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

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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) is different in microgravity such as urine calcium levels. Ground testing, in terms of major systems for eventual flight applications are of two major classes. First is ground testing of technologies for one or more future mission programs which have yet to be defined and approved (requirements have not been developed). Second is the ground testing necessary during the design, development, testing and evaluation (DDT&E) phases of specific hardware designs as part of an approved mission where requirements have been defined.

A. Ground Testing During Technology Development Phase The decision to include an integrated human-in-the-loop ground testing approach would, most likely, be made by a major sub-system such as ECLSS rather than a NASA prescribed approach. Due to the complex nature of the interdependencies of ECLSS technologies and the necessity of having to down-select technologies for continued maturation (the budgets will not support developing all candidate technologies to high levels of maturity), a strong integrated ground testing effort will serve to focus technology development efforts, coupled with targeted flight experiments for technologies believed to be microgravity sensitive, and provide a strong and cost-effective approach. NASA has defined a system to classify technology readiness in terms of its maturation from basic principles observed to flight-proven. The nine technology readiness levels (TRL) are shown in Figure 1. Technologies must continue to progress or mature through the TRLs so that technologies are “mature enough” to be available for selection by an approved flight mission or program (normally not less than TRL=6). Notice that the definition of TRL 6 is “System/subsystem model or prototype demonstration in a relevant environment (Ground or Space)”. In the case of ECLSS, since it is such a complex major sub-system, technologies must be tested in a “systems” environment with humans as an integral part of the system in a relevant environment, ground or space. As discussed earlier it is impractical from a cost standpoint to test a complete experimental ECLSS in space so a practical approach is high- fidelity ground test environments with focused space-based experiments for specific technologies with likely micro- gravity compatibility issues (such as multi-phase mixtures of gases and liquids and solids).

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Figure 1. Technology Readiness Levels (TRL).1

When a specific set of approved mission requirements is not available, a logical set of assumptions about future missions must be made to guide the content of work (such as a Mars mission, lunar mission, or an asteroid redirect mission) – these “assumptions” are viewed as surrogate “requirements” which must be under some form of configuration control. The high-fidelity ground testing then provides the “technology pull” function in the absence of an approved flight mission. Care must be taken that all components of an ECLSS mature at approximately the same rate so that a full system complement of ECLSS technologies are ready (maturity level of TRL=6) when needed to support a newly approved flight program. TRL 6 is defined as “System/sub-system model or prototype demonstration in an operational environment.”1 In general, when a technology achieves TRL 6, it is ready to be handed over to, or is a candidate technology for, a flight program. It is difficult to mature technologies to TRL 6 without the pull function of a set of requirements from an approved mission since as technologies move up the TRL scale their technology-specific cost rises which drive hard down-select decisions. In this environment, technologies are often not fully evaluated especially in a systems environment and thus cannot really achieve a TRL of 6. This has been described as the “valley of death” by NASA’s Office of the Chief Technologist (Figure 2). As a result, technologies often only achieve a TRL of 4 or 5 because they are not tested in a “systems environment”.

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Figure 2. The Valley of Death.2

Additionally, a set of agreed-to assumptions with which to make the technology comparisons and down-selects must be used (ECLSS has a periodically-updated Baseline Values and Assumptions Document3 for this express purpose).

