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36 Chapter 6. System-Level Considerations Earth orbit (Arya et al., 2016), and should also be able to be leveraged for deployment in Mars orbit. However, similar-sized structures have not been deployed on the surface of celestial bodies, e.g., Earth’s moon or Mars, where gravitational loading dominates (roughly 0.4 g or 3.71 m/s2 on the Martian surface). The largest solar array deployed on the Martian surface was for the Phoenix lander, providing roughly 3.5 m2 of active area (White et al., 2007). This is a factor of 30 smaller than the scale required for generation of oxygen and fuels required for Mars ascent. Unfortunately, the Phoenix solar array cannot be scaled up in a facile way to provide the necessary area for the present task (900 m2 for a rectenna array used for power beaming or up to 120 m2 for an integrated PEC design operating at a 10% efficiency for conversion of sunlight to methane and oxygen), and other structural architectures must be considered. Packaging and deployment represents an unresolved challenge for the realization of Martian ISRU using solar energy. In general, uniaxial packaging and deployment—where the structure is only compacted along one axis—is easier to implement than biaxial packaging and deployment—where the extent of the structure must be reduced along two orthogonal in-plane directions (Miura, 1985). For this reason, when the performance of the structure depends solely on the deployed area (as in the case of photovoltaic or PEC devices) multiple independent uniaxially compacted structures are often used, as opposed to a single larger biaxially compacted structure. When area continuity of the deployed structure is required, as in the deployment of the rectenna for the power beaming architecture, biaxial packaging and deployment techniques must be employed; this often results in engineering complexity. Typical uniaxial folding motifs include fan folding (e.g., in the Phoenix solar array), z-folding (e.g., in the ISS solar array wings), or rolling (e.g., in the second generation Hubble solar array). Key drivers for the structural design include deployed area, deployed shape accuracy, maximum allowable deflections, and dominant loading conditions. The first two motifs, i.e., fan folding and z-folding, require sharp geometrical kinks in the structure; these kinks can be implemented either using hinges (e.g., piano hinges as on the ISS solar array wings) or shallow curves that limit the minimum radius of curvature of the material being folded. Creases, i.e., concentrated bending stresses, should be avoided to prevent creep, plasticity, damage, and non-planarity when deployed (Papa & Pellegrino, 2008). (Note that this is a general design principle and may be forsaken in cases where avoiding creases adds complexity and the material system is proven to be robust under creasing loads, e.g., the edges of the ISS solar array wings, which are used for power routing and are designed to be folded and unfolded for many tens of cycles.) At this time, rolling appears to be a promising candidate for the folding architecture of the proposed Mars ISRU designs. Rolling limits the maximum bending stresses imposed on the structure during packaging without the need for mechanical hinges and without sacrificing packaging efficiency. Back-rolling, also known as curvature reversals, can be used to reduce shear stresses that occur during the rolling of long sheets of laminated materials (Arya et al., 2016).PDF Image | ISRU Challenge Production of O2 and Fuel from CO2
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