Heat generation of ball bearing for rocket-engine turbopump in low-temperature gas hydrogen

Abstract

Cryogenic fluid such as liquid hydrogen, liquid oxygen, and liquid methane is used for rocket propellant because of their performance. Cryogenic fluid turbopump equipped in rocket-engine is started to rotate after precooling bearings is completed. The ball bearing used for the turbopump have worse lubricating performance because the lubricant is not liquid material such as oil which coagulate at cryogenic temperature but solid material. Polytetrafluoroethylene (PTFE) is used for the lubricant in Japanese rocket engines [1]. One of the glass transition temperature of PTFE is around 100 K, and it is considered that the strength of PTFE become weak discontinuously above 100 K and poor lubrication problem would occur in bearings. Ball bearings for turbopump which need to perform high stiffness at high rotation speed don’t meet the performance suddenly when they have any slight damages. Therefore, bearings should be precooled before the engine start so that poor lubrication problems would not occur. However, there is little knowledge of the ball bearing performance in the environment of gas coolant. If ball bearings for turbopumps have good performance in the environment, turbopump can be started to rotate under the insufficient precool environment. It contributes reduction of time and propellant for the precool, and improvement of rocket performance and diversification of space transportation mission are expected. In this study, bearing rotational tests in gas coolant are conducted in the several conditions of the rotational speed and the thrust load in order to clarify the bearing performance for rocket engines in gas coolant.