B. Ground Testing During Design, Development, Test and Evaluation (DDT&E) Phases After A Flight Mission Has Been Approved. When a flight mission is approved, requirements are carefully defined and put under configuration control to meet the mission objectives. However, selection of technologies for the mission are also driven by cost, schedule and relative risk of each technology to be ready for flight as well as being able to meet the mission requirements. This nearly always results in selection of technologies which are at a technology readiness level (TRL) of 6 or greater so that program schedule, cost and performance objectives can be met. Thus, if technology development programs and projects do not develop objectives to deliberately and methodically mature technologies, the technologies do not mature adequately, and the flight programs when approved will not adopt them (refer to Figure 2). Rather, the flight program will select older but proven mature technologies which sometimes have difficulty meeting the requirements within the established mass, power, volume and reliability needs of the mission. The flight programs are concentrating on prototype development and testing, qualification testing, and flight certification testing. Even prototype hardware defined as, ‘hardware of a new design’, is subjected to a design qualification test program, but not necessarily designated for flight4. Due to schedule or cost pressures, prototypes will be re-classified as proto-flight hardware. The proto-flight hardware designation means - flight hardware of a new design subjected to a qualification test program combining elements of prototype and flight acceptance verification. Once the higher fidelity testing on the ground is done, the new hardware is then flown. However, this approach does include acceptance of an increased but unknown level of risk to the flight mission5. For sub-systems as complex as ECLSS, high-fidelity systems testing with humans in the loop is necessary for durations (24/7) consistent with the mission profiles. This is necessary because the possible complex interactions over time cannot be adequately predicted either computationally or by conducting component technology evaluations in a non-systems integrated environment (and humans are an integral and dynamic part of the system). Of course, corollary flight testing of technologies for those technologies expected to be impacted by microgravity provides a more complete picture from which to identify and deal with risks prior to mounting the mission.

C. “Test-like-you-fly, Fly-like-you-test” Approach Test like you fly is a philosophy which assures that the test environment, configuration, and operations reflect the way the system will be used. A “test-like-you-fly, fly-like-you-test” approach seeks to verify the system and uncover unexpected interactions and couplings; especially important in a very complex system like an ECLSS. The purpose is verifying the “ECLSS system” will behave as intended.

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An important part of verification planning is identifying system state data to be collected and analyzed. States are selected to provide visibility into potential failures. The trending of state data may uncover degradations, interactions, or functional deficiencies. These may be precursors to failures. Such information can lead to modifications in technology approaches or limits to the range of performance of specific components. Integrated ground testing not only verifies the ECLSS technologies, but should also be used to validate subsystem modeling and assumptions made during technology development phases. Extensive use of modeling has become commonplace and is crucial to eventual flight operations, but models must be fully evaluated with ground and flight test data in order to rely on them during flight operations. The closer to the operational environment the test conditions are, the higher the fidelity of the test. If elements, either hardware or software, are included that cannot be tested, the reliability of the system is effectively reduced, since those elements will not have been tested in an operational-like scenario. Therefore, during technology development phases as well as DDT&E phases, the system environment must be the guiding principle and technologies should be designed for early and repeated testability, and preliminary test plans should be developed to accurately mimic or bound the expected flight environment during testing. Determining when and in which environments to test an ECLSS system can be some of the more critical decisions, and ones that have the potential to significantly affect cost and schedule at the end of the development process. Early testing allows for a much more informed down-select process and ensures that the best technologies that are consistent with the eventual flight missions continue to mature. Fly (operate) like you test is a philosophy which avoids operating the flight system in an environment, configuration, or way which has not been verified. During technology development phases this informs the bounds or capabilities of technologies for eventual flight such that proposed flight architectures or operational regimes can be devised to more closely approximate what can be achieved during DDT&E. During DDT&E phases, specifically the system verification process, the ground testing exercises the system, in operational configurations and environments that, to the degree practical, accurately represent the flight environment and operational configurations. Nevertheless, once the system has been successfully operated in space, it could be tempting to extend the capabilities to new environments, new configurations, and new ways of operation. Great care must be taken to ensure that new environments or operational modes do not exceed the performance envelope of the original design and verification. Ground testing can provide guidance to evaluating this by conducting tests to emulate the proposed new configuration or new operating environment before implementation.

IV. History of Integrated, Human-in-the-Loop ECLSS Ground Testing Numerous tests evaluating various combinations and maturities of ECLSS technologies have been conducted over the years. Below is a brief summary.

A. Mercury / Gemini / Apollo Testing Mercury, Gemini, and Apollo missions ranged from hours to Apollo 17’s 12 days 14 hours in duration. ECLSS subsystems were “open-loop” rather than “regenerative” or recycling. However, in most cases, integrated vehicle ground testing with humans was carried out in simulated flight conditions (Figures 3,4,5, and 6).

