Nonlinear dynamical analysis for the control of a reentry vehicle in hypersonic flight

Existing reentry vehicles are prone to fall into inertially coupled nonlinear motions in case a failure of automatic flight control systems occurs in flight. In an attempt at devising a remedy control technique for recovering a level flight from nonlinear motions, the paper offers a methodology for making nonlinear dynamical analyses, taking the Japan's HOPE-X in hypersonic flight as an example. The dynamical analyses include equilibrium analyses by the continuation method and stability analyses of equilibrium points by Lyapunov's first method. The paper first proposes the improvement of the continuation method from the standpoint of an initial guess of equilibrium points. The improved method is applied to the HOPE-X in hypersonic flight to yield the results that the shapes of equilibrium branches are much different between subsonic and hypersonic speeds due to aerodynamics and at hypersonic speeds it is necessary to take damping coefficients into account in order to have a stable basic solution branch that includes a true level flight without any rotational motion. Based on the knowledge gained from these analyses, the paper finally discusses possible remedy control techniques to be implemented in the future.