I.

Introduction

Jet Propulsion, thrust imparting forward motion to an object, as a reaction to the rearward expulsion of a high-velocity liquid or gaseous stream.

A simple example of jet propulsion is the motion of an inflated balloon when the air is suddenly discharged. While the opening is held closed, the air pressure within the balloon is equal in all directions; when the stem is released, the internal pressure is less at the open end than at the opposite end, causing the balloon to dart forward. Not the pressure of the escaping air pushing against the outside atmosphere but the difference between high and low pressures inside the balloon propels it.

An actual jet engine does not operate quite as simply as a balloon, although the basic principle is the same. More important than pressure imbalance is the acceleration to high velocities of the jet leaving the engine. This is achieved by forces in the engine that enable the gas to flow backward forming the jet. Newton's second law (see Mechanics) shows that these forces are proportional to the rate at which the momentum of the gas is increased. For a jet engine, this is related to the rate of mass flow multiplied by the rearward-leaving jet velocity. Newton's third law, which states that every force must have an equal and opposite reaction, shows that the rearward force is balanced by a forward reaction, known as thrust. This thrusting action is similar to the recoil of a gun, which increases as both the mass of the projectile and its muzzle velocity are increased. High-thrust engines, therefore, require both large rates of mass flow and high jet-exit velocities, which can only be achieved by increasing internal engine pressures and by increasing the volume of the gas by means of combustion.

Jet-propulsion devices are used primarily in high-speed, high-altitude aircraft, in missiles, and in spacecraft (see Airplane; Guided Missiles; Space Exploration). The source of power is a high-energy fuel that is burned at intense pressures to produce the large gas volume needed for high jet-exit velocities. The oxidizer required for the combustion may be the oxygen in the air that is drawn into the engine and compressed, or the oxidizer may be carried in the vehicle, so that the engine is independent of a surrounding atmosphere. Engines that depend on the atmosphere for oxygen include turbojets, turbofans, turboprops, ramjets, scramjets, and pulse jets (see below). Nonatmospheric engines are usually called rocket engines (see Rocket).

II.

Atmospheric Jet Engines

All atmospheric engines depend on the flow of a large mass of air that is first compressed, then used to oxidize fuel, and finally expanded to low pressures through a nozzle in order to achieve a high jet-exit velocity.

A.

Turbojet Engines

The most widely used atmospheric engines are turbojets. After air has been drawn into the engine through an inlet, the air pressure is increased by a compressor before it enters the combustion chamber (see Air Compressor). The power required to drive the compressor is provided by a turbine that is placed between the combustion chamber and the nozzle.

Practically all airborne jet engines use an axial-flow compressor, in which the air flows generally in the direction of the shaft axis through alternate rows of stationary and rotating blades, called stators and rotors. The blades are arranged so that the air enters each row at a high velocity. As it flows through the blade passage the air is decelerated to a lower velocity, thereby increasing the pressure. Modern axial-flow compressors can increase the pressure 24 times in 15 stages, with each set of stators and rotors making up a stage.

The compressed air then enters the combustion chamber where it is mixed with fuel vapor and then burned. For best performance, the combustion temperature should be the maximum obtainable from the complete combustion of the oxygen and the fuel. This temperature, however, would make the turbine too hot; turbine inlet temperatures, which currently limit turbojet performance, cannot exceed about 1100° C (about 2000° F) because of the thermal limitations of the materials. To reduce the temperature of the turbine inlet, only part of the compressed air is burned. This is achieved by dividing the air as it enters the combustion chamber. Part of the air is mixed with the fuel and ignited; the remainder is used to cool the turbine.

In the turbine, which acts in opposite fashion to the compressor, the gases are partially expanded through alternate stator and rotor passages. At the entry to each blade row, the velocity is low, allowing the gas to expand and speed up in the passage while it turns the rotor. The turbine provides the power to drive the compressor, to which it is connected by a shaft through the center of the engine, and it also provides the power for the fuel pump, generator, and other accessories.

The gases, which are now at an intermediate pressure, are finally expanded through the rearward-facing nozzle to reach the desired high jet-exit velocity. The greatest thrust would be obtained if the nozzle expanded the gases to the pressure of the surrounding atmosphere. In practice, however, such nozzles would be too large and too heavy. Actual nozzles are made shorter in order to provide higher exit pressures and a somewhat reduced engine performance.

