Role of AP–HMX mass ratio in combustion and agglomeration of NEPE propellant

AP (Ammonium Perchlorate) and HMX (Octogen) are the two oxidizers most often used in Nitrate-Ester-Plasticized Polyether (NEPE) rocket propellants. How the AP–HMX ratio influences the agglomeration of NEPE propellants remains unclear. We experimentally investigated the effect of the AP–HMX ratio on the combustion and agglomeration of NEPE propellants using burning rate test, quenched surface analysis, microscopic observations, and the collection of condensed combustion products. It was found that with the decrease in AP content from 40wt% to 10wt%, the burning rate decreased from 14.2 mm/s to 9.2 mm/s because the adiabatic flame temperature of NEPE propellants decreased from 3 828 K to 3 736 K. Pockets bounded by AP particles appeared on the surface when AP content was 40wt%; however, the accumulations grew and covered the burning surface eventually as the AP–HMX ratio decreased. The time required for the accumulation to coalesce into agglomerates increased with decreasing AP content. Even with similar agglomerate sizes, the coalescence time increased by 83% when the AP content decreased from 40wt% to 30wt%. The agglomerate size in the Condensed Combustion Products (CCPs) increased from 100 μm to 200 μm, and the fraction of large agglomerations increased from 6.4% to 24.7% when the AP content decreased from 40wt% to 10wt%. Overall, the high flame temperature of the AP particles enhanced the decomposition of the surrounding binder, resulting in the rapid ejection of the aluminum particles into the gas, which had a separating effect on the accumulation, thus weakening the agglomeration.