Free flight static and dynamic aerodynamic characteristics for re-entry capsule at transonic speed

This paper studies the motion and aerodynamic characteristics of the re-entry capsule with stable fins at transonic speed by the wind-tunnel free-flight test in which the motion freedom is not limited. When the wind tunnel test model flights freely by the observational window, the high speed camera captures the motion image immediately by a single light path, and then the model motion trajectory and attitude angle can be acquired by image acquisition software. Finally, the pitching static and dynamic derivatives coefficient are derived by the parameter differential method from the recorded angular motion of the model of more than two cycles with the linear and nonlinear motion equations. The result shows that the static derivative coefficient C_mα obtained by the linear and nonlinear aerodynamic parameter identification is less than zero and the variation is not significant. From the results of nonlinear aerodynamic parameters identification, the value of static derivatives coefficient of the re-entry capsule is mainly determined by the linear term C_mα0, and the nonlinear term C_mα²α² is relatively smaller. The nonlinearity of the static derivatives coefficient is weak, which can be approximated by the linear aerodynamic model. The dynamic derivative coefficient of the re-entry capsule is nonlinear in the range of experimental angles of attack. It is determined by the linear term in the range of small angle of attack, and dominated by the nonlinear term in large angle of attack.