Thermally Switchable Microwave Absorptivity of Composite Propellant and Engineered Additives

Currently, most solid rocket motors (SRMs) exhibit fixed burning rates and essentially predetermined thrust curves once ignited. This lack of active throttle control limits operational flexibility across a wide range of missions, including hypersonic vehicles, deep-space probes, and satellites that would benefit from on-demand thrust shaping or restart capability. The proposed concept introduces a fundamentally novel, energy efficient approach to propulsion system control. Discussed is a microwave-controllable solid propellant based on an ammonium perchlorate composite propellant (APCP), incorporating additives that switch from microwave transparent to microwave absorptive under pyrolysis at the propellant burning surface to enable microwave thermal heating at the surface. Unlike approaches based on pre-applied conductive coatings or electrode-driven energetic formulations, this system forms its own conductive carbon layer in-situ, enabling direct deposition of microwave energy into the burning surface. Investigated are APCP formulations containing the char modifier polyimide along with a burn rate inhibitor to enhance char formation. Experiments conducted at 1 atm demonstrate that this char layer enhances local thermal feedback and produces measurable, power-dependent increases in regression rate, with the burning surface temperature and burn rate scaling proportionally with applied microwave power. Thermal imaging and microscopy confirm the temperature trends of the polyimide-derived char as a microwave absorber. The results establish a new pathway for controllable solid propulsion using material systems compatible with conventional APCP manufacturing while offering dynamic burn-rate modulation.