Nozzle throat erosion characterization study using a solid-propellant rocket motor simulator

Understanding of the nozzle throat erosion processes and developing methods for mitigation of erosion rate can allow higher operating pressure of rocket motors. To evaluate the individual effects of oxidizing species on the chemical erosion rates of rocket nozzles (e.g., G-90 graphite nozzle) a solid-propellant rocket motor simulator (RMS) has been designed and tested. The RMS simulates the mass fractions of the oxidizing product species (like H₂O, OH, CO₂) and product composition temperature generated from a non-metallized composite propellant called Propellant S at the throat region. This instrumented RMS test setup incorporates the use of a real-time X-ray radiography system for measurement of the instantaneous contour of the rocket nozzle. The instantaneous erosion rates of the G-90 nozzle throat were deduced from the recorded X-ray images. Significant nozzle throat erosion rates were observed in the RMS operating at relatively low pressures around 500-750 psia (3.45-5.17 MPa). Comparison of average erosion rates to RMS tests conducted at similar operating conditions (Le., 500-750 psia) with a swirl reducer and improved gas supply system showed an erosion rate reduction as much as 32%. Operation of the RMS was extended to nominal pressures of 1,000 psia (6.89 MPa) and the measured erosion rates showed higher than expected values. Qualification tests of solid-propellant grains for both non-metallized Propellant S and metallized Propellant M were conducted by motor firings as a portion of this study at average chamber pressures of 1,120-1,200 psia (7.72-8.27 MPa). These tests showed normal operations of the rocket motors without any anomalies. From the recorded data of these motor firings, nozzle throat erosion rates were deduced from the ratio of the instantaneous rocket motor chamber pressure and thrust levels. Nozzle erosion phenomena of the recovered G-90 samples were studied. Surface analysis of recovered nozzles used in the firings of non-metallized Propellant S revealed an increased surface roughness in comparison to those tested with metallized Propellant M, which generated a protective aluminum oxide layer over the nozzle surface.