Terminal guidance strategy for a hybrid thrust-tether lunar landing scheme

A hybrid thrust-tether lunar landing scheme and its terminal guidance strategy are proposed in this paper, which has potential application in avoiding the dusts aroused by the plume of thrusters. The combined lander is made up of a descent stage and a rover, which are connected by a tethered device. An innovative combination of fuzzy and variable-structure controllers is introduced to guide the terminal landing, which is more robust than some classical guidance laws derived from the linearized dynamics. At the beginning of this phase, the combined lander carries out the targeting guidance law from the height of 250 m to the desired landing site. When the combined lander arrives at the height of about 20 m, the tethered device is triggered to release the rover which is controlled by the tensioning force provided by the motor and windlass. In releasing the rover, the descent stage is required to hover above the lunar surface at a certain height until the rover meets safe landing conditions. After the rover cuts off the tether, the descent stage will be driven by the deputy thrusters as far away from the rover as possible. A typical scenario is implemented numerically to demonstrate the stabilization of the horizontal initial velocity even in nonzero azimuth angle case. To investigate the robustness of the closed-loop guidance law, a Monte-Carlo simulation is performed to create all the scenarios parameterized by the errors in initial position and velocity which is the result of last powered descent phase.