Whereas the engines in propeller-powered aircraft generally achieve their maximum efficiency at much lower speeds and altitudes, jet engines achieve maximum efficiency at speeds close to or even well above the speed of sound. Jet aircraft generally cruise most efficiently at about Mach 0.8 (981km/h (610mph)) and at altitudes around 10,000–15,000m (33,000–49,000ft) or more.
The idea of the jet engine was not new, but the technical problems involved did not begin to be solved until the 1930s. Frank Whittle, an English inventor and RAF officer, began development of a viable jet engine in 1928,[1] and Hans von Ohain in Germany began work independently in the early 1930s. In August 1939 the turbojet powered Heinkel He 178, the world's first jet aircraft, made its first flight. A wide range of different types of jet aircraft exist, both for civilian and military purposes.
History
The Heinkel He 178 was the first aircraft to fly on turbojet power, in August 1939
After the first instance of powered flight, a large number of jet engine designs were suggested. René Lorin, Morize, Harris proposed systems for creating a jet efflux.[2]
After other jet engines had been run, Romanian inventor Henri Coandă claimed to have built a jet-powered aircraft in 1910, the Coandă-1910. However, to support this claim, he had to make substantial alterations to the drawings which he used to support his subsequently debunked claims.[3] In fact the ducted-fan engine backfired, setting the aircraft on fire before any flights were ever made, and it lacked nearly all of the features necessary for a jet engine - including a lack of fuel injection, and any concern about hot jet efflux being directed at a highly flammable fabric surface.[3]
During the 1920s and 1930s a number of approaches were tried. A variety of motorjet, turboprop, pulsejet and rocket powered aircraft were designed. Rocket-engine research was being carried out in Germany and the first aircraft to fly under rocket power was the Lippisch Ente, in 1928.[4] The Ente had previously been flown as a glider. The next year, in 1929, the Opel RAK.1 became the first purpose-built rocket aircraft to fly.
The turbojet was invented in the 1930s, independently by Frank Whittle and later Hans von Ohain. The first turbojet aircraft to fly was the Heinkel He 178, on August 27, 1939 in Rostock (Germany), powered by von Ohain's design.[5][6] This was largely a proof of concept, as the problem of "creep" (metal fatigue caused by the high temperatures within the engine) had not been solved, and the engine quickly burned out. Von Ohain's design, an axial-flow engine, as opposed to Whittle's centrifugal flow engine, was eventually adopted by most manufacturers by the 1950s.[7][8]
The first flight of a jet-propelled aircraft to come to public attention was the ItalianCaproni Campini N.1motorjet prototype which flew on August 27, 1940.[9] It was the first jet aircraft recognised by the Fédération Aéronautique Internationale (at the time the German He 178 program was still kept secret). Campini began development of the motorjet in 1932; it differed from a true turbojet in that the turbine was driven by a piston engine, rather than combustion of the turbine gases - which was a much more complex solution.
Boeing 707
The British experimental Gloster E.28/39 first flew on May 15, 1941, powered by Sir Frank Whittle's turbojet.[10] The United States Bell XP-59A flew on October 1, 1942, using two examples of a version of the Whittle engine built by General Electric. The Meteor was the first production jet, with the first orders for production examples being made on 8 August 1941,[11] the prototype first flying on 5 March 1943 and the first production aircraft flying on 12 January 1944,[12] while the first orders for production Me 262 aircraft were not issued until 25 May 1943,[13] and the first production Me 262 did not fly until 28 March 1944[14] despite the Me 262 program having started earlier than that of the Meteor, as Projekt 1065, with initial plans drawn up by Waldemar Voigt's design team in April 1939.
