Measurement of Aerodynamic Coefficients of Blunt Cones at the High Enthalpy Shock Tunnel Göttingen

Abstract

During atmospheric re-entry, intense heating of the shock layer leads to vibrational excitation, dissociation and even ionization of the gas near the forefront of the spacecraft, while further downstream, recombination reactions occur. These so called “high-temperature effects” may have a substantial impact on the forces and heat loads imposed on the vehicle. A well-known example is the “pitch-up anomaly” observed during the first Space Shuttle orbiter re-entry: An unforeseen increase of the pitching moment, which was later attributed mainly to high-temperature effects, necessitated the body flap to deflect twice the predicted amount to maintain a trimmed flight. The latest campaign at the High Enthalpy Shock Tunnel Göttingen (HEG) is presented. Its goal is to perform accurate measurements of lift, drag and pitching moment on blunt cones and determine the impact of high-temperature effects on these parameters. During previous experiments at JAXA’s HIEST shock tunnel, differences in the axial force coefficient between measurement and numerical prediction as well as indications of a possible pitch-up effect were found for high enthalpy flows around these geometries. This study aims to investigate whether these observations are facility-dependent or can be reproduced and attributed to high-temperature effects. The second objective is to assess and further improve the accuracy of the employed short-duration force measurement techniques. For the current study, two comparatively light-weight, spherically blunted cone models with varied nose radii were manufactured and positioned in the test section via a detachable support that left them unrestrained after nozzle flow startup (free-flight technique). Experiments were conducted at different angles of attack for low and high enthalpy flow conditions. Forces and moments were measured using two different approaches simultaneously: The first technique derives lift, drag and pitching moment from a high-speed video record of the model movement using an optical tracking algorithm, while the second uses readings of accelerometers mounted inside the model. These approaches are described in detail and first results of both measurement techniques are contrasted with one another and with numerical CFD calculations obtained using the DLR TAU Code. Finally, an outlook is given on how these results relate to the HIEST experiments.