The utilization of low speed facilities in transonic stability of reentry vehicles research - An evaluation

The customary practice of utilizing low speed facilities to extract flow characteristics of reentry vehicles in transonic regimes is examined. Using the Apollo capsule as a generic reentry configuration, the similarity between its wake in incompressible and compressible regimes is assessed from static measurements of lift, drag, pitching moment, surface pressure, wake size and periodicity of the wake flow at four nominal Mach numbers ranging from that of essentially incompressible to Mach 0.9. The results to date indicate that, for vehicle orientations of practical interest in which the heat-shield is facing the freestream, sufficient similitude is noted to warrant such an extrapolation at least as an exploratory tool in devising strategies for wake modification. Additional utility of low speed tests in acquiring wake features of an oscillating vehicle is conceptually feasible and discussed. Introduction A reentry capsule is shaped to be blunt so as to survive the intense aerodynamic heating in the hypersonic portion of the flight. However, during the transonic and subsonic phases of the reentry in what might be considered as off-design conditions, the vehicle faces a more pronounced stability and control problem. The dynamic vortex formation typical of the blunt body wake flow at the aforementioned speed regimes produces unsteady loading on the vehicle, and can sometimes thereby affect its handling characteristics severely. It is therefore important to determine the stability characteristics of the vehicle so as to ensure proper and timely deployment of parachutes prior to the onset of instability, as well as having a well documented *NSF-NATO Postdoctoral Fellow, Member AIAA. Associate Professor. 'Doctoral Candidate, Student Member AIAA. Research Engineer. Copyright © 1998 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. description of the flow-field in order to understand the associated fluid mechanics. The importance and difficulties in characterizing the unsteady wake in studying the stability of a reentry vehicle are illustrated and discussed in further details in Ref. 1. In the context of vehicle stability and parachute deployment issues which take place in high subsonic and transonic regimes, the flow-field characterization around and in the near-wake of a reentry capsule is often extracted from low speed tests where both visualization and detailed diagnostics can be carried out more readily. The utilization of a low speed facility to extrapolate flow-field information of a reentry capsule at higher Mach numbers, although attractive in many perspectives and often implemented, is a practice that deserves a methodical examination. Using the Apollo capsule as a representative shape of planetary reentry vehicles, the present paper presents the results of a first series of experiments conducted at the von Karman Institute (VKJ) in assessing this issue of fundamental and practical importance by comparing the global aerodynamic quantities and the flow structures of a non-oscillating capsule in the incompressible, high subsonic and transonic regimes. These findings would lead to a disclosure of the degree of similarity in the flow-field of the various regimes, as well as contributing to the fundamental knowledge of three-dimensional bluff body flow with compressibility effect. In the case of a two-dimensional symmetric or three-dimensional axisymmetric body in free flow at zero lift, the associated global time-averaged wake characteristics are strongly dependent of the drag exerted by the body as this quantity represents an integral of the wake motion. In the event of a lifting body, it is expected that the lift would play its role and manifest itself ultimately in the wake as well. Similarly on a more refined scale, the details of the flow around the body, which could at least be partially revealed by surface pressure measurements, constitute the initial distribution of vorticity shed downstream of the body thereby 1 American Institute of Aeronautics and Astronautics Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc. influencing the microscopies of the wake both on a time-averaged and temporal basis. With the above in mind, the global aerodynamic quantities lift, drag and moment coefficients about the center of gravity were measured at four different Mach numbers. As for the comparison of flow quantities, the investigation included surface pressure distributions as well as optical diagnostics around and in the near-wake of the capsule. In addition, the temporal characteristic of the flapping wake motion was mapped out. All of the subsonic and transonic measurements had been conducted in the VKI-S1 facility whereas their incompressible counterparts were measured in three separate low speed wind tunnels: VKI-L7, L7+ and L2A. The details of the various facilities are found in Ref. 2 Experimental Setup The Apollo Command Module Block I configuration with no protuberances was selected to represent a generic reentry vehicle in this investigation. A drawing of this configuration, with the various dimensions normalized with the maximum diameter (i.e., heat-shield) of the model, is shown in Figure 1. The model is at a =180° as illustrated, with the freestream flow in the direction from right to left. A clockwise rotation corresponds to the direction of decreasing angle of attack. Depending on the particular test, the model was supported either one-sided cantileverly or on both sides with a transverse rod through the theoretical center of gravity location shown in Figure 1. In this mounting arrangement the pitching moment at the model's theoretical center of gravity is therefore measured directly. The additional pertinent details for the various tests are tabulated in Table 1. Fig. 1 A scaled drawing of the Apollo model. Table 1: Summary of Test and Mounting Conditions in Various Facilities L7+