Jet Engines

A simplified view of how a jet engine works.

The Jumo 004 jet engine of World War II. Its main features carried over to later engines.

The turboprop used power from a jet engine to drive a propeller. Additional turbines, placed near the exhaust, tapped this power and spun rapidly. Turboprops drew attention between 1945 and 1960 but lost out because jet aircraft were faster.

Twin-spool jet engine compared with a conventional design. Note that the twin-spool version has two compressors, each driven by its own turbine. This arrangement gave more thrust with better fuel economy.

Jet fighters gained speed by burning fuel within an afterburner. This was a tube fitted to the end of the jet engine. Exhaust from that engine contained a great deal of hot air and allowed fuel to burn within the afterburner, for more thrust.

The J-31 (also known by its company designation, I-16) was the first turbojet engine produced in quantity in the United States. It was developed from the original American-built jet engine, the General Electric I-A, which was a copy of the highly secret British “Whittle” engine.

On April 15, 1952, a prototype Boeing B-52 Air Force bomber first flew with eight Pratt & Whitney J57 turbojets, each producing 10,000 pounds of thrust. The J-57 was an immediate success. In 1952, the engine won the Collier Trophy, one of aviation’s most prestigious awards, for “the greatest achievement in aviation in America.”

The development of the J-79 turbojet began in 1952 as a more powerful follow-up to the General Electric J47 turbojet. GE built more than 17,000 J-79s over 30 years, powering aircraft such as the F-104 Starfighter and F-4 Phantom II.

Two Jumo 004 engines powered the Me 262. This was the first jet fighter to fly in combat.

The Lockheed F-104 was the first fighter to fly at twice the speed of sound.

Gerhard Neumann built an engine for supersonic flight by overcoming “compressor stall.” Top: the problem. The compressor tended to pull in more air than the engine could swallow. Middle: Neumann gave attention to “strators.” In a conventional engine, these were fixed in position and helped air in the compressor to flow properly. Bottom: Neumann arranged for strators to turn, changing their position. This reduced the airflow in the compressor and prevented it from drawing in too much air.

Top: general layout of a turbofan engine. Note that a separate set of turbines drives the front fan, as in a turboprop. The term “high-bypass” means that most of the air in the exhaust comes from the fan and flows past the rest of the engine, rather than flowing through it.

In the early 1990s, GE developed the GE 90 turbofan engine to power the large, twin-engine Boeing 777. The GE 90 family, with the baseline engine certified on the 777 in 1995, has produced a world’s record thrust of 110,300 pounds in ground testing, has the world’s largest fan at 123 inches in diameter, composite fan blades, and the highest engine bypass ratio (9:1) to produce the greatest propulsive efficiency of any commercial transport engine.

Before World War II, in 1939, jet engines existed only as laboratory items for test. But at the end of the war, in 1945, it was clear that the future of aviation lay with jets. The new engines gave great power and thrust, but were compact in size. They also were simple in their overall layout.

A jet engine, down to the present day, pulls in air by using a compressor. It looks like a short length of an ear of corn, but instead of corn kernels, the compressor is studded with numerous small blades. The compressor rotates rapidly, compressing the air.

The compressed air flows into a combustor. Here fuel is injected, mixed with this air, and burned. This heats the air to a high temperature. The hot, high-pressure air then passes through a turbine, forcing it to spin rapidly. The turbine draws power from this hot airflow. A long shaft connects the turbine and compressor; the spinning turbine uses its power to turn the compressor.

The jet-engine principle was known early in the twentieth century. However, jet engines work well only at speeds of at least several hundred miles per hour. Racing planes were the first to reach such speeds, with a British seaplane setting a record of 407 miles per hour (655 kilometers per hour) in 1931 and an Italian aircraft raising this record to 440 miles per hour (708 kilometers per hour) in 1934.

A young German physicist, Hans von Ohain, was in the forefront. He started by working on his own at Gottingen University. He then went to work for Ernst Heinkel, a planebuilder who had a strong interest in advanced engines. Together they crafted the world’s first jet plane, the experimental Heinkel He 178, which first flew on August 27, 1939.

Building on this work, the German engine designer Anselm Franz developed an engine suitable for use in a jet fighter. This airplane, the Me 262, was built by the firm of Messerschmitt. It was the only jet fighter to fly in combat during World War II. But the Me 262 spent most of its time on the ground because it used too much fuel. It was a sitting duck for Allied attacks.

In England, Frank Whittle had no knowledge of Ohain’s ideas but invented a jet engine completely on his own. The British drew on his work and developed a successful engine for another early jet fighter—the Gloster Meteor. Britain used it for homeland defense but it did not see combat over Germany because it lacked high speed.

The British shared Whittle’s technology with the United States, enabling the engine-builder General Electric (GE) to build jet engines for America’s first jet fighter, the Bell XP-59. The aircraft company Lockheed then used a British engine in the initial version of its Lockheed P-80, America’s first operational jet fighter, which entered service soon after the war’s end. The British continued to develop new jet engines that used Whittle’s designs, with Rolls-Royce initiating work on the Nene engine during 1944. Rolls sold Nenes to the Soviets, and a Soviet-built version of the engine subsequently powered the MiG-15 jet fighter that fought U.S. fighters and bombers during the Korean War.

