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Description
The demisability of optical payloads is challenging due to the high performance materials used. Two high ground risk aspects, glass materials used in lenses and mirrors, and titanium bipods, were tested and carefully modelled in this work.
A number of previous works have attempted to assess the demisability of glass materials, in particular Zerodur, although little material demise testing has been performed. In this work, a set of material demisability tests were carried out in conjunction with an assessment of the glass viscosity-temperature curve. The tests demonstrate that, although material deformation can be observed earlier, material removal is driven by the viscosity of the material, and not by a melting temperature. The rate of material removal can be correlated directly with the viscosity, and a model has been derived which successfully rebuilds the material loss in a set of tests performed in the DLR L2K high enthalpy wind tunnel. This model, implemented in the SAMj destructive re-entry simulation tool, demonstrates that Zerodur is significantly less demisable than estimated by current models.
A titanium bipod designed to support an 8kg mirror was also tested in the L2K facility, and the demise of the bipod was clearly demonstrated to be linked to the length scales of the different parts of the bipod. The cutouts which enable the flexure of the bipod melted first, promoting a fragmentation of the bipod into smaller parts, and the small length scale of the legs and foot resulted in the demisability being relatively good. Using 3D printing, bipods with a set of demise enhancing features were produced. A hollow version of the bipod, and then a bipod with a set of holes allowing the flow to pass through were tested. Both models satisfied the load and flexure requirements of the original bipod design. The hollow bipod was seen to improve the demisability slightly due to the lower mass of the object. The design with holes, however, demonstrated a substantial improvement in the demisability of the bipod, consistent with halving the length scale of the object. This suggests that flow holes are an effective design-for-demise technique, and it is strongly recommended that more research is put into this concept.
