A fully-reusable two-stage-to-orbit (TSTO) launch vehicle with ethanol-fueled rocket-based combined cycle (RBCC) engines is a promising option for future space transportation that is currently studied in Japan. In this paper, a conceptual design study is conducted for such a vehicle using a multi-objective, multidisciplinary design optimization (MDO) technique. An MDO framework considering the coupling between vehicle geometry, propulsion system, aerodynamics, and flight trajectory is constructed, and they are optimized simultaneously with the aim of maximizing the payload mass, minimizing the gross mass of the combined vehicle (booster and orbiter), and minimizing the horizontal takeoff velocity. This multi-objective optimization enables a more comprehensive and exploratory design study compared with the gross-mass minimization problem subject to a predetermined mission plan and a fixed takeoff-velocity constraint. The optimization is executed via a novel algorithm, and a set of Pareto optimal solutions with a good spread is obtained. In addition to discussing some representative solutions, knowledge is extracted by applying sensitivity analysis to the solutions. The results provide some insight into the underlying features and trends of the design problem of TSTO space planes with RBCC engines.
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