The next generation of visible quality, space-based sensors are being driven to provide wider area coverage and improved resolution. This push requires larger aperture systems (2.5-6m) operating at higher altitudes (GEO or LEO). To be viable, these goals must be obtained with payloads significantly lighter and less expensive than the current technology. One approach is to develop lightweight, graphite fiber based composite optical substrates which can be used to obtain very lightweight mirror substrates; but they have some very fundamental fabrication issues which make stability to optical tolerances very difficult to obtain. The materials have problems with moisture expansion, resulting in anisotropic bulk material properties and problems with stability over time and temperature. The moisture absorption problems can be potentially eliminated by using a vapor barrier (a thin coating of copper or nickel) to seal the substrates. This thin metal layer creates a different problem, since the different coefficient of thermal expansion of the metal tends to distort the substrate over large temperature ranges. SSG proposes an innovative Continuous Fiber Reinforced Ceramic (CFRC) Silicon Carbide (SiC) ultralightweight composite mirror substrate. The excellent bulk material properties of the CFRC SiC material and the fabrication geometries possible will allow the fabrication of large, lightweight optical substrates which can offer weight savings of > 5x compared to existing technologies (using a 1m diam IR mirror substrate as a baseline) while eliminating the technical problems with long term stability of delamination associatd with the GrEp-based composites. The CFRC SiC material is currently being developed as a millimeter wave reflector substrate. The extension of this monolithic SiC/SiC composite for optical components is currently being pursued. During Phase I the viability of the proposed CFRC SiC mirror substrate will be demonstrated with fabrication of ultralightweight 0.25 m diam. SiC mirror substrates. These pieces will be fabricated to meet performance and weight requirements agreed upon by AFPL personnel, and cryogenically tested to show thermal stability.
Keywords: silicon carbide fiber reinforce ceramic ultralightweight optics
