In order to understand a mechanism of an axial self-excited vibration which sometimes occurs in a rocket turbopump, pressures and temperatures of the turbopump were measured when the turbopump caused the axial self-excited vibration. The obtained data showed that the phase of the oscillating pressure in the balance piston chamber was delayed at about 20 degrees compared to the phase of the fluctuating rotor displacement. The experimental result means that the damping ratio of the axial thrust balancing system is negative and the system is unstable under the condition of the self-excited vibration as predicted by an analytical model. Furthermore, many experiments using other turbopumps were simulated with the analytical model, and it was found that the estimated damping ratios were always smaller than a certain value when the self-exited vibration happened. It became possible to predict whether the axial self-excited vibration occurs with the analytical model.
