Synergistic coupling of micro-explosive and catalysis combustion enabled by high‑nitrogen energetic compounds: An innovation in HTPB regression rate modulation for hybrid propulsion

Hybrid rockets present an attractive option for small, low-cost launch vehicles, space tourism and exploration, owing to their features of intelligent remote control of thrust and switch, rapid response capabilities, cost-effectiveness, environmental friendliness. However, it suffers from low regression rate ( r f ) of solid fuels relating to difficult vaporization of fuel and low heat feedback from diffusion-dominated combustion. Herein, high‑nitrogen energetic compounds (HNEC) sodium azotetrazole (STZ) and 4,4′,6,6′-tetraazido-1,3,5-triazine (TAHT) were employed into hydroxyl-terminated polybutadiene (HTPB) fuels to amplify its r f and thermal decomposition with minimizing formulation complexity, respectively. Here the STZ was prepared by one-step oxidation coupled with spray-drying and TAHT was prepared by two-step method of hydrazine uncoupling and diazotization. The combustion regression process of fuels at oxygen flow were measured at hybrid propellent combustion platform to determine the r f and micro-explosion combustion. The r f of the 5 wt% STZ and TAHT formulation reached 2.84 and 3.46 mm/s at an oxygen mass flux ( G ox ) of 200 kg/m 2 ·s, increasing by 300.00 % and 387.32 % compared to pure HTPB. Upon the cessation of combustion, the scanning electron microscope (SEM) image of the HNEC material showed a porous layer structure, ensuring sufficient contact between fuel and oxygen and accelerating r f . Furthermore, HNEC had a catalytic decomposition effect on HTPB via the differential scanning calorimetry (DSC) tests, where the decomposition end temperature ( T end ) of the heat classification was maximum advanced by 71.03 (5 wt% STZ) and 20.64 °C (5 wt% TAHT). Above analysis results indicate that cost-effective HNEC overcome the thermal feedback limitations of conventional fuels through a micro-explosion and catalyze synergistic mechanism, as well will offer a novel direction for combustion rate control in hybrid rocket propulsion.