Jet engine
A Pratt & Whitney F100 turbofan engine for the F-15 Eagle and the F-16 Falcon is tested at Robins Air
Force Base, Georgia, USA. The tunnel behind the engine muffles noise and allows exhaust to escape. The mesh cover at the front of the engine (left of photo) prevents foreign objects (including people) from being pulled into the engine by the huge volume of air rushing into the inlet.
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, ramjets and water jets, but in common usage, the term generally refers to a gas turbine Brayton cycle engine used to produce a jet of high speed exhaust gases for special propulsive purposes. Jet engines are so familiar to the modern world that gas turbines are sometimes mistakenly referred to as a particular application of a jet engine, rather than the other way around.
Contents
1 History
2 Types
3 Type comparison
4 Turbojet engines
5 Turbofan engines
6 Major components
6.1 Air intakes
6.1.1 Subsonic inlets
6.1.2 Supersonic inlets
6.2 Compressors
6.3 Combustors
6.4 Turbines
6.5 Turbopumps
6.6 Nozzles
6.7 Cooling systems
6.7.1 Air systems
6.7.2 Rocket engines
6.8 Fuel system
6.8.1 Fuel control unit (FCU)
6.9 Fuel pump
6.10 Engine starting system
6.11 Ignition
6.12 Lubrication system
7 Advanced designs
7.1 J-58 combined ramjet/turbojet
7.2 Pre-cooled turbojets
7.3 Nuclear-powered ramjet
7.4 Scramjets
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History
See also: Timeline of jet power
Jet engines can be dated back to the first century AD, when Hero of Alexandria 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 11th century. Rocket exhaust was initially used in a modest way for fireworks but gradually progressed to propel some quite fearsome weaponry; and there the technology stalled for hundreds of years.
The problem was that rockets are simply too inefficient to be useful for general aviation. Instead, by the 1930s, the piston engine in its many different forms (rotary and static radial, aircooled and liquid-cooled inline) was the only type of powerplant available to aircraft designers. This was acceptable as long as only low performance aircraft were required, and indeed all that were available.
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.<1> 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 earliest attempts at jet engines were hybrid designs in which an external power source supplied the compression. In this system (called a thermojet by Secondo Campini) the air is first compressed by a fan driven by a conventional piston engine, then it is mixed with fuel and burned for jet thrust. The examples of this type of design were the Henri Coandă''s Coandă-1910 aircraft, and the much later Campini Caproni CC.2, and the Japu-11 engine intended to power 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.
Jet engine airflow simulation
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. The first patents for jet propulsion were issued in 1917. Limitations in design and practical engineering and metallurgy prevented such engines reaching manufacture. The main problems were safety, reliability, weight and, especially, sustained operation.
The W2/700 engine flew in the Gloster E.28/39, the first British aircraft to fly with a turbojet engine, and the Gloster Meteor.
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''s team experienced near-panic when the engine would not stop, 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 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.
Ohain approached 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 HeS 1 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 HeS 3 of 1,100 lbf (5 kN), which was fitted to Heinkel''s simple and compact He 178 airframe and flown by Erich Warsitz in the early morning of August 27, 1939, from Marienehe aerodrome, an impressively short time for development. The He 178 was the world''s first jet plane.
Meanwhile, Whittle''s engine was starting to look useful, and his Power Jets Ltd. started receiving Air Ministry money. In 1941 a flyable version of the engine called the W.1, capable of 1000 lbf (4 kN) of thrust, was fitted to the Gloster E28/39 airframe specially built for it, and first flew on May 15, 1941 at RAF Cranwell.
A picture of an early centrifugal engine (the DH Goblin II) sectioned to show its internal components
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 e