Constant Thrust Adaptive Convex Programming Guidance for Lunar Powered Descent

Compared with throttleable engines, constant thrust engines exhibit inherent advantages in architectural simplicity, operational reliability, mass efficiency, and lifecycle cost reduction. If using a constant thrust engine can accomplish the lunar pinpoint landing, it would undoubtedly enhance mission economy. So, an adaptive constant thrust iterative convex programming guidance (CTICPG) is proposed for the multi-constraint propellant-optimal guidance problem for lunar powered descent with a unthrottled engine. First, the first-order hold (FOH) is used to discretize the continuous-time trajectory optimization problem. Then, to avoid the linearization error introduced by successive convexification, lossless and approximate convexification methods for constant thrust magnitude constraint, spherical gravity model, tilt angle rate constraint and final tilt angle constraint are presented. Subsequently, the lossless convex programming (LCP) algorithm is extended to obtain the CTICPG that can achieve lunar soft landing and pinpoint landing. Afterward, in order to enhance robustness and engineering practicability, the real-time estimation algorithm of engine parameters is introduced into CTICPG, thereby obtaining the adaptive CTICPG. Finally, numerical simulations are performed to verify the results, which show that CTICPG can achieve the soft landing and pinpoint landing.