Abstract: A method combining the advantages of chimera method and structured grid into a unified algorithm is developed to predict unsteady fluid flow involving moving geometries. It provides an efficient and time accurate calculation. The computational trajectory results of test case are validated against the available experimental data of a generic wing-pylon-store configuration at Mach 0.95. The results demonstrate that the method can provide an accurate and viable approach for this problem. After testing, a light munition separation performance is analysed at Mach 0.6 under two different ejector forces. The results showed that accurate store properties and ejector forces are critical for an accurate numerical simulation. The forward ejector force has a significant effect on the computational results, most notably for the predicted pitch and yaw angles, indicating the dependence of accurate trajectory simulations on accurate ejector force properties and accurate store properties.