Regression rate behavior of hybrid rocket solid fuels

An experimental investigation of the regression-rate characteristics of hyd roxyl-terminated polybutadiene (HTPB) solid fuel burning with oxygen was co nducted using a windowed, slab-geometry hybrid rocket motor. A real-time, x -ray radiography system was used to obtain instantaneous solid-fuel regress ion rate data at many axial locations. Fuel temperature measurements were m ade using an array of 25-mu m fine-wire embedded thermocouples. The regress ion rates displayed a strong dependence on axial location near the motor he ad-end. At lower mass Bur levels, thermal radiation was found to significan tly influence the regression rates. The regression rates were also affected by the addition of activated aluminum ponder. 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 op erating conditions and port geometry for both pure HTPB and for HTPB loaded with certain fractions of activated aluminum. Thermocouple measurements in dicated that the fuel surface temperatures for pure HTPB were between 930 a nd 1190 Ii. The HTPB activation energy was estimated at 11.5 kcal/mole, sug gesting that the overall regression process is governed by physical desorpt ion of high-molecular weight fragments from the fuel surface.