Manufacturing Process Maturation for Propulsion Technology: Structural Insulators

MDA missile systems employ solid rocket motors and divert and attitude control systems (DACS) to propel and guide kinetic energy weapons. These systems create hot, high pressure gases that impose high heat loads on surrounding metal structures and electronic controls. Exist-ing designs use ablative insulators (e.g. EPDM) and monolithic ceramics (e.g. NZP) to limit the temperatures in surrounding components. While these materials offer attractive low thermal dif-fusivities, they also exhibit poor high temperature strengths and toughness that have resulted in unanticipated failures during ground testing. Clearly these insulators must be improved if MDA systems are to improve their reliability. Several possibilities exist for improved high temperature structural insulators including cermets, ceramic matrix composites (CMCs), and reinforcement of off-the-shelf insulators. Ad-ditionally it may be possible to use existing weak materials by modifying the design to limit the operational stresses. Cermets of interest include low conductivity matrix compositions such as HfN, HfO2, ZrN, and ZrO2 reinforced with discontinuous refractory metals such a tantalum and tungsten alloys. CMC solutions include various combinations of rayon-based carbon, Nicalon, and Nextel fibers with matrix materials that include CVI SiC, PIP SiC, and sol-gel oxides. The CMCs can employ needled or continuous fiber preforms. Off-the-shelf insulators include oxide micro balloons imbedded in an oxide ceramic, FiberForm and MinK. While FiberForm (carbon felt) and MinK (oxide felt) are weak, they can be strengthened by densification with CVI SiC or PIP SiC for FiberForm and a sol-gel oxide matrix with MinK. Design modifications that may allow existing insulators to work include layered materials tailored to the thermal environment, segmented insulators to limit thermal stresses, and the exploitation of contact resistance and tor-tuous heat paths. Since there are several possibilities for improved structural insulators, the proposed Phase I effort focuses first on an analytical study to identify one or more approaches that offer the most potential. Once identified, one or more attractive insulators will be purchased and characterized. The objectives of the program include definition of design requirements, calculation of theoreti-cal thermostructural properties, analysis of transient temperatures and stresses, fabrication of one or more sample materials, and characterization of thermal and structural properties. The Phase I effort will be performed by a team of Materials Research & Design (MR&D), Aerojet, a selected material fabricator, and Southern Research Institute. MR&D will manage the program, compute material properties, analyze the thermostructural response, and in conjunction with the team se-lect attractive materials. Aerojet will provide realistic design requirements and SoRI will measure critical properties.