Control of an orbiting flexible square platform in the presence of solar radiation

A mathematical model for the solar radiation forces and moments acting on a square plate (platform) in orbit is obtained by considering the plate mode shapes as combinations of free-free beam shape functions. The moment expressions for a plate of arbitrary reflectivity coefficient are obtained as a function of the solar incidence angle. It is seen that only the first three flexible modes of the plate generate a first order net moment about the center of mass, and that the solar radiation pressure does not influence the flexible modes of the plate for small amplitude vibrations. The solar radiation disturbance model is then included in the dynamic model of a square plate nominally oriented along the local vertical and having the major surface of the plate normal to the orbital plane. The roll angle of the plate is seen to increase steadily due to the solar radiation pressure whereas the pitch and yaw motions oscillate with an amplitude of approximately 0.2° for a 100 m square thin aluminum plate in synchronous orbit. To control the shape and orientation of the plate two point actuators are assumed—one whose force axis is normal to the plane of the plate, the second with a force axis in the plane of the plate. The control law and the feedback gain values are obtained based on linear quadratic Gaussian methods. Transient responses and control requirements are simulated for local vertical and horizontal orientations.     

Single-Axis Solar Orientation of a Satellite of the Earth

The possibility of using the mode of single-axis solar orientation is considered for a satellite placed into a nearly circular orbit with an altitude of 900 km and bearing a solar sail. The satellite (together with the sail) has an axisymmetric structure, its symmetry axis being the principal central axis of the maximum moment of inertia. The center of the sail pressure lies on this axis and is displaced with respect to the satellite's center of mass. The symmetry axis of the satellite is set to the Sun so that its center of mass would be located between the Sun and the pressure center and would rotate around this axis with an angular velocity of a few degrees per second. The satellite's axis of symmetry makes a slow precession under the action of the gravitational moment and the moment of light pressure forces. Though the maximum magnitudes of these moments are comparable, the moment of the light pressure forces dominates and controls the precession in such a way that the symmetry axis orientation to the Sun remains unchanged.     

使用太阳模拟器进行热平衡试验的附加外热流影响分析

航天器的结构复杂化与表面热光学性能差异,使得航天器热平衡试验中对大型太阳模拟器的需求越来越大。文章根据离轴式太阳模拟器的结构,分析了使用太阳模拟器进行热平衡试验时,真空容器中附加外热流的来源及其对试验的影响,并通过建立热控星模型和容器与热控星的联合模型进行仿真计算,给出温度分布结果,进而提出相应的试验设计改进建议。     

A more accurate modeling of the effects of actuators in large space structures

The present paper deals with finite actuators. A nonspinning three-axis stabilized space vehicle having a two-dimensional large structure and a rigid body at the center is chosen for analysis. The torquers acting on the vehicle are modeled as antisymmetric forces distributed in a small but finite area. In the limit they represent point torquers which also are treated as a special case of surface distribution of dipoles. Ordinary and partial differential equations governing the forced vibrations of the vehicle are derived by using the Hamilton's principle. Associated modal inputs are obtained for both the distributed moments and the distributed forces. We show that the finite torquers excite the higher modes less than the point torquers. Modal Cost Analysis proves to be a suitable methodology to this end.     

The influence of effects of the heliospheric interface on parameters of backscattered solar <Emphasis Type="Italic">L</Emphasis>α radiation measured at the Earth’s orbit

A new kinetic model of distribution of interstellar hydrogen atoms in the heliosphere is suggested in this paper. It takes into account global effects associated with charge exchange of interstellar atoms near the heliospheric boundary. The constructed model allows one to find efficiently the detailed distribution of hydrogen atoms over space and velocities. For the axisymmetric steady-state case a comparison is made of the parameters of interstellar hydrogen atoms that were obtained using the classical hot model, two modifications of the improved hot model, and a global self-consistent kinetic gas-dynamic model of the heliospheric interface. The results of calculations of the spectral moments of scattered solar Lα radiation are presented. They were derived on the basis of different models of distribution of hydrogen atoms in the heliosphere.     

One method of gravitational attitude control of a rotating satellite

The mode of spinning up a low-orbit satellite in the plane of its orbit is studied. In this mode, the satellite rotates around its longitudinal axis (principal central axis of the minimum moment of inertia), which executes small oscillations with respect to the normal to the orbit plane; the angular velocity of the rotation around the longitudinal axis is several tenths of a degree per second. Gravitational and restoring aerodynamic moments were taken into account in the equations of satellite’s motion, as well as a dissipative moment from eddy currents induced in the shell of the satellite by the Earth’s magnetic field. A small parameter characterizing deviation of the satellite from a dynamically symmetric shape and nongravitational external moments are introduced into the equations. A two-dimensional integral surface of the equations of motion, describing quasistationary rotations of the satellite close to cylindrical precession of the corresponding symmetrical satellite in a gravitational field, has been studied by the method of small parameter and numerically. We propose to consider such quasistationary rotations as unperturbed motions of the satellite in the spin-up mode.     

