Engineering:Turbojet

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The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and a turbine (that drives the compressor). The compressed air from the compressor is heated by burning fuel in the combustion chamber and then allowed to expand through the turbine. The turbine exhaust is then expanded in the propelling nozzle where it is accelerated to high speed to provide thrust.[1] Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s.[2]

Turbojets have poor efficiency at low vehicle speeds, which limits their usefulness in vehicles other than aircraft. Turbojet engines have been used in isolated cases to power vehicles other than aircraft, typically for attempts on land speed records. Where vehicles are "turbine-powered", this is more commonly by use of a turboshaft engine, a development of the gas turbine engine where an additional turbine is used to drive a rotating output shaft. These are common in helicopters and hovercraft.

Turbojets were widely used for early supersonic fighters, up to and including many third generation fighters, with the MiG-25 being the latest turbojet-powered fighter developed. As most fighters spend little time traveling supersonically, fourth-generation fighters (as well as some late third-generation fighters like the F-111 and Hawker Siddeley Harrier) and subsequent designs are powered by the more efficient low-bypass turbofans and use afterburners to raise exhaust speed for bursts of supersonic travel. Turbojets were used on the Concorde and the longer-range versions of the Tu-144 which were required to spend a long period travelling supersonically. Turbojets are still common in medium range cruise missiles, due to their high exhaust speed, small frontal area, and relative simplicity.

History

Heinkel He 178, the world's first aircraft to fly purely on turbojet power, using an HeS 3 engine

The first patent for using a gas turbine to power an aircraft was filed in 1921 by Frenchman Maxime Guillaume.[3] His engine was to be an axial-flow turbojet, but was never constructed, as it would have required considerable advances over the state of the art in compressors.[4]

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

In 1928, British RAF College Cranwell cadet[5] Frank Whittle formally submitted his ideas for a turbojet to his superiors. In October 1929 he developed his ideas further.[6] On 16 January 1930 in England, Whittle submitted his first patent (granted in 1932).[7] 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 later concentrated on the simpler centrifugal compressor only, for a variety of practical reasons. A Whittle engine was the first turbojet to run, the Power Jets WU, on 12 April 1937. It was liquid-fuelled. Whittle's team experienced near-panic during the first start attempts when the engine accelerated out of control to a relatively high speed despite the fuel supply being cut off. It was subsequently found that fuel had leaked into the combustion chamber during pre-start motoring checks and accumulated in pools, so the engine would not stop accelerating until all the leaked fuel had burned off. Whittle was unable to interest the government in his invention, and development continued at a slow pace.

In Germany, Hans von Ohain patented a similar engine in 1935. His design, an axial-flow engine, as opposed to Whittle's centrifugal flow engine, was eventually adopted by most manufacturers by the 1950s.[8][9]

On 27 August 1939 the Heinkel He 178, powered by von Ohain's design, became the world's first aircraft to fly using the thrust from a turbojet engine. It was flown by test pilot Erich Warsitz.[10] The Gloster E.28/39, (also referred to as the "Gloster Whittle", "Gloster Pioneer", or "Gloster G.40") made the first British jet-engined flight in 1941. It was designed to test the Whittle jet engine in flight, and led to the development of the Gloster Meteor.[11]

The first two operational turbojet aircraft, the Messerschmitt Me 262 and then the Gloster Meteor, entered service in 1944, towards the end of World War II, the Me 262 in April and the Gloster Meteor in July. Only about 15 Meteor saw WW2 action but up to 1400 Me 262s were produced, with 300 entering combat, delivering the first ground attacks and air combat victories of jet planes.[12][13][14]


One of the last applications for a turbojet engine was Concorde which used the Olympus 593 engine. However, joint studies by Rolls-Royce and Snecma for a second generation SST engine using the 593 core were done more than three years before Concorde entered service. They evaluated bypass engines with bypass ratios between 0.1 and 1.0 to give improved take-off and cruising performance.[15] Nevertheless, the 593 met all the requirements of the Concorde programme.[16] Estimates made in 1964 for the Concorde design at Mach 2.2 showed the penalty in range for the supersonic airliner, in terms of miles per gallon, compared to subsonic airliners at Mach 0.85 (Boeing 707, DC-8) was relatively small. This is because the large increase in drag is largely compensated by an increase in powerplant efficiency (the engine efficiency is increased by the ram pressure rise which adds to the compressor pressure rise, the higher aircraft speed approaches the exhaust jet speed increasing propulsive efficiency).[17]

