Chemical erosion of carbon-carbon/graphite nozzles in solid-propellant rocket motors

A comprehensive theoretical/numerical framework is established and validated to study the chemical erosion of carbon-carbon/graphite nozzle materials in solid-rocket motors at practical operating conditions. The formulation takes into account detailed thermofluid dynamics for a multicomponent reacting flow, heterogeneous reactions at the nozzle surface, condensed-phase energy transport, and nozzle material properties. Many restrictive assumptions and approximations made in the previous models have been relaxed. Both metallized and nonmetallized AP/HTPB composite propellants are treated. The predicted nozzle surface recession rates compare well with three different sets of experimental data. The erosion rate follows the trend exhibited by the heat-flux distribution and is most severe in the throat region. H₂O proved to be the most detrimental oxidizing species in dictating nozzle erosion, followed by much lesser contributions from OH and CO ₂, in that order. The erosion rate increases with increasing chamber pressure, mainly due to higher convective heat transfer and enhanced heterogeneous surface reactions. For nonmetallized propellants, the recession rate is dictated by heterogeneous chemical kinetics because the nozzle surface temperature is relatively low. For metallized propellants, the process is diffusion-controlled due to the high surface temperature. The erosion rate decreases with increasing aluminum content, a phenomenon resulting from reduced concentrations of oxidizing species H₂O, OH, and CO₂. The transition from the kinetics-controlled to diffusion-controlled mechanism occurs at a surface temperature of around 2800 K.