Moments applied in the manual assembly of space structures: EASE biomechanics results from STS-61B

Measurements of the level and pattern of moments applied in the manual assembly of a space structure were made in extravehicular activity (EVA) and neutral buoyancy simulation (NBS). The Experimental Assembly of Structures in EVA program included the repeated assembly of a 3.6 m tetrahedral truss structure in EVA on STS-61B after extensive neutral buoyancy crew training. The flight and training structures were of equivalent mass and geometry to allow a direct correlation between EVA and NBS performance. A stereo photographic motion camera system was used to reconstruct in three dimensions rotational movements of structural beams during assembly. Moments applied in these manual handling tasks were calculated on the basis of the reconstructed movements taking into account effects of inertia, drag and virtual mass. Applied moments of 2.0 Nm were typical for beam rotations in EVA. Corresponding applied moments in NBS were typically up to five times greater. Moments were applied as impulses separated by several seconds of coasting in both EVA and NBS. Decelerating impulses were only infrequently observed in NBS.     

辐射换热条件下空间薄壁圆管结构瞬态温度场、热变形有限元分析

对航天结构中常见的辐射换热条件下的空间薄壁圆管结构，构造了一种用于非线性瞬态温度计算的傅立叶－有限单元。圆管温度沿杆长用有限元离散，沿周向分布展成三角函数。圆管温度单元每个结点包含平均温度、余弦和正弦分布温度幅三个自由度，并且在每个时间步内实现了平均温度增量与沿截面温差增量的解耦。在结构热变形分析时圆管单元节点上既承受温度轴力，又承受截面温差导致的热弯矩。采用这种单元对非线性换热条件下的复杂空间结构进行离散可以较准确地反映结构的温度场与热变形。经与前人的解析解和三维有限元结果的验证，证明该单元是可靠的。对太阳能帆板梁与空间抛物面天线的热－结构分析说明这种新单元的应用价值。     

Modeling of the aerodynamic moment acting upon a satellite

We consider the issues of modeling the moments of aerodynamic forces acting upon a satellite with gravitational system of stabilization. It is assumed that satellite orbits are almost circular with heights in the range 550–750 km. Simplified analytical expressions are suggested for the aerodynamic moment in the case when a satellite moves in the regime of gravitational orientation. Accuracy of the obtained expressions is estimated to be compared with that of expressions derived under the assumption of constant coefficient of frontal resistance. An analysis is made of short-periodic variations of the atmosphere density occurring due to orbital motion of a satellite. It is demonstrated that these variations can result in a substantial change of the aerodynamic moment, and their approximation by a truncated Fourier series is suggested. Estimates of the accuracy of the suggested approximation are given.     

Concave corner flow problem

Many satellites, especially those equipped with solar panels, are of concave shape. The calculation of the aerodynamic forces on concave bodies exposed to a free molecular flow is complicated due to shielding effects of one surface by another and due to the multiple molecular collisions between these surfaces. In this paper the flow into concave corners is treated. It is shown that shielding has an influence on the surface forces, only for certain corner angles and flow incidence angles.     

基于解耦和动态补偿的卫星控制系统设计

针对带大型挠性单翼太阳帆板和偏置动量的三轴稳定卫星,研究如何消除星内外干扰对姿态控制精度影响的卫星控制系统的设计问题。首先对星内外干扰进行了数值分析,进而阐明了在经典控制方案中干扰力矩对姿态控制精度的影响,指出经典控制在克服干扰力矩对姿态控制精度影响方面存在的不足之处。然后根据卫星的动力学特点,提出并论证了在设定角动量交换系统标称值条件下基于解耦和动态补偿的卫星控制系统的设计方案。最后深入分析了在该方案下太阳帆板挠性模态的稳定性。     

Asymptotic theory for thrusting,spinning-up spacecraft maneuvers

We consider the problem of a spacecraft subjected to constant body-fixed forces and moments about all three axes during a spinning-up, thrusting maneuver. In applications, undesired forces and moments can arise due to thruster imbalances and misalignments and to center-of-mass offset. In previous works, approximate analytical solutions have been found for the attitude motion, and for the change in inertial velocity and inertial position. In this paper we find asymptotic and limiting-case expressions which we derive from the analytic solutions, in order to obtain simplified, practical formulas that lend insight into the motion. Specifically, we investigate how the motion evolves (1) as time grows without bound and (2) for geometric cases of the sphere, the thin rod, and the thin plate. Closed-forms or upper-bound limits are provided for angular velocities, Eulerian angles, angular momentum pointing error, transverse and axial velocities, and transverse and axial displacements. Summaries for the asymptotic limits (for zero initial conditions) are provided in tabular form. Results are verified by numerical simulations.     

