5
Technology Capability Objectives Supported by the ISS

The panel’s review of technology capabilities supported by the ISS was restricted to aspects of the ISS research plan that pertain to the physical sciences. This discussion is based primarily on presentations and documentation provided by NASA.a As noted in Chapter 1, the panel had little time in which to attempt a rigorous identification, assessment, and prioritization of the research and operational studies associated with the physical sciences that may be needed to support NASA’s space exploration objectives. The areas highlighted in this chapter represent the best efforts of the panel at this juncture to point NASA to topics that do not appear to have been thoroughly considered.

One of the significant issues for the panel’s evaluation of the ISS focus on physical science was the lack of a consistent set of technology issues and proposed approaches to satisfying these. For example, the Zero-Base Review appears to show that all materials science, combustion, and fluid physics funding will be (or has been) eliminated.b Yet some information provided to the panel implies that several ISS facilities and modules constructed for research in these disciplines would still be delivered to the ISSfor example, the fluids integrated rack and the combustion integrated rack. The panel also notes that the microgravity sciences glove box is already on the ISS.c This made it difficult to comment on the efficacy and appropriateness of the intended ISS research, since it was unclear what research is to be, or can be, carried out on the ISS. In this context, the panel has chosen to identify potential gaps in a number of broad areas, and examples of these are discussed below. Owing to the limitations of this study, this list cannot be considered comprehensive by any means, but the panel thinks that the issues identified are important in the context of both risk reduction and the design and testing of advanced technologies for exploration missions.

FIRE SAFETY

Fire in the constricted areas of a spacecraft can be devastating. It has been documented that five incidents with ignition potential have occurred during the 12 years of shuttle operations,1 and two fire incidents involving oxygen generators occurred on the Mir.2 When the operational durations of these vehicles are compared with the time needed for a voyage to Mars, it can be seen that fire would be a very serious risk in the Mars program. Past NRC studies have repeatedly called for research on fire mitigation, detection, and suppression.3,4

Fundamental research on combustion phenomena in microgravity has elucidated many fire issues peculiar to microgravity, yet there is still relatively little known about flame behavior that relates to fire

a  

NASA, “Pre-Brief Materials for the NRC Review of NASA Strategic Roadmaps: ISS Panel. Review Context and Background,” presentation dated September 19, 2005.

b  

E. Trinh, NASA Headquarters, “Human Systems Research and Technology. Summary of Zero Base Review (ZBR). Process and Results,” presentation dated September 12, 2005.

c  

P. Ahlf, Exploration Systems Mission Directorate, NASA, “ESMD ISS Utilization Requirements Analysis Processes and Results,” presentation to the Review of NASA Strategic Roadmaps: Space Station Panel, October 3, 2005, National Research Council, Washington, D.C.



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Review of NASA Plans for the International Space Station 5 Technology Capability Objectives Supported by the ISS The panel’s review of technology capabilities supported by the ISS was restricted to aspects of the ISS research plan that pertain to the physical sciences. This discussion is based primarily on presentations and documentation provided by NASA.a As noted in Chapter 1, the panel had little time in which to attempt a rigorous identification, assessment, and prioritization of the research and operational studies associated with the physical sciences that may be needed to support NASA’s space exploration objectives. The areas highlighted in this chapter represent the best efforts of the panel at this juncture to point NASA to topics that do not appear to have been thoroughly considered. One of the significant issues for the panel’s evaluation of the ISS focus on physical science was the lack of a consistent set of technology issues and proposed approaches to satisfying these. For example, the Zero-Base Review appears to show that all materials science, combustion, and fluid physics funding will be (or has been) eliminated.b Yet some information provided to the panel implies that several ISS facilities and modules constructed for research in these disciplines would still be delivered to the ISSfor example, the fluids integrated rack and the combustion integrated rack. The panel also notes that the microgravity sciences glove box is already on the ISS.c This made it difficult to comment on the efficacy and appropriateness of the intended ISS research, since it was unclear what research is to be, or can be, carried out on the ISS. In this context, the panel has chosen to identify potential gaps in a number of broad areas, and examples of these are discussed below. Owing to the limitations of this study, this list cannot be considered comprehensive by any means, but the panel thinks that the issues identified are important in the context of both risk reduction and the design and testing of advanced technologies for exploration missions. FIRE SAFETY Fire in the constricted areas of a spacecraft can be devastating. It has been documented that five incidents with ignition potential have occurred during the 12 years of shuttle operations,1 and two fire incidents involving oxygen generators occurred on the Mir.2 When the operational durations of these vehicles are compared with the time needed for a voyage to Mars, it can be seen that fire would be a very serious risk in the Mars program. Past NRC studies have repeatedly called for research on fire mitigation, detection, and suppression.3,4 Fundamental research on combustion phenomena in microgravity has elucidated many fire issues peculiar to microgravity, yet there is still relatively little known about flame behavior that relates to fire a   NASA, “Pre-Brief Materials for the NRC Review of NASA Strategic Roadmaps: ISS Panel. Review Context and Background,” presentation dated September 19, 2005. b   E. Trinh, NASA Headquarters, “Human Systems Research and Technology. Summary of Zero Base Review (ZBR). Process and Results,” presentation dated September 12, 2005. c   P. Ahlf, Exploration Systems Mission Directorate, NASA, “ESMD ISS Utilization Requirements Analysis Processes and Results,” presentation to the Review of NASA Strategic Roadmaps: Space Station Panel, October 3, 2005, National Research Council, Washington, D.C.

