Structural Analysis and Test of a Highly-Nonlinear, Stability-Critical System on Europa Clipper

The structural qualification of the Europa Clipper spacecraft included testing of a highly-nonlinear, stability-critical Linear Separation Assembly (LSA). Of note, the LSA was ultimately descoped from the final Europa Clipper flight configuration in favor of a Marmon Clamp-style separation device. Nonetheless at the time of spacecraft static testing the LSA was the flight configuration and thus was structurally tested. The LSA is a double-wall cylinder design with a frangible circumferential notch. The firing of a pyrotechnic device expands a detonation tube which then severs the structure at the frangible notch, releasing the spacecraft from the launch vehicle. Two segments of aluminum 0.025 inches thick is all that exists at the minimum cross-section of this notch around the periphery, across which the launch loads of a 6000kg spacecraft must be reacted. The throat of the notch is predicted to yield substantially under launch loads. If the throat yielding propagates far enough it would become an unstable plastic hinge, leading to catastrophic plastic buckling of the structure. Testing at the subassembly level was successfully carried out to validate the LSA; however, analysis indicates the boundary conditions of the subassembly-level test artificially relieved stresses within the LSA and did not adequately exercise this failure mode. Thus, the risk of this failure mode at spacecraft-level testing could not be retired based on the success of the subassembly-level test. Extensive analysis was performed on the LSA to ensure notch throat yielding is not detrimental in nature and that the structure would not buckle during spacecraft-level static testing. Studies in the linear regime were carried out attempting to bound the buckling behavior of the notch in cases where the notch were fully-elastic or fully-yielded. These studies were augmented with material assessments such as the Neuber method to determine the extent of the predicted material strain to compare against elongation limits. A fully-nonlinear model of the LSA was assembled to support the assessments of notch yielding and a novel postprocessing technique was developed to identify the load at which the structure begins to present signs of instability. Ultimately this concert of analytical methods was substantiated by the successful completion of the Europa Clipper spacecraft static test. This paper takes a structural engineering focus walking through the concerns identified through both subassembly and spacecraft-level analysis and testing of the LSA. The paper describes in detail the analytical methods employed and developed to demonstrate the structural capability of the LSA and the testing that substantiates the conclusions of that analysis.