Spaceflight Life Support Systems


(Excerpt from Chapter 2)

A case mentioned in Section 2.1.3.4 as an issue in human-automation systems referred to the debate in the space community of whether to support either purely robotic or manned missions for space exploration, the main argument in support of the former being that they are less expensive.

The cost of both robotic and manned missions is determined by requirements in mass, volume, and power [113]. Manned missions differ from robotic ones in that, in addition to science instruments, they also need to support the physiological demands and quality of life of a human crew [46, 114]. The subsystems that keep the crew alive while contributing to mission success are called life support systems (LSS). These subsystems add mass and volume to mission elements, resulting in the need for greater launch capacities, which as a consequence increase their overall cost [115]. In addition, the presence of a human crew has traditionally created the need for expensive management structures to minimize the risk of loss-of-mission (LOM) and loss-of-crew (LOC) events. For example, the Space Shuttle program management at the National Aeronautics and Space Administration (NASA) used to absorb 69% of the total budget allocated to generic operations and infrastructure functions [116].

Recent innovations in commercial spaceflight aim to considerably reduce costs while increasing autonomy of operations [117]. Besides mission requirements, mission duration also increases the cost of manned missions. If the LSS operates in open loop, i.e. byproducts are not recycled on board the spacecraft, the total mass of consumables must be launched, stored, and consumed throughout the duration of the mission. In consequence, the mass of consumables, as well as byproducts, increase with mission duration. Although early space exploration systems were able to revitalize air, they were unable to recycle water from urine nor to produce food, thus limiting the autonomy of the spacecraft to only 14 days [46]. One way to cope with this problem is to regenerate consumables by recycling byproducts. The components that provide these capabilities are called regenerative LSS; they include a suite of technologies based on physico-chemical and biological processes aimed to transform wastes and byproducts back into consumables. Regenerative LSS are meant to be autonomous and to help maximize crew time dedicated to mission objectives. However, their operation is not trivial: regenerative LSS processes require considerable e ort and time, and they constitute complex mass and energy transfer networks subject to the behaviors of their unit processes and to crew demands. As a consequence, they pose novel challenges for their integration and operation.

This section introduces spaceflight LSS as a domain of research and application for human-automation systems. It provides background on life support technologies used in the past and those considered for future manned missions. It describes the Water Recovery System (WRS) currently commissioned on the International Space Station (ISS) and lessons learned from an anomaly occurring during Expeditions 23 through 26. Finally, it presents the challenges in this domain for their integration, automation, and safe operation.

Background

During the Space Race, a total of 34 astronauts rode three different spacecrafts in the Mercury, Gemini, and Apollo Programs [118]. Twelve of them were enabled to walk and explore the Moon during this period. The aim of these programs was to test and determine how NASA would send a crew to the Moon, ensure they accomplish mission objectives, and guarantee their safe return to Earth. The Moon landings were conducted according to a mission profile similar to Figure 8.

Figure 8: Bat chart of the Apollo 17 moon landing mission [9].

The spacecraft that enabled the six landings on the Moon was composed of two modules with independent LSS [46]: the Command Module (CM), and the Lunar Module (LM). Figures 9 and 10, respectively, show the diagrams of their environmental control and life support systems (ECLSS). As Figure 9 shows, the liquid and solid byproducts of the physiological processes of the crew operated in an open cycle (bottom left), i.e. liquid wastes were dumped into space and the solids were stored on board. In addition, the power subsystem, shown as the fuel cell in the top right of Figure 9 produced water while generating electricity.

Figure 9: ECLSS of the Apollo Command Module [10]

Figure 10: ECLSS of the Apollo Lunar Module [10]

Here, fuel cells converted chemical energy from hydrogen and oxygen into electricity, while also generating water for the crew. In contrast, the byproduct from gaseous processes, i.e. respiration and transpiration, was processed in a closed cycle. In this case, air containing humidity was extracted from the cabin atmosphere by an air revitalization process that included a carbon and a lithium hydroxide filters. The regenerated air flowed back into the cabin through space suit connections. These LSS processes enabled humans to explore the Moon. Since then, no other nation has undertaken space exploration missions. While one problem is, of course, their apparent cost-benefit, another is the autonomy of human exploration systems. The question is: How can spacecrafts be made more sustainable?

Yet another question can be raised from Figure 10. The water subsystem of the LM was composed of a number of valves that had to be manually operated. Such tasks imposed additional workload on the crew that, if automated, could have freed crew time for other mission tasks. It is important to note that this still was an open cycle subsystem; future technologies may operate in closed cycle, increasing system complexity and further justifying the need for automation. The question is then: How specifically may automation enable the deployment and proper operation of increasingly complex LSS?

During the span of more than 60 years, various space agencies have studied regenerative technologies to increase spacecraft autonomy [46, 119]. Some regenerative technologies have been successfully demonstrated in ground-based experiments [120]. A few technologies have already started to mature in current hardware on the International Space Station (ISS). The ISS is today the platform used for the development of LSS technologies to enable future exploration missions to the Moon and to other destinations in the solar system. Private companies, such as Space Exploration Technologies (SpaceX) and Bigelow Aerospace, are expected to build capacities and destinations to join in these efforts.

The ISS is also the primary space-based ECLSS research platform. Among its purposes is to test, incorporate, and mature technologies to reduce the need for resupply missions and to enable long-term manned spaceflight beyond low earth orbit [121]. Its three key components are the Water Recovery System (WRS), the Oxygen Generating System (OGS), and the Carbon Dioxide Reduction Assembly (CRA) [122]. These processes are entirely physico-chemical and help to close the water and atmosphere regeneration cycles.

