HTPB-based Binder Surrogates and their Effects on Preliminary Rocket Motor Analyses

This paper presents a comparison of the hydroxyl-terminated polybutadiene (HTPB) models used during conceptual and preliminary rocket propulsion analyses. The HTPB molecular formulas and enthalpies of formation from various literature sources, obtained by different methods, are discussed. It is shown that the range of heats of formation and properties of HTPB-based binders presented in these references are very wide. The Benson additivity rule was used to estimate the HTPB enthalpy of formation and to compare it with values found in the literature. The HTPB binder models are set side by side in terms of solid rocket motor performance and observable combustion products, using chemical equilibrium software. Moreover, simple heat transfer and aluminum combustion models are used to present the impact of using different models of binders on design calculations. It isshown that ammonium perchlorate/ HTPB propellant thermochemical output data may not be valuable, if used without caution. Taking appropriate contingencies into account and understanding what type of model is being used is necessary. The objective of this paper is to turn the attention of the propellant and explosives community to a class of problems that are often overlooked during initial design phases due to propellant composition simplification.