A turbojet engine cannot start directly from rest; the engine must first be induced to spin by an external starting motor. The fuel is then ignited by a heated plug. Once the engine is running, however, combustion is maintained without spark plugs.

The thrust delivered by a turbojet decreases as the surrounding air temperature increases because the decreased density of the hot air reduces the mass flow through the engine. On hot days, takeoff thrust can be increased by injecting water at the compressor inlet and allowing the evaporating water to cool the air.

In military engines, bursts of speed or additional thrust for takeoff and climb can be provided by a second burner, or afterburner, installed between the turbine and the nozzle. In the afterburner, more fuel is added to burn the oxygen in the air that is not used in the combustion chamber; this process increases both the air volume and the jet velocity. The low efficiency of an afterburner, however, restricts its use to situations requiring a great burst of speed.

B.

Turbofans or Bypass Engines

The turbofan engine is an improvement on the basic turbojet. Part of the incoming air is only partially compressed and then bypassed in an outer shell beyond the turbine. This air is then mixed with the hot turbine-exhaust gases before they reach the nozzle. A bypass engine has greater thrust for takeoff and climb, and increased efficiency; the bypass cools the engine and reduces noise level.

In some fan engines the bypass air is not remixed in the engine but exhausted directly. In this type of bypass engine, only about one-sixth of the incoming air goes through the whole engine; the remaining five-sixths is compressed only in the first compressor or fan stage and then exhausted. Different rotational speeds are required for the high- and low-pressure portions of the engine. This difference is achieved by having two separate turbine-compressor combinations running on two concentric shafts or twin spools. Two high-pressure turbine stages drive the 11 high-pressure compressor stages mounted on the outer shaft, and 4 turbine stages provide power for the fan and 4 low-pressure compressor stages on the inner shaft. An example of an engine of this type is the JT9D-3 jet engine, which weighs about 3850 kg (about 8470 lb) and can develop a takeoff thrust of about 20,000 kg (about 44,000 lb). This is more than double the thrust available for the largest commercial planes before the Boeing 747.

Current research in turbojet and turbofan engines is largely directed to achieving more efficient operation of the compressors and turbines, to devising special turbine-blade cooling systems to permit higher turbine-inlet temperatures, and to reducing jet noise.

C.

Turboprop Engines

In a turboprop engine a propeller mounted in front of the jet engine is driven either by a second, or free, turbine or by additional stages from the turbine that supplies power to the compressor. About 90 percent of the energy of the expanding gases is absorbed in the turbine portion that drives the propeller, leaving only about 10 percent to accelerate the exhaust jet. The exhaust jet, therefore, contributes only a small fraction to the overall propulsive thrust. Turboprops have certain advantages for small and medium-sized planes at speeds of up to about 480 to 640 km/h (about 300 to 400 mph). They cannot compete, however, with turbojets or fanjets for very large planes and for higher speeds.

D.

Ramjets and Scramjets

The air rushing toward the inlet of an engine flying at high speeds is partially compressed by the so-called ram effect. If the air speed is high enough, this compression can be sufficient to operate an engine without either compressor or turbine. Ramjets can operate at speeds above 320 km/h (about 200 mph), although they become practical only at very high or supersonic speeds. Ramjets that operate at supersonic speeds are often called scramjets.

The ramjet has been called a flying stovepipe, because it is open at both ends and has only fuel nozzles in the middle. A straight stovepipe would not work, however; a ramjet must have a properly shaped inlet-diffusion section to produce high-pressure air at the combustion section, and it must also have a properly shaped exhaust nozzle. Because ramjets depend on the compression of the inrushing air for their operation, a vehicle powered by a ramjet must first be accelerated by other means to a sufficiently high speed.

E.

Pulse Jets

A pulse jet is similar to a ramjet, except that a series of spring-loaded shutter-type valves is located ahead of the combustion section. In a pulse jet, combustion is intermittent or pulsing rather than continuous. Air is admitted through the valves, and combustion is initiated, which increases the pressure, closing the valves to prevent backflow through the inlet. The hot gases are expelled through the rear nozzle, producing thrust and lowering the pressure to the point that the valves may open and admit fresh air. Then the cycle is repeated. The most widely known pulse jet was the German V-1 missile, or buzz bomb, used near the end of World War II, which fired at a rate of about 40 cycles per sec.

