ADVANCING HYBRID PROPELLANT DUCTED ROCKET MOTOR WITH HTPB-BORON-SILVER FUEL AND REGENERATIVE ENGINE PRINCIPLES

Boron is actively being investigated as an energetic additive in ducted rocket propulsion systems due to its exceptionally high energy density. A typical ducted rocket comprises two combustion chambers, one primary and one secondary, connected by a mixing chamber and operates on the regenerative combustion principle. However, the effective utilization of boron's energy content is significantly constrained by the formation of a boron oxide (B<sub>2</sub>O<sub>3</sub>) shell on the particle surface, which acts as a barrier and inhibits efficient combustion. Several studies in the existing literature suggest that ducted rocket systems are particularly well-suited for boron-based fuels, offering favorable conditions to harness their full energy potential. A laboratory-scale ducted rocket motor was utilized to evaluate the potential benefits of incorporating silver nanoparticles as an additive to enhance the combustion efficiency of boron-hydroxyl-terminated polybutadiene (B-HTPB)-based solid fuels. Three solid fuel grain formulations were tested: a pure HTPB sample, HTPB containing 5% boron particles, and HTPB with 5% boron and 1% silver nanoparticles. The material characterization of the feed particles and residual combustion product (RCP) collected from the secondary combustor of the ducted rocket motor were assessed through various analytical techniques, including scanning electron microscopy (SEM) and X-ray diffraction (XRD), to examine morphological and chemical changes. Heating values of the three fuel samples were determined using a bomb calorimeter, while thermogravimetric analysis (TGA) was conducted on the boron feed particles and RCP to determine the active boron content and thus evaluate the efficiency of boron combustion. XRD analysis confirmed significant boron combustion by indicating the formation of B<sub>2</sub>O<sub>3</sub> in boron-containing RCP samples. Based on thermogravimetric analysis, the residual active boron content in the boron-HTPB formulation was found to be lower than that in the boron-HTPB sample doped with silver nanoparticles, indicating reduced combustion efficiency in the latter. These results reflect the influence of regenerative effects occurring within the secondary combustor.