Improved technology for multiwatt radioisotope heater units

Aerospace radioisotope heater technology has been developed through analysis and testing of improved containment and ablation materials and configurations. To best meet the widest application range, an unvented 238Pu sphere/cube heater in two power sizes, 5 and 50 W(th), is recommended. The critical design constraint is helium pressure containment during reentry for up to 20 years. An ATJ-S graphite ablator combined with a heat-treated pyrolytic-graphite insulator wrapped around a T-111/Pt - 20 wt% Rh capsule is the optimum configuration using current technology. A HfB2 composite currently being developed would provide a major reduction in ablation rates. Computer-optimized geometry and layer thicknesses are described. The design approach was verified by fabrication and key development testing of prototype heater units.