Research On Solar Sail Halo Orbit Design And Station-keeping

With the successful launch of Japan's IKAROS solar sail in 2010,the research on solar sail technology has set off a great upsurge all over the world.Solar sails,which use solar radiation pressure(SRP)to generate propulsion without consuming chemical propellant,have the advantages that traditional spacecraft cannot possess,and enable completely new mission applications that traditional spacecraft cannot achieve.For example,due to the special geometrical relationship relative to the primaries,solar sail halo orbits in the vicinity of artificial Lagrange points can be utilized to various missions such as solar wind monitoring,celestial observation,communication relay,etc.Since halo orbits are inherently unstable,station-keeping is required for spacecraft to avoid deviating from their reference orbits.Different from traditional spacecraft,station-keeping for solar sails can be achieved using SRP force,while without consuming any chemical propellant.However,station-keeping for solar sails is challenging due to the inherent characteristics of SRP.In this dissertation,station-keeping for solar sails are researched in the aspects of orbit design,orbit control methods and orbit control technologies.First,the equations of motion of the solar sail circular restricted three-body problem(SSCR3BP)and solar sail elliptic restricted three-body problem(SSER3BP)in the Sun-Earth system are derived.According to the momentum exchange of photons,the non-dimensional SRP models for perfectly reflecting solar sails,non-perfectly reflecting solar sails and solar sails with reflectivity control devices(RCDs)are derived.An exponential model of optical degradation is derived based on the fact that the optical properties of solar sails degrade with time.The above models provide the model foundation for orbit design and station-keeping design.In previous studies,the reference orbits for station-keeping are mainly designed in the SSCR3BP,and thus the Earth's orbit eccentricity would lead to a large perturbation to the station-keeping.In order to improve the fidelity of orbit design,this dissertation presents a general strategy to design families of solar sail halo orbits in the SSER3BP.Since the Earth's orbit eccentricity is taken into account in the reference orbit so that the station-keeping does not need to counteract the disturbance due to the eccentricity.The seed orbits that satisfy the constraint for periodicity are designed using improved 3^rd order approximation method and differential correction in the SSCR3BP.Subsequently,Then,the families of halo orbits in the SSER3BP are generated using a variable step numerical continuation method with the eccentricity and the lightness number as continuation parameters.The linear stability of the orbits is analyzed and demonstrated by numerical simulation,which shows that the in-plane motion is unstable while the out-of-plane motion is neutrally stable,and the orbital stability increases with the amplitude of the orbits.Finally,Simulations are undertaken to compare and analyze the station-keeping performances.The results show that a reference orbit designed in the SSER3BP is significantly more efficient for station-keeping than a reference orbit designed in the SSCR3BP.The orbit control and attitude control of a solar sail are coupled with each other,and the model of motion is a typical nonlinear and coupling system.Therefore,traditional linear control methods could fail when applied to station-keeping of a solar sail,especially when considering large orbital errors or significant deviations from the nominal control variables.To solve this problem,this dissertation presents an indirect estimation based active disturbance rejection control(ADRC)for solar sail station-keeping.In contrast to previous applications of ADRC to spacecraft station-keeping,the proposed approach estimates the deviation of the orbit state from the reference orbit,instead of estimating the orbit state directly.Since the deviation of the orbit state varies more slowly than the state itself,the indirect estimation significantly improves the estimation accuracy.In addition,since the model of motion is non-affine,i.e.,the control variables appear nonlinearly in the dynamic equations,ADRC can only provide the required control acceleration but not the required control variables,such as the attitude angles.In previous studies,the required control variables are approximately obtained by linearizing the control acceleration.However,this dissertation uses a Newton iteration based method to solve for the exact required control variables.The simulation results show that the proposed indirect estimation based ADRC can significantly improve the station-keeping accuracy compared to ADRC that utilizes direct estimation of the states in previous studies and more adaptive to different errors compared to the traditional LQR method.In addition,conventional solar sails cannot control the magnitude of SRP force and hence the acceleration in the normal direction.RCD solar sails are able to control the magnitude of SRP force to some extent.However,the normal control acceleration provided and hence the station-keeping performance are highly limited.Therefore,this dissertation presents a hybrid propulsion scheme which combines a solar sail with a one-degree-of-freedom solar electric propulsion(SEP).The equations of motion of the solar-sail/SEP spacecraft are derived,and then the station-keeping control law is designed using the indirect estimation base ADRC method.The simulation results show that the normal control acceleration is extremely smaller than the tangential control acceleration for an RCD solar sail,while the solar-sail/SEP spacecraft can provide comparable control accelerations in both the normal direction and the tangential direction.In addition,the presented solar-sail/SEP system can significantly improve the convergence and robustness compared to the RCD solar sails.Finally,considering the optical degradation for an RCD solar sail,this dissertation presents a station-keeping method based on on-line reference orbit update,which combines orbit design and station-keeping in-situ.A simple estimation method,which is easy to implement,and an unscented Kalman filter(UKF)method,which are more accurate,are used to estimate the optical coefficients on-line,respectively.The reference orbit is updated using on-line numerical continuation when the optical properties have degraded by a prescribed amount.The simulation results show that this strategy provides discrete updates to the reference orbits such that the perturbation due to the optical degradation is maintained within a small range,and thus the presented strategy enables a long-term effective station-keeping even in the presence of a large optical degradation.