Cryogenic Tribology of Turbopumps for Rockets

The high-speed bearings and rotating-shaft seals of turbopumps which supply extremely low temperature propellants such as liquid oxygen (LO2) and liquid hydrogen (LH2) to liquid rocket engines are very important parts. However, the operating conditions of turbopump bearings and shaft seals are characterized by poor lubrication because of the low viscosity due to oxidation (LO2) or reduction (LH2). Therefore, turbopump bearings and shaft seals must be highly durable and reliable under extreme operating conditions in LO2 and LH2 environment. As a means of reducing launch costs, reusable rocket engines are required for future space transportation systems, and the durability of bearings and shaft seals in reusable turbopumps must be much greater than that of currently expendable turbopump bearings and shaft seals. For example, the required life of the reusable turbopump bearings and shaft seals of the space shuttle main engine (SSME) is 7.5 hours, which is 15 times longer than that required for the expendable LE-7 engine of the Japanese H-II rocket. However, a serious wear problem has been reported for the turbopump bearings of the SSME and hybrid ceramic bearings are now used in an attempt to reduce serious bearing wear. In order to develop turbopump bearings and shaft seals, many tribological problems for extremely low tomperatures and high rotating speeds must be solved. The cryogenic tribological problems experienced in the development of the turbopump bearings and shaft seals of the LE-5 engine for the H-I rocket and the LE-7 engine for the H-II rocket are introduced in this report. Additionally, the performance of a self-lubricating hybrid ceramic bearing developed to increase bearing life is described.