Photographs of co-annular nozzle, a maneuvering nozzle, and the.
 external geometry. Computer drawing of a convergent and a convergent-divergent
 nozzle and a turbine engine.

Most modern passenger and military aircraft are powered by gas turbine engines, which are also called jet engines. There are several different types of gas turbine engines, but all turbine engines have some parts in common. All gas turbine engines have a nozzle to produce thrust, to conduct the exhaust gases back to the free stream, and to set the mass flow rate through the engine. The nozzle sits downstream of the power turbine.

A nozzle is a relatively simple device, just a specially shaped tube through which hot gases flow. However, the mathematics which describe the operation of the nozzle takes some careful thought. As shown above, nozzles come in a variety of shapes and sizes depending on the mission of the aircraft. Simple turbojets, and turboprops, often have a fixed geometry convergent nozzle as shown on the left of the figure. Turbofan engines will sometimes employ a co-annular nozzle as shown at the top left. The core flow will exit the center nozzle while the fan flow exits the annular nozzle. Afterburning turbojets and turbofans often have a variable geometry convergent-divergent (CD) nozzle as shown on the left. In this nozzle, the flow first converges down to the minimum area, or throat, then is expanded through the divergent section to the exit at the right. The variable geometry causes these nozzles to be heavy, but provides efficient engine operation over a wider airflow range than a simple fixed nozzle. Rocket engines usually have a fixed geometry CD nozzle with a much larger divergent section than is required for a gas turbine. You can explore the design and operation of nozzles with our interactive nozzle simulator program which runs on your browser.

All of the nozzles we have discussed thus far are round tubes. Recently, however, engineers have been experimenting with nozzles with rectangular exits. This allows the exhaust flow to be easily deflected, as shown in the middle of the figure. Changing the direction of the thrust with the nozzle makes the aircraft much more maneuverable.

Because the nozzle conducts the hot exhaust back to the free stream, there can be serious interactions between the engine exhaust flow and the airflow around the aircraft. On fighter aircraft, in particular, large drag penalties can occur near the nozzle exits. A typical nozzle-afterbody configuration is shown in the upper right for an F-15 with experimental maneuvering nozzles. As with the inlet design, the external nozzle configuration is often designed by the airframer. The internal nozzle is usually the responsibility of the engine manufacturer.

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byTom Benson
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