No abstract

Due to the non-spherical perturbation of the Moon, the lifetime of ultra low-altitude Lunar spacecraft may be quite short. In this paper, we analyze the lifetime of about 50 km-altitude Lunar spacecraft with different initial orbit. The lifetime in low inclination orbits is much shorter than the ones in the near polar orbits. To extend the lifetime and keep the spacecraft in an appropriate range, an orbit maintenance strategy based on low-thrust propulsion system is proposed. The influence of the orbit initial conditions (e.g., semi-major axis, inclination, right ascension of the ascending node) on lifetime extension are discussed and the effect of the low-thrust magnitude in orbit maintenance is analyzed. According to the numerical simulation results, the lifetime of about 50 km-altitude 100 kg Lunar spacecraft with 10 kg fuel and 20 mN thruster can be extended from 7.958 days to over a 109.1725 days, which demonstrates the effectiveness of the strategy. Furthermore, a global perspective for ultra low-altitude Lunar spacecraft lifetime extension problem is provided in this paper, which can be applied to Moon mission designs extensively.

We propose two approaches based on feedforward control and model-predictive control, respectively, to solve the station-keeping problem of an electric-propulsion geostationary Earth orbit (GEO) satellite, whose thrusters are mounted on two robotic arms on its anti-nadir face. This novel configuration enables a wider range of thrust direction, making it possible to regard the thrust direction as control variables. To solve this control problem, we present the quick feedforward controller (QFFC) and the fuel-optimal model predictive controller (FOMPC). The QFFC is developed based on the analysis of GEO dynamics and the thruster configuration. The FOMPC applies an optimization algorithm to solve the nonlinear model predictive control (NLMPC) problem with the initial value given by the QFFC. Numerical simulations suggest that both controllers could achieve stable station-keeping over multiple objective elements with fewer thrusters and fewer maneuvers. The QFFC has higher control accuracy and lower computational requirements than the FOMPC, whereas the FOMPC could significantly save fuel consumption. The robustness assessment and other discussions of the controllers are also presented.