Description: The use of multidisciplinary optimization in the case of hypersonic vehicles is very important for proper sizing of the vehicle. The key disciplines are aerodynamics, inlet performance, propulsion and structures. One of the serious concerns in the preliminary design stage is the vehicle stage performance based on the assumed takeoff gross weight. Particularly, effective specific impulse is an important parameter in the attainment of a desired vehicle trajectory. Since the specific impulse is a direct function of the vehicle thrust and drags, minimizing the drag force is essential. On the other hand, the same thing can be achieved by maximizing thrust through the improvement of the inlet performance and the fuel volumetric efficiency. The traditional sizing method attains closure on a vehicle design by photographically scaling the baseline to achieve the required fuel fraction. This occurs at the interval of the fuel required and the fuel available curves. It is possible to attain closure by bringing curve up. As a first step, the fuel available will be held constant in the approach. Hence bringing the fuel curve down is the primary requirement. This is accomplished by maximizing the specific impulse (Isp) at a critical point in the trajectory by modifying the baseline configuration geometry. The forebody of the vehicle is only considered for simplicity. The effect of forebody on the aftbody aerodynamics is neglected. The aftbody effects are included in the propulsion contributing analysis (CA) and the trim content.