Reconstruction of Wind Tunnel Tests using CFD for a Reusable First Stage during Rocket Retro-propulsion
Creators: name:  T. BYKERK 
name:   S. KARL 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: REUSABLE SYSTEMS
FLIGHT PHYSICS
DYNAMICS

description: 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 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 and ground based testing. This is primarily due to the complex fluid mechanical and thermo-chemical interaction between the exhaust plumes and the free stream. During retro-propulsion phases, large parts of the vehicle are immersed in its exhaust gases. Large recirculation regions and zones of flow reversal significantly affect the heat load distribution and aerodynamic characteristics. Furthermore, the divergence of the exhaust jets at low ambient pressures gives rise to strong plume-plume interactions in the vicinity of the launcher base. This paper focuses on the presentation and discussion of steady-state numerical simulation results which reconstruct selected wind tunnel tests during both powered and unpowered descent phases. The CFD simulations cover a Mach number range from 0.3 up to 7, with dry air used to model the exhaust plume. Typical flow topologies and features are discussed, with quantitative results coming from a comparison of force balance and surface pressure measurements between experimental results and CFD simulations. Schlieren images from the experiments are used to evaluate the plume structure and shock stand-off distances. For the unpowered descent cases the agreement between schlieren images, force and pressure measurements is extremely strong. During powered descent the basic plume structure is captured well despite the highly unsteady and fluctuating flow field. Pressure and force measurements gave mixed results, but overall the studies show that RANS simulations perform well compared to experimental data.