Graphite/polyimide technology overview and Space Shuttle Orbiter applications

The materials, production processes, testing procedures, and repair techniques for composite structures on the Orbiter are reviewed. The Orbiter currently features graphite/epoxy payload bay doors, as well as graphite/polyimide elevons, vertical tail, and aft body flap, which can withstand up to 600 F temperatures. Graphite/polyimide is also being considered for areas of the baseline aluminum structure and the thermal protection system in order to achieve a 20-30% weight reduction, i.e., over 7.5 tons. An analysis of the allowable stresses on FRC imide thermal protection system tiles bonded directly to the structure has shown satisfactory performances in high temperature and cryogenic conditions, although failure modes are yet to be determined. Ultrasonic emissions have been used for defect detection in nondestructive evaluations, together with tension, notch, shear, and compression tests. Field repairs have not been perfected, and require the evolution of high temperature adhesives that are curable under vacuum pressure and heat blankets.