In Pursuit of Improving Airburst and Ground Damage Predictions: Recent Advances in Multi-Body Aerodynamic Testing and Computational Tools Validation

An airburst from a large asteroid during entry can cause significant ground damage. The damage depends on the energy and the altitude of airburst. Breakup of asteroids into fragments and their lateral spread have been observed. Modeling the underlying physics of fragmented bodies interacting at hypersonic speeds and the spread of fragments is needed for a true predictive capability. Current models use heuristic arguments and assumptions such as pancaking or point source explosive energy release at pre-determined altitude or an assumed fragmentation spread rate to predict airburst damage. A multi-year collaboration between German Aerospace Center (DLR) and NASA has been established to develop validated computational tools to address the above challenge

MODELLING CAPSULE STABILITY ACCOUNTING FOR SHAPE CHANGE

Earth return missions or exploration missions use mostly capsule-like shapes, which enter the atmosphere at very high velocities. Some of these missions, like sample return, do not use any parachute or other stabilizing aerodynamic or RCS devices. Therefore, the capsule stability has to be guaranteed solely by the spacecraft configuration from hypersonic conditions down to the subsonic regime at landing. This task is very challenging and requires reliable design tools. However, both experimental and numerical tools still have shortcomings in full simulation or modelling of the flight environment. Therefore, further improvement of these tools by means of complementary application is essential. Most of the exploration missions use an ablative thermal protection system, which experiences shape changes during the hypersonic flight regime. This may lead to a change of the pressure distribution and movement of the center of gravity of the vehicle. Since the vehicle does not have control devices, it can lose its aerodynamic stability and the situation may become critical. The prediction of the Thermal Protection System (TPS) recession over the complete surface with the existing tools is not possible. Therefore, the aerodynamic design should consider it in the margin policy and the flight qualities and risk analysis need to be performed accordingly. The ESA TRP MODSHAPE (Modelling Capsule Stability accounting for Shape Change) addresses the aforementioned challenges and this paper gives an overview of the planned activities and summarizes the main challenges and goals

Mehrkörperaerodynamik während der Fragmentation beim atmosphärischen Wiedereintritt

"Sommer mit Sicherheit" - so war die erste Sommerakademie der zivilen Sicherheitsforschung überschrieben, die vom 23. bis 27. Juli 2018 in Bad Pyrmont stattfand. Der vorliegende Band stellt die Beiträge vor, die während dieser Woche des interdisziplinären Arbeitens in den Gruppen präsentiert, diskutiert und schließlich für diese Publikation weiterentwickelt wurden. Während der Akademie wurde zu folgenden übergeordneten Themen gearbeitet: Sicherheitswahrnehmung und Polizeiarbeit Sicherheitstechnologien Kommunikation in (Un-)Sicherheitslagen Kritische Infrastrukturen, Risikobewertung und Katastrophenmanagement. Weitere Informationen: www.sifo-dialog.d

Experimental determination of aerodynamic coefficients of simple-shaped bodies free-flying in hypersonic flow

The aerodynamic drag, lift and pitching moment of basic geometric bodies in hypersonic flows has been measured. Therefore, simple geometries like spheres, cubes and cylinders were studied at a Mach number of 7.0. Furthermore, the influence of the flight attitude on aerodynamic coefficients and the static stability behaviour of these objects have been investigated. This should enhance the understanding of atmospheric re-entry of rotating objects as well as the improvement of available codes to predict re-entry trajectories of satellites, meteoroids and space stations. Experiments were carried out in the Hypersonic Wind Tunnel (H2K) at the German Aerospace Center (DLR) in Cologne using a free-flight technique. This technique allows a continuously rotation and avoids the influence of stings or balances to determine the forces and moments onto an object. The model motion is tracked by a nonintrusive stereo tracking system with two high-speed cameras. Markers are utilized on the model surface to apply a digital image correlation algorithm. Thus, the three-dimensional trajectories and attitudes of the free-flying objects are reconstructed. After a careful post-processing of the high-resolution motion data, the aerodynamic coefficients are determined. Furthermore, the unsteady flow structures are visualized with high-speed schlieren photography. As a result, the aerodynamic coefficients of cubic and cylindrical bodies are a function of the pitch angle

Aerodynamic Coefficients of Free-Flying Cubes in Hypersonic Flowfield

Aerodynamic coefficients of a cube depending on a broad-range attitude change have been measured using the free-flight technique in a hypersonic flowfield. Experiments were performed therefore in the hypersonic wind tunnel H2K at the German Aerospace Center (DLR) in Cologne. The free-flight technique in H2K has allowed achieving a continuous rotation of the cube without any sting interferences in a broad angular range of 90°. This motion of the model during the free flight has been acquired using a nonintrusive high-speed stereo tracking system. A marker-based tracking algorithm has been applied to reconstruct the three-dimensional flight trajectories and attitudes to determine the resulting forces and moments. By using high-speed schlieren photography, the unsteady flow structures have been recorded. Pitch-angle-dependent aerodynamic coefficients of rotating cubes have been observed

Interactions between asteroid fragments during atmospheric entry

The current work explores the interactions between asteroid fragments and the associated flow topology to motivate a physically consistent representation of the fragmentation process following a fragmentation event during atmospheric entry. Multibody aerodynamic simulations run with computational fluid dynamics (CFD) solvers were used to generate a lookup table of forces detailing the interactions of two spheres. Trajectory simulations parsing the resulting database to determine the relative motions of any two spherical fragments were then validated with hypersonic wind tunnel experiments. A following series of fragment interaction simulations yielded categorization of the fragments' final relative states and an estimate of the total time of interaction. The fragment interaction model was nondimensionalized to permit study over a wide range of possible asteroid impacts. The interaction parameters are presented with explicit semi-analytic equations, defining the asteroid fragment-flow interaction model and thereby eliminating the need to perform a separate fragment interaction simulation for each fragmentation event in an atmospheric entry model. Finally, a set of illustrative examples demonstrates the efficacy of the model in a variety of fragmentation situations