Demisability and Structural Performance Assessment of Novel Composite Systems for Spacecraft External Panel Assembly

With the recent awareness of the space sector on the fragile near-Earth space region and the forecast of the booming number of satellited objects, various mitigation approaches are currently being evaluated and starting to be implemented to limit the impact of space activities, and achieve a safe and sustainable space environment. Unfortunately, knowledge uncertainties and technology gaps delay our capability to act immediately, especially to apply a design-for-demise (D4D) approach, which aims to modify a spacecraft design and conception process to achieve the safest destructive reentry possible by material substitution, specific geometries, or dedicated subsystems.<br/>As part of a Network Partnering Initiative launched by the Swiss Federal Institute of Technology in Lausanne (EPFL) and the European Space Agency (ESA), this work focuses on the design and experimental evaluations of novel composite components to improve the overall spacecraft demisability. The new system is compared to baseline critical systems, targeting higher altitude break-up while maintaining equivalent mission-relevant properties. The project’s main attention landed on a complementary dual strategy with the material substitution of a benchmark system composed of an external sandwich panel and its fasteners. First, a hybrid reinforcement made of carbon and demisable flax fibers is evaluated to replace aluminium panel skins or critical full carbon composite skins. The integration of a thermally conductive and reactive metallic matrix filler composed of aluminium-magnesium alloy micro-powder has also been investigated. Second, a novel short carbon fiber reinforced polyetheretherketone (CF/PEEK) bolted joint design is evaluated to replace critical titanium or steel alloys currently used.<br/>The demisability assessment is performed at material and lab scale component levels. This involves the measurement of material’s mechanical properties under static loading at room temperature, and dynamic loading over a temperature range to detect their softening point. Static and dynamic reentry simulation tests were carried out using a laboratory-scale high-temperature creep test and a plasma wind tunnel test to evaluate on one-hand, the thermo-mechanico-physical property change over typical uncontrolled reentry conditions, in particular the material break-up point, and on the other hand, the composite degradation. Results to date led to the selection of a promising reinforcement architecture. An optimal ply-by-ply carbon-flax hybrid/epoxy shows the best trade-off in terms of demisability and specific mechanical properties for the skin, with respectively a 180% ablation rate improvement starting at a lower temperature as compared to CFRP, while having an equivalent specific bending modulus to CFRP or 40% higher as compared to aluminium. The addition of the AlMg filler improved the matrix degradation rate by more than 10% while reducing its onset by 40°K. The study of the joints demonstrated that stainless-steel bolts present no sign of effective demise under testing up to 1070°K, whereas the novel CF/PEEK bolts start to demise before reaching 670°K, while having a superior specific tensile, σ/ρ, and shear strength, τ/ρ, within a typical space mission temperature range (120-400°K).<br/>The project's next phases will focus on the demisability sequence of a bolt-insert fastening system on a sandwich panel assembly to improve the technology readiness level of the concept.<br/>This multi-collaborative project aims to reduce current uncertainties regarding composite materials' demise and implement these technologies on a typical spacecraft platform to move a step forward toward casualty risk mitigation.