Robust Control Design for Flexible Guidance of the Aerodynamic Descent of Reusable Launchers

Abstract

Rocket reusability is a key-factor for a sustainable and cost-effective future of space missions. Despite successes in this technology, some of the open problems remain an impediment to further development. For example, the descent phase presents important challenges from a guidance and control perspective, because, due to model and environment uncertainty, the optimal re-entry trajectory is often unknown a priori. As a result, the previously designed (or offline) controller may be unable to track the reference trajectories provided by the adaptive (or online) guidance algorithm. This paper is an attempt to build up a methodology to address this issue. First, a systematic approach for H ∞ control design for a pre-defined nominal descent trajectory is described. The synthesis approach combines a disturbance rejection structured H ∞ design at fixed operating conditions with a blended gain-scheduling approach across different altitudes. Then, the problem of providing robustness over a family of reference trajectories is framed as a multi-plant control design approach. The proposed robust control synthesis approach shows a satisfactory performance with respect to system requirements in the face of a large variation in the tracked trajectories and provides a novel viewpoint on designing robust controllers to successfully deploy flexible guidance algorithms.