|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.|
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.
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 (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 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.
Note: Part of the text is from Wikipedia.
"After World War Two, piston engines continued to power civil airliners for many years, but in the field of military aircraft they were rapidly displaced by the gas turbine. Fighters and bombers switched to the turbojet, transports and maritime-patrol aircraft used turboprops, and helicopters benefited greatly from changing to turboshaft engines. The change meant more power for less weight, far greater reliability, no cooling problems and safer kerosene-type fuels.
With extraordinary reluctance, designers eventually recognized that the turbofan, offering a wide choice of bypass ratio (BPR - the mass flow of air in the bypass duct divided by that through the core), could with advantage replace the turbojet. In supersonic aircraft the need to minimize frontal area means that BPR is seldom as high as 1, and even then the installation must be done with great care. When the J79 turbojet of 79.63 kN thrust installed in the McDonnell Douglas F-4 Phantom was replaced in the British versions by the Rolls-Royce Spey turbofan of 91.25 kN the change made the aircraft slower in level flight, while giving improvements in take-off and climb performance!
Today the turbojet is almost extinct, except for some countries like China, where different criteria apply. Elsewhere, the trend has been towards achieving greater power with engines that are not only lighter but also smaller and dramatically simpler. For example, the Spey Mk 202, the engine of the RAF Phantoms, had a total of 17 stages of blading in the compressors (5+12 flow pressure+high pressure) and four stages of blading in the turbines (2+2). The next-generation RB. 199, engine of the Tornado, has 12 stages of compression (3+3+6) and again four stages of expansion through the turbines (1+1+2), whereas today's Eurojet EJ200, engine of the Eurofighter, has only eight compressor stages (3+5) and two turbine stages (1+ 1).
In general, the more stages of blading an axial-flow compressor has, the greater the overall pressure ratio (OPR) and the better the fuel economy (and thus, for a given aircraft tankage, the greater the range and endurance). One might therefore think that the simpler compressors have been achieved at the expense of more rapid fuel burn, but in fact the reverse is true. The OPR of the Phantom's Spey was 20, the figure for the Tornado engine is 23, and for the Eurofighter it has gone up to 26. Indeed, the next-generation fighter engine could have an OPR of 35, with only six or seven stages of blading.
Simpler engines mean greater reliability, better resistance to battle damage, easier maintenance, and several other advantages including lower cost, though cost is not as dominant as it is in the civil sector. In the immediate postwar era, up to 1970, it was normal practice not to introduce an engine to the airlines until hundreds or even thousands had gained experience in fighters and bombers. The two families then diverged. Airliners needed engines offering the lowest possible fuel consumption and lowest possible noise at airports, and these (surprisingly slowly) eventually led to today's engines with a BPR of from 5 to 9, with enormous fans. Combat aircraft need slim engines, as already noted, so military experience is seldom much help to civil engines (though the best-selling CFM56 has the core of a long-established military engine, the F101 used in the Rockwell B-1B Lancer).
Today, the military trend towards greater simplicity is being echoed by civil engines. Nearly 30 years ago, special turbojets and turbofans were being produced purely to lift VTOL (vertical take-off and landing) aircraft. They were used only at take-off and landing, so were made as simple as possible. Like other engines, they sometimes had two spools (low-pressure and high-pressure compressors, each driven by its own turbine), and the aerodynamicists found that by making the spools rotate in opposite daemons, it was possible to do away with at least some of the stator (fixed) blades ahead of the turbine rotors.
Apart from Concorde, which has a low-augmentation form of reheat (afterburning), civil aircraft do not burn fuel in the jetpipe downstream of the turbines. Supersonic aircraft have afterburning engines in order to increase the energy in the jet so that, properly expanded in a special nozzle, it can be ejected at highly supersonic speed, in order to achieve the highest flight Mach number possible. Such aircraft as the MiG-25 Foxbat and Lockheed SR-71 Blackbird can fly at Mach 3 (three times the speed of sound).
Today's fighters have augmentation, the name given to burning extra fuel downstream of the turbines of a turbofan engine. With a turbofan there is abundant oxygen in the mixed flow in the jetpipe, much of which has not passed through the core and thus has had no fuel already burned in it. In any case, the latest engines are so powerful that augmentation is needed only on rare occasions (for example, in close combat) when maximum thrust is needed, because it burns fuel rapidly and also shortens engine life.
Thus, many modem fighters are capable of making a cold (unaugmented) take-off. The first to do this were the Grumman F-14B and F-14D re-engined versions of the Tomcat naval fighter. Fitting the General Electric F110-400 engine not only transformed reliability but also increased dry thrust to 71.6 kN (16,080 lb), not far short of the thrust of the original TF30 engine in full afterburner. As a result, the F-14 can be catapulted off a carrier without using afterburner, giving an increase in mission range of no less than 62%. Moreover, the greater dry thrust results in a reduction in time to climb to patrol altitude of 61%!
