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.