Military jet engines
A jet engine is an engine that discharges a fast moving jet of fluid to generate thrust in accordance with Newton’s third law of motion. This broad definition of jet engines includes turbojets, turbofans, rockets and ramjets and water jets, but in common usage, the term generally refers to a gas turbine used to produce a jet of high speed exhaust gases for special propulsive purposes.
History
In 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. However, engineers were beginning to realize conceptually 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 powerplant would have to be developed. This was the motivation behind the development of the gas turbine engine, commonly called a “jet” engine, which would become almost as revolutionary to aviation as the Wright brothers’ first flight.
The key to a practical jet engine was the gas turbine, used to extract energy to drive the compressor from the engine itself. In 1929, Aircraft apprentice Frank Whittle formally submitted his ideas for a turbo-jet to his superiors. 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. Whittle would later concentrate on the simpler centrifugal compressor only, for a variety of practical reasons. In 1935 Hans von Ohain started work on a similar design in Germany, seemingly unaware of Whittle’s work. Whittle had his first engine running in April 1937. It was liquid-fuelled, and included a self-contained fuel pump. Von Ohain’s engine, as well as being 5 months behind Whittle’s, relied on gas supplied under external pressure, so was not self-contained. Whittle unfortunately failed to secure proper backing for his project, and so fell behind Von Ohain in the race to get a jet engine into the air.
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. 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. 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.
Austrian Anselm Franz 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. Jumo was assigned the next engine number, 4, and the result was the Jumo 004 engine. After many lesser technical difficulties were solved, mass production of this engine started in 1944 as a powerplant for the world’s first jet-fighter aircraft, the Messerschmitt Me 262. After the end of the war the German Me 262 aircraft were extensively studied by the victorious allies and contributed to work on early Soviet and US jet fighters.
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. Also, its engine components are robust; axial-flow compressors are more liable to foreign object damage.
British engines also were licensed widely in the US. Their most famous design, the Nene would also power the USSR’s jet aircraft after a technology exchange. American designs would not come fully into their own until the 1960s.
Types
There are many different types of jet engines, all of which get propulsion from a high speed exhaust jet. Some of the most important types are listed below.
| Type | Description | Advantages | Disadvantages |
| Turbojet | Generic term for simple turbine engine | Simplicity of design | Basic design, misses many improvements in efficiency and power |
| Turbofan | First stage compressor greatly enlarged to provide bypass airflow around engine core | Quieter due to greater mass flow and lower total exhaust speed, more efficient for a useful range of subsonic airspeeds for same reason, cooler exhaust temperature | Greater complexity (additional ducting, usually multiple shafts), large diameter engine, need to contain heavy blades. More subject to FOD and ice damage. Top speed is limited due to the potential for shockwaves to damage engine |
| Ramjet | Intake air is compressed entirely by speed of oncoming air and duct shape (divergent) | Very few moving parts, Mach 0.8 to Mach 5+, efficient at high speed (> Mach 2.0 or so), lightest of all airbreathing jets (thrust/weight ratio up to 30 at optimum speed) | Must have a high initial speed to function, 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 kept lit. |
| 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 |
| 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 |
| Rocket | Carries all propellants onboard, emits jet for propulsion | Very few moving parts, Mach 0 to Mach 25+, efficient at very high speed (> Mach 10.0 or so), thrust/weight ratio over 100, no complex air inlet, high compression ratio, very high speed (hypersonic) exhaust, good cost/thrust ratio, fairly easy to test, works in a vacuum-indeed works best exoatmospheric which is kinder on vehicle structure at high speed. | Needs lots of propellant- very low specific impulse typically 100-450 seconds. Extreme thermal stresses of combustion chamber can make reuse harder. Typically requires carrying oxidiser onboard which increases risks. Extraordinarily noisy. |
Turbojet engines
A turbojet engine is a type of internal combustion engine often used to propel aircraft. Air is drawn into the rotating compressor via the intake and is compressed, through successive stages, to a higher pressure before entering the combustion chamber. Fuel is mixed with the compressed air and ignited by flame in the eddy of a flame holder. This combustion process significantly raises the temperature of the gas. Hot combustion products leaving the combustor expand through the turbine, where power is extracted to drive the compressor. Although this expansion process reduces both the gas temperature and pressure at exit from the turbine, both parameters are usually still well above ambient conditions. The gas stream exiting the turbine expands to ambient pressure via the propelling nozzle, producing a high velocity jet in the exhaust plume. If the jet velocity exceeds the aircraft flight velocity, there is a net forward thrust upon the airframe.