The Messerschmitt Me 262 was the first operational jet fighter,[15] manufactured by Germany during World War II and entering service on 19 April 1944 with Erprobungskommando 262 at Lechfeld just south of Augsburg. An Me 262 scored the first combat victory for a jet fighter on 26 July 1944, the day before the British Gloster Meteor entered operational service. The Me 262 had first flown on April 18, 1941, but mass production did not start until early 1944, with the first squadrons operational that year, too late for any effect on the outcome of the World War II. While only around 15 Meteors were operational during WW2, up to 1,400 Me 262 were produced, with 300 entering combat. Only the rocket-propelled Messerschmitt Me 163 Komet was a faster operational aircraft during the war.[citation needed]
Around this time, mid 1944, the United Kingdom's Meteor was being used for defence of the UK against the V-1 flying bomb– the V-1 itself a pulsejet-powered aircraftand direct ancestor of the cruise missile– and then ground-attack operations over Europe in the last months of the war. In 1944 Germany introduced the Arado Ar 234 jet reconnaissance and bomber aircraft into service, though chiefly used in the former role, with the Heinkel He 162Spatz single-jet light fighter appearing at the end of 1944. USSR tested its own Bereznyak-Isayev BI-1 in 1942, but the project was scrapped by leader Joseph Stalin in 1945. The Imperial Japanese Navy also developed jet aircraft in 1945, including the Nakajima J9Y Kikka, a modified, and slightly smaller version of the Me 262 that had folding wings. By the end of 1945, the US had introduced their first jet fighter, the Lockheed P-80 Shooting Star into service and the UK its second fighter design, the de Havilland Vampire.
The Concorde was the longest running commercial SST providing service from 1976 to 2003
BOAC operated the first commercial jet service, from London to Johannesburg, in 1952 with the de Havilland Cometjetliner. This highly innovative aircraft travelled far faster and higher than propeller aircraft, was much quieter, smoother, and had stylish blended wings containing hidden jet engines. However, due to a design defect, and use of aluminium alloys, the aircraft suffered catastrophic metal fatigue which led to several crashes,[16] which gave time for the Boeing 707 to enter service in 1958 and thus to dominate the market for civilian airliners. The underslung engines were found to be advantageous in the event of a propellant leak, and so the 707 looked rather different from the Comet: the 707 has a shape that is effectively the same as that of contemporary aircraft, with marked commonality still evident today for example with the 737 (fuselage) and A340 (single deck, swept wing, four below-wing engines).
Turbofan aircraft with far greater fuel efficiency began entering service in the 1950s and 1960s, and became the most commonly used type of jet.
The Tu-144supersonic transport was the fastest commercial jet aircraft at Mach 2.35 (2,503km/h (1,555mph)). It went into service in 1975, but was withdrawn from commercial service shortly afterwards. The Mach 2 Concorde entered service in 1976 and flew for 27 years.
The fastest military jet aircraft was the SR-71 Blackbird at Mach 3.35 (3,661km/h (2,275mph)).
Other jets
Most people use the term 'jet aircraft' to denote gas turbine based airbreathing jet engines, but rockets and scramjets are both also propelled by jet propulsion.
Cruise missiles are single-use unmanned jet aircraft, powered predominantly by ramjets or turbojets or sometimes turbofans, but they will often have a rocket propulsion system for initial propulsion.
The fastest airbreathing jet aircraft is the unmanned X-43 scramjet at around Mach 9–10.
The fastest manned (rocket) aircraft is the X-15 at Mach 6.85.
The Space Shuttle, while far faster than the X-43 or X-15, was not regarded as an aircraft during ascent as it was carried ballistically by rocket thrust, rather than the air. During re-entry it was classed (like a glider) as an unpowered aircraft. The first flight was in 1981.
Jet-powered wingsuits exist – powered by model aircraft jet engines – but of short duration and needing to be launched at height.[18]
Aerodynamics
Because of the way they work, the typical exhaust speed of jet engines is transonic or faster, therefore most jet aircraft need to fly at high speeds, either supersonic or speeds just below the speed of sound ("transonic") so as to achieve efficient flight. Aerodynamics is therefore an important consideration.
Jet aircraft are usually designed using the Whitcomb area rule, which says that the total area of cross-section of the aircraft at any point along the aircraft from the nose must be approximately the same as that of a Sears-Haack body. A shape with that property minimises the production of shockwaves which would waste energy.
The different types are used for different purposes.
Rockets are the oldest type, and are mainly used when extremely high speeds are needed, or operation at extremely high altitudes where there is insufficient air to operate a jet engine. Due to the extreme, typically hypersonic, exhaust velocity and the necessity of oxidiser being carried on board, they consume propellant extremely quickly, making them impractical for routine transportation.