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Figure 3. Mercury spacecraft being prepared for simulated mission test in altitude chamber at Cape Canaveral. MERCURY PROJECT SUMMARY (NASA SP-45). “The astronaut as an integral part of the system during tests. The astronaut was considered part of the total system and functioned during systems test and mission simulations as he would during the actual mission. This resulted in a dual advantage. The system tested was closer to flight configuration when the astronaut was included, and the astronaut became intimately familiar with the spacecraft and spacecraft system.”

Figure 4. Gemini spacecraft testing Figure 5. Astronaut James A. Lovell Jr., Gemini 12 prime crew at the McDonnell Aircraft pilot, simulates using space food packet while in the Gemini Corporation 30-foot chamber. Spacecraft 12 in the 30-ft. Altitude Chamber at McDonnell 1964. NASA/CR—2003-208933.7 Aircraft Corporation, St. Louis, MO. Publication Date: Aug 15, 1966, NIX (Document) ID: S66-51054.

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B. Apollo

Figure 6. The Apollo command and service module test in Chamber A at the Johnson Space Center in 1968. S68-34803. The vehicle stack was operated at vacuum conditions with human crews to fully validate the ECLSS system operations.

C. Skylab Skylab was the first long-duration set of missions (1973 -1974) with three flights of 28, 59 and 84 days, respectively, and was the first to use a regenerative CO2 removal system shown in Figure 7 (the CO2 was dumped overboard while O2 was supplied from onboard tanks). Potable water was provided from onboard tanks.

Figure 7. Skylab Pressure Swing Molecular Sieve for CO2 removal.

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In addition to ground testing of the ECLSS subsystems a 56-day ground test was carried out in a chamber running at 5 psia total pressure and 70% oxygen in July 1972 to obtain medical data and evaluate medical experiment equipment for Skylab. The test was called the Skylab Medical Experiment Altitude Test (SMEAT) (Figure 8).

Figure 8. Astronaut Robert L. Crippen, Skylab Medical Experiment Altitude Test (SMEAT) commander, holds the training model of Skylab experiment T003, the aerosol analysis test. He was part of a three-man SMEAT crew who spent 56 days in the Crew Systems Division's 20-foot altitude chamber at the NASA Manned Spacecraft Center (MSC) beginning in July 1972 to obtain medical data and evaluate medical experiment equipment for Skylab. The two crew members not shown in this view were astronauts Karol J. Bobko, SMEAT pilot, and Dr. William E. Thornton, SMEAT science pilot. S72-43280 (15 June 1972).

D. Shuttle The Shuttle fleet (Columbia, Challenger, Discovery, Atlantis, and Endeavour) flew a total of 135 missions between April 12, 1981 and July 21, 2011. The longest Shuttle mission was STS-80 which lasted 17 days 15 hours. CO2 removal was accomplished with lithium hydroxide (LiOH) canisters, an expendable, which were periodically changed out during missions. A new regenerative CO2 removal system, the Regenerative Carbon Dioxide Removal System (RCRS) (Figure 9), was developed for the Shuttle Extended Duration Orbiter (EDO) and flew 14 times between 1992 and 2003 (only Columbia and Endeavour actually flew with this technology).

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Figure 9. Astronaut Kenneth D. Bowersox, pilot, performs in-flight maintenance (IFM) on the Regenerative Carbon Dioxide Removal System (RCRS) on the mid-deck of the Earth-orbiting Space Shuttle Columbia. STS050-20-012 (26 June 1992).

