The material chosen is a high-density composite, reinforced with carbon fiber bundles in a three-dimensional pattern. This material was selected by the design/de- velopment team for the aeroshell, graphite impact shell and other components. The key fabric mechanical and physical properties contributing to safety are: • superior high-temperature mechanical strength; • low thermal expansion; • tensile fracture by crack propagation between fiber bundles coupled with fiber pullout and minor amounts of fiber fracture; and • high impact toughness, causing the aeroshell on impact to fracture into several pieces with notable energy absorption. The insulator sleeve disc sets within the heat-source modules are non-load-bearing with respect to the graphite impact shell/fueled clad assembly, and are self-loaded so as to maintain their integrity through the launch vibration phase. These sets insulate the iridium fuel clad during launch, operations and accidents. Design Verification The 238PuO2 clads, GPHS heat sources and RTG assemblies have been subjected to a number of design verification tests to assess safety margins for various accident environments. These tests have included explosion overpressure, fire, fragment impact, ground impact and simulated re-entry arc jet tests. These tests have shown the RTG component design to be very resilient in the face of many environments, summarized in the safety analysis results presented in the following section. The RHUs have also been subjected to design verification tests. A separate extensive RHU safety analysis report has been completed. Radioisotope Heater Unit (RHU) An RHU is a radioisotope-fuelled system consisting of a pellet of 238PuO2 which produces one watt of heat through natural decay. It is encased into an alloy of 70% platinum-30% rhodium, further enclosed in an insulation system of pyrolytic graphite, and finally enclosed in a graphite reentry aeroshell. The outside dimensions are 2.54 cm in diameter by 3.3 cm in length. Each unit weight about 0.049 g. The Galileo spacecraft will have approximately 120 heater units to provide local heating of the spacecraft's instruments. Principal design features of the heater units are based on these safety considerations: containment of fuel under explosion overpressure and fragment conditions; and heat and impact resistance. Design features related to safety include: • platinum-30-rhodium ‘clad', selected on the basis of its compatibility with the fuel and its high melting temperature of 1910°C; • pyrolytic graphite insulators, providing thermal insulation around the clad and immediately inside the aeroshell; and • fine-weave pierced fabric aeroshell, providing reentry protection and added impact resistance. Alternatives The requirements for spacecraft electrical power and component heating can be satisfied by a variety of systems—solar panel, batteries and nuclear systems—depend-
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