Characterizing lightweight and dimensionally ultra stable structures for space applications

Abstract

Lightweight and dimensionally ultra stable materials and structures are becoming more and more important for spaced based applications. For scientific and earth observation missions, instruments with ultra high precision requiring ultra-stable structural materials are necessary to achieve the mission goals. Ultra stable glass ceramics exhibit a very good behavior in terms of dimensional stability, but its huge mass is a disadvantage. Composite materials like CFRP (Carbon Fiber Reinforced Plastic) are lightweight and their CTEs (coefficient of thermal expansion) are tunable to be low at a specific temperature. On the other hand these composite materials are changing their geometry during time and environmental situations caused by outgassing. Structures combining the stiffness and lightweight properties of CFRP and the dimensional stability of Zerodur are a key technology for future space missions. Building such structures and measuring their stability are both challenging tasks. Our focus is to measure the dimensional stability of lightweight and dimensionally stable structures for LISA/NGO and GRACE-FO. We characterized an optical breadboard consisting of two 5 mm thick Zerodur plates connected via a CFRP honeycomb structure leading to a weight reduction of 70%. For GRACE-FO the thermal stability of the triple mirror assembly (TMA) of the laser ranging instrument consisting of a 0.5 m CFRP spacer and Zerodur endfittings on each side was characterized. For these measurements a new facility was built. To measure the CTE of the structures a heterodyne interferometer is used to detect the expansion of the device under test. Two beams are reflected at two mirrors attached representatively to the structure on each side. The noise level of our interferometer was demonstrated to be below 2pm/sqrt(Hz) ensuring an ultra-high sensitivity of our measurements. The temperature is measured using PT100 sensors. Temperature changes are applied radiatively by a heating system in vacuum. In this paper, we present our new measurement facility and first measurements of the dimensional stability of the GRACE-FO TMA.