The optimal design of launch vehicles based on liquid rocket engines is critically dependent on ascent trajectories and thrust throttling. Recently many authors have incorporated trajectory optimization at conceptual design level but still have not gauged the potential of using thrust throttling at conceptual design phase. In this study we propose not only the trajectory optimization but also the thrust throttling at the conceptual design phase. This newly formulation problem is solved through hybrid optimization algorithm using Genetic Algorithm as global optimizer and Sequential Quadratic Programming as local optimizer starting from the solution given by Genetic Algorithm. The objective is to find minimum gross launch weight (GLW), optimal trajectory and thrust throttling profile for liquid fueled space launch vehicle (SLV). The improvement in system design using thrust throttling is studied in detail.