Planetary landing: modelling and control of the propulsive descent

In the propulsive phase, after parachute release, of planetary landing like Mars or Moon, horizontal motion is obtained by tilting the axial thrust, so that it aligns either to the negative velocity vector (gravity turn) or to the requested acceleration vector. The latter strategy is assumed here, as it allows pinpoint landing. As such, tilt angles (pitch and yaw) become proportional to the horizontal acceleration. Instead of designing a hierarchical guidance and control in which horizontal acceleration becomes the attitude control target, a unique control system can be designed based on the fourth order dynamics from angular acceleration to position. The paper shows that the combined dynamics can be (quasi) input-state linearized except the nonlinear factor of the tilt angles (the axial thrust imposed by vertical braking), and other less severe flaws. The paper shows that control design around the reference trajectory (tilt and position) given by the guidance can exploit the quasi linearization, but tracking error stability must be proved. The paper is restricted to closed-loop control strategies, and their effectiveness is proved through Monte Carlo simulations, and comparison with a small-tilt design.