Filter failure analysis for the SWAP instrument on-board of PROBA2

During its intensive testing campaign, SWAP (Sun Watcher using Active Pixel System detector and image Processing), an optical instrument developed by Centre Spatiale de Liege (CSL) and mounted onto the Belgian satellite PROBA2, was subjected to a multitude of load cases to verify the integrity of the instrument during and after the launch and during the operational phase. The front and rear filters of SWAP, 200 nm thin films, survived intensive vibration tests on the qualification model and acceptance tests on the flight model, both at an instrument level. During the acoustic test on the spacecraft, the front filter exploded while the rear one was undamaged. A new, strengthened filter, coated with a very fine metal mesh was mounted in the instrument. However, this new filter has less good optical properties due to optical diffraction. Therefore a profound analysis of the filter failure is necessary to provide the best possible optical accuracy for future missions with similar equipment. Three load cases are further studied in this paper: the acceleration of the rocket, the instrument depressurisation and the vibro-acoustic behaviour under the large sound and vibration levels. The acceleration and depressurisation are studied together in a two-level computational fluid dynamics (CFD) analysis to assess the effect of the venting of the instrument on the differential pressure over the filter. The main focus of this paper is on a numerical vibro-acoustic analysis, following the procedures used by CSL and ESTEC (European Space Research and Technology Centre). This analysis needs special attention because of the presence of a stochastic excitation (diffuse field) and the very nonlinear behaviour of the filter itself due to its small thickness. Also, an additional postprocessing step based on filter displacements, rather than differential pressure, as done in the ESTEC procedure, is discussed.