a land vehicle powered by a rocket engine.Because rockets have higher thrust/weight ratios than any jet engine, very large accelerations and high speeds are possible...
. A turbofan powered car
ThrustSSCThrust SSC is a British-designed and built jet-propelled car developed by Richard Noble, Glynne Bowsher, Ron Ayers and Jeremy Bliss....
currently holds the
land speed recordThe land speed record is the fastest speed achieved by any wheeled vehicle on land. There is no single body for validation and regulation; what is used in practice is the Category C flying start regulations, officiated by regional or national organizations under the auspices of the Fédération...
.
Jet engine designs are frequently modified to turn them into gas turbine engines which are used in a wide variety of industrial applications. These include electrical power generation, powering water, natural gas, or oil pumps, and providing propulsion for ships and locomotives. Industrial gas turbine can create up to 50,000 shaft horsepower. Many of these engines are derived from older military turbojets such as the Pratt & Whitney J57 and J75 models. There is also a derivative of the P&W JT8D low-bypass turbofan that creates up to 35,000 HP.
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:
- Cold Section:
- Air intake (Inlet) — For subsonic aircraft, the air intake to a jet engine consists essentially of an opening which is designed to minimise drag. The air reaching the compressor of a normal jet engine must be travelling below the speed of sound, even for supersonic aircraft, to allow smooth flow through compressor and turbine blades. At supersonic flight speeds, shockwaves form in the intake system, these help compress the air, but also there is some inevitable reduction in the recovered pressure at inlet to the compressor. Some supersonic intakes use devices, such as a cone or a ramp, to increase pressure recovery.
- Compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
or FanA mechanical fan is an electrically powered device used to produce an airflow for the purpose of creature comfort , ventilation, exhaust, cooling or any other gaseous transport....
— 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.
- Diffuser section: - This section is a divergent duct that utilizes Bernoulli's principle to decrease the velocity of the compressed air to allow for easier ignition. And, at the same time, continuing to increase the air pressure before it enters the combustion chamber.
- Hot section:
- Combustor
A combustor is a component or area of a gas turbine, ramjet or pulsejet engine where combustion takes place. It is also known as a burner or flame can depending on the design...
or Can or FlameholdersA flame holder is a component of a jet engine designed to help maintain continual combustion.All continuous-combustion jet engines require a flame holder. A flame holder creates a low-speed eddy in the engine to prevent the flame from being blown out...
or Combustion Chamber — This is a chamber where fuel is continuously burned in the compressed air.
-
- Turbine
A turbine is a rotary engine that extracts energy from a fluid or air flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum, with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they...
— 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
An afterburner is an additional component added to some jet engines, primarily those on military supersonic aircraft. Its purpose is to provide a temporary increase in thrust, both for supersonic flight and for takeoff...
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
A nozzle is a mechanical device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....
— 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
A de Laval nozzle is a tube that is pinched in the middle, making an hourglass-shape. It is used as a means of accelerating the flow of a gas passing through it to a supersonic speed...
. 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.
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.
Common types
There are two types of jet engine that are seen commonly today, the turbofan which is used on almost all commercial airliners, and
rocket engineA rocket engine or simply "rocket" is a jet engine[Rocket Propulsion Elements; 7th edition- chapter 1] that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
s which are used for
spaceflightSpaceflight is the use of space technology to achieve the flight of spacecraft into and through outer space.Spaceflight is used in space exploration, and also in commercial activities like space tourism and satellite telecommunications...
and other terrestrial uses such as ejector seats, flares, fireworks etc.
Turbofan engines
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'.
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 as well as higher thrust at low speeds. In addition, the lower exhaust speed gives much lower noise.
In the 1960
s 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 thrustSpecific thrust is a term used in gas turbine engineering to show the relative bulk of a jet engine and is defined as the ratio: net thrust/total intake airflow.-Low specific thrust engines:...
-net thrust divided by airflow) to keep jet noise to a minimum and to improve fuel efficiency. Consequently the
bypass ratioThe term bypass ratio relates to the design of turbofan engines, commonly used in aviation. It is defined as the ratio between the mass flow rate of air drawn in by the fan but bypassing the engine core to the mass flow rate passing through the engine core.A high bypass ratio gives a lower ...
