Coupling among Pitch Motion, Axial Vibration, and Orbital Motion of Large Space Structures

The parameter dependency of large space structures on the coupling among attitude motion, structural vibration, and orbital motion has been investigated, particularly for structures such as tethered satellite systems. Tethered satellites are noticed for their possible applications. From space solar power systems to deorbit system, the applications of tethered satellites spread out over various fields. In this paper, we investigate planar motion of a satellite model, which consists of two tip particles and a massless spring. For such a space structure, the characteristics of the planar motion are shown to be determined by the mass ratio of tip particles, the natural frequency ratio of axial vibration to orbital motion, and the axial length ratio of the spring to the orbital radius. Among these three parameters, the natural frequency ratio has the greatest influence on the coupling phenomenon. Furthermore, the parameter range over which the coupling phenomenon occurs is specified.     

Space Structures: Issues in Dynamics and Control

A selective technical overview is presented on the vibration and control of large space structures, the analysis, design, and construction of which will require major technical contributions from the civil∕structural, mechanical, and extended engineering communities. The immediacy of the U.S. space station makes the particular emphasis placed on large space structures and their control appropriate. The space station is but one part of the space program, and includes the lunar base, which the space station is to service. This paper attempts to summarize some of the key technical issues and hence provide a starting point for further involvement. The first half of this paper provides an introduction and overview of large space structures and their dynamics; the latter half discusses structural control, including control‐system design and nonlinearities. A crucial aspect of the large space structures problem is that dynamics and control must be considered simultaneously; the problems cannot be addressed individually and coupled as an afterthought.     

New Approach to Mission Design Based on the Fundamental Equations of Motion

The complexity of space missions has been steadily increasing as new missions are expected to be designed to fly under more and more stringent constraints. This paper shows the use of a new, simple methodology that is useful for complex mission design. It deals with determining the “exact” control force needed to cause a spacecraft to move at a given inclination in a circular orbit around an oblate rotating body whose gravitational field is nonuniform. The methodology uses the explicit Udwadia–Kalaba equations of motions showing the simplicity, accuracy, and power of the approach, and the ease with which the control forces can be explicitly obtained.     

Structural and Orbital Conditions on Response of Large Space Structures

Effects of orbital and structural initial conditions, such as initial structural axial deformation, pitch (attitude) angle, and orbit altitude have been investigated on the subsequent orbital motion (time variation of orbit radius), attitude or pitch motion (time variation of pitch angle), and axial motion (time variation of axial length) of a large space structure in low earth orbit (LEO) and geosynchronous earth orbit (GEO). The space structure is assumed to be only axially flexible and executing a planar motion under the action of the earth's gravitational force. In LEO it is observed that the initial values of the axial deformation and pitch angle have appreciable effects on the structure's subsequent axial and attitude motions, whereas they have negligible effects on the structure's orbital motion. In GEO, the effects are found similar to those in LEO, except that the initial values of pitch angle have no effect on axial deformation. Furthermore, the investigation shows that the response of the space structure is greatly affected by the change in the orbit altitude. The study also indicates that flexibility of the structure has significant influence on its pitch librational motion.     

Continuum Models of Space Station Structures

The equivalent continuum properties of a structure composed of repeated patterns of discrete elements with both displacement and rotation coordinates are determined. These nodal coordinates are transformed to rigid body and strain gradient variables using a polynomial representation. The set of independent strain gradient variables is identified by inspection and depends on the geometry of the structure being modeled. The procedure is applied to six example problems, including two in which the effect of structural damage is analyzed.     

Graph‐Theory Approach to Eigenvalue Problem of Large Space Structures

The dynamic analysis and control system design of large space structures involve the solution of the large‐dimensional generalized matrix eigenvalue problem. The computational effort involved is proportional to the third power of the dimension of the matrices involved. To minimize the computational time a graph‐theory approach to reduce a matrix to lower‐ordered submatrices is proposed. The matrix‐reduction algorithm uses the Boolean matrices corresponding to the original numerical matrices and, thus, the computational effort to reduce the original matrix is nominal. The computational savings directly depend upon the number of submatrices into which the original matrix is reduced. A free‐free square plate is considered as an example to illustrate the technique. In this example a matrix of 16th order is reduced to three scalars corresponding to three rigid‐body modes, and three matrices of order three and one matrix of order four.     

