Pyrolysis Behavior of Hybrid-Rocket Solid Fuels Under Rapid Heating Conditions

An experimental investigation of the thermal pyrolysis behavior of several hybrid-rocket solid fuels under rapid heating conditions was conducted to determine pyrolysis laws and to identify and quantify the products of fuel pyrolysis. The study focused on four fuel formulations: pure hydroxyl-terminated polybutadienes (HTPB), 80 percent HTPB/20 percent Alex, 80 percent HTPB/20 percent Al, and the Joint Industrial Research and Development fuel formulation. A rapid conductive-heating technique was developed and employed to determine Arrhenius-type pyrolysis laws. All four fuels displayed two sets of Arrhenius parameters, depending on the range of surface temperature. For pure HTPB, E(a) = 4.91 kcal/mol and A = 11.04 mm/s above 722 K, while E(a) = 13.35 kcal/mol and A = 3965 mm/s below 722 K. These results agree well with those obtained previously using a lab-scale hybrid motor operating under realistic conditions. The gas chromatograph/mass spectrometer tests of the nonmetallized fuels, using a flash-heating oven, indicated that the relative concentrations of the pyrolyzed species depended strongly on temperature. For pure HTPB seven major products were identified, with 1,3-butadiene representing the dominant product at all temperatures tested, up to 1073 K. The measured mole fractions of the pyrolysis products and deduced pyrolysis laws of the fuels studied can be utilized in a comprehensive model simulation for combustion performance predictions. (Author)