The recent success of several commercial launchers in landing, recovering and relaunching complete main stages has renewed the interest in the study of re-usable space transportation concepts. As shown by several systems, a restriction to the recovery of the first stage and the application of retro-propulsion for landing appears to be a promising concept for low-cost, robust and flexible vehicles. In this context, the RETPRO project aims at the further development and validation of experimental

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... and numerical simulation techniques which are needed for the aerodynamic and aerothermal design of re-usable launch systems with retro-propulsion. RETPRO (Validation of Wind Tunnel Test and CFD Techniques for Retro-Propulsion) is carried out under a programme of and funded by the European Space Agency through the Future Launchers Preparatory Programme (FLPP). The recovery of the first stage booster is a complex task which relies heavily on the successful implementation of a control system. This requires engineers to have intimate knowledge of vehicle aerodynamics, the effect of control surface deflections and the use of propulsion systems for vehicle deceleration and attitude control. This data is typically consolidated in an aerodynamic database, where derivatives are compiled according to Mach number, Angle of Attack, thrust setting and control surface inclination angle. Due to the complexity and cost of gathering this data from full scale flight tests, results from high-fidelity CFD predictions play a large role in the vehicle design. These datasets are complemented by ground test results which further serve as a basis for CFD accuracy assessment and validation. A close interaction of wind tunnel based and CFD investigations enables the reduction of prediction uncertainties and extrapolation of ground test data to flight scales. While there is substantial experience in load predictions for hypersonic capsule or winged type re-entry concepts, retro-propulsion maneuvers are a particularly challenging problem for both CFD [...]