Incremental Nonlinear Dynamic Inversion Flight Control for the DLR Reusability Flight Experiment ReFEx

Abstract

The Reusability Flight Experiment (ReFEx) is a technology demonstrator developed by the German Aerospace Center (DLR) for a launch vehicle with vertical takeoff and horizontal landing capabilities. Its objective is to verify key technologies critical to the reusability of a launch vehicle's first stage and boosters. During atmospheric flight, the vehicle's trajectory spans a wide range of Mach numbers and angles of attack. The vehicle encounters substantial shifts in dynamic pressure and highly nonlinear aerodynamic effects. These include sign changes in the control derivatives and singularities, where the lateral motion may become uncontrollable. The aerodynamic control surfaces, which are applied to control the vehicle's attitude, exhibit strong cross-coupling and varying effectiveness. Given the uncertainties in the vehicle's mechanical properties and aerodynamic data, a reliable control design capable of handling the uncertainties and disturbances is essential. The flight controller presented in this paper addresses these challenges by combining a cascaded control architecture based on nonlinear dynamic inversion (NDI) with an incremental NDI approach for control allocation. For fast guidance reference signal tracking the presented controller employs feed-forward control while uncertainties, disturbances and model mismatches are compensated by linear feedback controllers. A control allocation based on a damped-least squares optimization approach determines aerodynamic control surface deflections and prevents excessive control actions in the vicinity of singularities. Simulation results and Monte Carlo campaigns validate the selected control approach and demonstrate that the proposed flight controller is capable of tracking the trajectory in the presence of disturbances and uncertainties.