In order to increase the fuel regression rate of hybrid rocket engine, the concave-convex surface grain was adopted for the swirling-oxidizer-flow-type hybrid rocket engine. Burning experiments were carried out by changing the convex width from 9 to 18 mm, the concave width from 4.5 to 18 mm and the concave depth from 3 to 9 mm. The fuel regression rate of concave-convex surface grains with and without swirl increased up to about 1.7 and 2.0 times of the rates of cylindrical grains with and without swirl at the same oxygen mass flux conditions, respectively. The upstream side of the convex part increased the fuel regression rate. The local regression rates at the concave part were considerably lower than those at the convex part. Combustion oscillations caused by the Helmholtz resonance occurred.
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