Low-thrust trajectory optimization in a full ephemeris model

The low-thrust trajectory optimization with complicated constraints must be considered in practical engineering. In most literature, this problem is simplified into a two-body model in which the spacecraft is subject to the gravitational force at the center of mass and the spacecraft＇s own electric propulsion only, and the gravity assist （GA） is modeled as an instantaneous velocity increment. This paper presents a method to solve the fuel-optimal problem of low-thrust trajectory with complicated constraints in a full ephemeris model, which is closer to practical engineering conditions. First, it introduces various perturbations, including a third body＇s gravity, the nonspherical perturbation and the solar radiation pressure in a dynamic equation. Second, it builds two types of equivalent inner constraints to describe the GA. At the same time, the present paper applies a series of techniques, such as a homotopic approach, to enhance the possibility of convergence of the global optimal solution.

Low-thrust trajectory optimization for multiple target bodies tour mission

Spacecraft science missions to planets or asteroids have historically visited only one or several celestial bodies per mission.The research goal of this paper is to create a trajectory design algorithm that generates trajectory allowing a spacecraft to visit a significant number of asteroids during a single mission.For the problem of global trajectory optimization,even with recent advances in low-thrust trajectory optimization,a full enumeration of this problem is not possible.This work presents an algorithm to traverse the searching space in a practical fashion and generate solutions.The flight sequence is determined in ballistic scenario,and a differential evolution method is used with constructing a three-impulse transfer problem,then the local optimization is implemented with low-thrust propulsion on the basis of the solutions of impulsive trajectories.The proposed method enables trajectory design for multiple asteroids tour by using available low thrust propulsion technology within fuel and time duration constraints.

Optimization of multi-revolution low-thrust transfer based on modified direct method

A modified direct optimization method is proposed to solve the optimal multi-revolution transfer with low-thrust between Earth-orbits. First, through parameterizing the control steering angles by costate variables, the search space of free parameters has been decreased. Then, in order to obtain the global optimal solution effectively and robustly, the simulated annealing and penalty function strategies were used to handle the constraints, and a GA/SQP hybrid optimization algorithm was utilized to solve the parameter optimization problem, in which, a feasible suboptimal solution obtained by GA was submitted as an initial parameter set to SQP for refinement. Comparing to the classical direct method, this novel method has fewer free parameters, needs not initial guesses, and has higher computation precision. An optimal-fuel transfer problem from LEO to GEO was taken as an example to validate the proposed approach. The results of simulation indicate that our approach is available to solve the problem of optimal muhi-revolution transfer between Earth-orbits.

Artificial Equilibrium Points in the Low-Thrust Restricted Three-Body Problem When Both the Primaries Are Oblate Spheroids

This paper studies the existence and stability of the artificial equilibrium points (AEPs) in the low-thrust restricted three-body problem when both the primaries are oblate spheroids. The artificial equilibrium points (AEPs) are generated by canceling the gravitational and centrifugal forces with continuous low-thrust at a non-equilibrium point. Some graphical investigations are shown for the effects of the relative parameters which characterized the locations of the AEPs. Also, the numerical values of AEPs have been calculated. The positions of these AEPs will depend not only also on magnitude and directions of low-thrust acceleration. The linear stability of the AEPs has been investigated. We have determined the stability regions in the xy, xz and yz-planes and studied the effect of oblateness parameters A1(0A1?and ?A2(0A2<1) on the motion of the spacecraft. We have found that the stability regions reduce around both the primaries for the increasing values of oblateness of the primaries. Finally, we have plotted the zero velocity curves to determine the possible regions of motion of the spacecraft.

小推力航天器逃离太阳系的引力辅助轨迹设计

针对太阳系边界探测,利用形函数轨迹优化算法快速生成连续小推力航天器引力辅助逃离太阳系的三维轨迹,实现在有限燃料条件下的超远距离探测。通过对比有无引力辅助设计的仿真结果,证明了引力辅助技术在逃离太阳系过程中的有效性。仿真结果表明,基于贝塞尔形函数方法的连续小推力轨迹优化算法可以高效地设计出引力辅助逃离太阳系的合理三维初始轨迹,对于初始探测任务的快速设计和分析具有重要意义。

