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 centre 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.

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 manoeuvrable.