E. International Space Station (ISS) The ISS ECLSS is the first major regenerative life support system in a space vehicle. Both air and water are recycled to a large degree and have been operating since 2000. Water from humidity condensate and urine is recycled up to approximately 90%. The air revitalization system (ARS) uses a regenerative carbon dioxide removal assembly (CDRA) to scrub CO2 from the atmosphere with the CO2 processed to recover O2 using a Sabatier system. Trace gas contaminants are controlled with activated carbon adsorbing bed and an ambient temperature catalytic oxidation (ATCO) system. Before the ISS ECLSS hardware was launched to the International Space Station and put into operation, it underwent extensive testing on the ground and in space. Ground testing consisted of challenging the separate ECLSS subsystems in chambers at the Marshall Space Flight Center (MSFC) (Figure 10). Testing included life cycle tests to determine maintenance and change-out requirements, operational tests, such as those using volunteers to generate waste-water for the Water Recovery System, and integration tests of flight hardware for the Space Station (Figure 11). The ISS ECLSS was not subjected to integrated ground testing with humans directly in the loop in a continuous, long-duration fashion. A four-person ECLSS ground validation test was planned for ISS lasting for 30 to 90 days as part of the flight certification but was deleted during ISS cost convergence activities in 1994.8 The ISS regenerable ECLSS hardware did not have a qualification set of hardware and did not go through a qualification program, consequently ISS has had to deal with required hardware spares, frequent routine maintenances, ‘band aid’ work around/additions, and excessive use of crew time for maintenance and spare replacement. This non- qualification program resulted in low reliability and high maintenance components that are not acceptable for deep space vehicles and surface habitats (personal communication, Hank Rotter, April 2015).9 Some parameters were unknown prior to long stays on the ISS and could not have been anticipated. One example is the accumulation of Ca in astronauts’ urine as a result of the microgravity environment.

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Figure 10. Environmental Control and Life Support Systems Test Facility at MSFC, MSFC-0101372, 2001.

Figure 11. Facility at MSFC where volunteers exercise to provide human metabolic samples (humidity condensate, CO2, and urine) to challenge ISS ECLSS hardware in another chamber. (From: NASA Facts. 2008. Marshall Space Flight Center. FS–2008–05–83–MSFC, 8–368788).

V. ECLSS Ground Testing Ground testing of ECLSS technologies is not new; it has been going on at many locations since the late 1960’s. Examples of ground testing of ECLSS are described below.

A. Langley Research Center Testing May 1968 – 28 Day Closed Door Test conducted in the Langley Integrated Life Support System (ILSS). Regenerative technologies were tested to close the oxygen loop and partially close the water loop. A four-person crew continuously occupied the ILSS over the test duration. The technologies (4-bed molecular sieve, Sabatier, and water electrolysis) are either installed on the ISS or planned for installation in the future.

B. McDonnell Douglas Astronautics Company 90-Day Manned Test of a Regenerative Life Support System 1970 – Langley contracted with McDonnell Douglas to conduct a 90-day closed chamber test including a four person crew. Regenerative technologies were tested that closed both the oxygen and water loops. A radioisotope was used to power the urine flush water processor (phase change). CO2 was controlled with a Solid Amine Water Desorbed (SAWD) unit while Sabatier reactor and water electrolysis technologies completed the O2 loop closure. A 4-bed

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molecular sieve was a backup for the SAWD and an air evaporation closed cycle unit was a backup for the urine flush water processor.10,11 This Operational 90-Day Manned Test of a Regenerative Life Support System was completed in a Space Station Simulator (SSS) on September 11, 1970, This test was conducted by the Advance Biotechnology and Power Department of the McDonnell Douglas Astronautics Company (MDAC), Huntington Beach, California, under Contract NAS1-8997. This project was performed for the NASA-Langley Research Center. The test (MDAC 90-Day Test) was successfully completed on September 11, 1970 and was performed with a crew of four men in a McDonnell Douglas space station simulator (Figure 12). The atmosphere was maintained at a total pressure of 68.9 kilo newton/meter2 (kN/m3 or kilopascal (kPa) or 10 psia and composed of an O2 partial pressure of 21 kN/m3 or 3.05 psia or approximately 30% by volume.

Figure 12. McDonnell Douglas space station simulator used for the 1970 ECLSS manned test.

C. Russia BIOS-3 1972 to 1984 Russian BIOS-3 recognizing that humans are an integral part of the system, conducted a series of closed door tests with 2- and 3-persons with life support systems making extensive use of growth of crops for food and air revitalization for periods ranging from 4 to 6 months (Figure 13).

