Methodology for robust analysis of bolts slippage in cryogenic space missions

Abstract

The assessment of slippage in cryogenic space missions is fundamental from a mechanical point of view as it is one of the main failure modes of a bolt in a mechanical interface. It is usually performed on the basis of temperature maps obtained from the worst-case thermal design cases, with particular interest in the transient case during the cooldown. Traditionally, the thermal mapping has to be transferred to the detailed FEM model. This process requires a lot of interaction between the thermal and structural disciplines, which is often not easy. Moreover, the thermal mapping usually corresponds to the instant of maximum gradient between the clamped parts along the transient case. In this paper, a new methodology is proposed to speed up the evaluation of the temperature effect on the slippage from an analytical model correlated with the FEM model. Then, the interactions between the structural and the thermal responsible may be reduced. In addition, the proposed methodology evaluates the entire temperature curve of the transient case, rather than a single instant. In this way, the thermal effect on slippage can be evaluated in a robust and agile process, facilitating the definition of requirements in terms of the maximum allowable temperature gradient as a function of preload or vice versa. This methodology has been validated with the primary mirror of the ARIEL mission, which is a cryogenic European mission that aims to study exoplanets by making observations from a thermally stable orbit at L2 point of the Sun-Earth system. Therefore, the correct design of the primary mirror is essential for the successful science observations of the mission.