Performance of Off-the-Shelf Technologies for Spacecraft Cabin Atmospheric Major Constituent Monitoring - NASA Marshall Space Flight Center

INTRODUCTION

Monitoring the atmospheric composition of a crewed spacecraft cabin is central to successfully expanding the breadth and depth of first-hand human knowledge and understanding of space. Highly reliable technologies must be identified and developed to monitor atmospheric composition. This will enable crewed space missions that last weeks, months, and eventually years. Atmospheric composition monitoring is a primary component of any environmental control and life support (ECLS) system. Instrumentation employed to monitor atmospheric composition must be inexpensive, simple, lightweight, and provide robust performance. Such a system will ensure an environment that promotes human safety and health and that can be maintained with a high degree of confidence. Key to this confidence is the capability for any technology to operate autonomously, with little intervention from the crew or mission control personnel. A study has been conducted using technologies that, with further development, may reach these goals.
Integrated testing was conducted to demonstrate the capability of simple, inexpensive, off-the-shelf instruments to control the composition of a simulated spacecraft cabin atmosphere. Long-duration testing results providing insight into the logistical support required for the instruments under evaluation are also presented. The results build a case for evaluating other off-the-shelf monitoring devices for a variety of spacecraft cabin atmospheric quality monitoring purposes.

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BACKGROUND

Successfully controlling the atmospheric composition of a crew spacecraft cabin requires robust monitoring technologies. Control and monitoring are dependent on each other. Without reliable instruments to monitor atmospheric composition, controlling that composition cannot be done reliably. As mission duration and complexity increase, the need for reliable monitoring instrumentation becomes more important. Operational autonomy also becomes important as the focus of exploration moves beyond low-Earth orbit.
Spacecraft cabin atmospheric composition monitoring can be divided into two distinct classifications—major atmospheric constituents and trace chemical contaminants. Major constituents include nitrogen (N2), oxygen (O2), carbon dioxide (CO2), and water (H2O) vapor, which are typically monitored continuously. Trace chemical contaminants are produced from a variety of sources including equipment offgassing, human metabolism, and myriad onboard operations.4 These contaminants, if not maintained at low concentrations, can contribute to symptoms associated with sick building syndrome.
Studies on sick building syndrome have indicated that a total nonmethane volatile organic compound concentration of 25 mg/m3 may cause symptoms under certain conditions.5–7 Analysis of samples collected from crewed spacecraft has reported compounds representing the hydrocarbon, alcohol, aldehyde, ester, ether, aromatic, ketone, organosilicone, and halocarbon functional classes.8 In addition, inorganic trace contaminants, such as ammonia, carbon monoxide, and hydrogen, are present. Methane accounts for the largest proportion of the typical trace contaminant loading followed by alcohols, hydrogen, organosilicone compounds, and carbon monoxide. In all, these contaminants have accounted for 97 percent of the total trace contaminant concentration in previously monitored space flights. Monitoring trace chemical contaminants is accomplished via a combination of nearly real-time monitors deployed on board the spacecraft and postflight analysis of archive samples collected during the mission.9 In combination with monitoring cabin pressure and the operation of assorted environmental control equipment
that conditions the cabin atmosphere by maintaining a comfortable temperature and removing carbon dioxide, trace contaminants, and excess water vapor, a healthy environment can be maintained for any mission architecture

CONCLUSIONS

In these tests, the units performed well with no failures observed. To be flight qualified, additional testing will be needed. Package design as well as electrical certification must also be addressed. Before performing this work, it is inappropriate to suggest any of the technologies are truly flight ready. The positive results observed, however, show that further testing and development is warranted.

Oxigraf Model O2 Oxygen Analyzer

The Oxigraf model O2 oxygen analyzer was the most stable instrument in the test group. As shown in figure 11, the data fell well within the allowed ±5 percent range, and in fact, fell well within 1 percent of the expected value in all but one instance. This shows that the unit will be a low maintenance item, requiring calibration only once every 2 to 3 wk. The only maintenance performed on the unit was the replacement of the hydrophobic filter membrane found at the inlet. This cartridge required replacement approximately once every 3 mo as shown in the appendix.

Authors:

J.D. Tatara; Qualis Corporation; Huntsville, Alabama
J.L. Perry; Marshall Space Flight Center, Marshall Space Flight Center, Alabama

Performance of Off-the-Shelf Technologies for Spacecraft Cabin Atmospheric Major Constituent Monitoring (pdf)