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138 CHAPTER FIVE

Rolls Royce uses three spools on Although the exhaust loses some of its energy, and thus
its larger engines, which increases becomes cooler, going through the turbine, it is still very hot.
the efficiency of the engine but The first vane/blade stage of the turbine sees temperatures
also the complexity and cost. similar to those in the burner, on the order of 2800°F (1500°C).
So, this turbine stage requires special cooling, including the
film cooling from bleed air as is found in the burner. A turbine blade
is shown in Figure 5.18. Notice that it is hollow, which is to allow
internal cooling air, and there are small holes on the surface. These
holes allow a cool air pocket to form around the surface of the blade.
This pocket is thin but allows the blades to survive the hot
temperatures.
The pressure change across the turbine goes from high to low
pressure. In the compressor the pressure went from low to high. Because
of this, unlike the compressor, there is little problem with the turbine
blades stalling. The pressure change across a turbine stage, therefore,
can be much greater than the pressure change across an axial-flow
compressor stage. Even with the energy loss, the gas leaving the turbine
still has a great deal of energy and can be used for propulsion.

The Turbojet

The simplest form of the jet engine is the turbojet shown in Figure
5.10. Basically, a turbojet is the turbine engine with a diffuser and a
nozzle. The diffuser works to “condition” the air before it enters the
compressor. The compressor is optimized for a certain airspeed. Con-

Fig. 5.18. A turbine blade. (Photo courtesy of NASA.)