Ground Testing for Development of Environmental Control and Life Support Systems for Long Duration Human Space Exploration Missions
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48th International Conference on Environmental Systems ICES-2018-5 8-12 July 2018, Albuquerque, New Mexico Ground Testing for Development of Environmental Control and Life Support Systems for Long Duration Human Space Exploration Missions Donald L. Henninger, PhD1 NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas, 77058 ECLS systems for very long-duration human missions to deep space will be designed to operate reliably for many years and will never be returned to Earth. The need for high reliability/maintainability is driven by unsympathetic abort scenarios. Abort from a Mars mission could be as long as 450 days to return to Earth. Simply put, the goal of an ECLSS is to duplicate the functions the Earth provides in terms of human living and working on our home planet but without the benefit of the Earth’s large buffers – the atmospheres, the oceans and land masses. With small buffers, a space-based ECLSS must operate as a true dynamic system rather than independent processors taking things from tanks, processing them, and then returning them to product tanks. Vital is a development process that allows for a logical sequence of validating successful development (maturation) in a stepwise manner with key performance parameters (KPPs) at each step; especially KPPs for technologies evaluated in a full systems context with human crews on Earth and on space platforms such as the International Space Station (ISS). This paper will explore the implications of such an approach to ECLSS development and the types and roles of ground and space-based testing necessary to develop a highly reliable life support system for long duration human exploration missions. Historical development and testing of ECLS systems from Mercury to the ISS will be reviewed. Current work as well as recommendations for future work will be described. Nomenclature ARS = Air Revitalization System CCAA = Common Cabin Air Assembly CDRA = Carbon Dioxide Removal Assembly CO2 = Carbon Dioxide DDT&E = Design, Development, Testing & Evaluation DMSD = Dimethylsilanediol ECLSS = Environmental Control and Life Support Systems EDO = Extended Duration Orbiter HEOMD = Human Exploration Operations Mission Directorate IFM = In-Flight Maintenance ILSS = Integrated Life Support System ISS = International Space Station JSC = Johnson Space Center KPP = Key Performance Parameters KSC = Kennedy Space Center LEO = Low Earth Orbit LiOH = Lithium Hydroxide MCA = Major Constituent Analyzer MDAC = McDonnell Douglass Astronautics Company MEIT = Multi-Element Integrated Test MF = Multifiltration MSFC = Marshal Space Flight Center NASA = National Aeronautics and Space Administration ORU = Orbital Replacement Unit RCRS = Regenerative CO2 Removal System SAWD = Solid Amine Water Desorbed SFE = Solid Feed Electrolizer SSS = Space Station Simulator TCCS = Trace Contaminant Control System TOC = Total Organic Carbon TRL = Technology Readiness Level UFRO = Ultrafiltration Reverse Osmosis VCD = Vapor Compression Distillation WPA = Water Processor Assembly I. Introduction PACE-BASED Environmental Control and Life Support Systems (ECLSS) have been a critical part of all S human spaceflight missions to date. Early missions such as Mercury, Gemini, Apollo and Apollo-Soyuz were relatively short with Apollo 17 in 1972 lasting 12 days, 13 hours, 51 minutes, 59 seconds which was one of the longest human missions up to that time. The mission requirements for these relatively short missions allowed for the ECLSS to rely on stored consumables such as tanks of water and oxygen and an expendable (lithium hydroxide (LiOH)) for carbon dioxide (CO2) removal. Skylab was the first to use a “regenerative” approach in which CO2 was selectively adsorbed with a molecular sieve with the CO2 then vented to space vacuum over repeated cycles. This “no or reduced consumable” approach was also used during selected Shuttle missions. The International Space Station (ISS) was the first flight vehicle to use reduced-consumable regenerative approaches for both air and water. ECLS systems for very long-duration missions to the Earth’s moon or Mars will be designed to operate reliably for many years and will never be returned to Earth. Design for reliability and maintainability (as well as low maintenance) will be major drivers. The need for high reliability is driven by unsympathetic abort scenarios for deep space missions. Future human exploration missions beyond low Earth orbit (LEO) will be to establish a long-term human presence. Such mission campaigns will likely last many years. Further, missions beyond LEO have more difficult abort scenarios than missions in LEO. In the case of the Earth’s moon, an abort takes a few