DYNAMIC CHARACTERISTICS OF INFLATED SPACE STRUCTURES

Abstract

Development in space technologies has allowed us to more effectively manage food production, natural resources, environment and natural disasters. It has enabled global communication, navigation, search and rescue, defense and human exploration beyond earth. It has advanced our knowledge profoundly about our world and our universe. Hence, space technology development is a key element in the understanding and solution of some of the world's most urgent problems like drought, climate change, the greenhouse effect, ozone depletion and pollution. There is a need for the development of large, deployable, efficient structures for many future spacecraft missions. These missions include large deployable antennas, sunshields, solar arrays, and solar sails. Launching cost of satellite is very significant. It increases as the mass of payload and / or stowed volume of the payload increase. Inflatable structures have the advantage of low weight and small stowage volume. Therefore, inflatable structures have been a subject of renewed interest in recent years for space applications such as communication antennas, solar thermal propulsion, and entry / landing systems. Despite the advantages of inflatable strictures, many major technical challenges still remain and these must be adequately addressed before space inflatables can be actually used on future missions. The shape accuracy, which is defined as the degree to which the actual shape of the structure deviates from the intended shape, is an important factor affecting the performance of these large ultra thin membrane inflatable structures. Meeting the necessary surface accuracy requires better understanding of the mechanics of membrane structures. The physical conditions under which space structures operate are very severe. Inflated structures undergo severe environmental changes as the structures pass from orbital day to orbital eclipse. Such environmental changes subject the inflated structure to pressure fluctuations and consequent shockwaves that vibrates the structures. Therefore, space structures must be ii robustly controlled from a vibration standpoint because signal transmission to and from the earth mandates tight tolerances. The control action in space structures is exerted by a small number of actuators. Hence, it is desirable that the dynamic characteristics of the space structure be known to a high degree of accuracy in order to provide high control authority. The space structures consist of assembly of many structural components. Inflatable beam, torus and paraboloidal structures are the main components of a typical space inflatable structure. For example, an inflated beam and toroidal structure (torus) are often used to provide structural support. Similarly, inflatable paraboloidal structure acts as reflector in satellite antennas and as concentrator in solar concentrator applitions. One possible way to reduce the vibration would be to control the vibration of its main support structure - the inflated struts / beanis, torus or paraboloidaF reflector surface itself Motivated by these facts, vibration analysis of inflated beams,. torus. and paraboloidal. , reflector has been carried out. The focus of the present study is to develop an understanding of the inflatable structures technology for future space applications. The analyses of inflated structures are not straight forward because these structures are made of very thin and highly flexible material and have negligible inherent stiffness. These structures gain strength and proper shape due to inflation pressure. Due to this pressure, the structure is in a condition of pre-stress. Dynamic .response of inflated structure undergoing small amplitude vibration can be obtained by a linear dynamic analysis. However, a non-linear static analysis is required to find out the initial stress state and consequently the stiffness of the structure. Since, the initial deformations due to the applied inflation pressure on the structure are large, nonlinear strain-displacement equations are used. Large deformation static analysis is carried out in which the stiffness matrix is updated at each load step. The final updated stiffness matrix is used in the iii dynamic analysis with the assumption that the structure is undergoing small amplitude vibration. Due to the vibration of the structure, the inflation fluid gets compressed and rarefied. Therefore, a standing pressure wave is created in the fluid medium accompanied with a small change in pressure and density. The pressure variation in the fluid due to the vibration of the structure can be obtained by solving the linear acoustic equation by making the usual assumption of an inviscid ideal fluid. The interaction between this pressure wave and the structural vibration affects the vibration. of the structure. To construct a model describing the dynamics of the coupled system, it is necessary to incorporate the shell dynamics and the coupling due to acoustic-structure interactions. A commercial finite element package, ANSYS, is used to model the inflated structures. To highlight the effect of considering the vibration of the enclosed fluid, three different models have been studied and their results have been compared. The inflatable structure gains stiffness due to the pressure exerted by the enclosed fluid. The effect-..of inflation pressure is taken in all the three models. Shell elements are used to moderthe SOA- structure and the pre-stress effect due to inflation pressure is accounted for in the modal analysis. The first model (Ml) does not account for the mass of the enclosed fluid. The second model (M2) is a modification of model M l wherein the mass of the enclosed fluid is distributed evenly on the structure. The third model (M3) accounts for the dynamic interaction between the enclosed fluid and the structure. The space inside the inflatable structure is meshed with fluid elements and hence the stiffness and the inertia properties of the fluid are directly incorporated in the model. , • Model M2 simulates the effect of the enclosed fluid by the addition of a non-structural mass. This is based on the assumption that the enclosed fluid is moving in phase with the structure for each mode of vibration. However, this is not always true. Therefore, iv incorporating the effect of fluid by simply adding the mass of the enclosed fluid leads to erroneous value of the natural frequency. Therefore, it is more accurate to include fluid-structure coupling effect rather than equally distributing the added mass of enclosed fluid. When the dynamic interaction between inflated structure and enclosed fluid is considered, a rich dynamics is observed. Values of the natural frequencies change significantly and the modal sequence pattern also gets modified. Some additional modes are observed. It is concluded that including the effect of coupling between fluid and structures is essential for an accurate description of the dynamic behaviour of inflated structures. The effect of aspect ratio, inflation pressure, and different boundary conditions on the modal behaviour of the structure has also investiga—t-61 lit is observed that fluid structure coupling significantly influences the dynamic behaviour of the inflated, structures. Inflated structures possess many advantageous properties such. as low . weight, small stowage volume, moderate strength, and on-orbit deployability. Therefore, usage of inflatables in large space structures will incur lower cost compared to the. conventional metal or composite space structures. An inflated beam, torus and parabolic lenticular are integral parts of several proposed inflatable space structures. The goal of this research is to understand the dynamic characteristics of the inflated structures. The present work establishes that the dynamic characteristics of inflated structures are greatly influenced by the enclosed fluid. Simply modeling the effect of inflation pressure and accounting for the mass of the fluid as a distributed structural mass is not appropriate. For a better understanding of the rich dynamics of the inflated structures, the fluid-structure coupling must be modeled. It is hoped that the present modeling effort will offer valuable insights into the considerable complexities involved in modeling and testing of thin film inflatable structure assembly.