Turbojet

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.[2] 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.[3]

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[4] Frank Whittle formally submitted his ideas for a turbojet to his superiors. In October 1929 he developed his ideas further.[5] On 16 January 1930 in England, Whittle submitted his first patent (granted in 1932).[6] 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 1950's.[7][8]

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.[9] 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.[10]

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.[11][12][13]

Air is drawn into the rotating compressor via the intake and is compressed to a higher pressure before entering the combustion chamber. Fuel is mixed with the compressed air and burns in the combustor. The combustion products leave the combustor and expand through the turbine where power is extracted to drive the compressor. The turbine exit gases still contain considerable energy that is converted in the propelling nozzle to a high speed jet.

The first turbojets, used either a centrifugal compressor (as in the Heinkel HeS 3), or an axial compressor (as in the Junkers Jumo 004) which gave a smaller diameter, although longer, engine. By replacing the propeller used on piston engines with a high speed jet of exhaust, higher aircraft speeds were attainable.

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.[14] Nevertheless, the 593 met all the requirements of the Concorde programme.[15] 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).[16]

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.[17]

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,[18] and by 1949 the de Havilland Goblin, being type tested for 500 hours without maintenance.[19] It was not until the 1950s that superalloy technology allowed other countries to produce economically practical engines.[20]

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. 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.[21]

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.[22]

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.

Allowable turbine entry temperatures have increased steadily over time both with the introduction of superior alloys and coatings, and with the introduction and progressive effectiveness of blade cooling designs. On early engines, the turbine temperature limit had to be monitored, and avoided, by the pilot, typically during starting and at maximum thrust settings. Automatic temperature limiting was introduced to reduce pilot workload and reduce the likelihood of turbine damage due to over-temperature.

The net thrust ${\displaystyle F_{N}\;}$ of a turbojet is given by:[29][30]

${\displaystyle F_{N}=({\dot {m}}_{air}+{\dot {m}}_{f})V_{j}-{\dot {m}}_{air}V}$

where:

${\displaystyle {\dot {m}}_{air}}$	is the rate of flow of air through the engine
${\displaystyle {\dot {m}}_{f}}$	is the rate of flow of fuel entering the engine
${\displaystyle V_{j}\;}$	is the speed of the jet (the exhaust plume) and is assumed to be less than sonic velocity
${\displaystyle V\;}$	is the true airspeed of the aircraft
${\displaystyle ({\dot {m}}_{air}+{\dot {m}}_{f})V_{j}}$	represents the nozzle gross thrust
${\displaystyle {\dot {m}}_{air}V}$	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.[31]

The rate of flow of fuel entering the engine is very small compared with the rate of flow of air.[29] If the contribution of fuel to the nozzle gross thrust is ignored, the net thrust is:

${\displaystyle F_{N}={\dot {m}}_{air}(V_{j}-V)}$

The speed of the jet ${\displaystyle V_{j}\;}$ must exceed the true airspeed of the aircraft ${\displaystyle V\;}$if there is to be a net forward thrust on the airframe. The speed ${\displaystyle V_{j}\;}$ can be calculated thermodynamically based on adiabatic expansion.[32]

The operation of a turbojet is modelled approximately by the Brayton cycle.

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.[33] 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.[34]

• Air-start system
• Exoskeletal engine
• Jet car
• Turbine engine failure
• Turbojet development at the RAE
• Variable cycle engine

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

• Erich Warsitz, the world's first jet pilot: includes rare videos (Heinkel He 178) and audio commentaries
• NASA reciprocating Engine Description: includes a software model
• Possibilities of Jet Propulsion: 1941 survey with discussion of experimental designs of the 1920s and 1930s.
• Whittle Power Jet Papers – Correspondence from the archives of Peterhouse, Cambridge College relating to the development of Whittle's reciprocating engine in Cambridge Digital Library