days to return to Earth whereas an abort from a Mars mission could be as long as 450 days to return to Earth. Thus there is more emphasis on a regenerative ECLSS as well as the need for higher reliability over much longer durations. ECLS systems are a complex assembly of very dynamic and interrelated processers that must operate as a seamless well integrated system. The goal of an ECLSS is to duplicate the functions the Earth provides in terms of human living and working on our home planet. On Earth, we rely on a set of complex chemical and biological systems to sustain us. The Earth’s buffers of the atmosphere, oceans, and land masses provide us with a very reliable “Earth ECLSS.” In space, the challenge for a reliable ECLSS is how small can the equivalent buffers be and still have a stable and resilient system to sustain human crews. The buffers must be as small as possible to keep total mass of the ECLSS to a reasonable level. The ECLSS must operate as a true dynamic system rather than independent processors taking things from tanks, processing it, and then returning them to product tanks. Thus, control strategies are very important in such a fast-changing complex system. Key to developing such systems is a development process that allows for a logical development sequence validating successful development in a stepwise manner. Establishing key performance parameters (KPPs) for technologies tested alone or in concert with selected other technologies is critical. This applies also to evaluations with human crews on Earth and on space platforms such as the ISS. II. Implications Since human missions to deep space will be very long duration, all materials inside pressurized volumes of vehicles and habitats will be exposed to the crew. And, whatever organic and inorganic compounds are formed through chemical reactions and/or biological action will also potentially be exposed to the crews. A vehicle or habitat occupied by humans and an operating ECLSS will be subject to chemical and biological changes limited only by the laws of chemistry, physics and biology. Compounds which are created only very slowly and consequently accumulate very slowly, could become potential problems over multiple year missions. Further, predicting what will be present in a vehicle habitat over long periods of time is impossible without high-fidelity, long-term, and human-in-the-loop testing. Preferably, this testing should take place in as realistic environment as possible. The space environment would be ideal but to build such a long-duration test capability in space is expensive. Rather, such testing can and should be carried out in Earth-based, sealed chambers with the space based systems intended for flight and human crews to 2 International Conference on Environmental Systems interact with the environment. Of course, ground test capabilities cannot impart micro or partial gravity gradients on the systems to be tested so flight experiments are necessary to get a more complete assessment of future flight systems. III. Ground Testing Integrated ground testing is relatively benign in terms of both risk and cost – it does not leave the Earth and is easy to access, change, and repeat. Ground-based test facilities and chambers can be used to economically and repeatedly test various operational concepts, technologies, components, and systems in a variety of simulated environments. Ground-based testing allows both individual component and system-level testing for certification of advanced technologies and systems before use. Both simulators and field tests allow “build a little; test a little” to provide greater insight into “go/no-go” technical decisions. Test repeatability of hardware performance, development of maintenance procedures, and an understanding of operational support needs are necessary prior to commitment to long-duration missions. In addition, ground-based testing of actual flight hardware in simulated real “flight-like” conditions provides an opportunity to model expected as well as unexpected failure modes while qualifying and certifying hardware for flight. This is the “test like you fly and fly like you test” philosophy. Ground testing allows for a greater array of performance data since ground laboratories can be used for extensive measurements not possible on the ISS or in a flight environment. Since risk to humans is much lower than flight, ground testing allows for examination of extremes of performance and failure scenarios that are not safe to pursue on ISS. Ground tests provide opportunities to make more dramatic system changes than in-line ISS; for example reduced atmosphere pressures and oxygen concentrations, new urine pretreatments or potable water biocides, etc. However, testing on ISS is still needed to assess gas/liquid fluid behavior in microgravity and crewmember biology (and sometimes behavior)