Better thermal insulation is needed to insulate cryogenic propellants used by NASA for launch vehicles, spacecraft, and orbiting fuel depots. In particular, cryotank insulation during in-air pre-launch and launch ascent stages currently uses spray-on foam insulation (SOFI), which is extremely problematic.

A novel load-responsive multilayer insulation (MLI) uses a dynamic spacer that can self-support a thin, lightweight vacuum shell. Integrated multilayer insulation (IMLI) is a next-generation MLI insulation that requires an internal vacuum. A thin, flexible, lightweight vacuum shell has been invented that allows an internal vacuum to be maintained within the insulation blanket, based on thin metal overlapping plates that support a membrane vacuum overbag and are supported by underlying dynamic beam spacers. The flexible vacuum shell is to be used in conjunction with the Load Responsive MLI (LRMLI) previously designed by Quest. The system must be capable of maintaining high vacuum (1 × 10–5 torr or lower) without high levels of gas permeation. The system allows a multilayer vacuum insulation such as LRMLI to provide superior thermal performance for in-atmosphere operation. LRMLI has been shown to provide 64 times better thermal insulation than SOFI per thickness, and five times better performance in-air per mass. Furthermore, the insulation provides much higher thermal performance in low atmospheric conditions at high altitude or in vacuum (on-orbit).

LRMLI with the new vacuum shell flexible plate and membrane vacuum overbag concept can have a mass of 2.3 kg/m2, compared to an equivalent heat leak of SOFI with a mass of 11.8 kg/m2, or conventional MLI with heavy metal vacuum jacket with a mass of 10.7 kg/m2.

The invention consists of thin aluminum plates that, when in contact and supported by the outmost posts of an LRMLI blanket, are of sufficient strength to withstand atmospheric pressure exerted on the outer surfaces. These metal panels must be flexible to allow for tank thermal expansion and contraction, as well as variation in external pressure from 1 atmosphere to vacuum, with the resultant variation in dimension as the LRMLI posts compress and expand. The invention also includes a membrane vacuum overbag that is supported by the metal plates and contains the internal vacuum. A polymer foil laminate film is suitable and allows bonding or heat-sealing of seams. The polymer/foil laminate is then heat-sealed or bonded into a bag shape suitable to surround the LRMLI insulation blanket and thin vacuum shell panels that are inherently attached to a cryogenic tank. Once the assembly is complete and leak-tight, the membrane bag is then pumped down to high vacuum conditions.

This work was done by Scott Dye of Quest Product Development Corp. for Goddard Space Flight Center. GSC-16355-1


NASA Tech Briefs Magazine

This article first appeared in the October, 2014 issue of NASA Tech Briefs Magazine.

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