(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.
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.
Rocket engines
A common form of jet engine is the rocket engine.
Rocket engines are used for high altitude flights because they give
very high thrustThrust-to-weight ratio is a ratio of thrust to weight of a rocket, jet engine, propeller engine, or a vehicle propelled by such an engine. It is a dimensionless quantity and is an indicator of the performance of the engine or vehicle....
and their lack of reliance on atmospheric oxygen allows them to operate at arbitrary altitudes.
This is used for launching satellites,
space explorationSpace exploration is the use of astronomy and space technology to explore outer space. Physical exploration of space is conducted both by human spaceflights and by robotic spacecraft....
and manned access, and permitted landing on the moon in 1969.
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 ratioThrust-to-weight ratio is a ratio of thrust to weight of a rocket, jet engine, propeller engine, or a vehicle propelled by such an engine. It is a dimensionless quantity and is an indicator of the performance of the engine or vehicle....
.
An approximate equation for the net thrust of a rocket engine is:
Where is the thrust, is the
specific impulseSpecific impulse is a way to describe the efficiency of rocket and jet engines. It represents the impulse per unit of propellant. The higher the specific impulse, the less propellant is needed to gain a given amount of momentum...
, is a
standard gravityStandard gravity, usually denoted by g0 or gn, is the nominal acceleration due to gravity at the Earth's surface at sea level, defined to be precisely 9.80665 m/s2 . This value was established by the 3rd CGPM...
, is the propellant flow in kg/s, is the area of the exhaust bell at the exit, and is the atmospheric pressure.
General physical principles
All jet engines are reaction engines that generate thrust by emitting a
jetA jet is a coherent stream of fluid that is projected into a surrounding medium, usually from some kind of a nozzle or aperture. Jets can travel long distances without dissipating. In the Earth's atmosphere there exist jet streams that travel thousands of miles.Some animals, notably cephalopods use...
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') as well as in
duct engines (those commonly used on aircraft) by ingesting an external fluid (very typically air) and expelling it at higher speed.
Thrust
The motion impulse of the engine is equal to the fluid mass multiplied by the speed at which the engine emits this mass:
where is the fluid mass per second and 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. (In practice parts of the exhaust may be faster than others, but it is the
average momentum that matters, and thus the important quantity is called the
effective exhaust speed - here.)
However, when a vehicle moves with certain velocity , the fluid moves towards it, creating an opposing ram drag at the intake:
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 different from that of an air breathing jet engine, and thrust is independent of speed.
The jet engine with an intake duct is only useful if the velocity of the gas from the engine, , is greater than the vehicle velocity, , as the net engine thrust is the same as if the gas were emitted with the velocity . So the thrust is actually equal to
This equation shows that as approaches , a greater mass of fluid must go through the engine to continue to accelerate at the same rate, but all engines have a designed limit on this. Additionally, the equation implies that the vehicle can't accelerate past its exhaust velocity as it would have negative thrust.
Energy efficiency
Energy efficiency of jet engines installed in vehicles has two main components,
cycle efficiency - how efficiently the engine can accelerate the jet, and
propulsive efficiency -how much of the energy of the jet ends up in the vehicle body rather than being carried away as kinetic energy of the jet.
Even though overall energy efficiency is simply:
For all jet engines the
propulsive efficiencyIn aircraft and rocket design, overall propulsive efficiency is the efficiency, in percent, with which the energy contained in a vehicle's propellant is converted into useful energy, to replace losses due to air drag, or gravity, or to accelerate the vehicle....
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.(Note:) The exact formula for air-breathing engines moving at speed with an exhaust velocity is given in the literature as: is
And for a rocket:
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 ratioIn aeronautical engineering, the term overall pressure ratio is defined as the ratio of the stagnation pressure as measured at the front and rear of the compressor of a gas turbine engine...
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.
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 ConsumptionSpecific fuel consumption, often shortened to SFC, or TSFC is an engineering term that is used to describe the fuel efficiency of an engine design with respect to thrust output. It allows the efficiency of different sized engines to be directly compared.SFC for thrust engines Specific fuel...
