Rotor vibrations of turbopump due to cavitating flows in inducer

During the initial phase of the development of the LE7 liquid-oxygen turbopump, supersynchronous and subsynchronous shaft vibrations occurred. We experienced a very curious phenomenon, namely, that three types of shaft vibrations appeared in the tests of the turbopump under almost the same operating conditions. That is, only supersynchronous shaft vibration occurred, only subsynchronous shaft vibration occurred, or supersynchronous shaft vibration appeared just after subsynchronous vibration had disappeared concomitant with the decrease of inducer inlet pressure. First, it was clarified that the supersynchronous shaft vibration was caused by rotating cavitation which occurred in the inducer of the liquid oxygen main pump. However, considerable time was needed to identify the cause of the subsynchronous shaft vibration, because this vibration occurred under almost the same conditions as that which brought about the rotating cavitation. Finally, it was concluded that the subsynchronous shaft vibration was a cavitation-induced system oscillation, that is cavitation surge. In the present paper, we show how the subsynchronous shaft vibration was identified as cavitation surge. Some results of calculations will also be presented to explain the test results.