The X-Ray Interferometric Space Telescope (XRIstel) is a proposed single-spacecraft mission designed for high-resolution astrophysics. It targets angular resolutions on the order of 100 μas, representing an O(10^4) spatial resolution improvement over existing X-ray observatories. This capability enables direct imaging and spectro-astrometric observations of compact sources, such as supermassive black holes (SMBHs) and X-ray binaries. Operating in a Lissajous orbit around the Sun-Earth L2 point, the spacecraft undergoes a controlled rolling motion, while its optical payload relies on grazing-incidence and slatted beam combining mirrors operating at wavelengths between 1.24 and 8.68 nm.

Maintaining interference fringe visibility requires optical path length differences between optics and detector to remain within 0.1 to 1.0 nm. Conventional spacecraft structures prioritize strength and stiffness, but lack sub-nanometer thermo-mechanical stability. This study addresses the design and evaluation of an ultra-stable primary structure for XRIstel. The objective is to achieve sub-nanometer stability while satisfying Ariane 6 launch constraints, mass envelopes, and outgassing limitations.

Building on design approaches from heritage missions like LISA and JWST, a Python framework was developed for conceptual structural sizing. The tool evaluated candidate beam geometries, global topologies, and materials, including CFRP laminates, C/C-SiC, Aluminium 7075-T6, and CF/PEEK. Classical Laminate Theory (CLT) was applied to determine the quasi-isotropic properties for composite laminates. Candidate concepts were then modelled in COMSOL Multiphysics using (mostly) shell elements. Finite element simulations evaluated static launch accelerations, linear buckling stability, natural frequencies, thermal environment behaviour, sinusoidal excitation responses, and micrometeoroid impact analyses.

Thin-walled circular beams demonstrated superior behaviour compared to octagonal beam profiles, while a CFRP material consisting of Toray M40J fibres and PMT-F33 cyanate ester resin outperformed other material candidates. A multi-criteria trade-off identified an eight-beam CFRP structure with five support rings as the optimal baseline for detailed design. The concept yielded mass savings while meeting the strict outgassing requirements and launch constraints. A detailed design was developed, including further parameter refinements on wall thicknesses, beam diameters, number of support rings, diagonal beams, cross-volume structures, and the inclusion of multi-layer insulation and solar panels.

Detailed design analyses identify the multi-octagon structural configuration (MO-BR) as the best performing structure, fully satisfying launch survivability with margin and outperforming the single-octagon regarding all metrics. These include the stress levels, eigenfrequencies, total mass, CoG location, and sinusoidal excitation response. Nearly all requirements are satisfied, either fully or marginally, while the spacecraft lid design and the CFRP/aluminium connective joints are identified as a source remaining local stress concentrations and unwanted longitudinal response behaviour.

This study confirms the feasibility of developing a single-spacecraft XRI mission with an ultra-stable spacecraft structure based on CFRP as primary material. The optimized multi-octagon composite structure combines launch survivability with a realistic baseline for further development towards a sub-nanometer thermo-mechanical stable spacecraft.

Future steps of the structural design process are recommended to include the identification and evaluation of the thermal environment and the structure's response during operation. Additionally, the inclusion of optical benches, realistic connective joints, and a detailed spacecraft lid design are recommended in near-future design iterations.
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