Investigation of Potential Low Temperature Insulators

Abstract

The requirement for long-duration (greater than one or two years) space flights is rapidly approaching, and the passive type insulation systems (surface coatings, foams, cork, superinsulations, etc) used for cryogenic tankage In short-duration flights (less than 24 hr) will probably be replaced by active insulation systems (refrigeration systems, reliquefaction systems, etc). The most immediate problem is for intermediate-duration flights (greater than 1 day but less than 1 yr), and for this problem superinsulation or radiation shields appear to be the most attractive insulations. Systems such as NCR-2 and Linde SI-62 have apparent thermal conductivity values when measured under high vacuum between room temperature and -320°F of 1 to 3 × 10^-5 Btu-ft/hr-ft²-°F. Evacuated fiberglass and evacuated powder insulations have k-factors approximately 30 to 50 times higher than superinsulations, whereas plastic foams have k-factors 500 to 1000 times higher. If one compares these insulation systems on a kp or (kp) ^1/2 basis, the differences between systems are reduced, but the superinsulations are still significantly better than the other passive systems.