Carbon Foam-Based Lightweight Mirrors

Several current and upcoming space-based telescope and laser beam director designs, such as the Next-Generation Space Telescope (NGST) and the Airborne Laser (ABL), call for lightweight, high-stiffness optical components. Using conventionalmirror/director technologies, the cost and weight of these systems would be prohibitive. In addition, conventional materials, such as glass, offer insufficient stiffness, and their performance is adversely affected by vibration and thermal serviceconditions. The use of coal-based carbon foam as a structural and support material is proposed. The coal-based foam can be tailored with regard to stiffness and thermal expansion and can be graded in density and pore structure through its thickness toprovide localized stiffness and thermal expansion control while maintaining an overall weight-efficient structure. Carbon foams' unique combination of properties can provide the properties of high stiffness, light weight and low thermal expansion requiredfor mirror/directors as well as mounts, optical benches and support structures. Precursor selection and blending will be studied to optimize foam characteristics while processing parameters and alternate processing methods will be investigated. Productuniformity and manufacturing times and cost will be evaluated as prototype applications are developed and qualified. Carbon, when graphitic and highly ordered as in fiber or deposited film forms, can exhibit unmatched stiffness and thermal stability (e.g., near-zero thermal expansion). Carbon foams can provide similar performance to woven or knitted carbon fiber formsat a fraction of the cost and with significantly greater design/use flexibility. Optical systems based on these foams can be designed in terms of stiffness, thermal expansion, and density - even on localized scales. Demonstration that foam composites canbe used in space-based optical systems - controlling structure stiffness and thermal expansion and surviving exposure to launch conditions, thermal cycling, vibration, and high-temperature oxidation - will aid in carbon foam insertion into otherspace-based and/or optical structures. These include: mirrors and directors, optical benches, reaction structures, reinforcement for large-area structures, and thermal protection systems.