An application of robust design process for reduction in Thomas force of rocket turbopump turbines is presented in this paper. Because of its special specification, rocket turbopump turbines are designed as supersonic impulse turbines, and as a result, these blades have higher loading. In such a high loading blade, there is a possibility of rotor vibration problem generated by turbine instability force, that is, “Thomas Force”.
Actually in the past engine development in Japan, there was the shaft vibration problem due to “Thomas Force”. In order to reduce shaft vibrations, it is important to keep “Thomas Force” small. In this paper, the results of parametric design for the existing rocket turbopump turbine by using the robust design process are discussed.

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A finite-element model is derived for high-pressure fuel turbopump（HPFTP）rotor system. It considers the effects of gyro-moment, axial force and rotor dynamic fluid forces acting on the impellers, the inducer, the seal, and the turbine. The damped natural frequencies, real parts of eigenvalues, and mode shapes of the rotor system are analysed over the range of 0∼45,000rpm and the results are validated by comparing them with existing experimental data. The effects of gyro-moment, axial force and rotor dynamic fluid forces on the dynamical characteristics of the system are investigated. As a result, it is shown that axial force has a small effect and is considered negligible, wheareas gyro-moment and rotor dynamic fluid force have a relatively large effect on the dynamical characteristics of the rotor system. The frequency response curve of the unbalance response of HPFTP is also obtained using a numerical simulation and the results correspond to its actual vibration phenomenon.

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One of the main problems in development of the rocket engine is the vibration problem of the fuel turbopump（FTP）. In this study, both the rotor dynamic fluid forces and the damping effect at the bearing supports are considered, and the derivation of reduced order model of FTP by utilizing the complex modal analysis technique is demonstrated. By using the derived reduced order model, the variation of the modal damping ratio for the rotor dynamic fluid forces and the damping at the bearing support are explained. The resonance curve of the modal amplitudes for various unbalance distribution cases are compared, and it is validated that modal amplitude is predictable from the mode shape, the modal unbalance, and modal damping ratio. Furthermore, the peak amplitudes of the dominant modes are focused, and the influence of the rotor dynamic fluid forces and the damping at the bearing supports on them are explained.

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