Turbojet engines had a significant impact on commercial aviation. Aside from giving faster flight speeds turbojets had greater reliability than piston engines, with some models demonstrating dispatch reliability rating in excess of 99.9%. Pre-jet commercial aircraft were designed with as many as four engines in part because of concerns over in-flight failures. Overseas flight paths were plotted to keep planes within an hour of a landing field, lengthening flights. The increase in reliability that came with the turbojet enabled three- and two-engine designs, and more direct long-distance flights.[18]

High-temperature alloys were a reverse salient, a key technology that dragged progress on jet engines. Non-UK jet engines built in the 1930s and 1940s had to be overhauled every 10 or 20 hours due to creep failure and other types of damage to blades. British engines, however, utilised Nimonic alloys which allowed extended use without overhaul, engines such as the Rolls-Royce Welland and Rolls-Royce Derwent,[19] and by 1949 the de Havilland Goblin, being type tested for 500 hours without maintenance.[20] It was not until the 1950s that superalloy technology allowed other countries to produce economically practical engines.[21]

Early designs

Early German turbojets had severe limitations on the amount of running they could do due to the lack of suitable high temperature materials for the turbines.[citation needed] British engines such as the Rolls-Royce Welland used better materials, giving improved durability. The Welland was type-certified for 80 hours initially, later extended to 150 hours between overhauls, as a result of an extended 500-hour run being achieved in tests.[22]

J85-GE-17A turbojet engine from General Electric (1970)

General Electric in the United States was in a good position to enter the jet engine business due to its experience with the high-temperature materials used in their turbosuperchargers during World War II.[23]

Water injection was a common method used to increase thrust, usually during takeoff, in early turbojets that were thrust-limited by their allowable turbine entry temperature. The water increased thrust at the temperature limit, but prevented complete combustion, often leaving a very visible smoke trail. [24]


Components

An animation of an axial compressor.
Schematic diagram showing the operation of a centrifugal flow turbojet engine. The compressor is driven by the turbine stage and throws the air outwards, requiring it to be redirected parallel to the axis of thrust.
Schematic diagram showing the operation of an axial flow turbojet engine. Here, the compressor is again driven by the turbine, but the air flow remains parallel to the axis of thrust.

Nose bullet

Air intake

Main article: Engineering:Intake


The intake gains prominence at high speeds when it generates more compression than the compressor stage. Well-known examples are the Concorde and Lockheed SR-71 Blackbird propulsion systems where the intake and engine contributions to the total compression were 63%/8%[25] at Mach 2 and 54%/17%[26] at Mach 3+. Intakes have ranged from "zero-length"[27] on the Pratt & Whitney TF33 turbofan installation in the Lockheed C-141 Starlifter, to the twin 65 feet (20 m) long, intakes on the North American XB-70 Valkyrie, each feeding three engines with an intake airflow of about 800 pounds per second (360 kg/s).

Compressor

Main article: Physics:Compressor



Combustion chamber

Main article: Engineering:Combustor


Turbine

Main article: Engineering:Turbine blade

Hot gases leaving the combustor expand through the turbine. Typical materials for turbines include inconel and Nimonic.[28] The hottest turbine vanes and blades in an engine have internal cooling passages. Air from the compressor is passed through these to keep the metal temperature within limits. The remaining stages do not need cooling.


Nozzle

Main article: Engineering:Propelling nozzle


Afterburner

Main article: Engineering:Afterburner


Reheat was flight-trialled in 1944 on the W.2/700 engines in a Gloster Meteor I.[29]

Thrust

Thrust augmentation

Liquid injection was tested on the Power Jets W.1 in 1941 initially using ammonia before changing to water and then water-methanol. A system to trial the technique in the Gloster E.28/39 was devised but never fitted.[30]

Net thrust

The net thrust FN of a turbojet is given by:[31][32]

FN=(m˙air+m˙f)Vj−m˙airV

where:

m˙air is the rate of flow of air through the engine
m˙f is the rate of flow of fuel entering the engine
Vj is the speed of the jet (the exhaust plume) and is assumed to be less than sonic velocity
V is the true airspeed of the aircraft
(m˙air+m˙f)Vj represents the nozzle gross thrust
m˙airV represents the ram drag of the intake

If the speed of the jet is equal to sonic velocity the 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.[33]

