Jet Engines and Gas Turbines

People associate jets with high speed and for good reason—jets power ThrustSSC as well as the fastest airplanes. Like the engines that propel rockets through space, jet engines use Newton’s third law, which says that for every action there is a reaction. In the case of jets (and rockets), the action is a spewing of gaseous

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This 1953 photo shows some of the fast jet airplanes tested by the National Advisory Committee for Aeronautics (NACA, the predecessor of NASA). The airplane in the center is the X-3, and the others are, starting from left and going clockwise, X-1A, D-558-1, XF-92A, X-5, D-558-2, and X-4. (NASA)

molecules out the back of the engine. The backward momentum of the gas imparts a forward momentum on the craft—this is the reaction.

The two engines that propelled ThrustSSC to the land speed record were jet engines of the same type that propel military fighter planes. The heart of a jet engine is a heat engine called a gas turbine, and it works on the same thermodynamics principles as the old steam engines and the piston engines that run in most of today’s automobiles. The gas turbine is an internal combustion engine that burns a fuel—propane or jet fuel—and uses the hot expanding gas to rotate a turbine. The process is similar to the steam turbine described above.

A basic gas turbine has three parts: a compressor, a combustion chamber, and the turbine, as shown in the figure on page 94. In the combustion chamber, the fuel burns and produces the hot,

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energetic gas that rotates the turbine. The rotation of the turbine can do work, just like the rotation of a steam turbine or the rotation of a crankshaft in a piston engine, and the work of the turbine is to drive the compressor. Gas turbines use a compressor in a way similar to the piston engines that use high compression ratios and, occasionally, turbochargers or superchargers to increase power. Fuel combustion at normal (atmospheric) pressure produces weak gases that do not expand much, and therefore cannot do much work. Compressing the fuel and air mixture into a small space and then burning it produces a hot, expanding gas at high pressure that can do a lot of work.

In a jet engine, the high-pressure gas not only turns the turbine but also provides thrust. The gas passes through a nozzle and out the back of the engine at high speed. But gas turbines are not just limited to jet airplanes. Gas turbines also power fast ships and drive the motion needed to produce electricity in electric power plants. For these applications, the turbine’s rotation drives the main shaft directly, as in a steam turbine. Instead of being channeled through

a nozzle, the exhaust gases are simply vented outside, like a muffler and pipe system of a car.

A gasoline turbine is similar to a steam turbine (compare to the figure on page 80), but the energy comes from the burning of gasoline in the combustion chamber. The oxygen to support combustion comes from the intake, and a compressor squeezes the volume of air so that more will fit into the chamber. The hot, expanding gases from the combustion turn the turbine.

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Gas turbines are lighter and typically more powerful than piston engines. Large commercial airplanes are powered by jet engines, often seen hanging down from the wings. Many of these gas turbines have an additional component to the three parts already discussed: there is a fan mounted at the initial stage of the engine, which an observer can easily see when viewing the engine from the front. Powered by the turbine’s rotation, the fan increases the amount of air taken into the engine and provides more air for thrust.

Although gas turbines are generally more powerful than piston engines, piston engines are still common. The reason for this is the same as why piston engines appeared long before gas turbines. The turbine is not a recent idea—although people developed the jet engine in the 1930s and 1940s, the concept of a turbine goes back to the 18th century. One of the problems with making a high-speed gas turbine involves the design of a compressor strong enough to handle the job. Another big problem was that unlike

a piston engine, in which periodic combustions provide a power stroke, a gas turbine continuously burns fuel to rotate the turbine, and it operates under high pressures and temperatures, which puts

a lot of stress on its components. Like a race car engine operating at high rpm, jet engines must be made from special materials to withstand the constant heat and pressure. Before jet engines could become a reality, these materials had to be discovered or designed. They are not cheap, so gas turbines are generally too expensive to put into ordinary automobiles.

Carnot’s equation says that an increase in operating temperature results in an increase in efficiency for any heat engine. The materials in jet engines can withstand high temperatures, so this is an advantage in terms of thermodynamics. But the efficiency of a gas turbine decreases if it has to stop and start—gas turbines are meant to provide continuous thrust, as they do in jet engines, or continuously turn a heavy object, as they do in electric power plants. This is yet another reason why cars do not use gas turbines, because while highway driving would be fine, the stop-and-go of city traffic would eliminate the gas turbine’s advantage.

Although efficiency is important, power and speed are often

a jet engine’s main concerns. The power can be increased even

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Air force maintenance personnel test a jet engine and its afterburner. (United States Air Force/2nd Lt. Albert Bosco)

further by adding an afterburner, at the expense of decreased efficiency. An afterburner adds extra fuel to the exhaust and then ignites it, increasing the temperature and velocity of the exhaust, which increases the thrust. This process burns a lot more fuel and reduces the engine’s efficiency, but for a fighter pilot trying to out- maneuver an enemy, the extra power is worth the expense.

Jet Engines and Gas Turbines