Jet engine

(PD) Photo: MSgt Shelly Gill (USAF)
A Pratt & Whitney F100 turbofan engine for the F-15 Eagle and the F-16 Falcon being tested at Robins Air Force Base, Georgia, United States. The tunnel behind the engine muffles noise and allows exhaust to escape.

(PD) Image: National Aeronautics and Space Administration
Simulation of a jet engine in operation.

A jet engine is a reaction engine that discharges a high velocity jet of gas to generate thrust in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, scramjets, pulse jets and pump-jets. In general, most jet engines are internal combustion engines but non-combusting forms also exist.

In some common usage, the term jet engine generally refers to an internal combustion duct engine, which typically consists of an engine with an air compressor powered by a turbine, with the leftover power providing thrust via a propelling nozzle. These types of jet engines are primarily used by jet centrifugal aircraft for long distance travel. The early jet aircraft used turbojet engines which were relatively inefficient for subsonic flight. Modern subsonic jet aircraft usually use high-bypass turbofan engines which help give high speeds as well as giving better fuel efficiency over long distances than many other forms of transport.

History

Jet engines can be dated back to the first century AD, when Hero of Alexandria (a Greek mathematician) invented the aeolipile. This used steam power directed through two jet nozzles so as to cause a sphere to spin rapidly on its axis. So far as is known, it was little used for supplying mechanical power, and the potential practical applications of Hero's invention of the jet engine were not recognized. It was simply considered a curiosity.

Jet propulsion only literally and figuratively took off with the invention of the rocket by the Chinese in the 13th century. Rocket exhaust was initially used in a modest way for fireworks but gradually progressed to propel formidable weaponry; and there the technology stalled for hundreds of years.

(PD) Photo: Flightglobal
The Coandă aircraft built by Henri Coandă for the Paris Flight Salon of 1910 in Paris.

In 1910, Henri Coandă designed, built and piloted the first "thermojet"-powered aircraft, known as the Coandă-1910, which he demonstrated publicly at the second International Aeronautic Salon in Paris. The powerplant used a 4-cylinder piston engine to drive a compressor, which fed two burners for thrust, instead of using a propeller. At the Issy-les-Moulineaux airport 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 often referred to as the C.C.2.

In 1913, René Lorin 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 propeller efficiency. This seemed to peak as blade tips approached the speed of sound. If engine, and thus aircraft, performance were ever to increase beyond such a barrier, a way would have to be found to radically improve the design of the piston engine, or a wholly new type of propulsion would have to be developed. This was the motivation behind the development of the gas turbine engine, commonly called a "jet" engine, which would become almost as revolutionary to aviation as the Wright brothers' first flight.

The earliest attempts at jet engines were hybrid designs in which an external power source first compressed air, which was then mixed with fuel and burned for jet thrust. In one such system, called a thermojet by Secondo Campini but more commonly, motorjet, the air was compressed by a fan driven by a conventional piston engine. Examples of this type of design were Henri Coandă's Coandă-1910 aircraft, and the much later Campini Caproni CC.2, and the Japanese Tsu-11 engine intended to power Yokosuka MXY7 Ohka kamikaze planes towards the end of World War II. None were entirely successful and the CC.2 ended up being slower than the same design with a traditional engine and propeller combination.

The key to a practical jet engine was the gas turbine, used to extract energy from the engine itself to drive the compressor. The gas turbine was not an idea developed in the 1930s: the patent for a stationary turbine was granted to John Barber in England in 1791. The first gas turbine to successfully run self-sustaining was built in 1903 by Norwegian engineer Ægidius Elling. 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 Hungary, Albert Fonó in 1915 devised a solution for increasing the range of artillery, comprising a gun-launched projectile which was to be united with a ramjet propulsion unit. This was to make it possible to obtain a long range with low initial muzzle velocities, allowing heavy shells to be fired from relatively lightweight guns. Fonó submitted his invention to the Austro-Hungarian Army but the proposal was rejected. In 1928 he applied for a German patent on 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 Guillaume. His engine was an axial-flow turbojet.

