The implementation of partially reusable Hypersonic Airbreathing Vehicles (HAVs) into orbital launch systems has the potential to improve both the mass efficiency and cost of launches. A recent study by Smart & Tetlow investigated a combined 3-stage Rocket-Scramjet-Rocket launch vehicle. This previous study showed that the system was a viable concept through analysis of the second stage HAV flight. The purpose of the current project is to investigate possible designs for the 1000kg 3rd stage rocket vehicle. This analysis utilised several different methodsto arrive at a solution. Trajectory simulations were carried out through the use of CADAC, a Fortran-based industry tool designed for accurate simulation of aerospace vehicle flight trajectories. Numerical analysis of orbital mechanics and rocket equations allowed the determination of certain design parameters. Also, research into current and historical spacecraft systems is combined with analysis and design to create viable designs for the upper stage module and the vehicle staging system. A final design was compiled through the combination of these methods. There are major advantages to generating multiple solutions; the ability to verify results whilst conducting the analysis, and the constant refinement of models that occurs throughout the process. It is found that an optimum angle of attack of 13o allows the HAV to reach an altitude of 88.5km, considered for these purposes to be a zeroatmospheric region. With a 100kg LOX/LH2 Rocket engine, scaled down from the Pratt & Whitney RL-10A-3A, the vehicle develops total thrust of 63 kN, with a specific impulse of 437 s. The total impulse required for the Stage III ascent to orbit is 4234ms-1, generated over a total engine burn time of 42.79s. This flight requires a total propellant mass of 630kg which is the combined masses of LOX and LH2. The vehicle is designed with a total maximum length of 5m, and a nominal diameter of 1m. This vehicle will fit comfortably inside the Stage II HAV, from where it is deployed using a spring-powered separation system under zero-atmospheric conditions. The separation process takes less than 10s, after which point the Stage III engine ignites to begin the ascent to LEO (200km). The final mass ratios for this craft are very promising. The dry or structural mass is 178.7kg, which when fuel is taken into account, leaves a final payload mass of 191.3kg. This equates to a payload mass fraction of around 1.85%, a large improvement on current systems. The major goal of this project is to develop a viable, effective design, to verify that the proposed HAV-assisted launch vehicle is feasible.