基于调相机制的航天器碰撞规避策略设计

针对携带电推进等小推力推进系统工作的航天器,提出一种利用连续推力调相机制的在轨碰撞规避机动策略。首先,采用混沌多项式展开方法对初始状态不确定性的航天器碰撞概率进行分析,并以此作为碰撞规避策略的评价指标;进而推导了一种适用于小推力的轨道机动的解析解。利用这一解析结果可以提出一种新的避碰方法,即通过航天器轨道相位的微调减小航天器碰撞概率,并且在机动后能够迅速恢复原轨道相位。数值仿真表明:基于所提出的避碰机动策略,可以在较短时间内减小碰撞概率。数值仿真还揭示了避碰时间、燃耗和推力加速度之间的关系。

Multi-revolution low-thrust trajectory optimization using symplectic methods

Optimization of low-thrust trajectories that involve a larger number of orbit revolutions is considered as a challenging problem.This paper describes a high-precision symplectic method and optimization techniques to solve the minimum-energy low-thrust multi-revolution orbit transfer problem. First, the optimal orbit transfer problem is posed as a constrained nonlinear optimal control problem. Then, the constrained nonlinear optimal control problem is converted into an equivalent linear quadratic form near a reference solution. The reference solution is updated iteratively by solving a sequence of linear-quadratic optimal control sub-problems, until convergence. Each sub-problem is solved via a symplectic method in discrete form. To facilitate the convergence of the algorithm, the spacecraft dynamics are expressed via modified equinoctial elements. Interpolating the non-singular equinoctial orbital elements and the spacecraft mass between the initial point and end point is proven beneficial to accelerate the convergence process. Numerical examples reveal that the proposed method displays high accuracy and efficiency.

High-precision shape approximation low-thrust trajectory optimization method satisfying bi-objective index

The shape approximation method has been proven to be rapid and practicable in resolving low-thrust trajectory;however,it still faces the challenges of large deviation from the optimal solution and inability to satisfy the specific flight time and fuel mass constraints.In this paper,a modified shape approximation low-thrust model is presented,and a novel constrained optimization algorithm is developed to solve this problem.The proposed method aims at settling the bi-objective optimization orbit involving the twin objectives of minimum flight time and low fuel consumption and enhancing the accuracy of optimized orbit.In particular,a transformed high-order polynomial model based on finite Fourier series is proposed,which can be characterized as a multi-constraint optimization problem.Then,a novel optimization algorithm is specifically developed to optimize the large-scale multi-constraint dynamical equations of shape trajectory.The key performance indicators of the index include minimum flight time,low fuel consumption and bi-objective optimization of the two.Simulation results prove that this approach possesses both the high precision achievable by numerical methods and low computational complexity offered by shape approximation techniques.Besides,the Pareto front of the fuel-time bi-objective optimization orbit is firstly introduced to analyze an intact optimal solution set.Furthermore,we have demonstrated that our proposed approach is appropriate to generate the preliminary orbit for pseudo-spectral method.

基于EKF/PI滤波的小推力轨控期间自主导航算法

研究了以电推小推力作为主要的轨控推力来源的高轨卫星在轨控期间的自主导航算法。针对GNSS接收机在高轨的测速精度相对较差,以及轨控期间电推产生的推力较小,常规加速度计无法准确测量,从而导致在轨控期间自主导航精度下降的问题,提出了基于EKF+PI滤波的一体化自主导航算法。通过结合EKF滤波和PI滤波算法,经EKF滤波抑制GNSS接收机定位数据的测量白噪声,然后通过PI滤波估计电推加速度并反馈给EKF滤波算法,提高电推工作期间EKF算法的导航精度。仿真结果表明,基于EKF+PI滤波的一体化自主导航算法可以准确估计出电推加速度大小,同时可有效提高连续小推力轨控期间的自主导航精度。

相继增压系统切换过程对增压器辅推轴承轴向力影响与轴承优化研究

采用数值仿真方法研究基本增压器在系统切换过程中的轴向力变化规律,并提出了辅助推力轴承的改进设计方法。首先数值仿真研究了基本增压器的转子系统在切换时的压气机/涡轮关键典型截面气动参数分布情况,分析了在柴油机增压模式切换过程中基本增压器转子轴向力大小和方向变化规律,然后提出了增加轴承承力面及改变承力面材料的优化方案。结果表明:基本增压器辅助推力轴承在切换的5 s时间内轴向力发生翻转,最大约承受2273 N的反向轴向力,是造成切换过程辅助推力轴承磨损的关键原因;优化方案最大承载能力为优化前的4.8倍,显著提升了辅助推力轴承承载力。经增压器平台可靠性验证,改进后的增压器辅助推力轴承在相同时间内未出现碰磨现象。