In 1923, Edgar Buckingham of the U.S. National Bureau of Standards published a report expressing scepticism that jet engines would be economically competitive with propeller 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."

(PD) Photo: Gaius Cornelius (Wikimedia Commons)
This Whittle W.2/700 turbojet engine flew in the Gloster E.28/39, the first British aircraft to fly with a turbojet engine. It also flew in the Gloster Meteor aircraft.

In 1928, RAF College Cranwell cadet Frank Whittle 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 compressor feeding a single-sided centrifugal compressor. Practical axial compressors were made possible by ideas from A.A.Griffith 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.

(CC) Photo: Aerospaceweb.org
The, Heinkel HE 178, which was the world's first jet aircraft.

In 1935, Hans von Ohain started work on a similar design in Germany, 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 Heinkel, one of the larger aircraft industrialists of the day, who immediately saw the promise of the design. Heinkel had recently purchased the Hirth engine company, and Ohain and his master machinist Max Hahn were set up there as a new division of the Hirth company. They had their first Heinkel HeS 1 centrifugal engine running by September 1937. Unlike Whittle's design, Ohain used hydrogen as fuel, supplied under external pressure. Their subsequent designs culminated in the gasoline-fuelled Heinkel HeS 3 engine which was fitted to Heinkel's simple and compact Heinkel He 178 airframe and flown by Erich Warsitz in the early morning of August 27, 1939, from the Rostock-Marienehe aerodrome. The He 178 was the world's first jet plane.

The world's first turboprop aircraft was the Jendrassik Cs-1 designed by the Hungarian mechanical engineer György Jendrassik. It was produced and tested in the Ganz factory in Budapest 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 he started receiving Air Ministry money. In 1941, a flyable version of his engine was fitted to the Gloster E28/39 airframe specially built for it, and first flew on May 15, 1941 at RAF Cranwell.

(CC) Photo: Ian Dunster (Commons)
An early centrifugal jet engine (DH Goblin II) sectioned to show internal components.

A Scottish aircraft engine designer, Frank Halford, working from Whittle's ideas developed a "straight through" version of the centrifugal jet which became the de Havilland Goblin.

One problem with both of these early designs, which are called centrifugal-flow 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 superchargers, 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 for aircraft propulsion 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.

(PD) Photo: National Museum of the U.S. Air Force
A cutaway of the Junkers Jumo 004 jet engine.

Austrian Anselm Franz of Junkers' aircraft 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 compressed 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 Reich Air Ministry's numbering sequence, 4, and the result was the Jumo 004 engine. After many lesser technical difficulties were solved, mass production of this engine started in 1944 as a powerplant for the world's first jet-fighter aircraft, the Messerschmitt Me 262 (and later the world's first jet-bomber aircraft, the Arado Ar 234). 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 II. Nonetheless, it will be remembered as the first use of jet engines in service.

In the United Kingdom, their first axial-flow engine, the Metrovick F.2, was first flown in 1943. Although more powerful than the centrifugal designs at the time, its complexity and unreliability was considered a drawback in wartime. The work at Metrovick led to the Armstrong Siddeley Sapphire engine which would be built in the United States of America 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 U.S. 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 damage than axial-flow compressor engines.

Although German designs were more advanced aerodynamically, the combination of simplicity and Britain's war-time availability of requisite rare metals for the necessary advanced metallurgy (such as tungsten, chromium and titanium) for high-stress components such as turbine blades and bearings meant that Whittle-derived designs were generally more reliable than their German counterparts. British engines were also widely manufactured under license in the United States, and were sold to Soviet Russia who reverse-engineered them with the Rolls-Royce Nene engine 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 engine performance until the 1960s, although the General Electric J47 engine provided excellent service in the F-86 Sabre 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 Comet and Avro Canada Jetliner. By the 1960s, all large civilian aircraft were also jet powered, leaving the piston engine in low-cost niche roles such as cargo flights.

