The tribology of valve seals in cryogenic rocket engines

Highly efficient rocket engines use powerful ergols like liquid hydrogen (LH2 at 20 K) and liquid oxygen (LOX at 90 K). The now of such fluids is controlled by seals operating under severe conditions of temperature and pressure. The seals must operate reliably during a large number of actuations, with contact pressure up to 35 MPa and reciprocating sliding speed up to 0.3 m/sec. Under these conditions, the lubrification of the seals becomes a critical issue for the design of the valves. A tribological model of friction and wear in cryogenic valve seals is presented. The model provides guidelines for the selection of materials and for the development of the mechanical design. A limited, however sufficient, number of individual experiments has been merged and analysed considering actual operative design parameters : loading, type of motion, geometry and environment. Regression analysis has been used to approximate the friction coefficient and the wear rate to a polynominal of the second degree in two variables, sliding speed and contact pressure. The model has been derived from pin-on-disk tests on two pairs of materials (PCTFE and a 15 % graphite-filled polyimide sliding against A286 stainless steel) which were carried out at room and liquid nitrogen temperature, and both in continuous and reciprocating motion. The good agreement between experimental data and mathematical approximation by means of second-degree polynomials is presented graphically. In conclusion, a model is described that can be used to calculate the friction coefficient and the wear rate under the exact conditions of any particular application.