Stability Margin Evaluation of DI Control System for Suborbital Spaceplane During Re-entry Phase

This paper reports the application of DI (Dynamic Inversion) theory, a nonlinear control theory, to suborbital spaceplanes. The effectiveness of the control law based on the DI theory for suborbital spaceplane FuJin, which is underdeveloped by mainly SPACE WALKER Inc. is confirmed by conducting numerical simulations of the longitudinal system for the re-entry phase. In addition, evaluate the stability margin, which is issue when applying the DI theory. In addition to the positive stability margin, it is revealed that there is the negative stability margin. Furthermore, a parametric analysis of the state variables is conducted to clarify the dependence of the stability margin on the state variables. It is found that positive stability margin is less dependent on the quantity of states variables, while negative stability margin is highly dependent on altitude, angle of attack and elevator deflection angle. In the longitudinal system, negative stability margin is not issue, and since positive stability margin also has no large fluctuation, it is possible to secure the stability margin by tuning the gain. In the future, this analysis will be extended to lateral and directional systems, and evaluation will be conducted in phases other than the re-entry phase.