Relentless improvements in the turboprop pushed the internal combustion piston engine out of the mainstream entirely, leaving it serving only the smallest general aviation designs. 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, only then did the fuel efficiency finally exceed that of the best piston and propeller engines. The dream of fast, safe, economical travel around the world finally arrived.

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 jet	For propelling water rockets and jetboats; squirts water out the back through a nozzle	Can run in shallow water, high acceleration, no risk of engine overload (unlike propellers), less noise and vibration, highly maneuverable, 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	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 and Caproni Campini N.1.
Turbojet	A tube with a compressor and turbine having a common shaft (with a burner in between) and a propelling nozzle for the exhaust. Uses high exhaust gas velocity to produce thrust. Has much higher core flow than bypass type engines	Simplicity of design, efficient at supersonic speeds (~Mach 2)	A basic design, relatively noisy.
Low-bypass Turbofan	One- or two-stage fan bypasses some air directly to nozzle/afterburner, avoiding combustion chamber. The rest is routed through combustion chamber and then turbine. Compared with its turbojet ancestor, is more efficient and somewhat less noisey. Used in high-speed military aircraft, some smaller private jets, and older civilian airliners such as the Boeing 707, McDonnell Douglas DC-8, and their derivatives.	As with the turbojet, has aerodynamic design, with only modest increase in diameter over the turbojet required for bypass fan and combustion chamber. Capable of supersonic speeds with minimal thrust decrease at high speeds and altitudes, yet more efficient than the turbojet at subsonic flight.	Noisier and less efficient than high-bypass turbofan, with less static (Mach 0) thrust. Added complexity for dual shaft designs. More inefficient than turbojet at about Mach 2.
High-bypass Turbofan	First stage compressor drastically enlarged to provide bypass airflow around engine core, and provides significant amounts of thrust. Compared to low-bypass turbofan and no-bypass turbojet, it 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.	About 10 to 20 percent less noisy than turbojet engine due to greater mass flow and lower exhaust velocity.. More efficient for a useful range of subsonic airspeeds for same reason. Cooler exhaust temperature. Less noisy and much better efficiency than low bypass turbofans.	Greater complexity (more ducting, usually multiple shafts) and the need to use heavy blades. Fan diameter can be extremely large, especially in high bypass turbofans. More subject to ice and foreign object damage (FOD). Top speed limited by potential for shockwaves damaging engine. Thrust lapse at higher speeds requires huge diameters, resulting in more drag.
Rocket engine	Carries all propellants and oxidants on-board, emits jet for propulsion	Few moving parts, Mach 0 to Mach 25+, efficient at high speed (about Mach 5.0 or higher), thrust/weight ratio over 100, simple air inlet, high compression ratio, high speed (hypersonic) exhaust, good cost/thrust ratio, works in a vacuum which is kinder on vehicle structure, small surface area to keep cool, and no turbine in hot exhaust stream.	Needs lots of propellant — very low specific impulse — typically 100-450 seconds. Extreme thermal stresses of combustion chamber can make reuse harder. Typically requires carrying an oxidizer on-board which increases risks. Extraordinarily noisy.
Ramjet	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 nozzle	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.	High initial speed needed to function, inefficient at slow speeds due to poor compression ratio, difficult to arrange shaft power for accessories, limited to small range of speeds, intake flow must be slowed to subsonic speeds, noisy, difficult to test, and difficult to keep lit.
Turboprop (Turboshaft is 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 / 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	Similar to a pulsejet, but combustion occurs as a detonation instead of a deflagration, may or may not need valves	Maximum theoretical engine efficiency	Extremely noisy, parts subject to extreme mechanical fatigue, hard to start detonation, not practical for current use
Air-augmented rocket	Essentially a ramjet where intake air is compressed and burnt with the exhaust from a rocket	Mach 0 to Mach 4.5+ (can also run exoatmospheric), good efficiency at Mach 2 to 4	Similar efficiency to rockets at low speed or exoatmospheric, cooling and inlet problems, relatively undeveloped and unexplored, very noisy, thrust/weight ratio similar to ramjets.
Scramjet	Similar to a ramjet without a diffuser; airflow through the entire engine remains supersonic	Few mechanical parts, can operate at very high Mach numbers (Mach 8 to 15) with good efficiencies	Still in development stages, must have a very high initial speed to function (Mach >6), cooling difficulties, very poor thrust/weight ratio (~2), extreme aerodynamic complexity, airframe difficulties, testing difficulties/expense
Turborocket	A turbojet with an additional oxidizer, such as oxygen, 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, oxidizers like liquid oxygen (LOX) can be dangerous. Much heavier than simple rockets.
LACE	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. Very high thrust/weight ratios possible (~14). Good fuel efficiency over wide range of airspeeds (Mach 0-5.5+). Features 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, Reaction Engines SABRE and ATREX. Requires liquid hydrogen fuel which has very low density and heavily insulated tankage.

