Regression rate and pyrolysis behavior of HTPB-based solid fuels in a hybrid rocket motor

An experimental investigation on the regression rate and pyrolysis behavior of hydroxyl-terminated polybutadiene-based solid fuels has been conducted. The overall objective was to obtain a better understanding of the physical processes governing solid-fuel regression and pyrolysis under different operating regimes. Experiments were conducted using a windowed, slab geometry hybrid motor and a conductive-heating induced thermal pyrolysis test rig. Gaseous oxygen was employed as the oxidizer in the 1-m long, lab-scale hybrid motor, which had realistic operating conditions. A real-time X-ray radiography system and an ultrasonic pulse-echo system were both used to obtain the local, instantaneous solid fuel regression rates. A semi-empirical approach was developed to analyze the experimental results and to correlate the regression rates with physically descriptive, dimensionless parameters.; For relatively high surface temperatures above 722 K, the activation energy of pure HTPB was 4.91 kcal/mole, indicating that the pyrolysis process was governed by formation and desorption of high molecular weight fragments from the fuel surface. The conductive-heating induced pyrolysis rates of HTPB, conducted at atmospheric pressure, were very similar to those measured in the hybrid motor tests at much higher pressures. This result implies that the regression rate of HTPB was governed primarily by thermal decomposition processes and not influenced by heterogeneous surface reactions.; Radiant heat transfer had a significant effect on the overall regression rate behavior of HTPB. Radiation from soot generally accounted for about 80 to 90% of the total radiant heat flux. Two separate expressions, one for the developing flow regime and one for fully-developed flow, were used to correlate the regression rate data. Both correlations show that standard hybrid boundary layer correlations must be modified to account for the effects of variable fluid properties across the boundary layer and radiation. Because of the flame distortion effect, the regression tended to vary with Reynolds number to the {dollar}-{dollar}0.125 power, rather than the standard {dollar}-{dollar}0.2 power.; Ultra-fine 'activated' aluminum powder (Alex) increased the fuel mass burning rate by up to 70% over that of pure HTPB. Conventional aluminum powder caused a slight regression rate increase, but was not as effective as the Alex.