A Challenge in Monitoring and Automation

The integration of various subsystems into a single life support system is a critical aspect of their design [46]. It primarily involves defining subsystem interfaces and determining the dynamics of mass and energy flows in, within, and out of the system. Although investigations continue to evaluate various single physico-chemical and biological technologies to increase loop closure, the challenges for their integration are still to be fully understood [46].

One challenge is the increasing complexity of their mass and energy flow networks. As discussed in Subsection 2.1.3.1, such complexity refers to unanticipated relationships that are said to "emerge" from the dynamic interaction of subsystems. In the case of LSS, these not only refer to mass and energy flows, but may also include unexpected chemical reactions taking place within the system. These emerging dynamics are usually discovered during test runs or during operation.

An example of such situations may be illustrated by an anomaly associated with the WPA and TOC measurements that occurred between June and November of 2010 on ISS [11, 48, 47]. TOC is a non-selective technique that provides a measure of the overall organic compounds contained in water samples; on ISS it is detected by manual measurements conducted with the Total Organic Carbon Analyzer (TOCA) [123]. The NASA Toxicology Group at Johnson Space Center (JSC) and the National Research Council established the maximum TOC concentration for ISS at 3000 [ppb]. By May 2009, after NASA had verified ight rules and procedures to regenerate water, the WRS was commissioned to recycle urine distillate and humidity condensate, allowing ISS to support a crew of six. During more than a year, TOC levels remained sufficiently stable, below 500 [ppb], such that stakeholders began wondering if the number of tasks related to the TOCA could be reduced. But on June 15, 2010, TOCA started to detect an unexpected and monotone increase of TOC in WPA-recycled water, as shown in Figure 11.

Figure 11: TOC increase in measurements of WPA water from ISS [11].

Only after Soyuz 22 brought back archived water samples in late September, 2010, teams at JSC and MSFC began analyzing the identity of the compound that produced such increase. For months, the crew on ISS and ight controllers on ground remained uncertain about how to proceed if TOC levels reached the 3000 [ppb] health-based limit. The crew and mission control attempted potential solutions, such as changing out critical multi-filtration systems and adjusting the temperature of the WPA catalytic reactor. However, nothing improved the TOC situation. Meanwhile, teams analyzing water samples identified the organic compound as dimethylsilanediol (DMSD)[11], a silicon-organic compound often obtained from the degradation of other silicon-based organic compounds. These are found in hygiene products, medications, sealants, lubricant oils, and a myriad of items also present on ISS. Although toxicologists determined that a 8000 [ppb] DMSD concentration (out of a 25,000 [ppb] maximum exposure limit) posed no risk to the crew, the source of DMSD remained unidentified. Finally on October 2010, DMSD concentration began a sudden drop toward nominal values without an apparent reason.

The case of the DMSD anomaly helps to illustrate the emergence of unknown situations in the operation of increasingly closed life support systems. Some of the lessons learned from the experience are:

1. Uninterrupted monitoring is recommended

Even without apparent extraordinary findings, monitoring provides insight into operations under nominal conditions. It also offers context that helps build human-operator situation awareness, which may influence their intervention during off-nominal conditions.

2. Archive samples complement in-flight monitoring

Despite having monitoring instruments on-board, not all chemical analyses will be possible in-flight. Archive samples provide a screening capability to identify unknown compounds and to perform forensic investigations to help determine the causes of unexpected situations.

3. Allowing for margin is critical

In-flight monitoring allows for operational margin during transitions to off-nominal conditions, given that it provides time to attempt troubleshooting, isolate and mitigate anomalies, analyze diagnostic archive samples, re ne health-based standards, and develop operational plans.

4. Unknown situations are to be expected

Although human-rated flight hardware undergoes extensive ground testing and an on-orbit-checkout period prior to crew utilization, unexpected events may develop even after two years of nominal on-orbit operations: "[Even with NASA having] the best intentions, the most comprehensive plans, the clearest fault trees, and the most logical hypothesis, the unexpected still happens" [11]. This lesson calls to incorporate redundancy in designs, to plan for failures that may never occur, and to expand perspectives on how to manage these complexities.

Some other challenges that NASA has posed as critical toward the integration of closed-loop LSS include [46]: (1) determination of health and safety requirements for waste treatment, (2) achievement of safe and reliable overall system operation, (3) investigation of control systems response to instabilities and anomalies, and (4) the capability to correct instabilities and anomalies by chaos and fuzzy logic.

Remarks

The field of human-automation systems is inherently multidisciplinary and finds its application in diverse domains. Some of the challenges between humans and automation are posed by issues that continue to evolve as new technologies and computational methods become available. The domain of life support system is not an exception.

Given their slow time responses, the interaction between humans and automation pose specific issues relevant to situation awareness. Fortunately, innovation in sensing technologies allows measurements of multiple environmental variables to assess the state of life support systems. However, such challenges require the development of methods to fuse their data, produce relevant information, and enable real-time decision making minimizing human errors. This research aims to offer an approach to this challenge by developing a solution in the domain of life support systems. Toward this purpose, the first question to be addressed was to develop a research platform that would allow experiments relevant to regenerative life support system. Chapter 3 presents the development of a small-scale and ground-based bioregenerative life support system as a response to this question.