Dynamics Of On-orbit Space Solar Power Station With Gravity Gradient Effects

With the development of economy,people have a growing demand for energy.As an inexhaustible clean energy source,solar energy has been widely concerned by human.Space solar power station,as a potential,continuous and efficient technology,has received much attention by many countries and research institutions.In order to ensure that space solar power station operates properly and efficiently on-orbit,the complex technical problems of space solar power station must be thoroughly investigated from multi-aspects.In this thesis,we will look at the dynamic behaviors of a large space solar power station working on-orbit.Unlike satellites with regular size,space solar power station has a super large structure with its size up to several kilometers or even tens of kilometers.This characteristic makes the effects of gravitational orbit-attitude coupling caused by gravity gradient on the orbital and attitude motions that can't be ignored any more.The magnitude of the coupling effects are governed by satellite's mass distribution,orientation and its size relative to the orbital radius,and would be amplified if satellite's size increases.In addition,another characteristic of space solar power station is ultra-low fundamental frequency of structural vibration,which may be on the order of 10-3 Hz or even lower.This characteristic makes the effects of dynamic stiffening and the coupling between gravity gradient and the structural deformation on the structural vibration that can't be ignored any more.The effects of dynamic stiffening on structural vibration are related to the ratio between the frequency of rotation angular velocity of structure and the fundamental frequency of structure.The stiffening effects would be amplified if the ratio increases,which could be significant for the structure of a space solar power station even in a very low rotation angular velocity.Due to ultra-low fundamental frequency of the structure,the coupling effects between gravity gradient and structural deformation on the structural vibration may be close to the effects of the elastic restoring force generated by the structural deformation on the structural vibration.So it is imperative to re-examine the dynamic characteristics of a space solar power station on-orbit by considering the above effects.Based on the above problems,the contents of this thesis can be divided into the following four parts:(1).By simplifying space solar power station into a particle,the orbital dynamic equation of a space solar power station working on-orbit is derived.The orbital dynamic characteristics of a space solar power station working in geosynchronous Laplace orbit are studied and compared with the orbital dynamic characteristics of a space solar power station working in geostationary orbit.Some advantages of geosynchronous Laplace orbit are proposed.(2).The effects of gravitational orbit-attitude coupling on the orbital and attitude motions of a space solar power station are studied.This part of work can be divided into two smaller parts.The first one is analyzing the coupling dynamic behaviors of the orbital and attitude motions through numerical simulations.The gravitational potential energy of space solar power station is expanded in a Taylor series in the small ratio(spacecraft size/orbital radius),which permits up to fourth order are retained.The gravitational orbit-attitude coupling dynamic equations contains various environmental disturbances and are with characteristics of high accurate.The second one is analyzing the gravitational orbit-attitude coupling mechanism of a space solar power station on-orbit.The coupling dynamic equations of the orbital and attitude motions of a space solar power station with arbitrary shape are firstly derived from Hamilton dynamics,and then the dynamic equations are analyzed through analytic methods.A resonance phenomenon of the orbital motion due to the coupling effects is discovered when the attitude motion satisfies some special conditions.The coupling effects can also result in secular orbital drift for space solar power station.The drift can be eliminated through selecting appropriate initial conditions for the orbital motion.A periodic interchange of energy between the orbital and attitude motions will occur when the mass distribution of space solar power station and the initial conditions of the attitude motion meet some conditions and there only exist conservation forces in the motions of space solar power station.In addition,a symplectic integration algorithm is used for computing the coupling dynamic characteristic of the orbital and attitude motions of SSPS in this thesis to ensure the correctness of the numerical results.(3).The effects of dynamic stiffening on the structural vibration of a space solar power station are investigated.The space solar power station dynamic equations of the rigid-flexible coupling system constituted of the attitude motion and the structural vibration are firstly derived by considering the effects of dynamic stiffening.Then a parametrical excitation model is obtained through discretization of the dynamic equations with the method of mode superposition.The dynamic characteristics of the rigid-flexible coupling system are analyzed through numerical simulations at last.Comparing to other existing models,the parametrical excitation model has a simple form and can describe the dynamic response of the coupling system more accurately.(4).The effects of gravity gradient on the structural vibration of a space solar power station on-orbit are investigated.The dynamic equation of the structural vibration is firstly derived by considering the effects,and then the dynamic equation is analyzed through analytic methods.Under the action of the effects,a static deformation of the structure will occur.When the fundamental frequency of the structure is less than(?)2 times the orbital angular velocity of space solar power station,the coupling effects between gravity gradient and structural deformation will cause buckling instability for the structure.The vibration of the structure will also be unstable when the orbital angular velocity,the fundamental frequency of the structure and the law of the attitude motion meet some certain conditions.In addition,it is also found that the dynamic characteristics of the structural vibration obtained from different modeling methods are quite different.And the differences are studied in this thesis.At last,the innovations and the conclusions of this thesis are summarized,and some future research suggestions are also given.