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Future manned habitats for a Lunar outpost or Martian base will require increased levels of self-sufficiency over Space Station Freedom to reduce the high costs and complexities of resupplying expendables, such as food for the crew. By growing food at these remote sites, not only will self-sufficiency be greatly increased, but significant benefits for crew life support will also be realized. Higher plants, such as those grown typically for food, are capable of consuming carbon dioxide (CO2), producing oxygen (O2), and reclaiming water (H2O) via transpiration. At NASA's Johnson Space Center (JSC) in Houston, Texas, the Regenerative Life Support Systems (RLSS) Test Bed project will use higher plants grown in a closed, controlled environment in conjunction with physicochemically-based life support systems to create an integrated biological/physicochemical RLSS.

Extended manned missions to the lunar and martian surfaces pose new challenges for active thermal control systems (ATCS's). Moderate-temperature heat rejection becomes a problem during the lunar day, when the effective sink temperature exceeds that of the heat-rejection system. The martian atmosphere poses unique problems for rejecting moderate-temperature waste heat because of the presence of carbon dioxide and dust. During a recent study, several ATCS options including heat pumps, radiator shading devices, and single-phase flow loops were considered. The ATCS chosen for both lunar and martian habitats consists of a heat pump integral with a nontoxic fluid acquisition and transport loop, and vertically oriented modular reflux-boiler radiators. The heat pump operates only during the lunar day. The lunar and martian transfer vehicles have an internal single-phase water-acquisition loop and an external two-phase ammonia rejection system with rotating inflatable radiators.

Candidate low-toxicity working fluids are evaluated for active internal thermal control systems in various NASA applications, such as human exploration missions and low-earth orbit spacecraft. The principal goal is to attain a lower freezing point than pure water (currently popular), for added protection against system blockage or bursting in either expected low temperature environments or in the event of failure. Fluids considered for moderate-temperature freeze protection include aqueous solutions of ethylene glycol, propylene glycol, denatured ethyl alcohol, glycerin, and potassium acetate. For very low-temperature freeze protection, the liquids Fluorinert 72, Hydrofluoroether 7100, D-Limonene, R-116, and R-134a are considered. Fluid performance with regard to pump power and heat exchange is evaluated based on comparison with water for fixed hardware and operating conditions.

Engineers at the Johnson Space Center (JSC) are using innovative strategies to design the TCS for the Bio-regenerative Planetary Life Support Systems Test Complex (BIO-Plex), a regenerative advanced life support system ground test bed. This paper provides a current description of the BIO-Plex TCS design, testing objectives, analyses, descriptions of the TCS test articles expected to be tested in the BIO-Plex, and forward work regarding TCS. The TCS has been divided into some subsystems identified as permanent “infrastructure” for the BIO-Plex and others that are “test articles” that may change from one test to the next. The infrastructure subsystems are the Heating, Ventilation and Air-Conditioning (HVAC), the Crew Chambers Internal Thermal Control Subsystem (CC ITCS), the Biomass Production Chamber Internal Thermal Control Subsystem (BPC ITCS), the Waste Heat Distribution Subsystem (WHDS) and the External Thermal Control Subsystem (ETCS).

Long duration human exploration missions to the Moon will require active thermal control systems which have not previously been used in space. The relatively short duration Apollo missions were able to use expendable resources (water boiler) to handle the moderate heat rejection requirement. Future NASA missions to the Moon will require higher heat loads to be rejected for long periods of time near the lunar equator. This will include heat rejection during lunar noon when direct radiation heat transfer to the surrounding environment is impossible because the radiator views the hot lunar surface. The two technologies which are most promising for long term lunar base thermal control are heat pumps and radiator shades. Heat pumps enable heat rejection to space at the hottest part of the lunar day by raising the radiator temperature above the environment temperature.

The thermal radiators are a major subsystem of the Space Station Freedom (SSF) Active Thermal Control System (ATCS). They dissipate to deep space the excess heat transported from the modules and truss mounted equipment. Condensation of the ATCS twophase working fluid occurs directly in small diameter tubes which are bonded to a thin aluminum face sheet in the flow-though radiator panels. The Permanently Manned Capability (PMC) configuration of the Space Station will have a total of 48 radiator panels grouped in 3 replaceable units of 8 panels on each side of the Space Station. Accurate prediction of radiator performance on orbit is important to keep the ATCS from getting too hot (exceeding its capacity) or getting too cold (freezing). For this reason, detailed models of the radiator system are being developed using the SINDA/FLUINT thermal and fluid systems analyzer.

