Cryogenic Moisture Uptake in Foam Insulation for Space Launch Vehicles

Rigid polyurethane or polyisocyanurate foams used for existing space launch vehicles such as the space shuttle and Delta 4, and planned for use on future space vehicles, were tested under cryogenic conditions and found to gain an extraordinary amount of water. A cryogenic moisture uptake apparatus was developed to determine the amount of water taken into the specimen under actual use propellant loading conditions. After 8 h of test duration with a simulated launch pad environment on one side and liquid nitrogen temperature on the other, these foams gained at least 30% (new condition) to 75% (weathered condition) by mass. This effect can translate into an additional mass of over 1000 kg for space vehicles at liftoff. To determine the thermal performance and moisture uptake of foam insulation systems, three different materials were tested, including NCFI 24-124, NCFI 27-68, and BX-265. Results are presented for testing of both aged specimens and weathered specimens. The trends of increasing mass gain are clear for both aging exposure and weathering exposure durations up to 24 months. The water accumulation in these flight quality, closed cell polyurethane foams is shown to be water vapor driving into the subsurface due to the extreme thermal gradient imposed by the cryogen.