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ATMOSPHERE REVITALIZATION OVERVIEW

As space exploration objectives extend the boundaries of human presence in our Solar System beyond low Earth orbit, the challenge associated with sustaining human life in the hostile space environment becomes ever greater. The technological approach to this challenge must build upon the foundation of NASA’s legacy of crewed space exploration programs during which many space-faring crews have explored space and returned safely to Earth. Beginning with Project Mercury and continuing through the International Space Station (ISS) program, the technological solutions for environmental control and life support (ECLS) systems have evolved from those required to support a single astronaut for minutes or hours to those supporting crews of three or more continuously for months and ultimately years.
The Vision for Space Exploration (VSE) has outlined space exploration objectives that challenge the NASA’s heritage in spacecraft ECLS system design and operation. Specific objectives that compose the VSE are the following:
1) Implementing sustained, affordable space exploration programs
2) Extending human presence throughout the solar system
3) Developing innovative technologies, knowledge, and infrastructure in support of exploration objectives
4) Promoting international and commercial participation to further the United States’ scientific, security, and economic interests
New requirements and performance standards will emerge as these objectives are defined in more detail and the pace quickens toward their implementation. The primary goal of the NASA’s efforts to develop the next generation of ECLS systems is to identify and mature the most promising process technologies available that address the most critical needs for making the VSE reality. To this end, the Exploration Life Support (ELS) Project is dedicated to identifying and maturing atmosphere revitalization (AR) process technologies for application to NASA’s next generation crewed spacecraft is a leading initiative. Narrowing the technological gap and mitigating risk to help make the United States’ space exploration dreams come true are the ELS Project’s principal goals.

Atmosphere revitalization aboard crewed spacecraft—past and present.

NASA enjoys a rich heritage in the developing and deploying  process technologies for the AR systems on board crewed spacecraft and space habitats such as Skylab and the International Space Station. Over the years that have spanned Project Mercury through ISS, NASA has employed a variety of process technologies and hardware embodiments to condition and purify spacecraft cabin atmospheres. The primary processes enabled by AR system equipment can be described as separations or reactions. The most common separations processes used include physical adsorption, absorption, and filtration. Chemical adsorption, oxidation, reduction, and electrochemical are some of the reactive processes employed. Removing carbon dioxide, trace volatile organic compounds (VOCs), and particulate matter (PM) from the cabin atmosphere are typical AR system functions. Other functions include resource recovery and recycling and atmospheric gas production, distribution, and storage.

AR processes used by the NASA to meet crewed space exploration

challenges includes removing carbon dioxide (CO2) with expendable granular lithium hydroxide (LiOH) canisters for the Mercury, Gemini, Apollo, and Shuttle Programs. Molecular sieves, regenerated by vacuum swing and/or thermal swing have been used for removing CO2 from the cabin atmosphere for the Skylab and ISS Programs. The CO2 removal equipment on board the ISS has been designed to ultimately accommodate the greatest approach toward a closed AR system whereby CO2 is reacted with hydrogen (H2) produced by a water electrolysis unit. The interaction between these processes recover a significant amount of water that otherwise must be supplied from Earth. Some experience has been gained with immobilized amines that are regenerated by a vacuum swing process. While such a process has been demonstrated on board several Shuttle missions, it is not presently used on any of NASA’s operational crewed spacecraft. Gaining an increased understanding of the amine-based process is a significant focus area for the ELS’s AR process technology developmental effort.

While the NASA takes great pains to keep each space vehicle clean when it is built and prepared for flight, it is impossible to completely eliminate all the trace atmospheric contaminant sources. These sources include offgassing from materials of construction and myriad chemicals used by the crew for personal hygiene and for conducting experiments. The crewmembers themselves also generate a significant amount of gaseous contaminants as well as contribute to the major share of PM released into the cabin atmosphere. To prevent these

contaminants from building up to unhealthy concentrations, NASA typically uses granular activated carbon and a variety of oxidation catalysts to remove the trace chemical contaminants from a cabin atmosphere. GAC derived from coconut shells is normally used. Some GAC is specially treated to remove contaminants such as ammonia (NH3) and formaldehyde (CH2O). For broad spectrum VOC control, NASA uses thermal catalytic oxidation in combination with GAC to remove methane (CH4) and carbon monoxide (CO) during long duration missions.

Tasks 

To bring the VSE’s goals to reality, a variety of AR system process technology maturation tasks are being pursued. Principle functions being addressed by these tasks include CO2 partial pressure control, trace VOC control, PM removal, atmospheric gas storage and distribution, resource recovery, and supporting infrastructure. Candidate technologies include the following: CO2 partial pressure control—improved chemisorbent media, improved physical adsorption media, novel adsorbent media substrates and bed packing geometries, vacuum swing adsorption processes, combined vacuum/temperature swing adsorption processes, and reversible reactive processes. Trace VOC control—expendable chemisorbent media, regenerable adsorbent media, novel adsorbent media substrates and bed packing geometries, improved thermal oxidation reactor designs including novel catalyst substrates and reactor geometries, highly efficient recuperative heat exchanger designs. PM removal—inertial separation (10 microns), electrically-enhanced filtration (<0.1 micron), electrostatic precipitation (<0.1 micron), and HEPA filtration (0.3 micron) system designs. Atmospheric gas storage, supply, and distribution—storage tank recharging equipment; high pressure, cryogenic, and supercritical storage; high pressure oxygen production. Resource recovery, storage, and recycling—carbon dioxide reduction processes to methane and carbon products, atmospheric gases from in-situ resources Supporting infrastructure—improved blowers, valves, and process monitoring and control instrumentation; well-developed robust design practices. Near term emphasis process technology maturation tasks is address AR process technology needs for the Orion Program. The process technologies being considered as likely solutions to AR these near-term challenges are: O2 partial pressure control—physical adsorption and reactive processes regenerated by vacuum swing Trace VOC control—expendable chemisorbents for ammonia, regenerable adsorbents for volatile organic compounds (VOCs), and improved ambient temperature CO oxidation catalysts Atmospheric gas storage and distribution—high pressure, cryogenic with minimum zero boil off Supporting infrastructure—imp roved blowers, valves, and process monitoring and control instrumentation Many process technologies being investigated by the ELS Project’s AR Element are expected to find a niche in multiple flight programs as the United States continues exploring the boundaries of human endurance on board the ISS and returns to the Moon to stay. In this endeavor, the ELS Project plays a key role in bringing the vision into focus.