Influence of pre-ignition technology on combustion and explosion characteristics of composite propellants

The slow cook-off test is widely used for analyzing the safety of propellants under accidental thermal stimulation. To examine the influence of the pre-ignition technology of a composite propellant on thermal safety and combustion characteristics, a quantitative evaluation experiment of slow cook-off reaction intensity of a typical composite propellant charge was designed and executed, considering hydroxyl-terminated polybutadiene (HTPB) and hydroxyl‑terminated block copolyether (HTPE) propellants containing ammonium perchlorate (AP) and aluminum (Al). The experimental method for quantitative evaluation of the slow cook-off reaction intensity is highly essential to investigate the safety of ammunition. The influence of pre-ignition on reaction intensity was analyzed at different temperatures, the combustion behavior of the heated propellant was studied using a closed bomb experiment, and the influence of pre-ignition on combustion evolution was analyzed. Finally, the process was characterized by theoretical modeling. The results show that the slider speed was 169.3 m/s when the HTPB/AP/Al propellant was ignited at 234 °C. The slider speed was reduced to 43.4 m/s and 61.8 m/s by pre-ignition at 120 °C and 214 °C, respectively. When the spontaneous ignition temperature of HTPE/AP/Al propellant was 212 °C, the slider speed was 110.5 m/s. Pre-ignition at 120 °C and 192 °C reduced the slider speed to 39.4 m/s and 43.7 m/s, respectively. Pre-ignition can effectively reduce the reaction intensity. Under the same conditions, the reaction strength of HTPE/AP/Al propellant is marginally lower than that of HTPB/AP/Al propellant. In the slow heating process before ignition, with the increase in ignition temperature, when HTPB/AP/Al propellant was pre-ignition at 120 °C and 214 °C, the pressurization time was 9.8 ms and 6.4 ms, respectively, while before ignition, the pressurization time decreased to 5.5 ms. At 120 °C and 192 °C, the pressurization time of HTPE/AP/Al propellant was 50.9 ms and 36.6 ms, respectively, while before ignition, the pressurization time decreased to 26.9 ms. The reduction in ignition temperature decreased the pressure growth rate of propellant during combustion and reduced the pressurization time. A calculation model of the propellant combustion process was established to characterize the growth process of the combustion pressure at different ignition temperatures. This model can effectively predict the combustion evolution of composite propellants after the slow cook-off ignition and aid in ensuring propellant safety.