Reaction evolution modeling and quantitative assessment of reaction intensity in solid propellants under slow cook-off conditions

Solid propellants are critical in aerospace systems, but their thermal safety under slow cook-off conditions remains a challenge. This study investigates the reaction evolution mechanisms of HTPB/AP/Al (hydroxyl-terminated polybutadiene/ ammonium perchlorate/ aluminium powder) composite propellants under confined heating via integrated modeling and experimental diagnostics. A reaction evolution model was developed based on energy conservation and confined combustion dynamics, validated through a custom slow cook-off test system. Experiments systematically analyzed the effects of casing constraints (0–4 mm groove depths) and ignition temperatures (spontaneous and 120 °C pre-ignition) on reaction intensity. Key findings reveal that reducing casing constraints decreases peak internal pressure by ∼60 % (from 348.31 MPa to 140.86 MPa), while pre-ignition technology lowers pressure by ∼25 % (from 123.34 MPa to 93.24 MPa). A novel quantitative framework integrating maximum pressure, casing fragment energy, and shockwave overpressure was proposed to classify reaction grades (deflagration and combustion). These results provide actionable guidelines for optimizing propellant safety design, particularly in mitigating combustion-to-detonation transitions. This work advances the understanding of slow cook-off dynamics and offers a scalable methodology for enhancing thermal safety in solid rocket motors.