Highly Insulating Structures for Thermal Preservation of Sample Return Mission Samples

To maximize reliability, Earth entry vehicles (EEV) for robotic sample return missions will comprise an aeroshell, a crushable layer that will absorb the energy of the ballistic impact landing, and a sample container inside the crushable layer; parachutes will not be used. In current work for NASA, Ultramet is designing, fabricating, and testing a lightweight engineered open-cell foam for use as the crushable layer to absorb the impact energy and minimize the gravitational loading on the sample container. Modeling results based on high strain rate crush testing indicate that engineered foams can reduce the mass of a 44-kg EEV by 10 kg. The downselected foam is based on Ultramet’s off-the-shelf glassy carbon foam, which has a very low thermal conductivity (0.08 W/m∙K at room temperature), but even lower conductivity will be needed for cryogenic preservation of samples. In previous work, Ultramet pioneered the use of aerogel-filled foam (AFF) insulation for both cryogenic and high temperature applications. At 300°C, Ultramet’s glassy carbon foam has a thermal conductivity of 0.163 W/m∙K, but by filling the pores with certain aerogels, the conductivity drops to 0.11 W/m∙K. As the temperature is reduced to –88°C, the conductivity of the AFF drops by nearly an order of magnitude to 0.02 W/m∙K at ambient pressure, and at a pressure of 0.01 mtorr it drops by almost another order of magnitude to 0.003 W/m∙K. In this project, Ultramet will bring together the two technologies to develop an AFF that can simultaneously provide thermal protection for cryogenic samples and absorb the landing impact energy. Specifically, Ultramet will fabricate carbon foams with optically dense aerogels filling the pores to block ligament-to-ligament radiative heat transfer. The crush strength of the AFFs will be measured at strain rates relevant to EEV design (~300 sec-1). The resulting data will be incorporated into Ultramet’s existing model for predicting the mass of the EEV.