Search Results

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.

In preparation for the contract award of the Crew Exploration Vehicle (CEV), the National Aeronautics and Space Administration (NASA) produced two design reference missions for the vehicle. The design references used teams of engineers across the agency to come up with two configurations. This process helped NASA understand the conflicts and limitations in the CEV design, and investigate options to solve them.

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.

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.

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.

When permanent bases are established on the moon, various methods may be employed to reject the heat generated by the base. One proposed concept is the use of a heat pump operating with a vertical, flow-through thermal radiator which is mounted on a Space Station type habitation module. Since the temperature of the lunar surface varies over the lunar day, the sink temperature for heat pump heat rejection will vary. As a result, the heat pump power demand will also vary over the lunar day. This variable power requirement could be provided by a fixed horizontal solar photovoltaic (PV) array placed on the lunar surface, since its power production will vary sinusoidally with the time of day. Using a dedicated PV array to power the heat pump may represent a favorable mass trade-off compared to enlarging the size of the base's central power grid due to power system simplification and improvements in efficiency.

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.

Work defining advanced life support (ALS) technologies and evaluating their applicability to various long-duration missions has continued. Time-dependent and time-invariant costs have been estimated for a variety of life support technology options, including International Space Station (ISS) environmental control and life support systems (ECLSS) technologies and improved options under development by the ALS Project. These advanced options include physicochemical (PC) and bioregenerative (BIO) technologies, and may in the future include in-situ-resource utilization (ISRU) in an attempt to reduce both logistics costs and dependence on supply from Earth. PC and bioregenerative technologies both provide possibilities for reducing mission equivalent system mass (ESM). PC technologies are most advantageous for missions of up to several years in length, while bioregenerative options are most appropriate for longer missions. ISRU can be synergistic with both PC and bioregenerative options.