Terminal Trajectory Planning and Optimization for an Unpowered Reusable Launch Vehicle

Abstract

A guidance scheme that employs a trajectory-planning algorithm has been developed for the terminal area energy management phase of an unpowered reusable launch vehicle. The trajectory-planning scheme computes a reference flight profile by piecing together several flight segments that are defined by a small set of geometric parameters. Iterating on three geometric parameters and propagating a test matrix of trajectories produces a set of feasible reference profiles that bring the vehicle from its current state to a desired approach and landing target state. Path constraints on states and controls are easily implemented in the propagation scheme, and simple numerical sorting is used to identify the optimal feasible path. Open- and closed-loop guidance commands are readily available once the best reference trajectory is de termined. The trajectory-planning algorithm is able to quickly generate new reference profiles for test cases with large variations in initial vehicle energy, initial heading, and vehicle drag. The effectiveness of the trajectory-planning algorithm is de monstrated by several numerical simulations. Next, optimal TAEM trajectories are obtained by using an inverse dynamics approach and nonlinear programming methods. These optimal paths exhibit reduced control effort (such as smaller bank angle profiles) when compared to the guided results, and suggest that significant improvements can still be achieved over a guidance method based on a geometric reference path.