The pulsing effect can also be achieved in a valveless engine, or wave engine, in which the cycling depends on pressure waves traveling back and forth through a properly scaled engine. A pulse-jet engine delivers thrust at zero speed and can be started from rest, but the maximum possible flight speeds are below 960 km/h (600 mph). Poor efficiency, severe vibration, and high noise limit its use to low-cost, pilotless vehicles.

F.

Hydraulic Jet Propulsion

Jet-propulsion devices are not limited to using gases as the working fluid; liquids, such as water, may also be used. A simple example of a liquid jet device that operates on the reaction principle is the spinning lawn sprinkler.

Attempts to develop hydraulic jet propulsion for ships were made by British and Swedish engineers as early as the 1920s. In such a system, water is inducted at the forward end of the ship, passed through high-pressure pumps, and then exhausted at the stern through one or more nozzles that produce high-speed water jets. Both highly efficient pumps and high speeds are required to make hydraulic jets competitive with other means of ship propulsion. Although water-jet propulsion has not proved successful for large vessels, it is currently employed in some high-speed boats and pleasure craft.

III.

History

Jet power as a form of propulsion has been known for hundreds of years, although its use for propelling vehicles that carry loads is comparatively recent. The earliest known reaction engine was an experimental, steam-operated device developed about the first century bc by the Greek mathematician and scientist Hero of Alexandria. Known as the aeolipile, Hero's device did no practical work, although it demonstrated that a jet of steam escaping to the rear drives its generator forward. The aeolipile consisted of a spherical chamber into which steam was fed through hollow supports. The steam was allowed to escape from two bent tubes on opposite sides of the sphere, and the reaction to the force of the escaping steam caused the sphere to rotate.

The development (1629) of the steam turbine is credited to the Italian engineer Giovanni Branca, who directed a steam jet against a turbine wheel, which in turn powered a stamp mill. The first recorded patent for a gas turbine was obtained in 1791 by the British inventor John Barber.

In 1910, seven years after the first flights by the American inventors Orville and Wilbur Wright, the Romanian scientist Henri Marie Coanda designed and built a jet-propelled biplane, which took off and flew under its own power with Coanda as pilot. Coanda used an engine that he termed a reaction motor, but, discouraged by the lack of public acceptance of his aircraft, he abandoned his experiments.

During the next 20 years the gas turbine was developed further in both the United States and Europe. One result of the experimental work of that period was the perfection in 1918 of a turbosupercharger driver by an exhaust gas turbine for conventional aircraft engines. In the early 1930s many patents covering gas turbines were awarded to a number of European engineers. The patent granted the British aeronautical engineer Sir Frank Whittle in 1930 is generally conceded to have outlined the first practical form of the modern gas turbine. In 1935 Whittle applied his basic design to the development of the W-1 turbojet engine, which made its first flight in 1941.

Meanwhile, the French aeronautical engineer René Leduc had exhibited (1938) a model of the ramjet in Paris, and a jet airplane that was powered by an axial-flow turbojet designed by the German engineer Hans Joachim Pabst von Ohain made its first flight in 1939. In the following year, under the direction of the aeronautical engineer Secundo Campini, the Italians developed an airplane powered by a turboprop engine with a reciprocating-engine-driven compressor. The first American-built jet airplane, the Bell XP-59, was powered by the General Electric 1-16 turbojet, adapted from Whittle's design in 1942. The first jet engine of exclusively American design was produced by Westinghouse Electric Corp. for the U.S. Navy in 1944.

From a principle first described in 1906, the pulse jet was developed by the German engineer Paul Schmidt, who received his first patent in 1931. The V-1, or buzz bomb, first flown in 1942, was powered by pulse jet. Also in the mid-1940s the first commercial airline flights using turboprop engines occurred. In 1947 the Bell X-1 experimental airplane, powered by a four-chambered liquid-rocket engine and carried to the stratosphere in the belly of a bomber for launching, was the first pilot-operated craft to break the sound barrier. Subsequently the Douglas Skyrocket experimental airplane, powered by a jet engine in addition to a liquid-rocket engine, broke the sound barrier at low altitude after taking off under its own power.

The first commercial jet airplane, the British Comet, was flown in 1952, but this service was stopped after two serious accidents in 1954. In the U.S., the Boeing 707 jet was the first jet airplane to be tested commercially, in 1954. Commercial flights began in 1958.

The continuous development of jet propulsion for air power has resulted in such advances as piloted aircraft capable of attaining speeds several times greater than the speed of sound, and intercontinental ballistic missiles and artificial satellites launched by powerful rockets.