,
Specific impulseSpecific impulse is a way to describe the efficiency of rocket and jet engines. It represents the impulse per unit of propellant. The higher the specific impulse, the less propellant is needed to gain a given amount of momentum...
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.
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 can in some cases actually can give
lower energy efficiency.
It can be seen that the subsonic turbofans such as General Electric's CF6 uses a lot less fuel to generate thrust for a second than Concorde's turbojet, the 593. However, since energy is force times distance and the distance per second is greater for Concorde, the actual power generated by the engine for the same amount of fuel is higher for Concorde at Mach 2 cruise than the CF6- Concorde's engines are more efficient for
thrust per mile, indeed, the most efficient ever.
Thrust-to-weight ratio
The thrust to weight ratio of jet engines of similar principles varies somewhat with scale, but mostly is a function of engine construction technology. Clearly for a given engine, the lighter the engine, the better the thrust to weight is, the less fuel is used to compensate for drag due to the lift needed to carry the engine weight, or to accelerate the mass of the engine.
As can be seen in the following table, rocket engines generally achieve very much higher thrust to weight ratios than duct engines such as turbojet and turbofan engines. This is primarily because rockets almost universally use dense liquid or solid reaction mass which gives a much smaller volume and hence the pressurisation system that supplies the nozzle is much smaller and lighter for the same performance. Duct engines have to deal with air which is 2-3 orders of magnitude less dense and this gives pressures over much larger areas, and which in turn results in more engineering materials being needed to hold the engine together and for the air compressor.
Comparison of types
TurbopropTurboprop engines are a type of aircraft powerplant that use a gas turbine to drive a propeller. The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller...
s 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.
TurbojetTurbojets are the oldest kind of general purpose jet engines. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s, although credit for the first turbojet is given to Whittle who submitted...
s 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 nozzleA de Laval nozzle is a tube that is pinched in the middle, making an hourglass-shape. It is used as a means of accelerating the flow of a gas passing through it to a supersonic speed...
. This is why they are suitable for supersonic and higher speeds.
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
- 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
MachMach number is the speed of an object moving through air, or any fluid substance, divided by the speed of sound as it is in that substance...
0.4 (0.4 times the speed of sound), low bypass turbofans become more effective at about Mach 0.75 and turbojets become more effective than mixed exhaust engines when the speed approaches Mach 2-3.
Rocket engineA rocket engine or simply "rocket" is a jet engine[Rocket Propulsion Elements; 7th edition- chapter 1] that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
s have extremely high exhaust velocity and thus are best suited for high speeds (
hypersonicIn aerodynamics, hypersonic speeds are those that are highly supersonic. Since the 1970s, the term has generally been assumed to refer to speeds of Mach 5 and above...
) 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.
Altitude and speed
With the exception of
scramjetA scramjet is a variation of a ramjet distinguished by supersonic combustion. A scramjet, like a ramjet, essentially consists of a constricted tube through which inlet air is compressed by the high speed of the vehicle, a combustion chamber where fuel is combusted, and a nozzle through which the...
s, jet engines, deprived of their inlet systems can only accept air at around half the speed of sound. The inlet system's job for transonic and supersonic aircraft is to slow the air and perform some of the compression.
The limit on maximum altitude for engines is set by flammability- at very high altitudes the air becomes too thin to burn, or after compression, too hot. For turbojet engines altitudes of about 40 km appear to be possible, whereas for ramjet engines 55 km may be achievable. Scramjets may theoretically manage 75 km. Rocket engines of course have no upper limit.
Flying faster compresses the air in at the front of the engine, but ultimately the engine cannot go any faster without melting. The upper limit is usually thought to be about Mach 5-8, except for scramjets which may be able to achieve about Mach 15 or more, as they avoid slowing the air.
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.
J-58 combined ramjet/turbojet
The
SR-71 BlackbirdThe Lockheed SR-71 is an advanced, long-range, Mach 3 strategic reconnaissance aircraft developed from the Lockheed A-12 and YF-12A aircraft by the Lockheed Skunk Works as a Black project. The SR-71 was unofficially named the Blackbird, and called the Habu by its crews, referring to an Okinawan...
