During development testing of the High Pressure Oxygen Turbopump (HPOTP) of the Space Shuttle Main Engine (SSME) to produce 109 percent of the rated thrust level, subsynchronous rotor whirl was encountered. This whirl was attributed to bearing wear reducing the radial bearing stiffness that caused the rotor second bending mode critical speed to enter the operating speed range. To eliminate this whirl, the pump end bearing loads were reduced to increase bearing life and damping added between the rotor and housing. This was achieved by converting impeller annular seals into “damping” seals that react part of the applied load and also damp the rotor response. Furthermore, the second rotor critical speed was increased by the added stiffness of the seal conversion and stiffening the rotor shaft. The bearing load reduction was verified by strain gaging the pump end bearing support into a load cell. These strain gages also were used to directly measure bearing ball wear during engine tests.

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