Pinpoint lunar landing with non-throttleable engine

The technology of pinpoint landing can ensure that the spacecraft reaches the target safely and reliably, which expands the detection areas of interest. Current guidance algorithms for pinpoint landings require a throttleable engine. However, the operatable capabilities of the developed engines are difficult to meet the demand. Focusing on this problem, a method for pinpoint lunar landing with non-throttleable engine is proposed. The braking guidance is split into longitudinal and lateral. The thrust direction is adjusted to deviate from the longitudinal plane by an angle to ensure the thrust magnitude component in the longitudinal plane meets the required value of the desired downrange and thus powered explicit guidance (PEG) is adopted. For lateral motion, a method of open-loop guidance is proposed by switching the symbol of the bank angle. The condition for the minimum number of switches is established, and the numerical algorithm is employed to solve the switch time of thrust. The proposed algorithm is computationally efficient and can be implemented onboard. Its effectiveness is verified by Monte Carlo trials.