The rate of flow of fuel entering the engine is very small compared with the rate of flow of air.[31] If the contribution of fuel to the nozzle gross thrust is ignored, the net thrust is:

FN=m˙air(Vj−V)

The speed of the jet Vj must exceed the true airspeed of the aircraft Vif there is to be a net forward thrust on the airframe. The speed Vj can be calculated thermodynamically based on adiabatic expansion.[34]

Cycle improvements

The efficiency of a gas turbine is increased by raising the overall pressure ratio, requiring higher-temperature compressor materials, and raising the turbine entry temperature, requiring better turbine materials and/or improved vane/blade cooling. It is also increased by reducing the losses as the flow progresses from the intake to the propelling nozzle. These losses are quantified by compressor and turbine efficiencies and ducting pressure losses. When used in a turbojet application, where the output from the gas turbine is used in a propelling nozzle, raising the turbine temperature increases the jet velocity. At normal subsonic speeds this reduces the propulsive efficiency, giving an overall loss, as reflected by the higher fuel consumption, or SFC.[35] However, for supersonic aircraft this can be beneficial, and is part of the reason why the Concorde employed turbojets. Turbojet systems are complex systems therefore to secure optimal function of such system, there is a call for the newer models being developed to advance its control systems to implement the newest knowledge from the areas of automation, so increase its safety and effectiveness.[36]