Going on from there, military engines are now so powerful that the latest fighters can supercruise, the term for flying at sustained supersonic speed without the use of the afterburner. Earlier supersonic aircraft could exceed Mach I on the level only in maximum afterburner, when the rate at which fuel was burned was so high that supersonic speed could not be sustained for longer than about a minute. Today such aircraft as the Lockheed Martin F-22 Raptor or Eurofighter Eurofighter Typhoon can accelerate to supersonic speed (with or without using augmentation) and then sustain such a speed indefinitely in dry thrust.
Apart from dramatically reducing the rate of fuel consumption, the ability to supercruise also reduces the IR (infrared) signature by some 75%. Clearly there is little point in making a 'stealth' aircraft, almost invisible to hostile radars, if its IR emissions proclaim its presence like a lighthouse. Many (indeed, most) of today's air-to-air guided missiles (AAMs) home in on a source of IR radiation, and a fighter in afterburner finds it much more difficult to throw a heat-homing AAM off the scent than a stealthy one in dry thrust, even in supercruise.
Of course, there is a relationship between the speed of an aircraft and that of its propulsive jet. For over 40 years, visionaries - and even a few professional aircraft designers - have considered military aircraft that are capable of hypersonic speed. This term is usually taken to mean Mach numbers several times greater than 1, such as Mach 5, which at high altitude equates to 2868 kts (5310 km/h). In my opinion there is no way a Mach 5 aircraft could supercruise, if by that it meant using a turbofan in dry thrust.
A Mach 5 aircraft will have to have an engine running continuously in full augmentation or, preferably, a ramjet. The trouble with a ramjet is that such engines cannot start from rest. Back in 1951, the Republic Aviation began beavering away at the XF-103, a fighter to cruise at Mach 3.7 (3,930km/h). This would have had a Wright J67, an afterburning turbojet based on the Bristol Siddeley Olympus, installed in a vast duct with a valve which, at high airspeed, could be switched over to bypass the J67 and convert the propulsion system into a ramjet. In August 1957 program was cancelled.
The SR-71 Blackbird had Pratt & Whitney J58 engines, which at Mach 3 behaved like ramjets, the J58s merely getting in the way of the hurricane passing through the nacelles. Whatever kind of engine might be invented, it is sure to have a nozzle whose profile and area can be varied. This by itself is quite a challenge, but beautiful examples can be seen on the latest fighters. Dr Viktor Chepkin is sad that his ALAI engines in the MiG 1.42/1.44 have so far stayed on the ground, because in his opinion, this augmented turbofan is a world-beater. Anyone who has watched the Su-27 Flanker perform, will know that his engines are pretty impressive, and he says "The AL-41 is a totally new generation engine".
Dr Chepkin is one of the Russians who have taken the bull by the horns and boldly combined a fully variable nozzle with the ability to vector (point in different direction). Vectored thrust was pioneered 50 years ago, initially by simply having a switch-in deflector to direct the jet either to go out the back or else through a separate nozzle pointing downwards. This was flown in Meteor RA490 in 1954, but it was a brutishly crude arrangement, and I would have hated to be the pilot if one engine had diverted while the other did not.
Since then, vectored thrust has taken many forms. Some aircraft, such as the Bell Model 65 of 1954 and the supersonic German VJ 101C of 1963, adopted the seemingly obvious method of mounting complete engines on pivots, so that they could point downwards or backwards. Bristol Siddeley designers adopted the more subtle method of fitting a turbofan with two pairs of nozzles, two for fan air and two more at the back for the hot jet. All four were mechanically linked (by motorcycle chain!) to swivel in unison, and the result was the Harrier. Despite the scorn of the USAF, which apparently thought there would always be a handy 10,000ft (3km) runway in any future war, some Harriers actually got into service, and proved so crucial in the Falklands that vectored-thrust versions of the Joint Strike Fighter have equal priority with those needing long runways.
Of course, another form of vectored thrust is to fit a thrust reverser, to slow the aircraft rapidly after a conventional high-speed landing. Reversers are universal on big jetliners (even on small ones, except the BAe I46/RJ and Fokker F28), but are rare on combat aircraft. Offhand, I can think of only the Viggen and Tornado. So far nobody has been clever enough to make an engine that can vector its thrust in all three modes: for combat agility, for VTOL or STOVL (short take-off and vertical landing) and to slow a conventional landing.
