Regression rate behavior of hybrid rocket solid fuels

An experimental investigation of the regression-rate characteristics of hydroxyl-terminated polybutadiene (HTPB) solid fuel burning with oxygen was conducted using a windowed, slab-geometry hybrid rocket motor. A real-time, x-ray radiography system was used to obtain instantaneous solid-fuel regression rate data at many axial locations. Fuel temperature measurements were made using an array of 25-μm fine-wire embedded thermo-couples. The regression rates displayed a strong dependence on axial location near the motor head-end. At lower mass flux levels, thermal radiation was found to significantly influence the regression rates. The regression rates were also affected by the addition of activated aluminum powder. A 20% by weight addition of activated aluminum to HTPB increased the fuel mass flux by 70% over that of pure HTPB. Correlations were developed to relate the regression rate to operating conditions and port geometry for both pure HTPB and for HTPB loaded with certain fractions of activated aluminum. Thermocouple measurements indicated that the fuel surface temperatures for pure HTPB were between 930 and 1190 K. The HTPB activation energy was estimated at 11.5 kcal/mole, suggesting that the overall regression process is governed by physical desorption of high-molecular weight fragments from the fuel surface.