Characterization of Dynamic Thermal Control Schemes and Heat Transfer Pathways for Incorporating Variable Emissivity Electrochromic Materials into a Space Suit Heat Rejection System

The core research objectives addressed throughout my PhD work were as follows. 1. Analytically characterize the steady-state performance envelope and contamination impacts for utilizing flexible radiators in the lunar surface environment 2. Analytically assess the impact of emissivity modulation on sink (equilibrium) temperature 3. Provide a first-order electrochromic pixel resolution metric based on analysis and testing for thermal control in a lunar environment 4. Assess transient impacts of flexible/electrochromic radiator performance on human thermal comfort in a dynamic environment 5. Evaluate integration extensibility potential for Martian surface EVA Each of the research objectives was explored within publications ranging from conference posters to technical journal articles. At the conclusion of those investigations, we showed that the full suit, variable emissivity radiator EVA thermal control architecture provided the ability to actively maintain an astronaut’s thermal condition within the specified ranges of metabolic rate and external environment. The system’s capability was consistent across the lunar and Martian surface EVA conditions examined. When the use of the system is constrained to the operational environments developed throughout the work, no water sublimation is needed for the purpose of space suit thermal control. Excursions outside of the defined envelopes, however, in either metabolic rate or environmental loading, are possible and can introduce the need for supplemental thermal control. A hybrid system that takes advantage of the local environment through the full suit radiator and provides a separate mechanism to additionally heat or cool the suit would enable the suit to maintain thermal equilibrium across much wider operational envelopes. The evaluation methods used to assess the electrochromic-based architecture also have broader applicability to implementing alternative space suit thermal control schemes and to other technologies that could be used in a variety of spacecraft. For example, in the fundamental Stefan-Boltzmann radiation heat transfer equation, other variables could be modified to control heat rejection from a space suit. While only variations in emissivity were considered throughout this research, other technologies can similarly leverage changes in radiator temperature or area. Additionally, the use of variable emissivity electrochromics has a similar potential to increase the operational envelope of spacecraft ranging from CubeSats to lunar landers. The thermal vacuum testing completed during this research provided insight into the impact of heat deposition schemes on radiator performance and verifies that mixing high and low emissivity states of discretized areas provides a predictable net emissivity. The integration of electrochromics into a space suit thermal control system will utilize either a constant heat flux or constant temperature configuration. While both heat input configurations offered the same effective net emissivity, impacts of the thermal conductivity of the substrate material must be considered, as large variations in temperature across a surface may be undesirable for a given application. The initial goal of this research was to advance the TRL of the variable emissivity suit radiator architecture. At the conclusion of Metts’ (2010) work, the TRL was considered to be a 3/4, indicating that analytical proof-of-concept and component level validation in a laboratory environment had been completed. At the conclusion of this research, the architecture remains at a TRL 3/4 by definition. However, contributions to guidelines for use in lunar and Martian surface environments, first-order pixelation considerations, and first-order dynamic simulation assessment all represent a significant iteration within this TRL. The results of these evaluations provide further characterization of the variable emissivity architecture and greatly expand the understanding of the capabilities of the proposed application. Finally, the Martian surface evaluation presented is a significant step forward in the evaluation of EVA thermal control strategies where a convective atmosphere exists. The results showed that diurnal and seasonal variations in the local environment should be explicitly considered throughout a systems development, the used of an average constant temperature is generally inappropriate. This is a significant shift in the thinking from lunar evaluation where, due to the length of a lunar day, bulk environmental loads are mostly constant for a given surface location.