Turbojets are the second oldest type; they have a high, usually supersonic, exhaust speed and low frontal cross-section, and so are best suited to high-speed, usually supersonic, flight. Although once widely used, they are relatively inefficient compared to turboprop and turbofans for subsonic flight. The last major aircraft to use turbojets were Concorde and Tu-144 supersonic transports.
Low bypass turbofans have a lower exhaust speed than turbojets, and are mostly used for high sonic, transonic, and low supersonic speeds. High bypass turbofans are relatively efficient, and are used by subsonic aircraft such as airliners.
One difference is that jet engines respond relatively slowly.[citation needed] This complicates takeoff and landing maneuvers. In particular, during takeoff, propeller aircraft engines blow air over their wings and that gives more lift and a shorter takeoff. These differences caught out some early BOAC Comet pilots.[16]
In aircraft overall propulsive efficiency is the efficiency, in percent, with which the energy contained in a vehicle's propellant is converted into useful energy, to replace losses due to air drag, gravity, and acceleration. It can also be stated as the proportion of the mechanical energy actually used to propel the aircraft. It is always less than 100% because of kinetic energy loss to the exhaust, and less-than-ideal efficiency of the propulsive mechanism, whether a propeller, a jet exhaust, or a fan. In addition, propulsive efficiency is greatly dependent on air density and airspeed.
Mathematically, it is represented as [19] where is the cycle efficiency and is the propulsive efficiency. The cycle efficiency, in percent, is the proportion of energy that can be derived from the energy source that is converted to mechanical energy by the engine.
Dependence of the propulsive efficiency () upon the vehicle speed/exhaust speed ratio (v/c) for rocket and jet engines
For jet aircraft the propulsive efficiency (essentially energy efficiency) is highest when the engine emits an exhaust jet at a speed that is the same as, or nearly the same as, the vehicle velocity. The exact formula for air-breathing engines as given in the literature,[20][21] is
where c is the exhaust speed, and v is the speed of the aircraft.
For a long range jet operating in the stratosphere, the speed of sound is constant, hence flying at fixed angle of attack and constant Mach number causes the aircraft to climb, without changing the value of the local speed of sound. In this case:
where is the cruise Mach number and the local speed of sound. The range equation can be shown to be:
which is known as the Breguet range equation after the French aviation pioneer Louis Charles Breguet.
See also
Coanda-1910– AircraftPages displaying short descriptions of redirect targetsPages displaying short descriptions with no spaces
A jet engine is a type of reaction engine, discharging a fast-moving jet of heated gas that generates thrust by jet propulsion. While this broad definition may include rocket, water jet, and hybrid propulsion, the term jet engine typically refers to an internal combustion air-breathing jet engine such as a turbojet, turbofan, ramjet, pulse jet, or scramjet. In general, jet engines are internal combustion engines.
A ramjet is a form of airbreathing jet engine that requires forward motion of the engine to provide air for combustion. Ramjets work most efficiently at supersonic speeds around Mach 3 and can operate up to Mach 6.
A turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a combination of references to the preceding generation engine technology of the turbojet and the additional fan stage. It consists of a gas turbine engine which achieves mechanical energy from combustion, and a ducted fan that uses the mechanical energy from the gas turbine to force air rearwards. Thus, whereas all the air taken in by a turbojet passes through the combustion chamber and turbines, in a turbofan some of that air bypasses these components. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.
An aircraft engine, often referred to as an aero engine, is the power component of an aircraft propulsion system. Aircraft using power components are referred to as powered flight. Most aircraft engines are either piston engines or gas turbines, although a few have been rocket powered and in recent years many small UAVs have used electric motors.
Thrust-specific fuel consumption (TSFC) is the fuel efficiency of an engine design with respect to thrust output. TSFC may also be thought of as fuel consumption (grams/second) per unit of thrust, hence thrust-specific. This figure is inversely proportional to specific impulse, which is the amount of thrust produced per unit fuel consumed.
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. 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. 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.
A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. As in ramjets, a scramjet relies on high vehicle speed to compress the incoming air forcefully before combustion, but whereas a ramjet decelerates the air to subsonic velocities before combustion using shock cones, a scramjet has no shock cone and slows the airflow using shockwaves produced by its ignition source in place of a shock cone. This allows the scramjet to operate efficiently at extremely high speeds.