Thermal Effects on Very Large Space Structures

Radiation thermal effects are studied simultaneously on the orbital motion, attitude motion, and axial deformation of a very large, axially flexible space structure describing a planar pitch motion around the earth. The relative importance of the three known sources of radiation in space environment—direct solar, Earth's albedo, and direct Earth—are studied for low Earth orbit (LEO) and geosynchronous Earth orbit (GEO). Influences of the area‐to‐mass ratio of the structure on thermal effects have been investigated for elliptical orbit. Radiation thermal effects are found to be significant in causing structural deformation and in producing libration in the attitude angle of a large space structure. However, on the orbital parameters of the space structure, the thermal effects are negligible. In LEO, effects of the earth's albedo and its direct radiation on the structure are appreciable, whereas in higher altitude orbits such as GEO, these may be neglected. The area‐to‐mass ratio of the structure is realized to cause a drastic change in the thermal effects on a large space structure.     

Statistical Sensitivity Analysis of Lattice Space Structures

This paper presents a statistical sensitivity analysis for shape distortions of space structures. The approach is based on a statistical shape‐distortion analysis on the structural errors and an adjoint method of sensitivity analysis. The statistical shape‐distortion analysis allows the stochastic errors to be represented by member‐length tolerances. The sensitivity analysis is performed to predict the effects of member‐length errors for lattice space antennas on the surface accuracy. The formulas presented in this paper give an effective approach to predict the effects of member‐length errors on the shape distortions, to obtain effective structural elements to correct the shape distortions, and to design tolerance errors of the structural elements. Numerical examples for statically determinate and indeterminate two‐dimensional truss beams have been demonstrated to identify the members contributing most to the errors. These results show that the errors of the longitudinal elements of the structure are important for designing accurate truss structures. Moreover, the validity and effectiveness of the present approach have been investigated.     

Integrated Genetic Algorithm for Optimization of Space Structures

Gradient‐based mathematical‐optimization algorithms usually seek a solution in the neighborhood of the starting point. If more than one local optimum exists, the solution will depend on the choice of the starting point, and the global optimum cannot be found. This paper presents the optimization of space structures by integrating a genetic algorithm with the penalty‐function method. Genetic algorithms are inspired by the basic mechanism of natural evolution, and are efficient for global‐searches. The technique employs the Darwinian survival‐of‐the‐fittest theory to yield the best or better characters among the old population, and performs a random information exchange to create superior offspring. Different types of crossover operations are used in this paper, and their relative merit is investigated. The integrated genetic algorithm has been implemented in C language and is applied to optimization of three space truss structures. In each case, an optimum solution was obtained after a limited number of iterations.     

Hypervelocity Impact Penetration Phenomena in Aluminum Space Structures

All long‐duration spacecraft are susceptible to high‐speed impacts by meteoroids and pieces of orbiting space debris. Damage to critical spacecraft systems caused by such impacts can lead to spacecraft failure and loss of life. In order to develop adequate protection against penetration for crew compartments and other critical spacecraft systems, an aerospace design engineer must possess a full understanding of the penetration mechanics involved in the hypervelocity impact loading of a variety of structural components. This paper describes the results of an experimental investigation of the penetration phenomena associated with oblique hypervelocity projectile impact of aluminum dual‐wall structures. Equations that quantitatively describe these phenomena are obtained through a regression of hypervelocity impact test data. These equations characterize observed penetration phenomena as functions of the geometric and material properties of the impacted structure and the diameter, obliquity, and velocity of the impacting projectile. A review of the test data shows that oblique hypervelocity impact penetration phenomena are strongly dependent on impact obliquity and therefore can differ significantly from those associated with normal high‐speed impacts. It is concluded that the possibility of non‐normal impacts and their effects on structural integrity must be considered in the design of any structure that is to be exposed to the hazardous meteoroid and space debris environment.     

