Survey of Blunt Body Dynamic Stability in Supersonic Flow <s> A. Forced 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> A. Forced 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
Dynamic wind tunnel testing with a forced oscillation setup generally uses an axial sting that measures forces and moments, as well as the rates of change of these parameters with respect to changing pitch angle or angle of attack. In order to capture the dynamic behavior, a motor attached to the sting imparts a one-degree-offreedom oscillatory motion to the vehicle at a wide range of frequencies and mode shapes. Sinusoidal motion is typically used BIB002 . The vehicle is inclined at a wide range of angles of attack and at each condition undergoes a series of small amplitude pitch oscillations. The damping response of the vehicle is measured as a function of pitch amplitude, angle of attack, and Mach number. Advantages [22] of this technique are its direct measurement of the dynamic aerodynamic coefficients  , BIB002 , controllability and repeatability  , and its ability to match a wide range of reduced frequency parameters (also known as Strouhal numbers)  . Additionally, mass scaling is not generally required to obtain representative full scale behavior in the sub scale environment BIB002 . Drawbacks of forced oscillation tests are the sting effects on the damping and the inability to observe limit cycle behavior  . Also, due to the nature of the the test setup itself, only the average damping over a pitch cycle can be obtained BIB001 and the accuracy to which it can be obtained is reduced as the measurement techniques are reliant on three independent strain gauge force measurements BIB002 .