Nowadays the structural verification of unmanned spacecraft programmes follows typically a protoflight approach1, wherein the flight structure is tested to levels somewhat above limit stress (or load) but below yield strength, rather than the more conventional prototype approach. Often, such option implies exposing flight items to extensive fatigue loading (incl. on-ground vibration testing at different levels of integration).
Usually, as far as possible, the structures are designed to be hyperstatic (i.e. with multiple load paths) to ensure sufficient strength for enduring the loading environment even in fail-safe condition. In a hyperstatic design, when one of the load paths is lost due to local failure, the loads may redistribute among the remaining structure preventing the complete failure.
In many cases, owing to restrictive design and mission requirements, it is not possible to realize redundant structures and it is necessary to introduce one or more critical structural items like isostatic mounts in the main load path, being these widely used in unmanned spacecraft designs. These are typically monolithic parts constituting a single-point-failure of the design and must be considered Potential Fracture Critical Items under a reduced fracture control programme.
Normally their damage tolerance capacity is demonstrated via crack-growth analysis. For a crack-growth evaluation it is necessary to derive the stress spectra for each event phase of the operating life and afterwards define the complete stress spectra in the most critical areas.
Typically the isostatic mounts are designed to support relatively large masses (e.g. optical benches, etc.); these items have their design optimized for (a certain level of) flexibility in order to comply with alignment and stability (e.g. of the optical bench)
under extreme environmental conditions. These requirements drive the design, frequently resulting in parts with regions of reduced cross-section and relatively high stresses
