Similarity and key parameters of retropropulsion assisted deceleration in hypersonic wind tunnels

Abstract

Retropropulsion, a deceleration technique where the thrust is directed along the flight path, is the driving technology for vertically landing, reusable launchers. Experiments, which are indispensable for the full comprehension of retropropulsion flows, pose serious challenges to the operability of wind tunnels and interpretation of these subscale experiments. The key factors to facilitate meaningful results of supersonic and hypersonic retropropulsion experiments are addressed in this study. Similarity parameters for subscale experiments are evaluated, the interaction of retropropulsion flows with the test facility is investigated, and condensation within these flows is analyzed. For this purpose, computational fluid dynamics simulations were performed; subsequently, cold gas experiments for a generic retropropulsion launcher configuration with a single central nozzle were carried out in the hypersonic wind tunnel Cologne H2K at the Supersonic and Hypersonic Technologies Department of the DLR, German Aerospace Center. Based on these data, an improved similarity parameter related to the exit condition of the engine exhaust flow is proposed, and a universally applicable scaling law for supersonic retropropulsion wind-tunnel models is derived and validated. Various condensation mitigation techniques for the conducted experiments are presented, and their varying effectiveness is highlighted.