Investigation of Geometric Imperfection in Inflatable Aerospace Structures

Communication and solar array inflated structures must be deployed to a very precise geometric configuration in order to meet quality requirements of their application. The focus of this paper is on geometric imperfections associated with inflated structures. To further understand some of the elements, that derive imperfection in a parabolic inflated communication and solar array structures, a computational model is proposed. This computational approach is dictated by the geometric complexity, deformation sensitivity as function of load and boundary conditions, and nonlinear characteristics of inflated structure assemblies. The deformation of a single component depends on the flexibility/stiffness of other components due to their interaction. In order to simulate such deformations of the multicomponent inflated structure, in the present study, the computational model consists of main parabolic shape envelope (reflector and canopy), torus, and catenary’s support and uses geometric nonlinear finite element. Further, tuning of communications and solar arrays is a primary concern in the operation of these systems. To investigate the effects of pressure tuning on geometric imperfection of a parabolic inflated antenna, in this investigation, analyses using uniformly axisymmetric and asymmetric applied load are performed. The analyses assume an initial parabolic shape envelope with a perfect circular edge for the reflector and canopy. Error estimates, which quantify geometric imperfections, are computed. The results show that as the axisymmetric load increases, the surface deviation from the parabolic shape of the envelope also increases. An asymmetric load on the surface of the torus leads to variable tensile forces in the catenary along the circular edge of the envelope, which in turn cause a visible local asymmetrical deformation in the vicinity of the circular edge of the envelope. In general, an asymmetric load causes greater geometric imperfections and should be avoided     

Active Control of Structures in Nonlinear Response

The need to account for geometric and material nonlinearities in the active control of highly flexible, large, space structures is emphasized herein. The performance index of the control problem is minimized subject to equations of state and costate using a variable metric algorithm. Unlike the conventional techniques for active control, the present algorithm is able to exploit the sparsity and symmetry of the mass and stiffness matrices of the finite element models of structures. The algorithm thus has the potential of being able to control moderately large‐scale, finite element models of highly flexible, large, space structures in a cost‐effective manner. The proposed algorithm is validated in suppressing the nonlinear vibrations of an impulsively loaded, highly flexible beam, and the need for inclusion of nonlinearities is demonstrated.     

Welded Joints for Robotic, On‐Orbit Assembly of Space Structures

A preliminary design concept for a weldable joint for on‐orbit assembly of large space structures is described. The joint was designed for ease of assembly, for structural efficiency, and to allow passage of fluid (for active cooling or other purposes) along the member through the joint. The members were assumed to consist of graphite/epoxy tubes to which were bonded 2219‐T87 aluminum alloy end fittings for welding on‐orbit to nodes of the same alloy. A modified form of gas tungsten arc welding was assumed to be the welding process. The joint was designed for the thermal and structural loading associated with a 37 m diameter tetrahedral truss intended as an aerobrake for a mission to Mars. It was concluded that the assembly process could lock large loads into the truss members and that the assembly robot could be required to exert large forces while aligning pairs of nodes during assembly. It was also concluded that the connections between the composite struts and the aluminum fittings will be subjected to very high service stresses due to the effects of differential thermal expansion.     

Concept of Adaptability in Space Modules

The space program is aiming towards the permanent use of space; to build and establish an orbital space station, a Moon base and depart to Mars and beyond. We must look after the total independency from the Earth's natural resources and work in the design of a modular space base in which each module is capable of duplicating one natural process, and that all these modules in combination take us to conceive a space base capable of sustaining life. Every area of human knowledge must be involved. This modular concept will let us see other space goals as extentions of the primary project. The basic technology has to be defined, then relatively minor adjustments will let us reach new objectives such as a first approach for a lunar base and for a Mars manned mission. This concept aims towards an open technology in which standards and recommendations will be created to assemble huge space bases and spaceships from specific modules that perform certain functions, that in combination will let us reach the status of permanent use and exploration of space.     

Robust Stationkeeping Control for Libration Point Quasi-Periodic Orbits

Libration orbit stationkeeping controls are designed based on selected reference quasi-periodic orbit trajectories. The baseline trajectory is designed to meet science requirements and in the same time achieve minimum fuel consumptions. The success of finding libration point reference orbits is based on accurate numerical computation, dynamics, and space environment modeling. The linear quadratic regulator controller has been developed widely for maintaining a spacecraft in such libration orbit reference trajectories as close as possible. However, any dynamics models, including the circular restricted three-body dynamics, space environment, sensor, and actuator, are only approximations of real physical systems. Any noise and uncertainties can cause spacecraft’ motion to diverge due to the high instability region around libration points. This study investigates the modeling and designing of a passive robust μ controller and an active adaptive linear quadratic regulator in libration point stationkeeping controls around L1. The adaptive law in the linear quadratic regulator is used to estimate unknown gains of spacecraft’ subsystems. The results are compared for a family of libration orbits with reasonable ΔV yearly budgets under the influence of perturbations, noise, and unmodeled dynamics. The comparison with a publicly accessible work indicates that the controller developed in this work can provide comparable annual cost by nearly even including the worst case of perturbations.     