Efficient optimization method for variable-specific-impulse low-thrust trajectories with shutdown constraint

This paper develops a sequential convex programming(SCP)-based method to solve the minimum-fuel variable-specific-impulse low-thrust transfer problem considering shutdown constraint,with emphasize on improving the computational efficiency.The variable parameter engine is more applicable for many low-thrust scenarios,therefore,both a continuously variable model and a ladder variable model are adopted.First,the original problem is convexified by processing the constraint feasible domain,which is composed of the nonlinear dynamic equations and second-order equality constraint,into convex sets.Then,the approximation is generated to close the optimal solution of the low-thrust problem by iteratively solving the convexified subproblem.Moreover,the switching self-detection and adaptive node refinement methods are presented,which can improve the accuracy of the solution and accelerate the convergence during the approximation process and is especially necessary and effective in the scenarios with shutdown constraint.In numerical simulations,the comparison with the homotopic approach shows that the proposed method only needs 4%computational time as that of the homotopic approach,and two variable-specificimpulse examples further demonstrate the effectiveness and efficiency of the proposed method.

Target selection for a small low-thrust mission to near-Earth asteroids

The preliminary mission design of spacecraft missions to asteroids often involves,in the early phases,the selection of candidate target asteroids.The final result of such an analysis is a list of asteroids,ranked with respect to the necessary propellant to be used,that the spacecraft could potentially reach.In this paper we investigate the sensitivity of the produced asteroids rank to the employed trajectory model in the specific case of a small low-thrust propelled spacecraft beginning its journey from the Sun–Earth L2 Lagrangian point and heading to a rendezvous with some near-Earth asteroid.We consider five increasingly complex trajectory models:impulsive,Lambert,nuclear electric propulsion,nuclear electric propulsion including the Earth’s gravity,solar electric propulsion including the Earth’s gravity and we study the final correlation between the obtained target rankings.We find that the use of a lowthrust trajectory model is of great importance for target selection,since the use of chemical propulsion surrogates leads to favouring less attractive options 19%of times,a percentage that drops to 8%already using a simple nuclear electric propulsion model that neglects the Earth’s gravity effects and thrust dependence on the solar distance.We also find that for the study case considered,a small interplanetary CubeSat named M-ARGO,the inclusion of the Earth’s gravity in the considered dynamics does not affect the target selection significantly.

Direct Optimization of Low-thrust Many-revolution Earth-orbit Transfers

Low-thrust Earth-orbit transfers with 10^- 5-order thrust-to-weight ratios involve a large number of orbital revolutions which poses a real challenge to trajectory optimization. This article develops a direct method to optimize minimum-time low-thrust many-revolution Earth-orbit transfers. A parameterized control law in each orbit, in the form of the true optimal control, is proposed, and the time history of the parameters governing the control law is interpolated through a finite number of nodal values. The orbital averaging method is used to significantly reduce the computational workload and the trajectory optimization is conducted based on the orbital averaging dynamics expressed by nonsingular equinoctial elements. Furthermore, Earth＇s shadowing and perturbation effects are taken into account. The optimal transfer problem is thus converted to the parameter optimization problem that can be solved by nonlinear programming. Taking advantage of the mapping between the parameterized control law and the Lyapunov control law, a technique is proposed to acquire good initial guesses for optimization variables, which results in enlarged convergence domain of the direct optimization method. Numerical examples of optimal Earth-orbit transfers are presented.

Overview of Earth-Moon Transfer Trajectory Modeling and Design

TheMoon is the only celestial body that human beings have visited.The design of the Earth-Moon transfer orbits is a critical issue in lunar exploration missions.In the 21st century,new lunar missions including the construction of the lunar space station,the permanent lunar base,and the Earth-Moon transportation network have been proposed,requiring low-cost,expansive launch windows and a fixed arrival epoch for any launch date within the launch window.The low-energy and low-thrust transfers are promising strategies to satisfy the demands.This review provides a detailed landscape of Earth-Moon transfer trajectory design processes,from the traditional patched conic to the state-of-the-art low-energy and low-thrust methods.Essential mechanisms of the various utilized dynamic models and the characteristics of the different design methods are discussed in hopes of helping readers grasp thebasic overviewof the current Earth-Moon transfer orbitdesignmethods anda deep academic background is unnecessary for the context understanding.