Figure 13. Dr. Josef Gitelson discusses test conditions with test subjects inside the BIOS-3 test facility in Krasnyarsk, Russia circa 1972.

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D. Series of ISS ECLSS Tests at the Marshall Space Flight Center (MSFC) These tests were carried out where ECLSS equipment was challenged with human-generated metabolic products but not continuous (24/7) human-in-the-loop. 1990: MSFC conducted “metabolic control tests”, Space Station Freedom air revitalization and the water recovery systems each integrated and subjected to “human-generated” metabolic challenges (the humans were not in the same chamber as the ECLSS equipment, but in an adjacent chamber where groups of volunteers came and went on a regular basis); the tests ranged from 4 to 30 days. 1991 – 1994: Continuation of Space Station Water Recovery Test; as hardware was improved it was integrated and tested at MSFC testing facility with Humans-in-the-Loop (humans in an adjacent chamber where groups of volunteers came and went on a regular basis). March 1996: Space Station Phase III Integrated Atmosphere Revitalization Test at MSFC closed module testing facility. The test included the solid feed electrolyzer (SFE), carbon dioxide removal assembly (CDRA), trace contaminant control system (TCCS), and the major constituent analyzer (MCA); all hardware was challenged with human-generated metabolic products but the humans were in an adjacent chamber where volunteers came and went on a regular basis. November 1997: ISS System-Level Trace Contaminant Injection Test at MSFC, test for determining the fate of a variety of trace gas contaminants in a cabin atmosphere using the TCCS and MCA (humans in an adjacent chamber where groups of volunteers came and went on a regular basis).

E. Series of ECLSS Tests at the Johnson Space Center (JSC) - Lunar Mars Life Support Test Project 15-, 30-, 60-, and 91-Day Human-in-the-Loop ECLSS Tests, 1995 – 1997 March 1996 – 15-day test completed in the 10 Foot Chamber, with 1 human test subject, demonstrating the use of higher plants (10 m2 of wheat) to provide the air revitalization requirements of a single person for 15 days. July 1996 – 30-day test completed in the 20 Foot Chamber (Figure 14) with 4 human test subjects, utilizing physicochemical technologies for air revitalization, water recovery, and thermal control (humans occupied the test facility around the clock during the test). March 1997 – 60-day test completed with 4 human test subjects utilizing functionally similar ISS life support subsystems (Figure 15 is a ISS prototype water recovery test article.) Human test subjects occupied the test facility around the clock during the test. December 1997 – 91-day test completed with 4 human test subjects with a combination of physicochemical and biological life support hardware. Figure 16 is the bioreactor used for oxidizing organic compounds from the waste water. Humans occupied the test facility around the clock during the test. This 91-day test is the longest closed human-in-the-loop test ever done by the U.S.12

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Figure 14. 20 Foot Chamber. Figure 15. Ultrafiltration–reverse osmosis

(UFRO) for water recovery.

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Figure 16. Bioreactors for water recovery.

VI. Where Are We Today and Where Are We Going? The International Space Station (ISS) is today’s only operational manned space system and has been in continuous manned operation in low earth orbit (LEO) since November 2000. In September 2014, the Human Exploration and Operations Mission Directorate (HEOMD) Risk and Knowledge Management team in coordination with the ISS, conducted a point-in-time, cumulative summary of key lessons learned derived from the design, fabrication, integration, test, assembly, and operations of the International Space Station (ISS)6. Each lesson learned is accompanied by a corresponding application to future exploration programs. Some notable “lessons learned” related to integrated testing and are listed below:

Lesson: Fidelity of Integrated End-to-End Testing System-level, end-to-end test scenarios must replicate nominal as well as off-nominal operational and environmental conditions, duty cycles, and durations necessary to expose latent defects and verify the hardware/software. Less than adequate test fidelity will lead to on-orbit or in-flight repair and potentially endanger the crew and mission. Application to Exploration: It seems that budget issues always end up limiting the scope and duration of test programs. On the other hand, any deep space mission must arguably have a no-compromise, no-shortcut, high fidelity end-to-end system- level test program. Program managers must demonstrate leadership in bringing all stakeholders to a common understanding of the importance of the activity.

