Jet propulsion is the production of motion in one direction by the release of a high-pressure stream of gas in the opposite direction. Jet propulsion powers rockets, guided missiles, and many airplanes.
Jet-propelled airplanes can reach much higher speeds than propeller-driven ones. Some jet aircraft fly faster than sound travels through the air. Jet propulsion also makes flight possible at extremely high altitudes—and even in outer space.
Jet engines cause less vibration than do piston engines, which are used in some airplanes to turn propellers. This smoothness of operation results in a safer and more comfortable ride. Jet engines are generally smaller and lighter in weight than piston engines that produce the same amount of thrust (forward-driving force). However, a jet engine burns more fuel than a piston engine that creates an equal amount of thrust.
About A.D. 60, the scientist Hero of Alexandria built a small toylike device that was probably the first jet engine. The first flight of an airplane powered by a jet engine occurred in Germany in 1939. Since then, jet propulsion has powered aircraft of all types, including supersonic airliners and spacecraft that have journeyed to other planets.
How jet propulsion works
The principle of jet propulsion
can be demonstrated with a garden hose connected to a water supply. When the nozzle at the end of the hose is closed, the water pushes in all directions against the inside of the nozzle. It also pushes back against the water in the hose that is trying to squeeze into the nozzle. When the nozzle is open, some of the water squirts out through the opening. This action upsets the balance of pressure inside the nozzle. It releases the pressure pushing forward just inside the nozzle opening. But the water that is still in the nozzle continues pressing backward and to the sides. If you let go of the nozzle, the unbalanced, backward-pushing pressure will propel it backward. The nozzle will move in the direction opposite that of the jet of water escaping from the nozzle.
The principle of jet propulsion was first described by the English scientist Sir Isaac Newton in his third law of motion, published in 1687. This law states that for every action there is an equal and opposite reaction. In the above example, squirting water out the end of the nozzle is the action. The equal and opposite reaction is the backward movement of the nozzle. Jet propulsion drives an aircraft engine in much the same way. Gas pressure builds up inside the engine. The pressure pushing in one direction is released in a powerful stream of jet exhaust. The action of this exhaust escaping from the rear of the engine causes an equal and opposite reaction that pushes the engine forward. See Motion (Newton’s laws of motion) .
Jet engines and rockets both use the principle of jet propulsion. However, they use different sources of oxygen to burn fuel. Jet engines use the oxygen in the air. For this reason, airplanes with jet engines cannot fly outside Earth’s atmosphere. Rockets carry their own supply of oxygen to use in burning their fuel. As a result, rockets can fly in the airless expanse of outer space. See Rocket (How rockets work) .
Power in a jet engine
is produced by burning fuel in a combustion chamber. The hot gases created by the burning of the fuel with a supply of air then rush out through a nozzle, creating jet thrust.
Air enters the jet engine through an inlet duct. It is then compressed until it has from 3 to 40 times as much pressure as that of the outside air. The compressed air flows into the combustion chamber. There, part of the air is mixed with a fine spray of jet fuel. Kerosene or a mixture of kerosene and gasoline are the most commonly used jet engine fuels. The mixture of fuel and air is ignited and the fuel burns, releasing a large amount of energy. The temperature of combustion ranges from about 3600 to 4000 °F (2000 to 2200 °C). These high temperatures could damage the jet engine. However, the remainder of the compressed air is mixed with the hot combustion gases to lower the temperature of the gases by about 50 percent. This process cools the walls of the combustion chamber and other parts of the engine. The mixture of gases then escapes through a nozzle at an extremely high speed.
The first jet-propulsion engines created thrust by accelerating a small amount of gas to great speeds. The exhaust of these engines contained much unused energy. Modern jet engines approach the efficiency of propeller engines, which move a large amount of air at a low speed, leaving little unused energy in the air.
The amount of thrust produced by a jet-propulsion engine is about the same at all flight speeds. But the amount of thrust generated by a propeller drops off as the flight speed increases. Thus, jet-propelled aircraft can reach much higher flight speeds than propeller-driven aircraft can.
The thrust produced by an aircraft engine is expressed in pounds or newtons. For example, each of the four huge jet engines used on many Boeing 747 airliners produces 51,600 pounds (232,000 newtons) of thrust.
