As one of its long term objectives NASA continues to support work on radioactive heat sources and a recent report by Atomics International (USAEC Rept. NAA–SR–12578) provides some interesting preliminary design data for a proposed test capsule capable of holding a large (undisclosed) heat source and operating in a space environment for more than five years with a surface temperature of approximately 1090°C.

The total assembly, in the form of a cylinder with hemispherical ends is 7.37 inches long and 1.72 inches in diameter. The isotope heat source is centrally disposed, surrounded by a tantalum honeycomb and finally welded into a thick walled tantalum alloy shell which provides the strength for long term helium containment and for impact survival. An external covering of 10 per cent rhodium-platinum alloy, 0.020 inches thick, protects this shell from oxidation. Metallic diffusion between the tantalum and platinum alloys is inhibited by a thin layer of vapour phase deposited alumina. The outer surface of the platinum alloy is coated with a layer of iron titanate to increase heat emmissivity.

Paradoxically enough the design of this capsule was complicated because of its low operating temperature, no reliable creep data on rhodium-platinum alloys tested below 1200°C having yet been published. The designers extrapolated downwards with respect to temperature and outwards by a factor of 45 with respect to time. In concluding that hoop stresses in the rhodium-platinum sheath should be kept below 75 pounds per sq. inch they emphasise the uncertainties involved in their assumptions.

The compatibility studies scheduled in this report will be made in vacuum of 10⁻⁸ Torr for periods up to 2200 hours. In both geometry and environment these test conditions depart from those existing inside the proposed capsule. It will be interesting therefore to read in subsequent reports whether the reactions between tantalum, alumina and rhodium-platinum tested in close association inside the capsule in a helium atmosphere differ significantly from those which occur in vacuum where the reaction products are continuously removed.