Turbine

The turbine provides the torque necessary to drive the compressor and any other auxilliary equipment (hydraulics, cabin controls, electrical) required for engine and aircraft operation. Turbines can also be used to drive fans, rotors, or propellers in other airbreathing engine configurations. Because of the extreme environment, the turbine blades are made of some of the most advanced materials available. Even using these materials, the turbine inlet temperature is limited by the strength of the blade materials. Since the turbine inlet temperature drives engine performance, there is a great desire to operate at temperatures near the limits of the material. Advanced blade cooling schemes have been developed toward this goal.

A turbine works like a compressor in reverse. Static nozzles convert pressure to velocity, and a moving section converts that velocity to rotational motion. Again, there are two types, both centrifugal and axial, but axial turbines are far more common.

Considerations in the design of the blades is different, however. Because the turbine is removing pressure instead of adding it, there is a favorable pressure gradient. Thus, much more extreme angles are possible without flow separation, and therfore a turbine generally has far fewer stages than a compressor. However, stress on the blades is much higher, due to the more extreme angles and due to the heat of the flow. The buckets must also be able, at these conditions, to withstand the forces of rotation. Indeed, turbine blade design is currently one of the limiting factors on engine design. Buckets must be periodically replaced because they stretch under centrifugal force, a phenomenon known as "creep". Newer methods of casting, including dirctionally solidified and single crystal castings, are used to lessen these problems.

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