Mechanical design of CubeSat structures using composites and polymers
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This study is about the mechanical design of structures for a small satellite of the CubeSat type. CubeSat frames are usually made of aluminum alloys, this study looks into the possibilities of using polymers and polymer based composites in a custom structural design for the NTNU test satellite (NUTS). This is done for the potential weight reductions of the structural framework and supporting structures. This can be beneficial also in other CubeSat project that has a thigh weight budget. The satellite will use backplane circuit board layout in contrast to common stacking using spacers. The design integrates these parts in the structural support. At the same time it aims to make a frame that is flexible and compatible with different layouts and internal components. The study also covers the component layout, general mechanical concept development and materials selection of the satellite primary and secondary structures. This includes integration of components like magnetorquers, solar cells, batterypack, antenna structures and small required components like launch deployment - switches and ?springs.The environment parameters and loads of the launch and orbit stages combined with requirements to CubeSats and spacecraft structures are reviewed. This entails the effect the environment has on polymers and polymer composite materials. In the launch phase this includes thermal changes, wear and friction and also quasi-static loads, vibrations and shocks. In the orbit environment it includes thermal changes, radiation, erosion, micrometeorites, outgassing and surface charging. These effects are considered in the further design and sizing of the structures. Relevant materials are suggested based on listed requirements, collected material properties recorded in space experiments and recommendations from standards. Issues with using composite and polymer materials in design of small structures are discussed. Structural analyses are researched and applied using FEM based analyses. Some advanced analysis methods were researched as tools to run response analysis in relation to shock and vibration loads and also orbit thermal loads. Work is also done looking at relevant production, prototyping and testing methods for the different structures. This has included vacuum bagging , press molding and milling of aerospace carbon/epoxy prepreg.The results of the study presents new design concepts for parts in the assembly, and some candidate materials like glass and carbon fiber reinforced epoxy, cyanate and bismaleimide resins. A detailed CAD model of the CubeSat concept assembly is produced along with drawings and prototypes of parts of the concept. The results of the FEM analysis show that the structure should support the defined loads using the current geometry and materials, some higher stressed areas were also identified. An advanced thermal model was built and its function is explained along with how to attain better results. The functions of setting up dynamic response analyses are also explained, to get satisfying results from these models more knowledge of the damping characteristics of the materials are needed. Modal analyses show responses of some parts, and how their natural frequencies can be increased. The main findings in designing with composites are challenges with fasteners and combinations with mixed material assemblies due to thermal expansion and galvanic corrosion. Issues in using polymer materials in a CubeSat is related to surface properties and wear effects during launch, and also the materials reaction to the space environment related effects. There is potential for mass savings, but the concept needs some further refinement. The production methods explored are thought to be applicable in further work.