Types of jet engines
There are four major types of jet engines: (1) turbojet, (2) turboprop, (3) turbofan, and (4) ramjet. They differ chiefly in the portion of their total thrust that they produce directly by jet propulsion. Turboprops and turbofans generate most of their thrust by turning propellers or propellerlike fans rather than by pure jet propulsion. Jet engines also differ in the way they compress the air that enters the intake ducts.
Turbojet
was the first type of jet engine used to power an airplane. All other types of jet engines are variations of the turbojet.
An inlet duct scoops air into the turbojet and carries the air to the compressor. The job of the inlet duct becomes more complicated in jet fighters and other aircraft that fly faster than the speed of sound. At an altitude of 40,000 feet (12,000 meters), sound normally travels at about 660 miles (1,060 kilometers) per hour. Supersonic flight (flight that exceeds the speed of sound) causes shock waves to develop in the air as it rushes through the inlet duct. These shock waves remove energy from the air, which can reduce the engine’s thrust. However, a turbojet can reduce the loss of energy caused by such shock waves by continually adjusting the shape of the inside of the inlet duct.
The compressor raises the pressure of the air in the engine. Turbojets are equipped with either an axial-flow compressor or a centrifugal-flow compressor.
An axial-flow compressor consists of several wheels with many small, winglike blades attached, as in an electric fan. These wheels are arranged one behind another along a shaft that runs through their centers and turns them at high speeds. Between each pair of wheels is a set of stationary blades. The air flows through the compressor parallel to the shaft. Each row of blades squeezes the air, increasing its pressure. Some axial-flow compressors raise the air pressure to a level about 40 times that of the air entering the inlet duct.
A centrifugal-flow compressor squeezes the air by taking it in near the center of a rapidly spinning wheel and throwing the air out toward the rim. Such compressors typically have a larger diameter than axial designs. As a result, they are used less frequently in turbojet engines than axial-flow compressors are.
After the air leaves the turbojet compressor, it enters the combustion chamber. There, from 25 to 40 percent of the compressed air is mixed with jet fuel and the fuel is burned. Combustion increases the temperature and pressure of the gases. The rest of the air from the compressor is then mixed with these gases to cool them.
As the hot gases rush out of the combustion chamber, they pass through the blades of a turbine (rotor device). The combustion gases spin the turbine wheels, which rotate the shaft that turns the compressor. A small amount of cold air is bled from the compressor through holes in the turbine blades to keep them from melting.
After passing through the turbine, the combustion gases rush out through the nozzle. In a typical turbojet, the exhaust gases leave the nozzle at a speed of about 1,000 miles per hour (1,600 kilometers per hour). Nozzles designed for flight speeds below the speed of sound narrow gradually to the opening. Nozzles designed for supersonic flight narrow and then flare out again. This widening of the nozzle helps accelerate the gases beyond the speed of sound.
Some turbojets are equipped with devices called afterburners. An afterburner greatly increases the thrust of the engine. In a turbojet, the afterburner sits between the turbine and the nozzle. The gases leaving the turbine are still rich in oxygen. In the afterburner, additional jet fuel is mixed with these gases and burned, greatly raising the temperature. The energized gases then accelerate through the nozzle, reaching extremely high speeds. This high-speed exhaust generates a great deal of thrust. However, the process of accelerating the gases uses a large amount of fuel. For this reason, turbojets are used only for short periods, such as during emergency maneuvers, rapid take-offs, or steep climbs.
Turbojets are used primarily to power military aircraft. The Northrop F-5E jet fighter uses two turbojets with afterburners. Each turbojet generates 3,500 pounds (15,600 newtons) of thrust. The afterburners boost the thrust to 5,000 pounds (22,200 newtons).
Turboprop
is basically a turbojet that uses nearly all its power to turn a propeller. The arrangement of the compressor, combustion chamber, and turbine in a turboprop is similar to that in a turbojet. However, the turboprop also has a second turbine just to the rear of the turbine that turns the compressor. Combustion gases spin the second turbine, which is sometimes called the power turbine. This spinning motion is transferred by a shaft and a gearbox to the propeller.
There is still a little energy left in the combustion gases after they have turned the power turbine. These gases shoot out of the nozzle, adding a small amount of jet thrust to the thrust produced by the propeller.