Uses

Jet engines are usually for the propulsion of jet aircraft, cruise missiles and unmanned aeronautical vehicles. In the form of rocket engines they are used for spacecraft, fireworks and model rocketry.

Jet engines have also been used to propel high speed cars, particularly drag racers, with the all-time record held by a rocket car. A turbofan powered car (ThrustSSC) currently holds the land speed record.

Jet engine designs are often converted into gas turbine engines are used in a wide variety of industrial applications. These include electrical power generation, driving water, natural gas, or oil pumps, and providing propulsion for ships and locomotives. Industrial gas turbines can create up to 50,000 shaft horsepower.

Major components

(CC) Image: Jeff Dahl (Commons) and Airplane Flying Handbook (FAA)
A typical gas turbine jet engine (also known as a turbojet. Air is compressed by the fan blades as it enters the engine, and it is mixed and burned with fuel in the combustion section. The hot exhaust gases provide forward thrust and turn the turbines which drive the compressor fan blades.

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 or Fan: The compressor is made up of stages. Each stage consists of vanes which rotate, and stators which remain stationary. As air is drawn deeper through the compressor, its heat and pressure increases. Energy is derived from the turbine (see below) and 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 or Can or Flameholders or Combustion Chamber: This is a chamber where fuel is continuously burned in the compressed air.

Turbine: The turbine is a series of bladed discs that act like a windmill, gaining energy from the hot gases leaving the combustor. Some of this energy is used to drive the compressor, and in some turbine engines (i.e., turboprop, turboshaft or turbofan engines), energy is extracted by additional turbine discs and used to drive devices such as propellers, bypass fans or helicopter rotors. One type, a free turbine, is configured such that the turbine disc driving the compressor rotates independently of the discs that power the external components. Relatively cool air, bled from the compressor, may be used to cool the turbine blades and vanes, to prevent them from melting.

Afterburner or Reheat: Produces extra thrust by burning extra fuel, usually inefficiently, to raise the nozzle entry temperature at the exhaust. Owing to a larger volume flow (i.e., lower density) at exit from the afterburner, an increased nozzle flow area is required, to maintain satisfactory engine matching, when the afterburner is alight.

Exhaust or Nozzle: Hot gases leaving the engine exhaust to atmospheric pressure via a nozzle, the objective being to produce a high velocity jet. In most cases, the nozzle is convergent and of fixed flow area.

Supersonic nozzle: If the nozzle pressure ratio (nozzle entry pressure/ambient pressure) is very high, it may be worthwhile to use a convergent-divergent (de Laval) nozzle to maximize the thrust. 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 optimum efficiency or performance. The performance and efficiency of an engine can never be taken in isolation. For example, the fuel/distance efficiency of a supersonic jet engine has reaches a maximum at about Mach 2, whereas the drag for the vehicle propelled by the engine increases which creates much extra drag in the transonic region. Thus, the highest fuel efficiency for the overall vehicle occurs typically at about Mach ~0.85.