's
Pratt & Whitney J58The Pratt & Whitney J58 was a variable cycle turbojet aircraft engine used on the Lockheed A-12, and subsequently on the YF-12 and SR-71 Blackbird aircraft. It was essentially a hybrid turbojet/ramjet engine....
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. 80% of the SR-71's thrust at high speed was generated in this way, giving much higher thrust, improving
specific impulseSpecific impulse is a way to describe the efficiency of rocket and jet engines. It represents the impulse per unit of propellant. The higher the specific impulse, the less propellant is needed to gain a given amount of momentum...
by 10-15%, and permitting continuous operation at Mach 3.2. The name coined for this setup is
turbo-ramjet.
Hydrogen fuelled air-breathing jet engines
Jet engines can be run on almost any fuel. Hydrogen is a highly desirable fuel, as, although the energy per
moleThe mole is a unit of amount of substance: it is an SI base unit, and one of the few units used to measure this physical quantity. The name "mole" was coined in German by Wilhelm Ostwald in 1893, although the related concept of equivalent mass had been in use at least a century earlier...
is not unusually high, the molecule is very much lighter than other molecules. 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. Also, although not duct engines, hydrogen-fueled rocket engines have seen extensive use.
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 the form of
liquid hydrogenLiquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H
2 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 would end up being very large, and difficult for most airports to accommodate. Finally, pure hydrogen is not found in nature, and must be manufactured either via
steam reformingFossil fuel reforming, hydrogen reforming or catalytic oxidation, is a method of producing useful products, such as hydrogen or ethylene from fossil fuels. Fossil fuel reforming is done through a fossil fuel processor or reformer. At present, the most common fossil fuel processor is a steam...
or expensive
electrolysisIn chemistry and manufacturing, electrolysis is a method of using an electric current to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially highly important as a stage in the separation of elements from naturally-occurring sources such as ores using an electrolytic...
. Nevertheless, research is ongoing and hydrogen-fueled aircraft designs do exist that may be feasible.
Precooled jet engines
An idea originated by Robert P. Carmichael in 1955 is that hydrogen-fueled engines could theoretically have much higher performance than hydrocarbon-fueled 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 Reaction Engines SABRE, that might permit single-stage-to-orbit launch vehicles, and
ATREXThe 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 6.0....
, which could 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 A2The Reaction Engines Limited A2 is a design study for a hypersonic airliner. The airliner is intended to provide environmentally-friendly, long range and high capacity commercial transportation. It is being examined as part of the LAPCAT programme of the European Union...
).
Nuclear-powered ramjet
Project PlutoProject Pluto was a United States government program to develop nuclear powered ramjet engines for use in cruise missiles. Two experimental engines were tested at the United States Department of Energy Nevada Test Site in 1961 and 1964.-History:...
was a nuclear-powered ramjet, intended for use in a
cruise missileA cruise missile is a guided missile that 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. Cruise missiles are generally designed to carry a large conventional or nuclear warhead many...
. 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).
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
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.
Scramjets start working at speeds of at least Mach 4, and have a maximum useful speed of approximately Mach 17. Due to
aerodynamic heatingAerodynamic heating is the heating of a solid body produced by the passage of fluid over a body such as a meteor, missile, or airplane. It is a form of forced convection in that the flow field is created by forces beyond those associated with the thermal processes...
at these high speeds, cooling poses a challenge to engineers.
Environmental considerations
Jet engines are usually run on fossil fuel propellant, and are thus a source of carbon dioxide in the atmosphere. Jet engines can use
biofuelBiofuel is defined as solid, liquid or gaseous fuel obtained from lifeless or living biological material and is similar to fossil fuels, which are derived from long dead biological material. Also, various plants and plant-derived materials are used for biofuel manufacturing...
s or hydrogen, although the production of the latter is usually made from fossil fuels.
Some scientists believe that jet engines are also a source of
global dimmingGlobal dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4%...
due to the water vapour in the exhaust causing cloud formations.
