Explicit near-optimal guidance and pulse-modulated control for lunar descent and touchdown

This research considers the descent path of a space vehicle, from periselenium of its operational orbit to the lunar surface. The trajectory is split in two arcs: (a) approach, up to an altitude of 50 m, and (b) terminal descent and soft touchdown. For phase (a), a new local-flat near optimal guidance is used, which is based on iterative projection of the spacecraft position and velocity and availability of closed-form expressions for the related costate variables. Attitude control is aimed at pursuing the desired spacecraft orientation, and employs an adaptive tracking scheme that compensates for the inertia uncertainties. Arc (b) is aimed at gaining the correct vertical alignment and modest velocity at touchdown. A predictive bang-off-bang guidance algorithm is proposed that is capable of guaranteeing the desired final conditions, while providing the proper allocation of side jet ignitions. Actuation of side jets is implemented using pulse width modulation, in both phases. Monte Carlo simulations prove that the guidance and control architecture at hand drives the spacecraft toward safe touchdown, in the presence of nonnominal flight conditions.