A hybrid rocket can be used in various applications and is an attractive propulsion system.However,serious erosion of nozzles is common in motor firing operations,which could restrict the application of hybrid rocket motors.Usually,the serious erosion is attributed to the high concentration of oxidizing species in hybrid motors,while the details of flowfields in the motors are not paid special attention to.In this paper,first the thermochemical erosion of C/C nozzle is simulated coupled with the flowfields in a 98%H2O2/hydroxyl-terminated polybutadiene(HTPB)hybrid rocket motor.The simulation is made on a typical axisymmetric motor,including a pre-combustion chamber,an aft-combustion chamber and nozzle structures.Thermochemical reactions of H2O,CO2,OH,O and O2with C are taken into account.Second,the change of flowfields due to fuel regression during motor firing operations is considered.Nozzle erosion in different flowfields is evaluated.Third,the results of nozzle erosion in the coupled simulation are compared with those under uniform and chemical equilibrium flow and motor firing test results.The results of simulation and firing tests indicate that the thermochemical erosion of nozzles in hybrid motors should be calculated coupled with flowfields in the motor.In uniform and chemical equilibrium flowfields,the erosion rate is overestimated.The diffusion flame in hybrid motors protects the nozzle surface from the injected oxidizer and high temperature products in flowfields,leading to a relatively fuel-rich environment above the nozzle.The influence of OH and the geometry of motor should also be considered in the evaluation of nozzle erosion in hybrid motors. A hybrid rocket can be used in various applications and is an attractive propulsion system. Host, serious erosion of nozzles is common in motor firing operations, which could restrict the application of hybrid rocket motors. Usually, the serious erosion is attributed to the high concentration of oxidizing species in hybrid motors, while the details of flowfields in the motors are not paid special attention to.In this paper, first the thermochemical erosion of C / C nozzle is simulated coupled with the flowfields in a 98% H2O2 / hydroxyl-terminated polybutadiene (HTPB) hybrid rocket motor. The simulation is made on a typical axisymmetric motor, including a pre-combustion chamber, an aft-combustion chamber and nozzle structures. Thermochemical reactions of H2O, CO2, OH, O and O2 with C are taken into account. Second, the change of flowfields due to fuel regression during motor firing operations is considered. Nozzle erosion in different flowfields is evaluated. Third, the results of nozzle erosion in the couple d simulation are compared with those under uniform and chemical equilibrium flow and motor firing test results. The results of simulation and firing tests indicate that the thermochemical erosion of nozzles in hybrid motors should be calculated coupled with flowfields in the motor. Uniform and chemical equilibrium flowfields, the erosion rate is overestimated. The diffusion flame in hybrid motors protects the nozzle surface from the injected oxidizer and high temperature products in flowfields, leading to a relatively fuel-rich environment above the nozzle. The influence of OH and the geometry of motor should also be considered in the evaluation of nozzle erosion in hybrid motors.