Under normal circumstances, the pumping action of the compressor prevents any backflow, thus facilitating the continuous-flow process of the engine. Indeed, the entire process is similar to a four-stroke cycle, but with induction, compression, ignition, expansion and exhaust taking place simultaneously, but in different sections of the engine. The efficiency of a jet engine is strongly dependent upon the overall pressure ratio (combustor entry pressure/intake delivery pressure) and the turbine inlet temperature of the cycle.
It is also perhaps instructive to compare turbojet engines with propeller engines. Turbojet engines take a relatively small mass of air and accelerate it by a large amount, whereas a propeller takes a large mass of air and accelerates it by a small amount. The high-speed exhaust of a jet engine makes it efficient at high speeds (especially supersonic speeds) and high altitudes. On slower aircraft and those required to fly short stages, a gas turbine-powered propeller engine, commonly known as a turboprop, is more common and much more efficient. Very small aircraft generally use conventional piston engines to drive a propeller but small turboprops are getting smaller as engineering technology improves.
The turbojet described above is a single-spool design, in which a single shaft connects the turbine to the compressor. Higher overall pressure ratio designs often have two concentric shafts, to improve compressor stability during engine throttle movements. The outer high pressure (HP) shaft connects the HP compressor to the HP turbine. This HP Spool, with the combustor, forms the core or gas generator of the engine. The inner shaft connects the low pressure (LP) compressor to the LP Turbine to create the LP Spool. Both spools are free to operate at their optimum shaft speed.
Turbofan engines
Most modern jet engines are actually turbofans, where the low pressure compressor acts as a fan, supplying supercharged air to not only the engine core, but to a bypass duct. The bypass airflow either passes to a separate ‘cold nozzle’ or mixes with low pressure turbine exhaust gases, before expanding through a ‘mixed flow nozzle’.
Forty years ago there was little difference between civil and military jet engines, apart from the use of afterburning in some (supersonic) applications.
Civil turbofans today have a 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 little consequence. Multi-stage fans are normally required to achieve the relatively high fan pressure ratio needed for a high specific thrust. Although high turbine inlet temperatures are frequently employed, the bypass ratio tends to be low (usually significantly less than 2.0).
Comparative suitability for turboshaft, low bypass and turbojet to fly at 10 km attitude in various speeds.
Comparative suitability for (left to right) turboshaft, low bypass and turbojet to fly at 10 km attitude in various speeds. Horizontal axis - speed, m/s. Vertical axis carries only logical meaning.
Efficiency as a function of speed of different jet typesEfficiency as a function of speed of different Jet types. Although efficiency plummets with speed, greater distances are covered, it turns out that efficiency per unit distance (per km or mile) is roughly independent of speed for Jet engines as a group.
Manufactures
The main manufactures of military jet engines today are :
* Pratt & Whitney (US; F-16, F-22)
* General Electric (US; B-1, B-2)
* Rolls-Royce (UK; Harrier)
* Tumansky (Soviet Union; Mig-25, Mig-29)
* Lyulka/Saturn (Soviet Union; SU-27, SU-37)
* Klimov (Soviet Union; Mig-17)
* Turbo-Union (UK, Germany, Italy; Tornado)
* EuroJet (UK, Germany, Italy, Spain; Eurofighter Typhoon)
* SNECMA (France; Mirage-2000, Rafale)
