Aerodynamic Analyses of Retro Propulsion Assisted Descent and Landing of Launcher Configurations

Kurzfassung

Reusable launchers which are descending and landing vertically with the aid of firing the engines against the flight velocity, the so-called retro propulsion, are considered the next step in the evolution of European launchers to make them more cost efficient and competitive. In the RETALT project key technologies for these retro propulsive landing configurations have been investigated. This thesis is focused on the steady and unsteady aerodynamic phenomena occurring during the three main flight phases of the descent and landing trajectory, namely, the reentry burn, the aerodynamic phase, and the landing burn. The reentry burn is a deceleration maneuver at high altitudes with several active engines, which serves to lower the dynamic pressure and heat loads in the aerodynamic phase. The aerodynamic phase is the unpropelled phase of the trajectory. The landing burn is the final retro propulsive maneuver which brings the velocity of the vehicle down to zero at touch down. The reentry burn was tested in the Hypersonic Wind Tunnel Cologne (H2K), the aerodynamic phase was tested in the Trisonic Wind Tunnel Cologne (TMK), and the landing burn was tested in the Vertical Free-Jet Facility Cologne (VMK). The exhaust plumes in the propelled phases were simulated with cold gas jets with pressurized air. Proper Orthogonal Decomposition (POD) of high speed Schlieren recordings and spectral analyses of high frequency pressure measurements were performed and an average modal solution was proposed to describe the strongly unsteady flow field, especially in the propelled phases. For the aerodynamic phase, an analytical model was developed for the design of the Aerodynamic Control Surfaces (ACS), which was validated against results of wind tunnel experiments and showed good agreement. The reentry burn and the landing burn were investigated regarding their steady and unsteady flow features. For the reentry burn the main scaling parameters are the thrust coefficient and the total pressure downstream of the bow shock. Configurations with different numbers of engines can be scaled with the total thrust coefficient. The main scaling parameters for the landing burn are the Ambient Pressure Ratio (APR) and the Momentum Flux Ratio (MFR). Strongly dominant frequencies were found especially during the landing approach, which are in the range of critical Strouhal numbers found for near-wake flows of ascent configurations. The normalized pressure fluctuations during the landing approach are one order of magnitude larger than for these near-wake flows.