Aerothermal Analysis of Re-usable First Stage during Rocket Retro-propulsion

Kurzfassung

Re-usable launch vehicles have a potential to significantly change the launch service market once low refurbishment costs and high reliability are guaranteed. Therefore, DLR conducted a system analysis testing a re-usable launch system capable of bringing 7 t to GTO. One concept is a retro-propulsion decelerated vertical landing vehicle. The idea is to decelerate the returning first stage by providing thrust in the opposite direction of motion. The retro-propulsion maneuver is carried out between 70 and 36 km and covers a Mach number range from 9.5 to 5.1. The thrust is provided by re-igniting three of the nine main engines. In this study, steady RANS simulations with DLR TAU are conducted at specific points along the retro-propulsion trajectory and for a point during landing. The influence of different flow and engine configurations on the thermal loads are examined. The wall temperature evolution and integral heating over the retro-propulsion maneuver are investigated by coupling a heat flux database with a simple structural model. The sidewall heat loads are mainly affected by the hot exhaust plume impinging on the surface. The maximum heat loads occur at the impingement area and increase as the first stage descends reaching up to 20 kW/m2. Over the retro-propulsion maneuver the sidewall temperature rises by about 100 K to a maximum temperature of 400 K depending on the sidewall thickness distribution. This temperature increase is also supported by NASA infrared images of SpaceX Falcon 9 that showed a maximum sidewall temperature of 450 K after the retro-propulsion maneuver. The baseplate heat loads are caused by a "base-impinging plume jet" which results from the interaction shocks of the plumes exhausting from the three active nozzles. The maximum heat load of 70 kW/m2 is reached at the beginning of the maneuver and decreases with deceasing altitude as the interaction shocks become weaker. The temperature evolution on the baseplate along the SRP trajectory revealed only a minor increase by approximately 20 K depending on the baseplate thickness. From the results it can be concluded, that during the supersonic retro-propulsion maneuver the heat loads are redistributed compared to the non-propulsive phase before and after. With deactivated engines the heat loads are concentrated around the aft of the first stage. During the retro-propulsion maneuver the baseplate and aft are protected by the plume, which in turn causes higher thermal loads on the upper part. This means that the amount of TPS needed is significantly reduced for the retro-propulsion re-entry.