The quest for enhancing the performance of rocket launcher has led to the development of rocket nozzles with a high expansion ratio. Even though high area ratio rockets give better efficiency at high altitude conditions, the same nozzle when operated at the low altitude results in separation of flow well within the nozzle. This unsteady and asymmetric separation of flow from the divergent wall leads to uncontrollable side loads and high dynamic loads. If this problem were to overcome, the rocket engine performance will improve substantially. In the paper, flow separation in the main nozzle is eliminated with secondary injection at the lip of the main nozzle. This prevents the entry of atmospheric air into the separation zone and full-flowing conditions are achieved in the main nozzle at a relatively lower chamber pressure of main nozzle. The impact of inlet pressure of the main nozzle, the secondary injection, its pressure, and the angle of introduction on the main nozzle flow are studied using computational methods. It was observed that the injection of the secondary nozzle flow parallel to the main nozzle gives better results. The increase in secondary injection pressure seems to be favourable in eliminating flow separation in the main nozzle.