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
Jet engines are usually very reliable and have a very good safety record. However, failures do sometimes occur.
Compressor blade containment
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 within the engine casing. Verification of a jet engine design involves testing that this system works correctly.
Bird strike
Bird strikeA bird strike is a collision between an airborne animal and a man-made vehicle, especially aircraft. It is a common threat to flight safety, and has caused a number of accidents with human casualties...
is an aviation term for a collision between a bird and an aircraft. It is a common threat to aircraft safety and has caused a number of fatal accidents. In 1988 an
Ethiopian AirlinesEthiopian Airlines is an airline headquartered on the grounds of Bole International Airport in Addis Ababa, Ethiopia. It is the national airline of Ethiopia, operating scheduled international passenger and freight services to over 50 destinations worldwide, as well as domestic services to 32...
Boeing 737The Boeing 737 is a short to medium range, single aisle, narrow body jet airliner. Originally developed as a shorter, lower-cost twin-engine airliner derived from Boeing's 707 and 727, the 737 has nine variants with the -600, -700, -800 and -900 currently in production.Originally envisioned in...
sucked pigeons into both engines during take-off and then crashed in an attempt to return to the
Bahir DarBahir Dar is a city in north western Ethiopia, and the capital of the Amhara Region .Administratively, Bahir Dar is considered a Special Zone, placing it midway between Addis Ababa and Dire Dawa which are organized as chartered cities , and cities like Mek'ele and Dessie, which are...
airport; of the 104 people aboard, 35 died and 21 were injured. In another incident in 1995, a
Dassault Falcon 20The Dassault Falcon is a family of business jets, manufactured by Dassault Aviation.Aircraft include:* Dassault Falcon 10 Scaled down Falcon 20...
crashed at a
ParisParis is the capital of France and the country's most populous city. It is situated on the river Seine, in northern France, at the heart of the Île-de-France region...
airport during an emergency landing attempt after sucking
lapwingVanellinae are any of various crested plovers, family Charadriidae, noted for its slow, irregular wing beat in flight and a shrill, wailing cry. Its length is 10-16 inches. They are a subfamily of medium-sized wading birds which also includes the plovers and dotterels. The Vanellinae are...
s into an engine, which caused an engine failure and a fire in the airplane
fuselageThe fuselage is an aircraft's main body section that holds crew and passengers or cargo. In single-engine aircraft it will usually contain an engine, although in some amphibious aircraft the single engine is mounted on a pylon attached to the fuselage which in turn is used as a floating hull...
; all 10 people on board were killed. In 2009, on
US Airways Flight 1549US Airways Flight 1549 was a scheduled commercial passenger flight from New York City to Charlotte, North Carolina, that, on January 15, 2009, was successfully ditched in the Hudson River adjacent to Manhattan six minutes after departing from LaGuardia Airport.While on its initial climb out, the...
, a Airbus A320 aircraft sucked in one bird in each engine. The plane landed in the Hudson River after taking off from LaGuardia International Airport in New York City. There were no fatalities.
Modern jet engines have the capability of surviving an ingestion of a bird. Small fast planes, such as military
jet fightersA fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets by dropping bombs. Fighters are small, fast, and maneuverable...
, are at higher risk than big heavy multi-engine ones. This is due to the fact that the fan of a high-bypass
turbofanA turbofan is a type of aircraft gas turbine engine that provides propulsion using a combination of a ducted fan and a jet exhaust nozzle. Part of the airstream from the ducted fan passes through the core, providing oxygen to burn fuel to create power. However, the rest of the air flow bypasses...
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 ductA bypass duct is an annular passage that allows some of a turbofan's airflow to bypass the engine core, or gas generator. If the turbofan is unmixed, the bypass duct will terminate at the bypass nozzle...
, rather than through the core of the engine, which contains the smaller and more delicate compressor blades.
Military aircraftA military aircraft is any fixed-wing or rotary-wing aircraft that is in the current employ of a military power. Fixed-wing military aircraft are also known as Warplanes....
designed for high-speed flight typically have pure
turbojetTurbojets are the oldest kind of general purpose jet engines. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s, although credit for the first turbojet is given to Whittle who submitted...