The turboprop is smooth running, reliable, and economical, but it is limited to subsonic flight speeds. Turboprops are much smaller and lighter than piston engines that produce the same amount of power. The Cessna Conquest, which is a small business airplane, and the Beechcraft 1900, a 19-passenger commuter airliner, use turboprops.
Turbofan
is essentially a turbojet that uses part of its power to turn a large fan. The fan is powered by a turbine and is enclosed in a duct at the front of the engine. It pushes air back toward the engine. Most of the air is forced back along the outside of the engine, creating thrust. The rest of the air enters the engine, where it is compressed. Fuel is mixed with the compressed air and burned, and the hot gases are released to generate thrust. By combining the two propulsion methods, a turbofan achieves some of the efficiency of a turboprop aircraft without sacrificing the high-speed performance of a turbojet. Some turbofans have afterburners.
A further advantage of the turbofan is its low noise level. Jet noise becomes louder as the jet exhaust velocity increases. The speed of gases leaving a turbofan is lower than the speed of turbojet exhaust. Thus, a turbofan is quieter than a turbojet.
All modern airliners use turbofans. Most military jets, including the Lockheed Martin F-16A Fighting Falcon, also are powered by turbofans.
Ramjet
is the simplest type of jet engine. It is basically a turbojet without a compressor or turbine. Air entering a ramjet is slowed down in the inlet duct. This air is compressed by the ramming action of more air trying to enter the duct as the ramjet travels at high speed. Fuel is mixed with the compressed air and burned, and the combustion gases are accelerated through the nozzle to create thrust. Because of its simplicity, the ramjet is sometimes called the “flying stovepipe.” Ramjets are used in guided missiles designed to attack enemy aircraft.
The major disadvantage of the ramjet is that it functions poorly at subsonic flight speeds. If sufficient air flow is not achieved, the combustion gases will exit both ends of the engine, and the engine could explode. Such an occurrence is avoided by first accelerating a ramjet to supersonic operating speed by a turbojet or rocket. The ramjet engages after there is an adequate high-pressure flow of air through the engine.
In one type of ramjet, called a supersonic combination ramjet or scramjet, combustion takes place at supersonic air velocities through the engine. A scramjet has flown at nearly 10 times the speed of sound, and scientists believe scramjets may be able to fly at speeds approaching 15 times the speed of sound. The U.S. National Aeronautics and Space Administration (NASA) Hyper-X program is devoted to developing such aircraft.
Development of jet propulsion
The scientist Hero of Alexandria built a small jet engine about A.D. 60. It was powered by steam escaping from a hollow sphere through two nozzles that pointed in opposite directions. The escaping steam turned the sphere in much the same way that jets of water spin a rotating lawn sprinkler. But jet engines were not used to power aircraft for nearly 1,900 years after Hero’s invention.
The growing tensions that led to World War II (1939-1945) accelerated the development of jet engines to propel aircraft. The first flight of a jet airplane occurred in Germany in 1939. This airplane, the Heinkel He-178, was powered by a turbojet designed by Hans von Ohain, a German physicist. In Italy, the jet-propelled Caproni-Campini CC2 airplane was built and flown in 1940. Neither of these first two jet aircraft engines proved practical. A more successful turbojet soon was developed by Frank Whittle, an officer in the United Kingdom’s Royal Air Force. Whittle had patented a design for a turbojet in 1930. His engine powered the Gloster E. 28/39, an experimental airplane that first flew in 1941. The first successful jet fighter was the German Messerschmitt Me262, which began to fly combat missions near the end of World War II.
During World War II, Germany pioneered the use of jet propulsion for guided missiles. In 1947, the rocket-powered Bell X-1, built in the United States, became the first airplane to fly faster than the speed of sound.
During the 1950’s, turbojets and turboprops began to power some commercial airliners. Also at this time, ramjets propelled such early United States guided missiles as the Bomarc and Talos. During the 1960’s, the turbofan began to replace the turbojet on commercial and military aircraft. Because of the turbofan’s efficiency and low noise, it came into widespread use in the 1970’s. Today, researchers continue working to increase the efficiency of jet engines while reducing the cost and the amount of pollution the engines produce.
