Active throttling and dual-thrust capability of 3D-printed swirl-star fuel grains for hybrid rocket engines

Abstract The ability to rapidly and repeatably transition between boost and sustain thrust levels is fundamental for planetary landers and orbital manoeuvring vehicles, yet it remains challenging for hybrid rocket motors because of their inherently non-linear, diffusion-limited combustion. This study evaluates additively manufactured swirl-star and swirl-star-reverse (SSR) ABS fuel grains, with $$N = 5,7,9$$ star points, combined with a tangential swirl injector, in a 1-inch GOX/ABS hybrid motor. Sixty static-fire tests (three burn durations, seven grain configurations) and six dedicated boost-sustain throttling tests were performed. Empirical power-law regression-rate correlations were obtained for each configuration, with the regression exponent $$n$$ ranging from 0.567 (9SSR) to 1.080 (5SSR). Measurement uncertainty, propagated using the Kline-McClintock method, was estimated at $$\pm 5.3\%$$ for regression rate and $$\pm 5.1\%$$ for oxidizer mass flux, consistent with the observed repeatability (mean coefficient of variation $$2.8\%$$ ). Among the tested geometries, the 7SSR grain gave the most balanced combination of a high regression exponent $$(n = 0.911)$$ and a smooth, low-oscillation chamber-pressure transition between boost and sustain phases (pressure undershoot below $$5\%$$ ), while the 5SSR grain showed the highest exponent but an oscillatory pressure response, and the 9SSR grain showed a sluggish, low-exponent response. It should be noted that thrust was not measured directly; all throttling conclusions are based on chamber-pressure response, which serves as a proxy for thrust in a choked-nozzle configuration. These results are limited to a single 1-inch motor scale and a 2:1 throttling ratio (5 to $$2.5$$ g/s); they indicate that swirl-augmented port geometry is a promising, but not yet fully validated, route toward more linear, throttleable hybrid rocket combustion.