The regulation of ferrocene-based catalysts on heat transfer in high-pressure combustion of ammonium perchlorate/hydroxyl-terminated polybutadiene/aluminum composite propellants

The regulation of the burning rate pressure exponent for the ammonium perchlorate/hydroxyl-terminated polybutadiene/aluminum (AP/HTPB/Al) composite propellants under high pressures is a crucial step for its application in high-pressure solid rocket motors. In this work, the combustion characteristics of AP/HTPB/Al composite propellants containing ferrocene-based catalysts were investigated, including the burning rate, thermal behavior, the local heat transfer, and temperature profile in the range of 7–28 MPa. The results showed that the exponent breaks were still observed in the propellants after the addition of positive catalysts (Ce-Fc-MOF), the burning rate inhibitor ((Ferrocenylmethyl)trimethylammonium bromide, FcBr) and the mixture of FcBr/catocene (GFP). However, the characteristic pressure has increased, and the exponent decreased from 1.14 to 0.66, 0.55, and 0.48 when the addition of Ce-Fc-MOF, FcBr and FcBr/GFP in the propellants. In addition, the temperature in the first decomposition stage was increased by 7.50 °C and 11.40 °C for the AP/FcBr mixture and the AP/FcBr/GFP mixture, respectively, compared to the pure AP. On the other hand, the temperature in the second decomposition stage decreased by 48.30 °C and 81.70 °C for AP/FcBr and AP/FcBr/GFP mixtures, respectively. It was also found that FcBr might generate ammonia to cover the AP surface. In this case, a reaction between the methyl in FcBr and perchloric acid caused more ammonia to appear at the AP surface, resulting in the suppression of ammonia desorption. In addition, the coarse AP particles on the quenched surface were of a concave shape relative to the binder matrix under low and high pressures when the catalysts were added. In the process, the decline at the AP/HTPB interface was only exhibited in the propellant with the addition of Ce-Fc-MOF. The ratio of the gas-phase temperature gradient of the propellants containing catalysts was reduced significantly below and above the characteristic pressure, rather than 3.6 times of the difference in the blank propellant. Overall, the obtained results demonstrated that the pressure exponent could be effectively regulated and controlled by adjusting the propellant local heat and mass transfer under high and low pressures.