The spacecraft structure, like any other structure, is there primarily to guarantee the integrity of the spacecraft under any loading, such as during handling, testing and launch. In addition, it must allow the spacecraft to serve as an optical bench for a telescope and therefore the structure must provide the necessary thermo-elastic stability in orbit. In the case of XMM-Newton, this led immediately to the selection of ultra-high-modulus carbon-fibre composites (low thermal expansion) for the main structural elements. Another advantage of this material is its very high modulus of elasticity. This limits the total structural mass needed to ensure the structure's stiffness.
These two favourable qualities of this carbon-fibre material, plus its low mass, have led to its widespread use on XMM-Newton, albeit for different reasons in different parts. For instance, for the telescope tube and the mirror support platform, a carbon-fibre composite was necessary for thermo-elastic reasons. A strongly directional lay-up made it possible to meet the requirements, whereas the mass could be kept low. On the other hand, for the central cone of the Service Module, the stiffness required was the main reason for using a carbon-fibre composite.
In complex items, such as the mirror support platform, the joints that are necessarily made out of metal degraded the intended high thermo-elastic stability to such a level that active thermal control was necessary. Here, a carbon-fibre composite was selected for its high stiffness and strength. Other parts outside the optical path were made of aluminium for reasons of thermal conductivity (honeycomb for Service Module side panels), light-tightness and ease of production (telescope Sun shield, outgassing baffle).