Robust Powered Descent with Control Contraction Metrics

Powered descent (PD) is the final stage in Entry, Descent, and Landing where a spacecraft plans and executes a trajectory to a desired landing location. Increased demand for precise landing has driven the need for more robust and sophisticated guidance and control algorithms. While several advances have been made in optimization-based guidance, little progress has been made in developing robust nonlinear controllers. The main technical challenge for controller design is the nonlinear control dynamics in the standard PD model; making traditional methods not applicable. This work derives a robust, exponentially stable nonlinear controller using the Control Contraction Metric framework. The resulting controller takes the form of a nonlinear mass-scheduled proportional-derivative controller - an interesting result given no assumptions about the controller's structure are made. Theoretical guarantees for integrated control effort (i.e., fuel usage) and tracking performance are derived and verified via Monte Carlo simulation. Modifications that facilitate integrating the contraction-based controller into existing optimization-based guidance algorithms are also proposed and verified in simulation.