Survey of Blunt Body Dynamic Stability in Supersonic Flow <s> A. Center of Gravity Location <s> The transonic dynamic stability of one 70° and two 60° half-angle cone configurations has been investigated in a continuous flow wind tunnel using free-flight techniques. The parameters varied were Mach number and oscillation amplitude for the approximate ranges of 0.6 to 2.0 and 0° to 15°, respectively. The test data show two marked effects: 1) a decrease in dynamic stability at transonic speeds relative to that exhibited in the subsonic and supersonic regimes, and 2) transonic dynamic stability data sensitive to oscillation amplitude. Generally, measured coefficients were destabilizing for angles of attack less than 10° and stabilizing at greater amplitudes. The effect of using these data in a six-degree-of-freedom computer program to generate oscillation envelopes and corresponding dynamic stability data for a simulated Mars entry trajectory is demonstrated. <s> BIB001 </s> Survey of Blunt Body Dynamic Stability in Supersonic Flow <s> A. Center of Gravity Location <s> The successful deployment of the parachute recovery system for the Stardust sample return mission requires an understanding of the static and dynamic aerodynamic characteristics in the supersonic and transonic speed range. Aerodynamics obtained in a ballistic range at Mach num0ers 1.2 to 2.8 and angles of attack to 30 degrees are presented. The observed motion histories indicated a limit cycle of between 25 and 30 degrees. The major contributor to this was the pitch damping which was highly nonlinear with angle of attack, being dynamically unstable at small angles and dynamically very stable at higher angles. Introduction Experimental Setup The deployment of a parachute or the direct landing of a passive probe on the surface of a planet presents some unique requirements from an aerodynamic standpoint. The sample return capsule for the Stardust mission is an example of these requirements (Ref. 1). Here a parachute is to be deployed in the low supersonics or transonic speed range, but the vehicle must also be capable of direct impact at small angles of incident to the normal to ensure that the payload package is undamaged in case of parachute deployment problems. The ballistic range tests were conducted in the Aeroballistic Research Facility of the Air Force Laboratory, Armament Directorate, located at Eglin Air Force Base, Florida. The ballistic range is a one atmosphere range with 50 orthogonal shadowgraph stations distributed over 200 meters, Ref. 2. The models were fired from a 80 millimeter smooth bore powder gas gun at a nominal velocity of 1 km/set. In the course of the flight down the range the velocity decayed to about 0.4 km&c. Hence data were obtained for Mach numbers from 2.8 to 1.2. The dynamics of entry into a planetary atmosphere are such that after maximum dynamic pressure the pitching amplitude envelope normally begins to grow. The extent of this growth is dependent on the aerodynamic characteristics and the inertial properties of the vehicle, and in many cases these characteristics can lead to a large amplitude limit cycle. This paper will examine the aerodynamic and trajectory characteristics of the Stardust sample return capsule in the low supersonic to transonic speed range as determined from ballistic range tests. Three models were tested with the same external geometry shown in Fig. 1 but with three different center of gravity positions. The models were made of steel to make them as heavy as possible to keep the velocity losses to a minimum. The nominal physical properties for the models with the three different center of gravity positions are shown in Table I. Also shown for reference are the properties of the full scale vehicle. The measured physical properties for the 10 models tested are shown in Table II and the test conditions, number of observed stations and distance flown are shown in Table III. * Consultant, Associate Fellow ALAA + Aerospace Engineer, Aerothermodynatnics Branch, Experimental Data Senior Member AIAA $ Vice President of Engineering, Senior Member AIAA This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. The data consists of 10 sets (tests) of 40-45 orthogonal shadowgraph images and the corresponding times that the shadowgraphs were taken. A typical range shadowgraph is shown in Fig. 2. For im..-. <s> BIB002
With the intuition gained from static stability experiments, one of the first trade studies regarding dynamic stability involved varying the location of the center of gravity. Buell and Johnson  investigated this effect in two blunt bodies, one a 30 o sphere cone and the other having a parabolic forebody, and found no conclusive trend regarding the damping characteristics as a function of center of gravity location and angle of attack at subsonic speeds. Ericsson presented supersonic results in  for the same sphere cone configuration of Buell's study and found that the amplitude and angle of attack range of the instability region decreased as the CG was moved axially toward the nose. Ericsson attributes this behavior to a decrease in the adverse effects of flow separation and reattachment on the aftbody that accompany the forward movement of the CG, outweighing the lengthened lever arm for the aftbody pressure forces to act. In this case, improved static and dynamic stability are both obtained by making an alteration to the vehicle. For most other parameters investigated, however, making changes in order to improve dynamic stability characteristics come with the consequence of reducing static stability. Other studies  - BIB001 conducted at transonic or supersonic speeds over the years have also concluded that damping improves as the CG is shifted forward. In BIB002 and  it was observed that although moving the CG forward improved damping at low angles of attack, the effect lessens and a more rearward located CG may improve damping at higher angles of attack (Fig. 16) . In some studies, such as that by Uselton, Shadow and Mansfield  , moving the CG rearward at a Mach number of 3 had a much less pronounced effect than at transonic and lower supersonic Mach numbers. This may indicate that sensitivity to CG location increases as Mach numbers approach the low supersonic, transonic and subsonic regimes as a result of the more prevalent pressure forces in the near wake as flow speed around the corners decreases, allowing for earlier reconvergence of the flow and more substantial near wake and recirculation regions. 