Hans Joachim Pabst von Ohain was a German physicist, engineer, and the designer of the first aircraft to use a turbojet engine. Together with Frank Whittle and Anselm Franz, he has been described as the co-inventor of the turbojet engine. However, the historical timelines show that von Ohain was still a university student when, in January 1930, Whittle filed his first patent for a turbojet engine and successfully tested his first engine in April 1937, some 6 months before von Ohain. Additionally, prior to designing his engine and filing his own patent in 1935, von Ohain had read and critiqued Whittle's patents. Von Ohain stated in his biography that "My interest in jet propulsion began in the fall of 1933 when I was in my seventh semester at Göttingen University. I didn't know that many people before me had the same thought." Unlike Whittle, von Ohain had the significant advantage of being supported by an aircraft manufacturer, Heinkel, who funded his work.
The Heinkel He 178 was an experimental aircraft designed and produced by the German aircraft manufacturer Heinkel. It was the world's first aircraft to fly using the thrust from a turbojet engine.
An afterburner is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and combat. The afterburning process injects additional fuel into a combustor in the jet pipe behind the turbine, "reheating" the exhaust gas. Afterburning significantly increases thrust as an alternative to using a bigger engine with its attendant weight penalty, but at the cost of increased fuel consumption which limits its use to short periods. This aircraft application of "reheat" contrasts with the meaning and implementation of "reheat" applicable to gas turbines driving electrical generators and which reduces fuel consumption.
The bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A 10:1 bypass ratio, for example, means that 10 kg of air passes through the bypass duct for every 1 kg of air passing through the core.
A propelling nozzle is a nozzle that converts the internal energy of a working gas into propulsive force; it is the nozzle, which forms a jet, that separates a gas turbine, or gas generator, from a jet engine.
This article outlines the important developments in the history of the development of the air-breathing (duct) jet engine. Although the most common type, the gas turbine powered jet engine, was certainly a 20th-century invention, many of the needed advances in theory and technology leading to this invention were made well before this time.
A jet engine performs by converting fuel into thrust. How well it performs is an indication of what proportion of its fuel goes to waste. It transfers heat from burning fuel to air passing through the engine. In doing so it produces thrust work when propelling a vehicle but a lot of the fuel is wasted and only appears as heat. Propulsion engineers aim to minimize the degradation of fuel energy into unusable thermal energy. Increased emphasis on performance improvements for commercial airliners came in the 1970s from the rising cost of fuel.
The Heinkel HeS 3 was the world's first operational jet engine to power an aircraft. Designed by Hans von Ohain while working at Heinkel, the engine first flew as the primary power of the Heinkel He 178, piloted by Erich Warsitz on 27 August 1939. Although successful, the engine had too little thrust to be really useful, and work started on the more powerful Heinkel HeS 8 as their first production design.
In aerospace engineering, concerning aircraft, rocket and spacecraft design, overall propulsion system efficiency is the efficiency with which the energy contained in a vehicle's fuel is converted into kinetic energy of the vehicle, to accelerate it, or to replace losses due to aerodynamic drag or gravity. Mathematically, it is represented as , where is the cycle efficiency and is the propulsive efficiency.
A variable cycle engine (VCE), also referred to as adaptive cycle engine (ACE), is an aircraft jet engine that is designed to operate efficiently under mixed flight conditions, such as subsonic, transonic and supersonic.
An airbreathing jet engine is a jet engine in which the exhaust gas which supplies jet propulsion is atmospheric air, which is taken in, compressed, heated, and expanded back to atmospheric pressure through a propelling nozzle. Compression may be provided by a gas turbine, as in the original turbojet and newer turbofan, or arise solely from the ram pressure of the vehicle's velocity, as with the ramjet and pulsejet.
The period between 1945 and 1979 is sometimes called the post-war era or the period of the post-war political consensus. During this period, aviation was dominated by the arrival of the Jet Age. In civil aviation the jet engine allowed a huge expansion of commercial air travel, while in military aviation it led to the widespread introduction of supersonic aircraft.
Lutz Warsitz: The First Jet Pilot – The Story of German Test Pilot Erich Warsitz, Pen and Sword Books Ltd., England, 2009, ISBN978-1-84415-818-8, English Edition
Radinger, Will; Schick, Walter Schick (1996). Me 262 (in German). Berlin: Avantic Verlag GmbH. ISBN978-3-925505-21-8.
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