The factors that affect the optimisation of efficiency or performance are: 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 others.

Common types

On aircraft, the most common type of jet engine is the turbofan. There is some use of ramjets for guided missiles, and pump jets for propulsion of watercraft. Spaceflight and other terrestrial uses such as ejector seats, flares, fireworks etc.

Most modern aircraft 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 1960s there was little difference between civil and military jet engines, apart from the use of afterburning in some (supersonic) applications. Civil turbofans today have a low exhaust speed (low specific thrust -net thrust divided by airflow) to keep jet noise to a minimum and to improve fuel efficiency. Consequently the bypass ratio (bypass flow divided by core flow) is relatively high (ratios from 4:1 up to 8:1 are common). Only a single fan stage is required, because a low specific thrust implies a low fan pressure ratio.

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.

General physical principles

All jet engines are reaction engines that generate thrust by emitting a jet of fluid rearwards at relatively high velocity. 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 produce the jet by using propellant from tankage attached to the engine (as in a rocket) as well as in duct engines (those commonly used on aircraft) by intaking an external fluid (very typically air) and expelling it at higher speed.

Thrust

The net thrust (F_N) of a turbojet is given by:

$F_{N}=({\dot {m}}_{air}+{\dot {m}}_{fuel})v_{e}-{\dot {m}}_{air}v$

where:
ṁ air	= the mass rate of air flow through the engine
ṁ fuel	= the mass rate of fuel flow entering the engine
ve	= the velocity of the jet (the exhaust plume) and is assumed to be less than sonic velocity
v	= the velocity of the air intake = the true airspeed of the aircraft
(ṁ air + ṁ fuel)ve	= the nozzle gross thrust (FG)
ṁ air v	= the ram drag of the intake air

The above equation applies only for air-breathing jet engines. It does not apply to rocket engines. Most types of jet engine have an air intake, which provides the bulk of the fluid exiting the exhaust. Conventional rocket engines, however, do not have an intake, the oxidizer and fuel both being carried within the vehicle. Therefore, rocket engines do not have ram drag and the gross thrust of the rocket engine 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 velocity. (See Rocket engine)

If the velocity of the jet from a jet engine is equal to sonic velocity, the jet engine's nozzle is said to be choked. If the nozzle is choked, the pressure at the nozzle exit plane is greater than atmospheric pressure, and extra terms must be added to the above equation to account for the pressure thrust.

The rate of flow of fuel entering the engine is very small compared with the rate of flow of air. If the contribution of fuel to the nozzle gross thrust is ignored, the net thrust is:

$F_{N}={\dot {m}}_{air}(v_{e}-v)$

The velocity of the jet (v_e) must exceed the true airspeed of the aircraft (v) if there is to be a net forward thrust on the aircraft. The velocity (v_e) can be calculated thermodynamically based on adiabatic expansion.

Energy efficiency

(PD) Graph: Milton Beychok
Dependence of propulsion efficiency (η) upon the vehicle speed/exhaust velocity ratio (v/v_e) for air-breathing jet and rocket engines.

The energy efficiency ($\eta$) of jet engines installed in vehicles has two main components:

• propulsive efficiency ($\eta _{p}$): 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.
• cycle efficiency ($\eta _{v_{e}}$): how efficiently the engine can accelerate the jet

Even though overall energy efficiency $\eta$ is simply:

$\eta =\eta _{p}\eta _{v_{e}}$

For all jet engines the propulsive efficiency is highest when the engine emits an exhaust jet at a velocity that is the same as, or nearly the same as, the vehicle speed as this gives the smallest residual kinetic energy. The exact formula for air-breathing engines moving at speed $v$ with an exhaust velocity $v_{e}$ is given in the literature as:

$\eta _{p}={\frac {2}{1+{\frac {v_{e}}{v}}}}$

And for a rocket:

$\eta _{p}={\frac {2\,({\frac {v}{v_{e}}})}{1+({\frac {v}{v_{e}}})^{2}}}$

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

(CC) Graph: Kashif Khan
Specific impulse as a function of exhaust jet velocity for different jet engine types using jet fuel (kerosene) having a heat of combustion of about 46 MJ/kg. Also indicates what the graphed maximums would be for jet engines using hydrogen having a heat of combustion of 142 MJ/kg or using a hydrocarbon having a heat combustion of 42 MJ/kg.

