Explicit Near-Optimal Guidance and Pulse-Modulated Control for Lunar Descent and Touchdown

Abstract

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 a specified altitude, and (b) terminal descent and soft touchdown. For phase (a), a new locally 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 uses an adaptive tracking scheme that compensates for the inertia uncertainties. Arc (b) is aimed at gaining the correct vertical alignment and low velocity at touchdown. For phase (b) 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 non-nominal flight conditions.

Lunar descent and landing via two-phase explicit guidance and pulse-modulated reduced-attitude control

“…They are ignited using pulse width modulation [27][28][29]. This study represents a considerable extension of previous research by the same authors [30], focused on planar descent and touchdown. In fact, this work considers perturbed three-dimensional trajectories, which is a more realistic description of the actual mission scenario with respect to perturbed planar paths (assumed in Ref.…”

“…ON /DC is the minimum torque magnitude that can be produced by the side jets associated with set i; T ON , therefore it is a technological constraint [30]. The side jets are assumed to be monopropellant thrusters, where the propellant is fed by the action of an inert pressurant gas.…”

“…In fact, this work considers perturbed three-dimensional trajectories, which is a more realistic description of the actual mission scenario with respect to perturbed planar paths (assumed in Ref. [30]). This circumstance leads to the following major novelties with respect to previous research [30].…”

“…[30]). This circumstance leads to the following major novelties with respect to previous research [30]. First, the guidance algorithms in phases 1 and 2 extend the preceding schemes developed for planar descent paths.…”