Influence of impeller’s elastic deformation on the stability of balance piston mechanism of rocket engine turbopump

Turbopump of rocket engine is one of the most important components and has a role to rise pressure of propellant and send to combustion chamber. Turbopump is required to generate high power compared to its small size, so vibration problems often occurred. Axial vibration is one of these vibration problems. Rocket engine turbopump generates large axial thrust due to its high discharged pressure. However, bearings cannot support the axial thrust because turbopump operates in cryogenic condition. Balance piston (BP) mechanism, which is a self-balancing mechanism of axial thrust, is hired for rocket turbopump to deal with the large axial thrust. BP has two orifices at impeller back shroud, and turbopump rotor is designed to be movable in axial direction. Pressure of impeller back-shroud changes in accordance with the change of two orifices clearances when rotor moves in axial direction, and axial thrust is automatically balanced. Although BP mechanism is statically stable, it has a potential to be dynamically unstable due to compressibility of fluid. If BP mechanism becomes unstable, self-excited vibration in axial direction occurs and turbopump can be destroyed due to contact of impeller and casings. In recent researches, stability of BP mechanism is investigated by one-dimensional model of BP. In this approach, rotor including impeller is modeled in lumped mass. However, in the case of open impeller with large diameter, rigidity of impeller decreases compared to closed impeller, and the elastic deformation of impeller affects the stability and vibration mode in axial direction. In such cases, conventional lumped mass model cannot be used. In this paper, influence of elastic deformation of impeller on the stability of axial motion is investigated by the model of BP mechanism including impeller's elastic deformation.