, or low-bypass turbofan engines, increasing the risk that ingested materials will get into the core of the engine to cause damage.
The highest risk of the bird strike is during the takeoff and
landingthumb|right|A [[Mute Swan]] alighting. Note the ruffled feathers on top of the wings indicate that the swan is flying at the [[Stall |stall]]ing speed...
, in low
altitudeAltitude is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object...
s, which is in the vicinity of the
airportAn airport is a location where aircraft such as fixed-wing aircraft, helicopters, and blimps take off and land. Aircraft may be stored or maintained at an airport...
s.
Uncontained failures
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 crash of
United Airlines Flight 232United Airlines flight 232 was a scheduled flight from Stapleton International Airport, in Denver, Colorado, to O'Hare International Airport in Chicago, Illinois, and then would continue on to Philadelphia International Airport in Philadelphia, Pennsylvania...
was caused when hydraulic fluid lines for all three independent hydraulic systems were simultaneously severed by shrapnel from an uncontained engine failure. Prior to the United 232 crash, the probability of a simultaneous failure of all three hydraulic systems was considered as high as a billion-to-one. However, the statistical models used to come up with this figure did not account for the fact that the number-two engine was mounted at the tail close to all the hydraulic lines, nor the possibility that an engine failure would release many fragments in many directions. Since then, more modern aircraft engine designs have focused on keeping shrapnel from penetrating the cowling or ductwork, and have increasingly utilized high-strength
composite materialComposite materials are engineered materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct on a macroscopic level within the finished structure.- History :The most primitive composite materials were straw...
s to achieve the required penetration resistance while keeping the weight low.
See also
- Air turboramjet
An air turboramjet is a type of jet engine intended for high speed flight. The engine is somewhat similar to a ramjet, but has a turbocompressor at the inlet, however the compressor turbine is driven by hot gas, usually hydrogen, which is fed from a heat exchanger in the combustion chamber, rather...
- Balancing machine
A balancing machine is a measuring tool used for balancing rotating machine parts such as rotors for electric motors, fans, turbines, disc brakes, disc drives, propellers and pumps. The machine usually consists of two rigid pedestals, with suspension and bearings on top supporting a mounting...
- Jet engine performance
In fixed-wing aircraft driven by one or more jet engines, the performance of the jet engine is important to the operation of the aircraft. Performance of the jet engine includes measurement of thrust, fuel consumption, noise and engine emissions....
- Jet aircraft
A jet aircraft is an aircraft propelled by jet engines. Jet aircraft generally fly much faster than propeller-powered aircraft and at higher altitudes — as high as 10,000 to 15,000 meters . At these altitudes, jet engines achieve maximum efficiency over long distances...
- Jetboat
A jetboat is a boat propelled by a jet of water ejected from the back of the craft. Unlike a powerboat or motorboat that uses a propeller in the water below or behind the boat, a jetboat draws the water from under the boat into a pump-jet inside the boat, then expels it through a nozzle at the...
- Variable Cycle Engine
A Variable Cycle Engine is an engine that is designed to operate efficiently under mixed flight conditions, such as subsonic,transonic and supersonic.The next generation of Supersonic transport may require some form of VCE...
- Pulse jet
- Turborocket
A turborocket is a type of aircraft engine combining elements of a jet engine and a rocket. It typically comprises a multi-stage fan driven by a turbine, which is driven by the hot gases exhausting from a series of small rocket-like motors mounted around the turbine inlet...
- Rocket engine nozzles
A rocket engine nozzle is a propelling nozzle usually of the de Laval type used in a rocket engine to expand and accelerate the combustion gases, from burning propellants, so that the exhaust gases exit the nozzle at hypersonic velocities.-History:...
- Spacecraft propulsion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by exhausting a gas from the...
- Water injection (engines)
In internal combustion engines, water injection, also known as anti-detonant injection, is a method for cooling the combustion chambers of engines by adding water to the cylinder or incoming fuel-air mixture, allowing for greater compression ratios and largely eliminating the problem of engine...
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