Defining touch temperature limits for skin contact with both hot and cold objects is important to prevent pain and skin damage, which may affect task performance or become a safety concern. Pain and skin damage depend on the skin temperature during contact, which depends on the contact thermal conductance, the object's initial temperature, and its material properties. However, previous spacecraft standards have incorrectly defined touch temperature limits in terms of a single object temperature value for all materials, or have provided limited material-specific values which do not cover the gamut of likely designs. A new approach has been developed for updated NASA standards, which defines touch temperature limits in terms of skin temperature at pain onset for bare skin contact with hot and cold objects. The authors have developed an analytical verification method for safe hot and cold object temperatures for contact times from 1 second to infinity.

The addition of metallic nanoparticles to a base heat transfer fluid can dramatically increase its thermal conductivity. These nanofluids have been shown to have advantages in some heat transport systems. Their enhanced properties can allow lower system volumetric flow rates and can reduce the required pumping power. Nanofluids have been suggested for use as working fluids for spacecraft Active Thermal Control Systems (ATCSs). However, there are no studies showing the end-to-end effect of nanofluids on the design and performance of spacecraft ATCSs.In the present work, a parametric study is performed to assess the use of nanofluids in a spacecraft ATCSs. The design parameters of the current Orion capsule and the tabulated thermophysical properties of nanofluids are used to assess the possible benefits of nanofluids and how their incorporation affects the overall design of a spacecraft ATCS. The study shows that the unique system and component-level design parameters of spacecraft ATCSs render them best suited for pure working fluids. The addition of nanoparticles to typical spacecraft thermal control working fluids actually results in an increase in the system mass and required pumping power. The addition of metallic nanoparticles to a base heat transfer fluid can substantially increase its thermal conductivity. These nanofluids have been shown to have advantages in some heat transport systems. Their thermal properties allow the system volumetric flow rate to be reduced, thus reducing the required pumping power. Nanofluids have been suggested as working fluids in spacecraft Active Thermal Control Systems (ATCSs). However, spacecraft ATCSs are unique in that they have stringent temperature control requirements and use specialized heat transfer devices. In the present work, a parametric study was performed to assess the use of nanofluids in spacecraft ATCSs. The tabulated thermophysical properties of various nanofluids, the design requirements of NASA's Orion ATCS and the performance parameters of its key heat transfer components were used to assess the effects of the incorporation of nanofluids. The study shows that the unique system and component-level design parameters of spacecraft ATCSs do not lend themselves to the use of nanofluids. The addition of nanoparticles to typical spacecraft internal flow loop working fluids actually results in an increase in either the system mass or the required pumping power, the opposite of the hoped-for effect. The intermediate results obtained in the study also suggest that that the addition of nanoparticles to an external ATCS loop is not likely to result in a significant overall system benefit.

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