Fuel/propellant consumption

A closely related (but different) concept to energy efficiency is the rate of consumption of propellant mass. Propellant consumption in jet engines is measured by Specific Fuel Consumption, Specific impulse or Effective exhaust velocity. They all measure the same thing, specific impulse and effective exhaust velocity are strictly proportional, whereas specific fuel consumption is inversely proportional to the others.

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 in the table (just below) that the subsonic turbofans such as General Electric's CF6 turbofan uses a lot less fuel to generate thrust for a second than does the Concorde's Rolls-Royce/Snecma Olympus 593 turbojet. 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 than the CF6. Thus, the Concorde's engines are more efficient in terms of thrust per mile.

Engine type	Scenario	SFC lb/(lbf·h)	SFC g/(kN·s)	Specific impulse (s)	Effective exhaust velocity (m/s)
NK-33 rocket engine	Vacuum	10.9	309	331	3,240
SSME rocket engine	Space shuttle vacuum	7.95	225	453	4,423
Ramjet	Mach 1	4.5	127	800	7,877
J-58 turbojet	SR-71 at Mach 3.2 (Wet)	1.9	53.8	1,900	18,587
Rolls-Royce/Snecma Olympus 593	Concorde Mach 2 cruise (Dry)	1.195	33.8	3,012	29,553
CF6-80C2B1F turbofan	Boeing 747-400 cruise	0.605	17.1	5,950	58,400
General Electric CF6 turbofan	Sea level	0.307	8.696	11,700	115,000
Note: SFC = Specific fuel consumption

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.

Jet engine or rocket engine	Mass, kg	Jet or rocket thrust, kN	Thrust-to-weight ratio
RD-0410 nuclear rocket engine	2,000	35.2	1.8
J-58 (SR-71 Blackbird jet engine)	2,722	150	5.2
Concorde's Rolls-Royce engine with reheat	3,175	169.2	5.4
Pratt & Whitney F119	1,800	91	7.95
RD-0750 rocket engine, three-propellant mode	4,621	1,413	31.2
RD-0146 rocket engine	260	98	38.5
Space Shuttle's SSME rocket engine	3,177	2,278	73.2
RD-180 rocket engine	5,393	4,152	78.6
F-1 rocket engine (Saturn V first stage)	8,391	7,740.5	94.1
NK-33 rocket engine	1,222	1,638	136.8
Merlin 1D rocket engine	440	690	160
Note: All rocket thrusts are vacuum thrusts unless otherwise noted

Comparison of types

(PD) Diagram: Federal Aviation Administration
Air enters the turbofan engine and splits into two paths. Some air bypasses the engine core and is called the bypass air. The rest of the air flows through the core engine and is combusted with fuel to form hot exhaust gas. The exhaust gas jet includes the bypass air and the hot engine exhaust gas.

Turbojets accelerate a much smaller mass of intake air and burned fuel, but they emit it at the much higher speeds which are made possible by using a de Laval nozzle to accelerate the engine exhaust. This is why they are suitable for aircraft traveling at supersonic and higher speeds.

Turbofans have a mixed exhaust consisting of the bypass air and the hot combustion product gas from the core engine. The amount of air that bypasses the core engine compared to the amount flowing into the engine determines what is called a turbofan’s bypass ratio (BPR).

While a turbojet engine uses all of the engine's output to produce thrust in the form of a hot high-velocity exhaust gas jet, a turbofan's cool low-velocity bypass air yields between 30 percent and 70 percent of the total thrust produced by a turbofan system.

