Optimization of Bistable Clamp for Aerospace Thermal Systems

Large spacecraft, launch vehicles, and hypersonic craft use a thermal protection system (TPS) consisting of tiles to mitigate potential thermal hazards while traveling through the atmosphere at high velocities. Historically, these tiles have been attached via silicone adhesive that mates tiles to the craft permanently. The proposed mechanism is a macroscale interlocking bistable compliant clamp that securely houses and rapidly exchanges thermal tiles attached to the TPS. Compliant mechanism design presents a significant learning curve due to large deflections requiring nonlinear mechanics models, and unexplored design space for analysis of deflection of flexible links and mechanisms. Despite their design challenges, compliant mechanisms present many desired characteristics not found in traditional multi-joint rigid-body components, including lower part cost, scalability, precise motion, zero off-gassing, and reduction in weight and friction. Here we report methodology, designs, and results that demonstrate use of optimization techniques to autogenerate revised compliant structures by comparing them against a specified fitness metric. This proposed method is less time-consuming and more efficient than traditional computational or “trial-and-error” methodology and exports parameterized structures in manufacturable file format. This paper's proposed bistable compliant clamp was fabricated in polylactic acid (PLA), assessed with finite element analysis (FEA), and mechanically cycled on a tensile tester. Results identified preliminary performance metrics regarding stress concentrations, retention force, and input force. It was theorized that these metrics depend on seven geometric parameters and could be optimized to maximize retention force and minimize input force, while keeping acceptable material-based stress concentrations and bistable characteristics. The design methodology implemented topology optimization techniques to identify potential parameter solutions that improve one or more of the defined performance metrics. Redesigned models were fabricated and tested against the original prototype to validate the algorithm's ability to enhance mechanisms' performance. The proposed compliant mechanism serves as a solution for a re-attachable TPS tile system and a methodology to optimize compliant structures that do not require extensive expertise in compliant design theory. This result also demonstrates optimal design of unit cells that may be tessellated to form an adhesive metasurface.