Today's supersonic fighters use vectoring purely to enhance in-flight agility. indeed, Eurofighter GmbH (it is a German company) is even at the Millennium still desperate to try to avoid putting vectoring nozzles on their otherwise superb aircraft. Despite presumably having watched Comrade Pugachev and his colleagues demonstrate the superb maneuverability of the MiG-29 Fulcrum and Su-27 Flanker, their position in late 1999 purported to be "We think, in the fullness of time there may well be a naval Eurofighter, and if so, that version might be improved by fitting vectored nozzles". Fortunately ITP in Spain has developed an excellent vectoring nozzle for the EJ200 engine, so it will be available when the penny finally drops. Indeed some Eurofighter folk have told me such nozzles might come "at the first mid-life update".
The technology of gas-turbine engines has never shown the slightest sign of approaching a plateau, far less a barrier. Since the dawn of gas-turbine aviation in 1940, the power available from a given bulk of engine has doubled roughly every 30 years, while specific fuel consumption (rate of fuel consumption for a given power output) has consistently fallen below the most sanguine expectations. If you look at a Gloster Meteor you see nacelles that housed engines of 15.6kN (3,500 lb) thrust. Today, nacelles of the same overall size could house engines with a dry thrust of 156 kN (35,000 lb).
Such power has led to modern fighters becoming impressively large. The McDonnell Douglas F-15 Eagle seemed big, with a wing area of over 55.7m2, but today the F-22 has a wing with an area of 78 m2, precisely the same as that of the World War Two Vickers Wellington heavy bomber. The wing of the MiG 1.44 has an area of 90.5m2. My own feeling is that future fighters ought to be smaller, and Saab (now in partnership with British Aerospace) has shown the way to go with the Gripen. This has the same engine as the Boeing F/A-18 Hornet, but half as many (just as its predecessor the Saab Draken had the same engine as the English Electric Lightning but half as many).
The easiest way to make fighters even smaller is to leave out the pilot. This has a considerable effect, because replacing the cockpit by 'black boxes' not only saves space and weight, and eliminates the environmental system, but also gives the designer greater flexibility in the overall layout. For example, he can put the engine inlet where the windscreen used to be, which most designers consider is a route to enhanced stealth characteristics, without significantly harming pressure recovery in the inlet during fight maneuvers.
Equally important, leaving out a human crew enables the whole aircraft to be approximately half as big, whilst at the same time allowing the designers to go for a maximum acceleration in the vertical plane of at least 20g. Such an aircraft could literally 'fly rings round' a fighter limited to today's 9g. Just how such aircraft would fly their missions depends on the task, and is outside the scope of this article. Suffice it to say, the future UAV (unmanned air vehicle) would almost certainly be single-engined, and could have an engine with a maximum thrust anywhere from 35,000lb (156kN) down to 350lb (1.56kN). Indeed, aircraft used solely as sensor platforms or decoys might have engines of a mere 35 lb (0.156kN) st, replacing today's much slower UAVs powered by tiny piston engines.
Eliminating the pilot, as well as any fins, will do much to enhance stealth qualities. This will focus increased pressure on the need to devise truly stealthy propulsion system For many years, designers have made it impossible for hostile, to 'see' the face of the engine, by suitably kinking the inlet duct (or, as noted, putting it above the fuselage). The propulsive nozzle is harder, and here there is a need to minimize thermal, visual and even acoustic signatures. The Lockheed Martin F-117 Nighthawk nozzles are flattened slits in the trailing edge of the wing, while those of the B-2 are tucked inside deep channels above the rear part of the wing, upstream of movable trailing edges.
I have numerous documents, all published openly in the United States, which purport to explain how the B-2 is even stranger - far stranger - than it appears. Most are articles published in commercial magazines, some are openly published US Patents, while a few are open USAF publications by Wright Aeronautical Laboratory and Air Force Systems Command's Astronautics Laboratory. They deal with such topics as electric-field propulsion, and electrogravitics (or anti-gravity), the transient alteration of not only thrust but also a body's weight. Sci-Fi has nothing on this stuff.
The literature goes back to Faraday, but the idea of electrogravitics really took off in the 1920s when an American physicist, Townsend T. Brown, carried out extensive experiments. He may have been the first to recognize that a capacitor (a dielectric material sandwiched between positive and negative plates) experiences a force tending to move it in the direction of the positive face. He found that the electrostatic charge induced a gravity field between the two plates. Soon he was making capacitors rotate on whirling arms, and measuring the loss in weight of capacitors with the positive face turned uppermost.
In 1953, Brown demonstrated to the USAF a whirling rig of 50ft (15.2m) diameter, which at 150,000 volts (150kV) became a mere blur. The subject was immediately classified, and for the next 40 years, while 'black' research in this field made astonishing progress, it was not reported. Though private individuals continued to experiment, and to take out unclassified patents, not much surfaced. Exceptions were Electrogravitics Systems (February 1956) and The Gravitics Situation (December 1956), published for subscribers only by Aviation Studies (International). This was a London-based 'think tank' run by two very bright young men: R G 'Dicky' Worcester and John Longhurst. Unlike the established journals, they published reports and informed comment without the slightest regard for questions of 'security'. The only time they were taken to court, they won their case and collected heavy damages.
