Research Of Low-thrust Orbit Transfer,Ultra Low-altitude Lunar Spacecraft Lifetime Extension And Gravity Assist

With the develpment of engineer of aerospace,the classification and the destination of the space missions are becoming more abundant,the range of space activities is also expanding.According to the distance between space activities and the Earth,the space activities can be classified as the ones near the Earth,the ones in the cislunar space and the ones in the interplanetary space.This paper picks up an important problem from each of these three aspects for research:1.This paper researches the problem of near-optimal low-thrust spiral orbit transfer.A modified semi-analytical method is proposed to solve and optimize the low-thrust orbit transfer problem.Equinox elements are adopted as the state vector to avoid the non-essential singularity caused by Kepler elements.Combining analytical steering law and the variation equations of angular momentum and eccentricity vector,the near-optimal direction of the thrust in the current state is derived.The relative weight ratios of different elements are introduced.They are optimized by reinforment learning so the different elements can be adjusted dynamically.The nearoptimal minimum-time low-thrust transfer orbit can be obtained.The designed of relative efficiency factor is also improved in this paper to balance the time and the fuel consumption.The near-optimal minimum-fuel low-thrust transfer orbit can be calculated by selecting appropriate relative efficiency factor.The numerical simulations with differents initial orbits and target orbits as well as results comparison with former works verify the effectiveness of the method proposed in this paper.2.Due to the non-sphere perturbation of the Moon,the lifetime of ultra low-altitude Lunar spacecraft may be quite short.An orbit maintenance strategy is proposed in this paper to extend the lifetime of the spacecraft.Not only for avoiding the collision also to keep the spacecraft in an appropriate range,electric propulsion system is employed aboard.In this paper,the influnce of different initial conditions on the lifetime extension is discussed including initial inclination,right ascension of the ascending node,magnitude of thrust and proportion of the fuel mass.According to the results of numerical simulations,the startegy is effective.With appropriate magnitude of the thrust,the lifetime of the spacecraft in low-inclination can be extended from several days to over a hundred days with 10% fuel.With the same fuel proportion,the ones in middle and high inclination orbits can be extended more than two hundred days or even more than a year.3.Gravity assist is widely applied in the deep space exploration because of its reliability and practicability.There are lots of researches in the literature about the nearly coplanar situations.In this work,a three-dimensional model of gravity assist model is developed in a semi-analytical manner on the basis of the geometry relationship between the parameters of spacecraft before gravity assist and the orbital elements after gravity assist.The parameters are the hyperbolic excess velocity vector of the spacecraft before fly-by Vin?,the height of fly-by H,the dihedral angle between approach plane and fly-by plane ? and so on.These equations can be used for analyzing the change of orbital elements in the process of gravity assist,discussing the influence of different parameters on them and deriving the condition that remains the semi-major axis unchanged.By these analyzing,the variation range of semimajor axis can be obtained.Curve fitting of the feasible region boundary of (?)i and contour plot of (?)i are utilized to analyze the pattern of (?)i changing with different parameters.This method can also be utilized to design deep space maneuver in the process of gravity assist to maximize the effect by ajusting the parameters of gravity assist.The problems researched in this paper have both theoretical significance and application value in the aerospace field.