See also

References

  1. ↑ "Turbojet Engine". NASA Glenn Research Center. http://www.grc.nasa.gov/WWW/K-12/airplane/aturbj.html. 
  2. ↑ Brown, Alan. "The Converging Paths of Whittle and von Ohain". Air & Space Forces Magazine. https://www.airandspaceforces.com/article/0106engines/. 
  3. ↑ Maxime Guillaume, "Propulseur par réaction sur l'air", FR patent Patent 534801, issued 13 January 1922
  4. ↑ Ellis, Guy (15 February 2016). Britain's Jet Age: From the Meteor to the Sea Vixen. Amberley. ISBN 978-1-44564901-6. https://books.google.com/books?id=lxqtCwAAQBAJ&pg=PT7. 
  5. ↑ "Chasing the Sun – Frank Whittle". PBS. https://www.pbs.org/kcet/chasingthesun/innovators/fwhittle.html. 
  6. ↑ "History – Frank Whittle (1907–1996)". BBC. https://www.bbc.co.uk/history/historic_figures/whittle_frank.shtml. 
  7. ↑ Frank Whittle, "Improvements relating to the propulsion of aircraft and other vehicles", GB patent Patent 347206, issued 1931-04-16
  8. ↑ Jenkins, Dennis R.; Landis, Tony R. (2008). Experimental & Prototype U.S. Air Force Jet Fighters. Specialty Press. ISBN 978-1-58007-111-6. 
  9. ↑ Foderaro, Lisa W. (10 August 1996). "Frank Whittle, 89, Dies; His Jet Engine Propelled Progress". The New York Times. https://www.nytimes.com/1996/08/10/world/frank-whittle-89-dies-his-jet-engine-propelled-progress.html. 
  10. ↑ Warsitz, Lutz (2009). The First Jet Pilot – The Story of German Test Pilot Erich Warsitz. England: Pen and Sword Books. p. 125. ISBN 978-1-84415-818-8. http://www.pen-and-sword.co.uk/?product_id=1762. 
  11. ↑ Listemann, Phil H. (6 September 2016). The Gloster Meteor F.I & F.III. Philedition. p. 3. ISBN 978-2-918590-95-8. https://books.google.com/books?id=DgakDAAAQBAJ&pg=PA3. 
  12. ↑ Heaton, Colin D.; Lewis, Anne-Marien; Tillman, Barrett (15 May 2012). The Me 262 Stormbird: From the Pilots Who Flew, Fought, and Survived It. Voyageur Press. ISBN 978-1-61058434-0. https://books.google.com/books?id=449Ob41RgZMC&pg=PT103. 
  13. ↑ Listemann 2016, p. 5.
  14. ↑ "The Day Germany's First Jet Fighter Soared into History". https://www.smithsonianmag.com/smithsonian-institution/day-germanys-first-jet-fighter-soared-history-180978152/. 
  15. ↑ Young; Devriese (11 May 1972). "Power for the second-generation SST". Flight International: 659. 
  16. ↑ J.D.Wragg (1998). "The Engine For TSR2". in Hunter, Alexander Freeland Cairns. TSR2 with Hindsight. Royal Air Force Historical Society. p. 120. ISBN 0951982486. 
  17. ↑ Hooker, S. G. (1963). "Power Plants for the Concord Supersonic Civil Airliner". Proceedings of the Institution of Mechanical Engineers 178 (1): 1224–1237. doi:10.1177/0020348363178001159. ISSN 0020-3483. https://journals.sagepub.com/doi/10.1177/0020348363178001159. 
  18. ↑ Larson, George C. (April–May 2010). "Old Faithful". Air & Space 25 (1): 80. 
  19. ↑ Bill Gunston (2006). World Encyclopedia of Aero Engines (5th ed.). Sutton Publishing. p. 192. 
  20. ↑ "sir alec | flame tubes | marshal sir | 1949 | 0598 | Flight Archive". https://www.flightglobal.com/pdfarchive/view/1949/1949%20-%200598.html. 
  21. ↑ Sims, C.T. (1984). "Superalloys 1984 (Fifth International Symposium)". TMS. pp. 399–419. doi:10.7449/1984/Superalloys_1984_399_419. 
  22. ↑ "Rolls-Royce Derwent | 1945". Flight (Flightglobal.com): 448. 25 October 1945. http://www.flightglobal.com/pdfarchive/view/1945/1945%20-%202113.html. Retrieved 14 December 2013. 
  23. ↑ Garvin, Robert V. (1998). Starting Something Big. p. 5. ISBN 978-1-56347-289-3. 
  24. ↑ "Turbojet Enhancements". NASA Glenn Research Center. https://www1.grc.nasa.gov/historic-facilities/special-projects-laboratory/turbojet-enhancements/. 
  25. ↑ Trubshaw, Brian; Edmondson, Sally (1999). Brian Trubshaw: Test Pilot. Sutton Publishing. Appendix VIIIb. ISBN 0750918381. 
  26. ↑ J. Thomas Anderson (August 19, 2013). How Supersonic Inlets Work: Details of the Geometry and Operation of the SR-71 Mixed Compression Inlet (Report). Lockheed Martin Skunk Works. Fig.26. http://www.enginehistory.org/Convention/2013/HowInletsWork8-19-13.pdf. Retrieved 16 May 2016. 
  27. ↑ Sóbester, András (2007). "Tradeoffs in Jet Inlet Design: A Historical Perspective". Journal of Aircraft 44 (3): 705–717. doi:10.2514/1.26830. ISSN 0021-8669. https://eprints.soton.ac.uk/46202/1/AIAA-26830-529.pdf. 
  28. ↑ "1960 | Flight | Archive". http://www.flightglobal.com/pdfarchive/view/1960/1960%20-%201525.html. 
  29. ↑ Bill Gunston (2006). World Encyclopedia of Aero Engines (5th ed.). Sutton Publishing. p. 160. 
  30. ↑ "1947 | 1359 | Flight Archive". https://www.flightglobal.com/pdfarchive/view/1947/1947%20-%201359.html. 
  31. ↑ 31.0 31.1 Cumpsty, Nicholas (2003). "3.1". Jet Propulsion (2nd ed.). Cambridge University Press. ISBN 0-521-54144-1. 
  32. ↑ "Turbojet Thrust". NASA Glenn Research Center. http://www.grc.nasa.gov/WWW/K-12/airplane/turbth.html. 
  33. ↑ Cumpsty, Jet Propulsion, Section 6.3
  34. ↑ "11.6 Performance of Jet Engines". http://web.mit.edu/16.unified/www/SPRING/propulsion/notes/node85.html#SECTION06364000000000000000. 
  35. ↑ Cohen, Henry; Rogers, Gordon Frederick Crichton; Saravanamuttoo, H. I. H. (1972). Gas Turbine Theory. London: Longman. pp. 72-73, fig 3.11. ISBN 0-582-44927-8. 
  36. ↑ Andoga, R.; Fozo, L.; Madarasz, L.; Judicak, J. (2010). "2010 IEEE 8th International Symposium on Applied Machine Intelligence and Informatics (SAMI)". IEEE. pp. 141–144. doi:10.1109/SAMI.2010.5423749. ISBN 978-1-4244-6422-7. 

Further reading

  • Springer, Edwin H. (2001). Constructing A Turbocharger Turbojet Engine. Turbojet Technologies. 



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