I was fascinated to read those reports, but had no wish to reside in The Tower, so I refrained from discussing clever airplanes with leading edges charged to millions of volts positive and trailing edges at millions of volts negative. In any case, it all seemed a bit farfetched, especially as it appeared that the gravity field could not only propel aircraft to supersonic speed with propulsive efficiency greater than / but could also lift them independently of the atmosphere.
Various snippets appeared suggesting that electrostatic fields could not only do wondrous things in the field of propulsion but could also reduce aerodynamic turbulence (at any Mach number), reduce radar cross-section and even virtually eliminate the sonic boom Indeed, back in 1952, Dr M Rose had noted in unclassified literature: "The positive field.. travelling in front... acts as a buffer which starts moving the air out of the way. This field acts as an entering wedge which softens the supersonic barrier..." From 1985, the name P A LaViolette emerges as author of a shoal of interesting electrogravitics articles in professional literature.
The first Northrop Grumman B-2 Spirit stealth bomber was rolled out on November 22,1988, and anyone with the slightest interest in aircraft could not fail to have noticed the unbelievable leading edge, with a deep profile coming to a knife-edge almost in line with the upper surface. In 1990, a NASA 'boffin' retired and perhaps foolishly talked to The Arkansas Democrat who did not understand his story and ran it under the headline "Ex-NASA expert says Stealth uses parts from UFO".
What really put the cat among the proverbial pigeons was a feature published in a March 1992 issue of Aviation Week & Space Technology, entitled "Black world engineers, scientists, encourage using highly classified technology for civil applications". For the first time in open literature, this article explained how the B-2's sharp leading edge is charged to "many millions of volts", while the corresponding negative charge is blown out in the jets from the four engines. There is more: though the General Electric F118 engines can operate as ordinary turbofans, in flight they act as flame-jet generators, pumping out gas greatly diluted by fresh air, all at millions of volts negative. The word 'flame' gives a rather false picture, because in fact the jet comes out not very much hotter than the surrounding atmosphere.
Unclassified articles have described in some detail how the leading edge is divided into eight sections, each individually ionized. The section on each wing immediately upstream of the engines cannot be thus ionized, because the air would then enter the engines and cancel out the negative charge in the jets. Accordingly, this is where the Hughes covert strike radars are installed. They would not be able to 'see' forwards if they were anywhere else.
Take-off thrust of the F118-100 at sea level is given as '19,000 lb (84.5 kN) class' by Northrop Grumman and as '17,300 lb (77.0 kN)' by the USAF. These are startlingly low figures for an aircraft whose take-off weight is said to be 336,500 lb (152,635 kg) and which was until recently said to weigh 376,000 lb (170,550 kg). Aircraft usually get heavier over the years, not 20 tones lighter. Even at the supposed reduced weight, the ratio of thrust to weight is a mere 0.2, an extraordinarily low value for a combat aircraft.
The USAF has never said anything about B2 speed. It has been tacitly assumed to be in the Mach 0.8 class, but according to the extensive open literature, the four FI 18 engines equate to about 25 MW (megawatts) of electrical power at take-off, but under the influence of the electrogravitic field the speed could soon become supersonic, the output of the air-diluted exhaust then rising to at least 100 MW.
Everyone who has heard a B-2 take off has been astonished at the quietness. Obviously the noise would not be in the same class as the F101 engines of the B-1B in full afterburner, but writers have used the words 'shocking', 'uncanny' and 'incredible' in describing B-2 departures. As for elimination of contrails (condensation trails) (normally a giveaway even for a stealth aircraft), the USAF said chlorofluorosulphonic acid was injected into the jets to eliminate contrails. Later it said this was done by 'regulating exhaust temperatures'. Such an explanation is nonsense; contrails are ice crystals from water vapor left when hydrocarbon fuel is burned, and can never be eliminated by 'regulating exhaust temperatures'. Another point to note is that the channels downstream of the jetpipes appear to be carbon-fiber composite, which is incompatible with normal jet temperatures (not because of the fiber but because of the adhesive sticking them together).
Other writers have commented on the size of the B-2 wing and noted that its stealth depends on the huge black skin being made of RAM (radar-absorbent material). This, say the physicists, is 'a high-k, high-density dielectric ceramic, capable of generating an enormous electrogravitic lift force when charged'. I could go on and on. We have come some way from the Lancaster and B-17, and I seem to have strayed some way from traditional jet engines."
Text is a rewrite of an article first published by Bill Gunston in Air International, January 2000
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