On the Dynamic Response of a Cryogenic Hybrid Journal Bearing due to Pneumatic Hammer

Hybrid bearings are critical components for enhancing the reliability and reusability of liquid rocket engine turbopumps in space propulsion systems. However, hybrid bearings with inadequately designed recesses (pockets) are susceptible to pneumatic hammer instability when operating with compressible fluids. This paper investigates test data revealing unexpected modulation of synchronous vibration components in a cryogenic hybrid bearing (60 mm in diameter, 25 mm in axial length, 0.049 mm in radial clearance, nine-recess) lubricated with cryogenic liquid nitrogen (LN2). During testing, with the bearing supplied with LN2 above its saturation pressure, pneumatic hammer instability occurred at an LN2 supply pressure of approximately 6 bar(g) when the shaft rotating speed reached 30 krpm. This instability induces persistent subsynchronous motions that reach limit cycles. The pneumatic hammer instability also modulates the synchronous motion amplitudes, creating distinctive sideband frequencies adjacent to the synchronous vibration components of the bearing housing. This phenomenon allows for the detection of even weak pneumatic hammer vibrations. Recess pressure is estimated from the relationship between bearing vibration amplitudes and rotor speeds. The current work suggests a strategy to prevent two-phase flow in hybrid bearings and ensure dynamically stable operation, even with deep recesses, when lubricated with cryogenic fluid.