Jet Propulsion

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.

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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.