Comparison of thermal insulation performance of fibrous materials for the advanced space suit.
The current multi-layer insulation used in the extravehicular mobility unit (EMU) will not be effective in the atmosphere of Mars due to the presence of interstitial gases. Alternative thermal insulation means have been subjected to preliminary evaluation by NASA to attempt to identify a material that will meet the target conductivity of 0.005 W/m-K. This study analyzes numerically the thermal conductivity performance for three of these candidate insulating fiber materials in terms of various denier (size), interstitial void fractions, interstitial void media, and orientations to the applied temperature gradient to evaluate their applicability for the new Mars suit insulation. The results demonstrate that the best conductive insulation is achieved for a high-void-fraction configuration with a grooved fiber cross section, aerogel void medium, and the fibers oriented normal to the heat flux vector. However, this configuration still exceeds the target thermal conductivity by a factor of 1.5.
Feasibility Demonstration of a Thermal Switch for Dual Temperature IR Focal Plane Cooling
A conical-shaped hydrogen gas-gap thermal switch using a metal hydride sorption bed to control the hydrogen gas supply was designed to provide the thermal isolation link between two cooling systems operating at different temperatures. Test results of the thermal switch show a nominal 1 K/W thermal resistance in the on-state conduction mode for temperatures above 100 K and for heat flows to 8 W, and a temperature dependent 600–900 K/W thermal resistance in the off-state conduction mode. In particular, the off-state resistance of the switch for operation between 60 K and 140 K was 660 K/W, and with a 0.92 K/W on-state resistance at 140 K, the off-state/on-state switching ratio of 717 was achieved. The switching time to convert to the on-state or off-state mode was on the order of 5 minutes and 12 minutes, respectively.
The thermal conductivity of textiles
A review is given of measurements of the thermal conductivity of natural clothing fibres and of formulae that have been proposed for the overall thermal conductivity of fibre-air mixtures in terms of their density and fibre conductivity. Measurements of the conductivity of wool felts ranging in density from 0.01 to 1.0 gm./cc. have been made, using Hercus and Laby's method for determining the conductivity of air. The conductivity of solid keratin (horn) has also been measured by the Lees disc method, and it is suggested that wool fibres and horn have identical conductivities of 4.62×10-4 c.g.s. units at 0.7% regain and 5.32×10-4 c.g.s. units at 10.7% regain. The data given for wool felts and horn are used to obtain an empirical relation between conductivity and bulk density. Results obtained with cotton fabrics indicate that published values of the thermal conductivity of cotton fibres are too high.