Combined effects of zirconium and RDX on combustion performance and combustion product formation in reduced-toxicity HTPB-based solid propellant formulations

This study investigates the influence of zirconium (Zr) and cyclotrimethylene-trinitramine (RDX) on the performance, combustion characteristics, and toxic gas emissions of unique tri modal AP to produce reduced-toxicity HTPB-based solid propellants. Three formulations were examined: a baseline (FP-10), FP-11 containing 5% Zr, and FP-12 containing both 5% Zr and 5% RDX. Mechanical testing revealed that the addition of Zr and RDX improved tensile strength by 4%, modulus by 8%, hardness by 1.5%, and viscosity by 1.3%, while slightly reducing elongation (–9.3%) and flow rate (–3%). Strand burner tests showed FP-11 achieved a 3.04% higher burn rate, 10% greater thrust, and notable increases in operating (+3.5%) and maximum (+5.64%) pressures relative to FP-10, attributed to the high exothermicity of Zr. FP-12 delivered only marginal performance gains but reduced toxic gas emissions (Al₂O₃, HCl, HNO) by 5%, offering a greener combustion profile. Numerical simulations in ANSYS FLUENT demonstrated excellent correlation with experiments confirming model robustness. Overall, Zr-enhanced propellants offer substantial gains in burn rate and peak pressure for thrust-intensive applications, while Zr–RDX blends provide a balanced performance with environmental benefits. These findings highlight the potential of tailored HTPB-based propellants for advanced propulsion in emerging space technologies.