Optimally Allocated Nonlinear Robust Control of a Reusable Launch Vehicle During Re-entry

A nonlinear robust control design approach is presented in this paper for a prototype reusable launch vehicle (RLV) during the critical re-entry phase where the margin for error is small. A nominal control is designed following the dynamic inversion philosophy for the reaction control system (RCS) and optimal dynamic inversion philosophy for the aerodynamic control actuation. This nominal controller is augmented next with a barrier Lyapunov function based neuro-adaptive control in the inner loop, which enforces the body rates of the actual system i.e. in presence of uncertainties to track the closed-loop body rates of the nominal plant. A fusion logic is also presented for fusing the RCS and aerodynamic control. The control design approach presented here assures robust tracking of the guidance commands despite the presence of uncertainties in the plant model. Extensive nonlinear six degree-of-freedom (DoF) simulation study, which embeds additional practical constraints such as actuator delay in the aerodynamic control actuation and constraints related to the RCS, shows that the proposed design approach has both good command following as well as robustness characteristics.