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Thermal conductivities for different volume ratios of cross-linked silica aerogel to RTV 655 were experimentally determined. Silica aerogel is an ideal insulator with a make up of 99% air. Cross-linked silica aerogel is a much stronger material while retaining the thermal properties of silica aerogel. RTV 655 can be shaped into any geometry and is stable under extreme temperature conditions. RTV 655 is a space qualified thermal insulator and the compound of aerogel-RTV 655 is being considered for use as a cryogenic propellant tank material for long duration space missions. Current cryogenic tanks, which are constructed using metals, experience self-pressurization due to incident solar radiation and extreme temperature changes. The heat transfer to the tank results in propellant vaporization. The compound cross-linked silica aerogel with RTV 655 is a new alternative for cryogenic tank constructions and may provide a means of significantly reducing the tank self pressurization rates. Several samples were made to test thermal properties of different volume ratios in an effort to assess the thermal conductivity of the combination of cross-linked silica aerogel and RTV 655. Thermal conductivity results are presented comparing a homogeneous compound consisting of micro scale aerogel particles embedded in RTV655 and a monolithic aerogel slab embedded in RTV 655. NomenclatureSymbols and Units k thermal conductivity, W/(m⋅K) α thermal diffusivity, m 2 /sec t time, sec ρ density of the material tested, g/cm 3 m mass of the material tested, gm V volume of the material tested, cm 3 VR volume ratio of cross linked silica aerogel-RTV 655Subscripts rtv room temperature vulcanized silicone 655 aerogel cross-lined silica aerogel