Aerodynamic Phenomena of Retro Propulsion Descent and Landing Configurations

The RETALT (RETro propulsion Assisted Landing Technologies) project aims to investigate vertically landing launcher configurations, which decelerate by means of retro propulsion manoeuvres during the descent and final landing phase. One key objective was to understand the complex steady and unsteady flow field and the resulting oscillating pressure loads on the re-entering vehicles.

In the course of the project, extensive wind tunnel test series have been performed in the three aerodynamic wind tunnel facilities at DLR in Cologne. The re-entry burn, with one to three active engines, was rebuilt in the Hypersonic Wind Tunnel Cologne (H2K), the aerodynamic descent phase was analysed in the Trisonic Wind Tunnel Cologne (TMK) and the landing burn was tested in the Vertical Free-jet Facility (VMK). Furthermore, detailed CFD analyses of the various flight phases have been performed by CFSE and DLR with the flow solvers NSMB and TAU.

In this paper the steady and unsteady flow field around the RETALT1 and RETALT2 vehicles in the various flight phases will be compared and the resulting loads will be analysed.
An analytical sizing method for aerodynamic control surfaces for the aerodynamic phase of RETALT1 is validated, for RETALT2 a strong hysteresis was found in the forces and moments in the aerodynamic phase. The plume length in the landing phase of RETALT1 follows a linear analytical approach. The flow field of RETALT1 in the retro propulsion phases during the landing burn and the re-entry burn is highly unsteady which is visible in the variance of the schlieren images and in the high frequency pressure measurements.