Research On Semi-monocoque Composite Service Cabin Structure In A Large Space Telescope

With the higher performance requirements,space telescope has developed with larger structure size,more equipment and greater total mass,thus higher requirements of structural configuration and mechanical properties are put forward to the supporting structure.Besides,with the development of on-orbit maintenance technology for space telescope in low Earth orbit,new requirements of on-orbit maintenance are raised to the supporting structure.This paper is based on a large space telescope with on-orbit maintenance fuctions.The space telescope has a semi-monocoque service cabin consisting of frame,skin panels and maintenance doors to connect and support outer components and equipment of the telescope.The maintenance doors can be opened and closed on orbit to provide access to inner modules during maintenance.The main structure of the service cabin is made of carbon fiber composite.According to the mechanical requirements and on-orbit maintainability requirements of the service cabin,this paper makes researches on related fields including composite joints,connecting mechanisms of maintenance door,composite optimization and ply design,integrated simulation and experiment.These reasearches make the service cabin meet the corresponding requirements at last.1.Research on connections of composite structure in the service cabin is conducted.First,connections of composite structure in the service cabin are designed.The frame is composed of longitudinal and transverse beams by hybrid bonded/bolted joints with multi-interfaces,and the skin is connected to the frame by many bolts and anchor nuts.Then a composite structure consisting of frame and skin is manufactured according to such structural features.A set of test stand is built and tensile tests are conducted on the structure.The results show that,bolt-hole clearance of the bolted connection of skin and frame can result in relative sliding between them,and reduce the stiffness of the structure.While the stiffness of the structure can be enhanced by increasing tightening torque of the skin-frame bolts and increasing numbers of the skin-frame bolts.Then a finite element model(FEM)of this composite structure is established,and the simulation model is verified by the experimental results.Analysis results of the FEM with different frame joints indicate that,stiffness of the structure with hybrid bonded/bolted joints is higher than that of the structure with bolted joints,and adhesive plays a leading role in the stiffness of the hybrid joints.Finally,finite element analysis of a single lap composite joint is conducted,and the results show that hybrid bonded/bolted joint has higher strength than bonded joint because the bolt continues taking load after adhesive damage.Hence hybrid bonded/bolted joint is suitable for application to main supporting frame structure in the service cabin by combining the advantages of high stiffness of bonded joint and high strength of bolted joint.2.Unloosening fastening devices and hinges of the maintenance door are designed and studied.When the maintenance doors are closed,they are connected to the frame by unloosening fastening devices to transfer load.In the unloosening fastening device,taper pin and taper sleeve match by conical surfaces,and they are connected and fastened by a coupling screw with flange head,thus clamping stagnation of connecting devices when opening arc door can be avoided and shear force can be transferred efficiently.A limit cover can prevent the screw falling off,and the limit cover can match electric tool by groove structure to prevent countertorque of the screw transferring into astronaut.When the maintenance doors are opened,they are connected to the cabin by hinges.Two types of hinges,namely double-bar linkage and hinge are designed and compared in this paper.The experiments of maintenance door show that,double-bar linkage has more than one degree of freedom which makes it inconvenient for people to close the door,while hinge has only one degree of freedom which makes it convenient for people to close the door,thus the type of hinge is chosen for futher development.To prevent the door drifting after opening,damping torque of the hinge is generated by cylindrical cams and disc springs.With the design of cam contour curves,there's no damping torque when the door is closed to avoid interference with operation to close the door;when the door is opened,the damping torque emerges and increases gradually,and keeps a constant value finally.3.Optimization and ply designs of the composite parts in the service cabin are conducted.Because of the complexity of the service cabin structure with many design variables,this paper proposes a simplified optimization method: according to different ply design principles for different parts,the composite material is equivalent to isotropic material or material with enhanced elasticity modulus in longitudinal direction;honeycomb core is equivalently modeled by sandwich plate theory;an optimization model is established with optimization objective of minimum total weight,design constraints of fundamental frequency and thicknesses,design variables of thicknesses of parts.After optimization analysis,the weight of the service cabin decreases obviously and meets the design requirement of weight.Finally ply designs of the composite laminate are conducted according to the thicknesses obtained from optimization,and make sure that the elasticity moduli in ply designs are close to the equivalent values used in optimization.4.Integrated simulation and experiment of the service cabin are conducted.An integrated FEM of the servive cabin is established and analyzed with composite ply parameters and bearing masses.The results of modal analysis show that,fundamental frequency of the service cabin is 16.2Hz which meets the stiffness requirement.The results of sinusoidal vibration analysis show that,stresses in the service cabin structure and acceleration response of the front endpoint are too large under the original input condition,and after the input condition is limited,they can meet the strength requirement that safety margin is bigger than 0.5.The results of static buckling analysis show that,local buckling phenomena of laminate skins with large areas can be improved by adding reinforcing ribs,then stability requirement that safety magin is bigger than 0.3 is met.Finally,the service cabin is manufactured and vibration tests are conducted on it.The experimental results show good agreement with simulation results,and there's no damage or abnormity in the structure,hence it's verified that the mechanical properties of the service cabin meet design requirements.