On the combustion of heterogeneous AP/HTPB composite propellants: A review

The combustion of ammonium perchlorate (AP) in a polymer binder matrix of hydroxyl-terminated-poly-butadiene (HTPB) is reviewed, covering the experimental and modeling approaches explored for over a half-century. AP is capable of self-deflagration and is a propellant in its own right as ammonia and perchloric acid decompose at the surface to sustain a flame in the gas-phase with an adiabatic flame temperature of roughly 1400 K. AP also has unique low temperature decomposition behavior and burning characteristics as a function of pressure attributed to the importance of condensed phase reactions. A polymer, most commonly HTPB, is used to bind AP crystals within a solid matrix and impart desired mechanical properties and additional fuel for inclusion in a solid rocket motor. Aside from providing an additional fuel source for the decomposing AP, the HTPB introduces diffusion flames into the system, which have the advantage of decreasing the combustion instability of the AP monopropellant. The objective of this paper is to review the critical physical processes of AP/HTPB propellant combustion and to explore the different experimental and computational avenues used to shed light on such a complex phenomenon. Experimental diagnostics and the simulation of solid propellant combustion have progressed immensely over the years with the introduction of more advanced laser diagnostics and imaging techniques, as well as an increase in computational resources and processing power available for computations; additional physics can now be modeled that would have otherwise been impossible to include before. A recommendation on future research is given based on the current state of the art.