Survey of Blunt Body Dynamic Stability in Supersonic Flow <s> B. Free Oscillation Testing <s> Free-oscillation tests of three Viking-type blunt conical configurations were conducted in a wind tunnel to determine the important variables influencing their dynamic stability characteristics at Mach 1.76. The effects of sting geometry are shown to be significant at low angles of attack, while the effects of cross plane motions and tunnel vibration characteristics appear to be negligible. Variation of the reduced-frequency parameter and model base geometry caused significant effects in the data. An approximate technique was developed for converting effective data from either forced-oscillation or free-oscillation tests to values based on the instantaneous angle of attack. <s> BIB001 </s> Survey of Blunt Body Dynamic Stability in Supersonic Flow <s> B. Free Oscillation Testing <s> With the retirement of the Space Shuttle, NASA is designing a new spacecraft, called Orion, to fly astronauts to low earth orbit and beyond. Characterization of the dynamic stability of the Orion spacecraft is important for the design of the spacecraft and trajectory construction. Dynamic stability affects the stability and control of the Orion Crew Module during re-entry, especially below Mach = 2.0 and including flight under the drogues. The Launch Abort Vehicle is affected by dynamic stability as well, especially during the re- orientation and heatshield forward segments of the flight. The dynamic stability was assessed using the forced oscillation technique, free-to-oscillate, ballistic range, and sub-scale free-flight tests. All of the test techniques demonstrated that in heatshield-forward flight the Crew Module and Launch Abort Vehicle are dynamically unstable in a significant portion of their flight trajectory. This paper will provide a brief overview of the Orion dynamic aero program and a high-level summary of the dynamic stability characteristics of the Orion spacecraft. <s> BIB002
Free oscillation uses a transverse rod positioned through the center of rotation of the vehicle. This rod is set in low friction bearings that allow the model to pitch freely in response to the oncoming freestream flow. Following an initial oscillatory perturbation, the vehicle's natural dynamic damping response is measured. By observing the time history of the oscillatory growth or decay, the dynamic aerodynamic coefficients can be determined with parameter identification techniques. This technique was matured during the Apollo program and is still in use today. It allows for explicit determination of the reduced frequency inherent to the vehicle (as opposed to sweeping through various possibilities as in forced oscillation technique) BIB002 , is simple in its implementation, and requires only one measurement (oscillation amplitude as a function of time)  . Similar to the forced oscillation setup, free oscillation testing yields an average value of the damping over one oscillation cycle and suffers from the effects of having a sting present to modify the wake flow. Also, it has additional scaling requirements in that the center of gravity of the vehicle must be coincident with the center of rotation and the pitch moment of inertia must be scaled properly from the expected full scale vehicle for representative behavior to be obtained BIB001 . Finally, damping due to bearing friction must be accurately quantified so that it can be separated from the aerodynamic damping of the vehicle.