This paper describes the process and analysis used to select a refrigerant and thermodynamic cycle as the basis of a vapor compression heat pump requiring a high temperature lift. Use of a vapor compression heat pump versus other types was based on prior work performed for the Electric Power Research Institute. A high lift heat pump is needed to enable a thermal control system to remove heat down to 275K from a habitable volume when the external thermal environment is severe. For example, a long term habitat will reject heat from a space radiator to a 325K environment. The first step in the selection process was to perform an optimization trade study, quantifying the effect of radiator operating temperature and heat pump efficiency on total system mass; then, select the radiator operating temperature corresponding to the lowest system mass. Total system mass included radiators, all heat pump components and the power supply system.

Independent temperature and humidity control may be required for a variety of reasons. One application under study at the NASA Johnson Space Center is the environmental control of completely sealed plant growth chambers. The chambers are used to optimize plant growth and to develop engineering prototypes of future plant growth chamber modules for long duration space travel. One chamber at the Johnson Space Center which is part of the Early Human Test Initiative was rebuilt and upgraded during 1994. Requirements called for a thermal control system which could supply the plants with a wide range of air temperatures and independently control humidity. A math model was developed using G189 thermal/environmental modeling software to simulate the internal environment of the plant growth chamber. The model was used in the design of the chamber thermal control system.

Growth of higher plants In closed environments requires a great deal of energy for lighting systems. Even the most efficient lights deliver only about a quarter of the energy they use as useful radiation for plant growth (photosynthetically active radiation or PAR). The remainder of the energy, as well as most of the PAR, ends up as waste heat which must be removed from the plant growth chamber. The thermal control system (TCS) which does this job can require a significant amount of volume, mass and power. Efficient and effective design of the TCS is therefore important to the overall feasibility of the plant growth chamber, either for terrestrial or aerospace purposes. As part of the Early Human Testing Initiative being conducted by the Crew and Thermal Systems Division at the Johnson Space Center, a plant growth chamber has been designed and built which has instruments for research and is outfitted for human testing.

Several factors are important in the development of active thermal control systems for planetary habitats. Low system mass and power usage as well as high reliability are key requirements. Ease of packaging and deployment on the planet surface are also important. In the case of a lunar base near the equator, these requirements become even more challenging because of the severe thermal environment. One technology that could be part of the thermal control system to help meet these requirements is a radiator shade. Radiator shades enhance direct radiative heat rejection to space by blocking solar or infrared radiation which lessens the performance of the radiator. Initial development work, both numerical and experimental, has been done at the Johnson Space Center (JSC) in order to prove the concept. Studies have shown that heat rejection system mass may be reduced by 50% compared to an unshaded low-absorptivity radiator.

In an era of tight fiscal constraints, research and development funds are not sufficient to study all possible avenues for technology development. Hence, development priorities must be set and funding decisions made based on the projected benefits which will arise from fully developing different technologies. In order to identify promising development initiatives for advanced thermal control systems, a study was conducted which quantified the potential mass savings of various technologies. Assessments were made for five reference missions considered to be likely candidates for major human space flight initiatives beyond the International Space Station. The reference missions considered were Space Station Evolution, Space Shuttle Replacement, First Lunar Outpost Lander, Permanent Lunar Base, and Mars Lander. For each mission a baseline active thermal control system was defined and mass estimates were established.

Many studies have been conducted to develop a thermal control system that can operate under the extreme thermal environments found on the lunar surface. While these proposed heat rejection systems use different methods to reject heat, each system contains a similar component, a thermal radiator system. These studies have always considered pristine thermal control system components and have overlooked the possible deleterious effects of lunar dust contamination. Since lunar dust has a high emissivity and absorptivity (greater than 0.9) and is opaque, dust accumulation on a surface should radically alter its optical properties and therefore alter its thermal response compared ideal conditions. In addition, the non-specular nature of the dust particles will may alter the performance of systems that employ specular surfaces to enhance heat rejection. To date, few studies have examined the effect of dust deposit on thermal control system components.