In this paper, minimum-fuel, three-dimensional trajectory optimization for lunar landing considering the desired landing site is dealt with during the powered descent phase which is one of the landing phases. The landing site is not usually considered while performing trajectory optimization of the soft lunar landing. However, after a de-orbit burn, the emergency situations, such as hazard avoidance maneuvers and navigation errors, could be occurred to change the trajectory to land at the alternative landing site. In this case, the trajectory has to be redesigned to land at the other landing site. Also, the analysis of the trajectory and control time histories, which are obtained from the optimization results considering the landing site, could be applied to other analyses such as divert capability, cross-range capability. Thus, the trajectory optimization considering the landing site is one of the most important problems for the soft lunar landing. Two major lunar landing problems are solved using a direct method for the trajectory optimization in this paper. One is to perform the trajectory optimization with the free latitude, longitude, and transverse velocities. The other is to redesign the trajectory changing the alternative landing sites and those results are discussed.
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