一种利用卫星星历计算机动加速度的方法

对于近地轨道卫星,由于受大气影响较大,轨道机动比较频繁,…轨道机动的快速检测以及推力的计算,对保障空间目标运行安全及故障分析意义重大。本文基于公开发布的卫星星历数据,提出了基于卫星星历识别轨道机动的方法,并通过轨道预报迭代反演和能量转化两种方式,实现了小推力条件下卫星轨道机动加速度的计算,两种计算方法计算结果有很好的一致性。应用该方法基于星历文件对多个星座目标进行机动识别与加速度解算,以解算结果对卫星轨道进行星历仿真,仿真结果与发布的星历基本一致。

组合动力飞行器模态转换段推阻匹配与控制律设计

针对组合动力飞行器在进行模态转换时,发动机推力的损失以及进气道进气时对飞行器周围气流的影响,导致飞行器本体动力学特性呈现出强烈的非线性、耦合性和模型不匹配性等问题。文中根据涡喷发动机和冲压发动机输出推力随流量变化的关系,提出一种流量分配方案补偿了发动机动力切换时的推力损失。考虑到组合动力飞行器模态转换段动力学的复杂性,采用L1鲁棒自适应控制实现飞行器的内环增稳,采用蒙特卡洛打靶方式验证控制律的鲁棒性。仿真实验证明,设计的控制律具有一定鲁棒性。

新型永磁辅助同步磁阻直线电机设计

同步磁阻直线电机是一种通过次级侧在不同位置引起磁阻变化、产生磁阻推力的交流电机,具有成本低、耗能小、推力易于控制等优点,但存在凸极比低、推力小等不足。针对传统直线电机推力小、推力波动大、成本高等问题,采用磁路法建立磁路模型、有限元分析等方法,设计了一种新型永磁同步直线电机,相对传统电机性能有了很大提升,尤其对推力精度要求高的场景,该新型永磁同步直线电机应用范围会更加广阔。

低压涡轮导向器开度对变循环发动机的影响

建立了变循环发动机整机模型并对可变几何低压涡轮特性进行修正,研究了低压涡轮导叶开度从-6°~6°时对各部件以及发动机整体性能的影响。结果表明:随低压涡轮导向器角度变大,低压涡轮进口折合流量增大,不论低压涡轮导向器开大或关小,高、低涡轮效率均下降;随导叶开度增大,高压涡轮膨胀比增大,高压轴功率增大,高压压气机(High pressure compressor,HPC)与核心机驱动风扇级(Core driven fan stage,CDFS)压比增大;双外涵模式下涡轮导叶角度为0°时单位推力最大,单外涵模式下涡轮角度为-1°时单位推力最大。

基于高斯伪光谱的星际小推力转移轨道快速优化

针对星际小推力转移轨道优化问题,给出了一种基于高斯伪光谱配点的快速优化算法。首先,基于归一化的改进春分点根数建立了星际小推力转移轨道的优化模型;然后,采用高斯伪光谱配点策略对优化模型进行离散化处理,推力方向限制和天体星历分别作为路径约束和事件约束,将轨道优化问题转化为一个大规模多约束参数优化问题;在此基础上,基于高斯伪光谱的配点特性,推导出性能指标和约束方程的解析雅可比矩阵,保证了雅可比矩阵计算的准确性和效率;最后,以利用太阳能电推进探测火星和水星为例,对所给算法进行了数值验证。数值结果表明:高斯伪光谱方法可有效用于星际小推力轨道的优化问题,并且与数值差分相比,解析的雅可比矩阵算法可提高计算效率67.78%。

快速绕飞卫星空间圆编队设计方法

连续小推力条件下,针对圆参考轨道卫星,推导了满足快速绕飞条件的空间圆编队动力学模型。分析了一个绕飞周期内的燃耗情况。在绕飞周期确定的条件下,给出了最小燃耗的计算方法,并利用数值方法验证了所推导公式的正确性。计算结果表明,在高度为500km的圆轨参考道卫星实现周期为10分钟、半径100m的空间圆绕飞,一个绕飞周期所需最小速度增量约为4.77m/s。文章提出的基本设计原理并不局限于空间圆编队,也可用于其它快速绕飞编队的设计工作。