The net thrust (F_N) generated by a turbofan is:

$F_{N}={\dot {m}}_{e}v_{e}-{\dot {m}}_{o}v_{o}+BPR\,({\dot {m}}_{c}v_{f})$

where:

ṁ e	= the mass rate of hot combustion exhaust flow from the core engine
ṁo	= the mass rate of total air flow entering the turbofan = ṁc + ṁf
ṁc	= the mass rate of intake air that flows to the core engine
ṁf	= the mass rate of intake air that bypasses the core engine
vf	= the velocity of the air flow bypassed around the core engine
ve	= the velocity of the hot exhaust gas from the core engine
vo	= the velocity of the total air intake = the true airspeed of the aircraft
BPR	= Bypass Ratio

Rocket engines have extremely high exhaust velocity and thus are best suited for high hypersonic speeds and high altitudes. The thrust and efficiency of a rocket motor improves slightly with increasing altitude (because the back-pressure decreases, thus increasing net thrust at the nozzle exit plane), whereas with a turbojet (or turbofan) the decreasing 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 scramjets, 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 have no upper limit.

Flying faster compresses the air in front of the engine, but ultimately the engine cannot go any faster. 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.

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 velocity exhaust jets emitted from engines such as high bypass turbofans are the quietest, whereas the fastest jets are the loudest.

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

Advanced designs

J-58 combined ramjet/turbojet

The SR-71 Blackbird's Pratt & Whitney J58 engines were rather unusual. They could convert in flight from being largely a turbojet to being largely a compressor-assisted ramjet. At high speeds (above Mach 2.4), the engine used variable geometry vanes to direct excess air through 6 bypass pipes from downstream of the fourth compressor stage into the afterburner. 80% of the SR-71's thrust at high speed was generated in this way, giving much higher thrust, improving specific impulse by 10-15%, and permitting continuous operation at Mach 3.2. The name coined for this setup is turbo-ramjet.

Hydrogen fuelled air-breathing jet engines

Jet engines can be run on almost any fuel. Hydrogen is a highly desirable fuel because its Heat of combustion 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 hydrogen 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 reforming or expensive electrolysis. 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 ATREX, 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 A2).

Nuclear-powered ramjet

Project Pluto was a nuclear-powered ramjet, intended for use in a cruise missile. Rather than combusting fuel as in regular jet engines, air was heated using a high-temperature, unshielded nuclear reactor. This dramatically increased the engine burn time, and the ramjet was predicted to be able to cover any required distance at supersonic speeds (Mach 3 at tree-top height).

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 heating at these high speeds, cooling poses a challenge to engineers.

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 strike 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 Airlines Boeing 737 sucked pigeons into both engines during take-off and then crashed in an attempt to return to the Bahir Dar airport; of the 104 people aboard, 35 died and 21 were injured. In another incident in 1995, a Dassault Falcon 20 crashed at a Paris airport during an emergency landing attempt after sucking lapwings into an engine, which caused an engine failure and a fire in the airplane fuselage; all 10 people on board were killed. A U.S. Airways Airbus A320 aircraft sucked in one bird in each engine and 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 fighters, are at higher risk than big heavy multi-engine ones. This is due to the fact that the fan of a high-bypass turbofan engine, typical on transport aircraft, acts as a centrifugal separator to force ingested materials (birds, ice, etc.) to the outside of the fan's disc. As a result, such materials go through the relatively unobstructed bypass duct, rather than through the core of the engine, which contains the smaller and more delicate compressor blades. Military aircraft designed for high-speed flight typically have pure turbojet, or low-bypass turbofan engines, increasing the risk that ingested materials will get into the core of the engine to cause damage.

The highest risk of the bird strike is during the takeoff and landing, in low altitudes, which is in the vicinity of the airports.

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 232 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 materials to achieve the required penetration resistance while keeping the weight low.