A framework to predict bearing loads of soft suspension reaction wheels during launch

Spacecraft stability requirements are continuously evolving due to the increasing sensitivity of instruments onboard. This imposes design limits on reaction wheel assemblies (RWAs), that are often identified as the primary source of in-orbit mechanical disturbances. These disturbances can be minimised by introducing a soft suspension system to the RWA’s standard flywheel, motor and housing components. However, in this configuration, the excitations during launch cause flywheel-casing contacts, inducing unknown motor bearing shock loads. Identifying critical load cases is not computationally viable with available finite element techniques. To characterise the vibro-impact dynamics problem, an experimental procedure that simplifies a soft suspension RWA is developed. The proposed setup comprises a mass suspended using an air cushion, variable stiffness spring attachments and a frame, respectively emulating the flywheel, soft suspension and housing. This assembly is mounted on a shaker/slip table and subjected to representative excitations. The test results are used to calibrate an analytical 1-degree of freedom (DOF) vibro-impact model. In particular, an impact oscillator is augmented with an empirically derived restitution coefficient, in order to account for neglected body flexibility. Following validation, the numerical vibro-impact model is extended to 2-DOF, with the capability of estimating bearing shock loads. The vibro-impact model is employed to investigate the effect of gap size between the impactor and case, on the mean shock and shock frequency, in order to estimate shock damage potential on bearings.