HTPB Heat of Formation: Literature Survey, Group Additive Estimations, and Theoretical Performance Effects

View Video Presentation: https://doi.org/10.2514/6.2021-1971.vid Hydroxyl-terminated polybutadiene (HTPB) is a common ingredient in rocket propellants, but its thermochemical properties (chemical composition, density, and heat of formation) are not well defined. A survey of the open literature and commonly utilized thermochemical databases indicated wide ranges for HTPB's thermochemical properties, especially its heat of formation (-593 to 821 kJ/kg). Six separate group additive schemes are utilized to estimate the heat of formation of HTPB and analyze the effects of hydroxyl functionalization and curing reactions. Good agreement is observed between the available group additive methods for HTPB isomer units, but cumulative differences result in significant disparities for larger, practical polymers. Increased hydroxyl functionality and the curing reaction are predicted to yield non-negligible decreases in the heat of formation of HTPB. The heat of formation of propellant-grade, IPDI-cured HTPB R-45M (C213.8H323.0O4.6N2.3) was computed by the only method available that can account for both hydroxyl functionality and polymer curing (Van Krevelen and Nijenhuis, 2008) as 342 kJ/mol or 114 kJ/kg. Chemical equilibrium analyses (CEA) were completed for solid propellants composed of ammonium perchlorate (AP) and HTPB, and for hybrid rocket engines based on HTPB burning with liquid oxygen (LOX) or nitrous oxide (N2O), where the heat of formation of HTPB was varied within a reasonable range (-500 to 500 kJ/kg). The CEA computations indicated that combustion gas properties (adiabatic flame temperature, specific heat ratio, and molecular weight) and theoretical propellant performance (characteristic velocity, specific impulse, and density specific impulse) can vary up to 5% within practical operating conditions for the range of HTPB heats of formation implemented. These results ultimately indicate the necessity for a modern experimental characterization of the thermochemical properties of HTPB polymers to improve the accuracy of theoretical propellant performance predictions and aid in future propulsion system design.