Unsteady Aerodynamics of the Retropropulsion Reentry Burn of Vertically Landing Launchers

During the vertical descent and landing of a launcher first stage with the aid of retropropulsion, commonly two main propulsive deceleration maneuvers are performed: the reentry burn in high altitudes at hypersonic to supersonic speeds and the landing burn shortly before touchdown at transonic to subsonic speeds. In the frame of the EU-funded H2020 project Retro Propulsion Assisted Landing Technologies (RETALT), the unsteady aerodynamics of those retropropulsion phases were studied. This paper presents results of experiments performed in the Hypersonic Wind Tunnel Cologne on the hypersonic reentry burn. The exhaust plume was simulated with pressurized air. Proper orthogonal decomposition was performed on high-speed schlieren videos, and spectral analyses of the time histories of the resulting modes were compared to the frequency content found in high-frequency pressure measurements. Dominant frequencies were found in the proper orthogonal decomposition modes for one and for three active engines. In the pressure measurements, dominant frequencies could only be observed for three active engines. The normalized pressure fluctuations are in the range of 0.002–0.012. Additionally, a good scaling of the pressures on the base area and in the wake of the configuration with the total pressure downstream of the bow shock could be confirmed, in the sense that the ratio of the local surface pressure to the total pressure downstream of the bow shock match for varying freestream Mach numbers.