Mechanical and chemical contributions to the erosion rates of graphite throats in rocket motor nozzles

Abstract

The paper presents an analytical approach, substantiated by experimental results, which predicts throat erosion in a rocket motor nozzle with fair accuracy over a wide range of graphite grades, pressures, and propellants. Both surface chemical reactions and mechanical removal are assumed to occur; the effectiveness of each depends on the composition of the reacting products in the combustion gases, the temperature, the pressure, the quality of the graphite, and the geometry of the nozzle. A simplified diffusion equation has been solved for the turbulent boundary layer close to the throat surface, and a suitable expression for the mass transfer coefficient that considers the geometry of the convergent portion of the nozzle has been employed to evaluate the chemical contribution. For the mechanical effect, a simple logarithmic function depending on porosity of the carbon and characteristic velocity of the propellant gases has been determined using dimensional analysis and experimental data. The results obtained under a nozzle material evaluation program, from both full-size and small-scale motors operating under different pressures and using various graphite grades and propellants, show a fairly close agreement with the theory. A new dimensionless number based on physical constants of the graphite throat has been defined for the graphical representation of results.