Optimal Trajectory Reconfiguration and Retargeting for Reusable Launch Vehicles

Abstract

Reconfigurable inner-loop control laws improve the fault tolerance of a vehicle to control effector failures. The control effectors can produce significant perturbations to the nominal forces and can also affect the flight conditions over which the vehicle can be controlled. Three-degree-of-freedom dynamical models typically include wing-body aerodynamic force effects but ignore the aerodynamic forces produced by the control surfaces. These trim effects are a component of 6-degree-of-freedom models; however, such models are cumbersome for the purposes of trajectory optimization. In this work, a method for including the trim effects as well as control-induced trajectory constraints in a 3-degree-of-freedom model is presented. The method uses nonlinear control allocation to determine the control effector positions, required to rotationally balance the vehicle, and uses that information to compute force perturbations and new control-induced trajectory constraints. The contributions of the control effectors to the vehicle lift and drag are computed, as well as constraints that define regions of the flight envelope where the vehicle cannot be rotationally balanced. An example is presented that includes control failure effects in a maximum downrange trajectory optimization problem. Constrained trajectories are planned by solving an optimal control problem using a Legendre pseudospectral method and state-dependent constraints, resulting from a control failure, are included. Results show that these methods hold the potential to enhance safety margins even when control effectors have failed.