This study investigated combustion characteristics of composite fuel grains designed based on a modular fuel unit strategy. The modular fuel unit comprised a periodical helical structure with nine acrylonitrile–butadiene–styrene helical blades. A paraffin-based fuel was embedded between adjacent blades. Two modifications of the helical structure framework were researched. One mirrored the helical blades, and the other periodically extended the helical blades by perforation. A laboratory-scale hybrid rocket engine was used to investigate combustion characteristics of the fuel grains at an oxygen mass flux of 2.1–6.0 g/(s·cm²). Compared with the composite fuel grain with periodically extended helical blades, the modified composite fuel grains exhibited higher regression rates and a faster rise of regression rates as the oxygen mass flux increased. At an oxygen mass flux of 6.0 g/(s·cm²), the regression rate of the composite fuel grains with perforation and mirrored helical blades increased by 8.0% and 14.1%, respectively. The oxygen-to-fuel distribution of the composite fuel grain with mirrored helical blades was more concentrated, and its combustion efficiency was stable. Flame structure characteristics in the combustion chamber were visualized using a radiation imaging technique. A rapid increase in flame thickness of the composite fuel grains based on the modular unit was observed, which was consistent with their high regression rates. A simplified numerical simulation was carried out to elucidate the mechanism of the modified modular units on performance enhancement of the composite hybrid rocket grains.

A paraffin-based fuel coupled with a nested helical matrix structure is a low-cost, high-performing alternative for hybrid rocket engine applications. The mechanical and combustion properties of the composite fuel grain can be enhanced by replacing the conventional polymer matrix with a metal skeleton. Three-dimensional printing was used to design an Al90Mg10 skeleton embedded with the paraffin-based fuel. The combustion characteristics of the composite fuel grain, including the ignition behavior, pressure oscillations, regression rate, and combustion efficiency, were comprehensively investigated. The properties of grains with and without a secondary perforated structure were compared. The flame structure and metal burning behavior of the hybrid rocket engine were monitored by endoscopic radioluminescence imaging, and the emission spectral characteristics of the plume were analyzed simultaneously. Good flammability makes the Al90Mg10 helical skeleton a promising candidate for enhancing the combustion performance of a paraffin-based fuel grain. The combustion process with rapid ignition was relatively stable, and no additional pressure oscillation frequency was observed. The metal-based composite fuel grains had a superior regression rate to that of a paraffin-based fuel grain (up to 100% higher using the perforated skeleton). Introducing a secondary structure into the fuel grain promoted the reaction and thereby enhanced the combustion performance.