Lesson: Integrated End-to-End Testing is Essential The ISS Multi-Element Integrated Test (MEIT) program (KSC) was cited by multiple interviewees as an absolutely essential requirement. Many significant problems were identified and corrected during the MEIT activity. Latent defects in design emerged that are not evident in sub-system and component-level testing. Problem solving on-orbit is costly (if even feasible), leads to delays, consumes crew time, and may endanger crew safety. Application to Exploration: Robust end-to-end testing should be a non-negotiable must-do in any exploration mission. The ISS concept of operation has the flexibility to change-out orbital replacement units (ORUs) and troubleshoot problems with real time communications. Exploration missions will have neither benefit. End-to-end testing is the final safeguard; beyond that the crew must rely upon spares (potentially flawed) and the ability to repair.

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Lesson: “You Can’t Do Too Much Testing” “The more you can take all your problems on the ground, you’re a lot better off because we didn’t think we’d find a lot of software problems at Houston, but we found a lot and prevented a lot of problems.” “You need to do the proper verification on the ground and it needs to be a rigorous verification if you’re going to go to deep space in that you really try to find all of the ins and outs and make the systems fail.” Application to Exploration: Deep space missions will require extensive ground testing and should draw upon ISS test and verification experiences.

A noteworthy problem on the ISS surfaced in 2010 associated with the water processor assembly (WPA). Total organic carbon (TOC) measurements of the potable water showed an increasing trend. The increased TOC could not be attributed to any specific contaminant at the time. The trend was ultimately reversed after replacing the two multifiltration (MF) beds. Analysis of water samples returned to Earth revealed the cause of the elevated TOC was dimethylsilanediol (DMSD).13 However, the reason for the TOC trend and subsequent recovery was not understood. A similar trend occurred in 2012 and again in 2013, 2014, and 2015. DMSD saturated both MF Beds in the series, requiring removal and replacement of both MF Beds with significant life remaining. Concentrations of DMSD were not near toxicity limits based on samples returned to Earth, but the elevated TOC caused by DMSD could also mask other contaminants in the water which could be at levels toxic to the crew so the MF beds were replaced. After much research work, it was determined that siloxanes from crew hygiene and medical products, lubricants, adhesives, etc. in the air on ISS collect on the hydrophilic coating of the common Cabin Air Assembly (CCAA) heat exchangers. The hydrophylic coating contains metal silicates and testing showed DMSD formation from deposition of linear siloxanes or polydimethylsiloxanes (PDMS) through hydrolysis to DMSD on the hydrophilic coating of the CCAA heat exchangers; the heat exchanger coatings catalyzed the hydrolysis reaction to DMSD which is much more soluble in water. The MF beds were not designed to remove DMSD since it was not known to be a problem before it manifested itself in flight. As stated earlier, ISS ground testing was carried out by challenging the ISS ECLSS with human-generated metabolic products; not by extended human-in-the-loop integrated testing. Hence, the polydimethylsiloxanes (PDMS) were not in sufficient concentration to become a problem during ground testing. What is the likelihood that it could have been detected (and dealt with prior to flight) in higher fidelity human-in-the-loop ground testing during development of the ISS ECLSS equipment? It turns out that increasing siloxane concentrations did appear in the Lunar Mars Life Support Test Project’s 91-day test in 1997 (Figure 17).

Figure 17. Lunar Mars Life Support Test Project – Phase III 91-day Test 1997.

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VII. Conclusions 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. The key to developing highly reliable (and maintainable) ECLSS systems is to be methodical about stepping through integrated levels of testing from the bench scale in the laboratory up through major systems such as air revitalization and water recovery and then to integrated testing with a full up ECLSS and humans as part of the test. Such tests must be conducted in a sealed ground test facility for extended periods of time to gain confidence in the ECLSS system destined for flight beyond low Earth Orbit. There are just too many potential unknowns that can easily translate into unacceptable levels of mission risk.

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