DEVELOPMENT OF A C/C-SIC SANDWICH OPTICAL BENCH

Kurzfassung

A new generation of ultra-stable optical bench made of a ceramic matrix composite (CMC) has been developed and validated in the frame of an ESA TDE project, which can conciliate the performances needed for challenging future space applications. The C/C-SiC sandwich optical bench demonstrator (600 x 600 x 60 mm³) has been developed in a close cooperation of DLR and Thales Alenia Space, France (TAS-F).Thereby, the expertise of TAS-F in the design, modelling, testing, and the integration of optical bench flight hardware, as well as industrial and commercial aspects, like reproducible production and system costs, could be ideally combined with the knowledge and long-term experience of DLR in the development of C/C-SiC materials and sandwich structures. C/C-SiC materials have been developed at DLR since the late 1980´s, originally for thermal protection systems of reusable spacecraft, and are manufactured using the liquid silicon infiltration (LSI) process, also developed at DLR, representing one of the fastest and most cost-efficient manufacturing method for CMC. This is the main reason, why LSI methods are commercially used for the large-scale industrial production of brake discs for automobiles, aircraft propeller brakes and jet vanes in rocket propulsion. C/C-SiC materials are based on carbon fibers embedded in carbon (C) and silicon carbide (SiC) matrix, leading to a unique class of material, offering high temperature and thermal shock resistance and abrasive stability. Even more important for optical benches, the combination of carbon fibers and SiC matrix leads to a very low coefficient of thermal expansion (CTE), close to zero. Since there is no polymer at all in the material, it offers a high stability in space environment, and outgassing is not an issue. The first phase of the TDE project was focused on a preliminary design of the C/C-SiC sandwich optical bench and a characterization campaign at coupon level. Based on the heritage of DLR and TAS-F, a sandwich design with a grid core and representative interfaces were selected, respectively. Sandwich samples were manufactured and successfully tested in 4-Pt. bending, in-plane shear and out of plane tension. Two types of interface concepts were developed and representative interface samples were successfully tested in screw tightening tests and out of plane tension. The interface components were entirely made of C/C-SiC, in order to ensure similar coefficients of thermal expansion of interface components and sandwich structure, and finally to avoid internal stresses and even minimal structural deformation. A material and structural model was developed, which was very helpful to define convenient sample geometries in order to obtain reliable and significant test results. The material model was evaluated by correlating simulation and test results, and it could be shown that the real structural behaviour was reproduced closely, and conservative failure criteria were obtained. In the second phase of the project, the design of the demonstrator was detailed, using the qualified material model. In an extensive test program at TAS-F, the thermal and mechanical performance of the C/C-SiC sandwich optical bench was demonstrated. This presentation will show the development and manufacture of the sandwich and interface samples as well as of the optical bench demonstrator. It will focus on the development of the material model and the test results obtained in mechanical and thermal test campaign. C/C-SiC sandwich structure in full scale geometry (600 x 600 x 60 mm³), without interfaces.