Fuzzy Active Control of Flexible Structures by Using Electromagnetic Actuators

Numerical and experimental investigations are carried out to assess the possibility of controlling a flexible beam by using an electromagnetic actuator (EMA). The advantage of EMAs is that they do not require contact with the structure so they can be applied to light and small mechanisms. Nevertheless, their open-loop instability and nonlinear dynamic behavior relating to excitation frequency can limit their fields of application. The EMA is designed and dimensioned as a function of the structure to be controlled. The effect of the EMA is considered a restoring force; consequently, the structure is still linear, which enables the calculation of the modal matrices of the structure. Moreover, an inverse model of the EMA is proposed to implement a linear action block for the frequency range used. The gap distance is estimated by using a modal approximation of the displacements resulting from the measurements. The control strategy is a fuzzy controller with displacements and velocities as inputs. Fuzzy controllers are used for their effectiveness in the presence of nonlinearities and uncertainties. The system is modeled and the characteristics of the model are identified experimentally. Several control configurations are assessed by using numerical simulations, and then the controller is tested experimentally in the context of impact perturbations. The results show the effectiveness and robustness of the developed control.     

Technical Requirements for Lunar Structures

The moon has recently regained the interest of many of the world’s space agencies. Lunar missions are the first steps in expanding manned and unmanned exploration inside our solar system. The moon represents various options; it can be used as a laboratory in low gravity, it is the closest and most accessible planetary object from the Earth, and it possesses many resources that humans could potentially exploit. This paper has two objectives: to review the current status of the knowledge of lunar environmental requirements for future lunar structures, and to attempt to classify different future lunar structures based on the current knowledge of the subject. The paper divides lunar development into three phases. The first phase is building shelters for equipment only; in the second phase, small temporary habitats will be built, and finally in the third phase, habitable lunar bases will be built with observatories, laboratories, or production plants. Initially, the main aspects of the lunar environment that will cause concerns will be lunar dust and meteoroids, and later will include effects due to the vacuum environment, lunar gravity, radiation, a rapid change of temperature, and the length of the lunar day. This paper presents a classification of technical requirements based on the current knowledge of these factors, and their importance in each of the phases of construction. It gives recommendations for future research in relation to the development of conceptual plans for lunar structures, and for the evolution of a lunar construction code to direct these structural designs. Some examples are presented along with the current status of the bibliography of the subject.     

Cable Structures and Lunar Environment

Lunar environmental characteristics, such as the lack of atmosphere, the smaller gravitational acceleration, and the weaker regolith, place different requirements on structural systems than the earth environment does. Some of these requirements are the internal pressurization of structures, emphasis on details, and careful design of foundation systems. Popular structural systems on the Earth environment, such as steel and reinforced concrete frames and trusses with traditional rigid connections may be inefficient for the lunar environment. Cable structures can be shown to meet the different and sometimes conflicting requirements of the lunar environment. The behavior of three different groups of cable structures in the lunar environment (differentiated by their small, medium and long spans) are studied in this paper. The structural systems can be designed to meet the main requirements in an efficient way. Foundation uplift problem is of particular interest, especially in the early lunar colonization stage. It was shown that with a slight modification in the cable system, the uplift problem can be solved, thus saving manpower and costs, while improving the overall system behavior.     

Flutter Control of Smart Composite Structures in Hygrothermal Environment

Flutter control of smart composite plates under subsonic airflow is investigated in hygrothermal environment. The active fiber composite (AFC) which is more effective and adaptive than the conventional monolithic piezoelectric material is used in the present analysis to control the undesirable response due to hygrothermal effect. The velocity and displacement feedback control algorithm are subsequently established to reduce actively the response of the plate. Numerical examples of isotropic and laminated composite plates with or without hygrothermal effect are presented. It is observed that the structures become weak in the presence of hygrothermal load in the form of reduced flutter boundary. The flutter boundary can be enhanced with the help of AFC. The parametric study is performed and it is observed that the flutter boundary can be enhanced with the help of a feedback-control system activating the AFC. Therefore, one can say that the AFC is effective to enhance the flutter boundary of the present aeroelastic structure in the hygrothermal environment.