A semiquantitative prediction of the erosion of graphite nozzle inserts

The erosion of graphite throat inserts is shown to be explicable on the basis of a chemical mechanism if account is taken of the chemical composition of the exhaust stream, the geometrical and temperature history of the insert, the ballistic effects of erosion, the rates of diffusion of the molecular species through the boundary layer, and the chemical kinetics of the surface reactions. Semiquantitative agreement with experiment is demonstrated for a noiialuminized propellant, and the effect of changing temperature and solids loading is predicted correctly. The results show that graphite erosion is kinetically limited by the two processes of diffusion and surface chemical reactions. Pressure is probably the most important single variable involved in erosion, and the relative importance of the two mechanisms is not strongly dependent on pressure or concentration of the corroding species. In general, the amount of erosion which is acceptable seems to be poorly defined. The limiting factor in determining the permissible amount of throat erosion is unlikely to be degradation of specific impulse, chamber pressure, thrust, or mass ratio. More probably, the limit on erosion will be dictated by the practical problem of fabricating inserts which will provide acceptably reproducible erosivities.