lecture_29

lecture_29 - 16.512, Rocket Propulsion Prof. Manuel...

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16.512, Rocket Propulsion Prof. Manuel Martinez-Sanchez Lecture 29: Rotordynamics Problems 1. Turbopump Rotor Dynamics Because of high power density and low damping in rocket turbopumps, these machines exhibit in their most extreme form a variety of vibration effects, which are either absent or masked by normal damping mechanisms in other turbo machines. The low damping is especially prominent in liquid hydrogen pumps, because of the very low viscosity and density of this medium. Oil squeeze film dampers are precluded in any cryogenic medium. The general frame work of Rotor Dynamics is now well established, through a combination of classical analysis and detailed numerical simulation [49, 50, 51]. Intensive efforts on the application of these theoretical methods to a specific rocket turbopump are described by Ek[52], and were instrumental in pointing the way to a series of improvements that resolved a serious development problem in the SSME. The greatest difficulty in this field remains the precise characterization of the fluid forces acting on the rotor at components such as seals, turbines, or impellers. Once these are specified, numerical models of great power and versatility can be brought to bear for analyzing the dynamics of a given configuration. Because of the remaining uncertainties in the basic forces. Ek’s 1978 recommendation [52] remains valid today: “Prediction of stability in a new design must be viewed with skepticism. A prediction of instability should, however, be taken very seriously”. 2. Forced and Self- Excited Vibrations Three are two types of rotor dynamics problems: (a) Resonances which usually occur when the rotating speed coincides with one of the natural (“critical”) frequencies of the rotor (including its supporting structure). These fall in the category of Forced Vibrations, in which an excitation force produces deflection responses of an amplitude which increases as the excitation frequency approaches a critical frequency. If the excitation is at exact resonance, the amplitude grows linearly in time at first, and then, if viscous damping exists, it approaches a limit which is inversely proportional to the damping factor. Removal of the excitation removes the response. The exciting forces are typically related to rotor mass imbalance or geometrical imperfections. Resonances rarely pose serious problems, unless the steady operating point lies very close to one critical. On the other hand, since the structure is made as light as practical, many natural modes usually exist, several of them either below or not far above the operating range. Efforts are made in the design phase to create a relatively wide range of resonance- free speeds around the normal operating point. Passage through criticals, if made rapidly enough, is not a severe condition.
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This note was uploaded on 11/07/2011 for the course AERO 16.512 taught by Professor Manuelmartinez-sanchez during the Fall '05 term at MIT.

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lecture_29 - 16.512, Rocket Propulsion Prof. Manuel...

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