It has been a challenge to investigate how to trace particles in a solid rocket motor (SRM) using aluminized composite solid propellant and submerged nozzle. In using CFD simulations, the boundary conditions for the ejecting particles constrain their trajectories, hence these affect the two-phase flow calculations, and thus significantly affect the evaluation of the slag accumulation. The RTR (X-ray Real-time Radiography) technique is a new method to detect the particles in a firing SRM. A method was developed to simulate the particle ejection from the propellant surface. The moving trajectories of metal particles in a firing combustion chamber were measured by using the RTR high-speed motion analyzer. Numerical simulations with different propellant-surface boundary conditions were performed to calculate particle trajectories. Through this study an appropriate surface velocity condition on the propellant surface was discovered. The method developed here can be used for the future CRM research.