A Multifunctional Shield for an Earth Satellite

Environmental hazards including the growing population of space debris and the transient flux of ionizing particles present a significant challenge to the structural integrity and functionality of satellites in low Earth orbit. Composite-stuffed aluminium foam sandwich panels (AFSPs) have been identified as one of several structures that may afford a low-mass means to provide protection to sensitive satellite interior components against hypervelocity impacts (HVIs) with a satellite's outer wall. Furthermore, composites of ultrahigh molecular weight polyethylene (UHMWPE) have been identified as a material that may afford both improved HVI and ionizing particle protection simultaneously. This work explores whether UHMWPE-stuffed AFSPs can perform as a multifunctional satellite shield by mitigating damage to satellite interior components due to both HVI, and damage from ionizing particles. Also explored is whether the HVI performance of the UHMWPE composite degrades as ionizing particles change its chemistry over time. To assess these materials, several experiments were conducted. A HVI testing campaign was performed at the Fraunhofer Ernst Mach Institut 2-stage Light Gas Gun to bound the ballistic limit of an UHMWPE-stuffed AFSP, which was compared to a baseline aramid-stuffed AFSP. Further samples including the UHMWPE composite alone and the UHMWPE-stuffed AFSP were characterised in terms of their ionizing particle absorptivity by exposure to protons of varied energies at the ANU Heavy Ion Accelerator Facility (HIAF). These test activities were used to validate computational models which further explore the performance of UHMWPE-stuffed AFSPs and compare against the baseline aramid-stuffed AFSP. HVI performance was characterised in detail by a smoothed particle hydrodynamics (SPH) finite element model including a non-linear orthotropic material model to describe the composites. This model was validated against the conducted HVI experiments and data reported in literature for both the UHMWPE- and aramid-stuffed AFSPs. The SPH model was also found to quantify the stochasticity of HVI event outcomes imposed by aluminium foam, which had been observed but not quantified in literature. The composite-stuffed AFSPs were found to have improved mean HVI performance relative to other baseline satellite walls, with the UHMWPE-stuffed AFSP outperforming the aramid-stuffed AFSP. However, the stochasticity in the HVI outcomes introduced by the aluminium foam was found to be significant, which may pose functional challenges for employing shields incorporating aluminium foam in orbit. A further numerical model was developed employing the SRIM-2013 software tool to predict damage to radiation-hardened and commercial-off-the-shelf representative microchips reported in literature. Damage was measured for both chips in terms of the environmentally induced soft error rate (SER), and total ionising dose (TID) applied over a notional satellite mission. Both of these properties were calculated considering a representative sun-synchronous orbital environment characterised by the SPENVIS software tool. This model was validated against the ANU-HIAF proton absorptivity experiments, showing excellent agreement for UHMWPE alone, and acceptable agreement for the UHMWPE-stuffed AFSP. The model incorporated material and structure dimensions, and chemical composition to demonstrate that UHMWPE-stuffed AFSPs lessen TID but not SER relative to other baseline satellite walls. The HVI performance of UHMWPE-stuffed AFSPs as the material properties of the UHMWPE degrade under imparted TID was also studied. UHMWPE tensile samples that had been irradiated to different environmentally representative doses via proton exposure at the ANU-HIAF were subjected to capstan tensile testing with optical strain measurement. The change in ultimate tensile stress (UTS) was measured by the tensile testing of several UHWMPE samples with a varied applied proton dose. The samples were observed to have decreased UTS with increasing proton dose, and the impact of the change on HVI performance was captured by appropriately changing the material properties in the SPH numerical model. No operationally significant change in HVI performance was noted. The reason for the observed change in UTS in UHMWPE composites due to proton exposure was further studied by the attenuated total reflectance infrared spectroscopy (ATR-FTIR) technique. This allowed for the investigation of change in UHMWPE chemical structure induced by the proton exposure. Notable chemical changes were observed which provided evidence to support the hypothesis that proton exposure was serving to sever carbon-carbon bonds that provide the mechanical strength of the UHMWPE composite. The results collectively show that UHMWPE-stuffed AFSPs outperform comparable satellite outer walls in both HVI shielding and lessening TID applied to internal satellite components, while retaining desirable mechanical properties for structural applications. However, this work finds that there are functional challenges in using AFSPs in HVI shields. The use of aluminium foam introduces stochasticity in HVI event outcomes, numerically quantified in this work, which introduces significant uncertainty in the operational HVI performance of composite-stuffed AFSPs in situ.<p></p>