Stability of an Axial-Thrust Self-Balancing System

Rocket pump is characterized by high speed and high delivery pressure. Therefore, balancing of axial thrust acting on the rotor assembly is one of the most important factors. To realize complete axial thrust balancing, a balance piston-type axial-thrust self-balancing system is often used in rocket pumps. Such a system is comprised of an inlet orifice (#1) located at the outlet part of the impeller, outlet orifice (#2) located at the small-radius position of the back shroud and a chamber between these two orifices. Those orifices made by edges of the casing and the impeller shroud look like rings. The rotor assembly is allowed to move axially less than 1 mm to control the clearances of the orifices. The rotor assembly moves toward the turbine part when unbalanced axial thrust is imposed on the rotor assembly in the direction from the inlet of the pump toward the turbine part. As a result, the clearance of the inlet orifice increases and that of the outlet orifice decreases. This results in an increase in the pressure in the chamber between the orifices and makes the axial thrust generated by the balance piston in the direction from the turbine part toward the inlet of the pump increase. In this way, unbalance axial thrust imposed on the rotor assembly can be compensated automatically. This axial thrust balance system acts dynamically as if it is a mass and spring system although there is no mechanical spring. Too much vibration in the axial direction causes metal to metal rubbing, resulting in the explosion of rocket turbopumps. Although large amplitude axial vibration has been observed in rocket engine turbopumps, the cause of the vibration has not yet been clarified. In the present study, the self-balancing system was modeled by combining the mechanical structure and the fluid system in a calculation program. Stability of the system was investigated using this program. Effects of geometry, fluids, etc., were examined and methods to stabilize the system in order to suppress the axial vibration were developed.