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The grain design of hybrid rocket motor is a design of proper fuel grain geometry and flow adjustment scheme based on the total impulse, thrust, chamber pressure, length and outer diameter of combustion chamber. And the laws of various parameters in the working process of motor are obtained through the internal ballistic calculation. Based on the overall indicators of the upper stage motor of launch vehicles, the grain design process of single wagon-wheel and the internal ballistic calculation of adjusting the oxidizer flow according to the thrust feedback under the preset thrust condition are established. According to the indicators of power system, the constant thrust is F=11kN, the total impulse is I≥1100kN·s, the outer diameter is Dt≤400mm and the chamber length is L≤1000mm. Furthermore, the comparison and optimization design on star-shaped and single wagon-wheel grain are carried out. The oxidizer is 98% hydrogen peroxide (HP), the fuel is hydroxyl-terminated polybutadiene (HTPB) based fuel (including 58% Al), and the optimal oxygen-fuel ratio (O/Fopt) is 1.3. According to the theory of constant regression rate of the burning surface, the changes of the burning line length and channel area of the single wagon-wheel grain with the regression of the combustion surface are calculated. The results show that when the expansion ratio is 100 and average combustion chamber pressure is 5 MPa, the burning area of single wagon-wheel grain increases by 16.9% compared with the star-shaped grain, the O/F is reduced during motor operation, and the average specific impulse of the engine is increased by nearly 2%. In this paper, a typical uniform-thickness single wagon-wheel fuel grain is adopted. According to the optimization design, when the spokes number is 6, the average specific impulse of the motor is the largest. With increasing the spokes angle coefficient, increasing the height of the spokes, and reducing the angle of the spokes, the burning area is increased and the channel area is reduced, thereby the regression rate and fuel flow are increased and the O/F is reduced during the working process. Then the average specific impulse and overall performance are improved. This research provides a reference for single wagon-wheel fuel grain of hybrid rocket motor, calculation of interior ballistic performance under preset thrust, and flow adjustment by thrust feedback.