Random Vibration Methodology and Analysis in Spacecraft Secondary Structures : Detailed Examination of Spring Assembly Dynamics

Sammanfattning

Separating a payload from a launch vehicle in space poses a unique engineering challenge due to the intense random vibration environment during ascent. One of the most critical components in the separation mechanism is the separation spring assembly, where a spring is used to gently separate the payload from its launch vehicle. To simulate and evaluate the mechanical behaviour of such devices, this study applies finite element modeling (FEM) to investigate dynamic responses, stress development, bolt load behaviour, and damping characteristics. The modeling approach considers combined static and random vibration loading, focusing on how structural modeling choices influence simulation outcomes. The study shows that simplified models, while computationally efficient, tend to overestimate internal stresses and underestimate bolt loads, potentially leading to non-conservative designs. Including a detailed spring model was essential for accurately predicting eigenfrequencies and stress distributions. It was also found that the best method for combining static and dynamic stress tensors is through direct summation of their individual components, resulting in a conservative and realistic representation of the stress state. It correctly accounts for worst-case sign combinations and the non-negative nature of stress results typical of random vibration analysis. Damping analysis showed that the commonly assumed 2\% structural damping ratio is only valid above 800 Hz, while significantly lower damping (around 0.6\%) was observed at lower frequencies, indicating that frequency-dependent damping models offer improved accuracy. The study further recommends using lumped mass representations only when the added components do not contribute to global stiffness, and emphasizes the use of RBE3 elements (a Nastran element that distributes loads without adding stiffness) for stiffness-neutral connections. Physical testing is advised to validate simplified configurations and extend findings to similar spacecraft secondary structures.