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lecture_27 - 16.512 Rocket Propulsion Prof Manuel...

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16.512, Rocket Propulsion Prof. Manuel Martinez-Sanchez Lecture 27: Turbines 1. Rocket turbine design emphasizes power density, because of the overriding concern for mass saving. Efficiency, while clearly a consideration, takes a less prominent role than in aircraft turbine design. This tends to favor low reaction turbines and when multi-staging, velocity compounding over pressure compounding. The degree of reaction of a turbine stage (stator nozzles plus rotor blades) is the fraction of the fluid static enthalpy drop which occurs in the rotor (see section 2. of this Lecture). In an impulse turbine , the degree of reaction is zero, meaning that the gas expands and accelerates as it turns in the stator passages, and then is merely redirected at constant thermodynamic state by the moving rotor blades. The stage velocity triangles are shown in Fig 1, which also includes the case of a 50% reaction turbine for comparison. In both cases, the flow leaves axially. The torque (and hence the power) is proportional to the change in the tangential component of the absolute velocity. Fig 1 shows that this change is the wheel speed R for the 50% reaction turbine, but is 2 ω ω R for the impulse turbine. It is also clear, however, that flow velocities are higher in the impulse case which will lead to larger viscous losses. Also, the lack of acceleration in the rotor passage will favor flow separation on the suction side. Altogether, the more powerful impulse stage is also less efficient.
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