The surrender of Germany, in 1945, unlocked a treasure trove of wartime discoveries and inventions. General Electric and Pratt & Whitney, another American engine-builder, added German lessons to those of Whittle and other British designers. Early jet engines, such as those of the Me 262, gulped fuel rapidly. Thus, an initial challenge involved building an engine that could give high thrust with less fuel consumption.

Pratt & Whitney solved this problem in 1948 with its “dual spool” concept. This combined two engines into one. The engine had two compressors—each rotated independently, with the inner one giving high compression for good performance. Each compressor drew power from its own turbine; hence there were two turbines, one behind the other. This approach led to the J-57 engine, which entered service with the U.S. Air Force in 1953.

This was one of the outstanding postwar engines. It powered U.S. Air Force fighters, including the F-100, the first to break the sound barrier without going into a dive. Eight such engines powered the B-52 bomber. Commercial airliners—the Boeing 707, the Douglas DC-8—flew with it. This engine also saw use in the U-2 spy plane, which flew over the Soviet Union and photographed its military secrets.

The dual-spool engine represented an important step forward, but engine designers soon wanted more. As they reached for increasing performance, they ran into the problem of “compressor stall.” This meant that at certain speeds while in flight, the compressor would pull in more air than the rest of the engine could swallow. Compressor stall produced a sudden blast of air that rushed forward within the engine. The engine lost all its thrust, while this air blast sometimes caused severe damage by breaking off compressor blades.

During the early 1950s, Pratt & Whitney rode merrily along with its J-57. Its competitor, GE, had a good engine of its own: the J-47, which powered the F-86 fighter and B-47 bomber. Still, GE’s managers wanted something better. They got it from the engineer Gerhard Neumann, who found a way to eliminate compressor stall. Neumann introduced the “variable stator.” This was a set of small vanes that protruded into the airflow within the compressor. Each such vane was like your hand that you stick into the outside air when you ride in a car. Like your hand, each vane could turn as if mounted to a wrist. When the vanes faced the airflow with their edges forward, they allowed the flow to pass them freely. But when the vanes were turned to present their broad faces to the flow, they partially blocked it. These vanes then reduced the amount of flow that was passing through the compressor, and kept it from gulping too much air.

This invention led to an important GE engine, the J-79. It became the first true engine for supersonic flight. With it, the Lockheed F-104 fighter flew at twice the speed of sound. In May 1958, U.S. Air Force pilots used this airplane to set a world speed record of 1,404 miles per hour (2,260 kilometers per hour) and an altitude record of 91,249 feet (27,813 meters). With supersonic flight in hand, the next frontier in jet-engine progress called for engines of very great power, suitable for aircraft of the largest possible size. The key concept proved to be the “turbofan,” also called the “fanjet.”

The “jet” of a jet engine is the hot stream of exhaust that blasts out the back to produce thrust. However, that exhaust carries power as well as thrust, which the turbines use to run the compressor. By using a larger set of turbines, it is possible to tap off still more of this power. The big turbine then turns a fan, which somewhat resembles an airplane propeller but has many long blades set closely together. The fan adds its thrust to that of the jet. This arrangement yielded the turbofan. It more than doubled the thrust of earlier engines. It also further improved fuel economy. In addition, turbofan engines were relatively quiet, in contrast to earlier jets that produced loud shrieks and screams. GE and Pratt & Whitney both built turbofans after 1965, with Rolls-Royce, offering versions of its own. All truly large airliners have used them, starting with the Boeing 747. These engines have also powered large U.S. Air Force cargo planes, including the C-5A and C-17.

The first aircraft to use these large engines was the Lockheed C-5, which entered development in 1965 and first flew in 1968. A key to its design was the engine—the GE TF-39 turbofan. It had a dual-spool layout as well as a variable stator, with its big fan providing 85 percent of the thrust. The dual-spool arrangement gave the fan its own turbine for power, separate from the rest of the engine. The compressor had 16 stages, or rows of blades.

These three design principles—dual-spool layout, variable stators, and the turbofan—remain in use to this day. All three can even appear in the same engine, as with the TF-39. The dual-spool design gives high thrust with good fuel economy. Variable stators allow efficient operation at all flight speeds. The big forward fan reduces noise, further improves fuel economy, and produces much of the thrust. In turn, the thrust of engines continues to increase. Germany’s engine for the wartime Me 262, the Jumo 004, delivered 2,000 pounds (8,900 newtons) of thrust. The J-57 was rated at 13,500 pounds (60,000 newtons) of thrust. The J-57 was similar in thrust but weighed considerably less, which made it much speedier. Early turbofans, around 1970, came in around 40,000 pounds (180,000 newtons) of thrust. But GE’s new GE 90 turbofan is rated at close to 90,000 pounds (400,000 newtons) of thrust! That is why today’s planes fly fast and are very large.

–T.A. Heppenheimer

Courtesy Centennail of Flight web site.


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