Attractive manifold-based adaptive solar attitude control of satellites in elliptic orbits

The paper presents a novel noncertainty-equivalent adaptive (NCEA) control system for the pitch attitude control of satellites in elliptic orbits using solar radiation pressure (SRP). The satellite is equipped with two identical solar flaps to produce control moments. The adaptive law is based on the attractive manifold design using filtered signals for synthesis, which is a modification of the immersion and invariance (I&I) method. The control system has a modular controller–estimator structure and has separate tunable gains. A special feature of this NCEA law is that the trajectories of the satellite converge to a manifold in an extended state space, and the adaptive law recovers the performance of a deterministic controller. This recovery of performance cannot be obtained with certainty-equivalent adaptive (CEA) laws. Simulation results are presented which show that the NCEA law accomplishes precise attitude control of the satellite in an elliptic orbit, despite large parameter uncertainties.     

MIC - a self deploying magnetically inflated cable system for large scale space structures

A new approach, termed MIC (Magnetically Inflated Cable) that enables large, lightweight very strong and rigid space structures is described. MIC would be launched as a compact package of coiled superconducting (SC) cables. After reaching orbit, the cables would be cryogenically cooled and electrically energized by a small power source. The resultant repulsion magnetic forces between the DC currents in the SC cables automatically cause the coiled launch package to self deploy into the final large space structure. The SC cables are held in place by a distributed network of high tensile strength tethers (e.g., Spectra material), creating a very stiff, rigid truss structure that strongly resists bending and torsional, etc. movements, without the need for gravity gradient stabilization. A linear quadrupole (LQ) MIC configuration is described that is suitable for large solar power satellites, space stations, space hotels, propellant tanks, manned Mars spacecraft, etc. The LQ has 2 long SC dipole loops, of horizontal width W, length L, and opposite magnetic polarity, which are vertically separated by distance W, producing a long truss structure of square cross-section (width W) with the 4 SC cables at the corners of the square. The SC currents are opposite in adjacent cables, yielding an outwardly directed net radial force on each cable. The ends of each SC loop experience outwards longitudinal forces. The magnetic forces are very strong, even for modest supercurrents. For example, a 4 meter square truss with I = 250 kiloamp has an outwards radial force of 220 kg per meter of cable. and 5250 kg outwards longitudinal force at the ends of each SC loop. The network of restraining tensile lines can support lightweight structures, including solar panels, propellant tankage, habitat modules, power transmission lines, etc. The design of a 1 kilometer long, 4 meter square cross section MIC truss for solar power satellites is described. The MIC launch package fits within the length/weight constraints of the shuttle bay, and includes all of the helium coolant lines, thermal insulation, and refrigeration equipment required.     

High-energy gamma radiation of solar flares as an indicator of acceleration of energetic protons

Using the SONG detector onboard the CORONAS-F satellite, gamma-ray emission of high energies (>100 MeV) was recorded during four solar flares. In the sequential spectra of gamma rays the peculiarity caused by generation and decay of neutral pions was isolated, which made it possible to determine with a high accuracy the moments of appearance in the solar atmosphere of protons accelerated up to energies above 300 MeV.     

Electrostatically inflated gossamer space structure voltage requirements due to orbital perturbations

This paper explores the concept of using electrostatic forces for deployment of gossamer space structures. The Electrostatically Inflated Membrane Structure (EIMS) uses two conducting membranes that are interconnected through membrane ribs. An absolute electrostatic charge is applied to the structure through active charge emission. This causes repulsion between layers of lightweight membranes that inflates the EIMS system and tensions the membranes. Assuming positive tensions, the EIMS system is modeled as a rigid system. Typical orbital perturbations are considered such as solar radiation pressure, differential gravity, and atmospheric drag which may compress the structure leading to shape destabilization. Restricting the analysis in this paper to flat membranes, the minimum potentials required to exactly compensate for the worst case scenario of differential solar radiation pressure at geostationary altitudes are estimated to be on the order of hundreds of volts. In low Earth orbit, voltage magnitudes of several kilovolts are required to reach an inflation pressure to offset the normal compressive drag pressure.     

Long-term variations of fluxes of solar protons and helium isotopes

The fluxes of hydrogen and helium isotopes in the solar wind are reconstructed over a long time scale since the present time up to 600 million years back. Abundances of helium isotopes, obtained in the helium isotopic analysis made for 8 lunar soil samples, were used as initial data in the reconstruction procedure. Samples were taken off from various levels of the 1.6-m core of lunar soil delivered by the automatic Luna-24 station in 1976. The data on modern hydrogen and helium fluxes were used as well. The developed reconstruction procedure allowed one to select various solar wind components in a ??gross?? composition. Proton flux variations over the interval of 600 million years do not exceed a value of 40 %. Helium flux variations reach a value of 1.5?C2 relative to the average value. Most likely, this circumstance is caused by considerable variations of a number of coronal mass ejections (CME) enriched by helium. The arguments in favor of solar activity polycyclicity on a long time scale are discussed.     

On the possibility of existence of an additional source of anomalous cosmic rays outside the solar system

On the basis of experimental data obtained at exposure of solid-state track detectors in the magnetosphere of the Earth during solar flares and in quiet Sun periods, an estimate of possible contribution of singly charged oxygen ions to the flare particle fluxes is made. A possibility is considered of the appearance in the vicinity of the solar system of singly ionized oxygen ions generated on stars.