Characterization of nozzle erosion behavior under rocket motor operating conditions

The performance deterioration of solid-rocket motors caused by nozzle throat erosion becomes more severe with increased operating pressure from higher rates of heat and mass transfer from the core flow to the nozzle surface. Understanding of the rocket nozzle throat erosion processes and developing methods for mitigation of erosion rate can allow motor operation pressures to be substantially higher than those of the existing propulsion systems. Two test rigs have been utilized in the study of nozzle throat erosion phenomena for G-90 grade graphite: an instrumented solid propellant motor (ISPM) and a solid-propellant rocket motor simulator (RMS). The X-ray translucent nozzle assembly used for the RMS and ISPM allows the real-time imaging of the nozzle throat station. It also has a feature for incorporating a nozzle boundary-layer control system to mitigate nozzle throat erosion rates. The RMS is a gaseous reactant combustor, which allows for control of product species compositions, their flow rates, and combustor operating pressure. The erosion process of G-90 graphite was also evaluated in the ISPM using both non-metallized and metallized composite solid propellants at nominal chamber pressures up to 26 MPa. A dimensionless nozzle throat erosion rate correlation in terms of the effective oxidizer mass fraction, chamber pressure, Reynolds number, and nozzle radius of curvature was developed for non-metallized propellant and RMS motor tests. The calculated erosion rates from the correlation showed agreement within ± 0.05 mm/s of the experimentally determined values. A separate correlation for nozzle erosion rates under metallized propellant heating was also developed. This correlation showed diffusion-limited behavior, since the oxidizer concentration effect is not important.