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Review of NASA Plans for the International Space Station safety issues in spacecraft. Indeed, our scientific understanding of fire on Earth is still emerging, and specialized studies in microgravity have been invaluable to that effort. In this context, the ISS is unique in providing sufficient time in a microgravity environment to achieve definitive evaluation of spacecraft fire phenomena—unachievable by short-duration microgravity facilities such as drop towers and unrealizable by computation that lacks sufficient spatial resolution. Thus, should fire safety gaps be identified that must be filled in order to reduce the fire risks on exploration missions to acceptable levels, the ISS is the only facility that could be used to conduct the studies. Such risk-reducing studies might include the effects of fire on humans and equipment and the design implications for suppressing, escaping, or correcting the damage of fire. The presentations by NASA to the panel were ambiguous in describing the projects in the proposed continuation of the ISS program that are deemed essential for human space exploration (Mars).d Fire safety prioritization appears to have focused on fire detection and fire suppression without evidence that these are the most critical fire safety areas for future missions.e In addition, the two areas that appear to have been retained in ISS research planningdetection and suppression—were not presented in sufficient detail to allow the panel to assess whether NASA’s plans to address them are adequate. NASA needs to set design performance objectives for fire detection, suppression, and prevention and then demonstrate the certainty of achieving them. Fire poses an uncertain risk for the success of long-term spaceflight. Some of the areas of importance that were reduced or eliminated from the ISS program include these: Tests for flammability and material screening. NASA currently has several tests for flammability.f However, these fire tests may not provide the proper levels of risk reduction for the new exploration program. For example, it has been noted that NASA-STD-6001 Test 1 results do not map quantitatively to results in the low- or partial-gravity environments of an exploration vehicle or habitat.5 Although NASA has performed extensive research to establish a replacement for the upward flammability test in a 1-g environment, it is not clear to the panel that this has been accomplished. NASA appears to be canceling the FEANICS program that is designed to verify the new test with ISS experiments. Previous reviews have stated the need for achieving prediction of surface flame spread in microgravity and fractional gravity.6 In order to rationally choose construction materials for spacecraft interiors that minimize the risk of, and danger from, fire and combustion, risk programs that include a better understanding of flame spread in microgravity are a primary consideration.7 Oxygen system safety. Combustion involving pure oxygen sources is a particular hazard in spacecraft and can result in temperatures capable of turning most materials into fuels. It was noted in a previous NRC report that the aspects of ignition, flame spread, and extinguishment that are unique to oxygen fires are critical research areas for human exploration.8 Thus, the use of oxygen generators and high-O2 atmospheres are matters of special concern. Smoldering and pyrolysis. Smoldering and transition to flaming combustion are significantly different processes in microgravity than on Earth.9 The mechanisms that could enhance smoldering d   The projects indicated for continued funding appear to be smoke detection (DAFT and SAME) and smoke suppression (FLEX), the latter using the combustion integrated rack. The project related to material flammability and development of a new NASA test method (FEANICS) appears to have been dropped from the ISS but might be carried out on a limited basis on the ground. e   Assessment of Directions in Microgravity and Physical Sciences Research at NASA (NRC, 2003, p. 87) lists a number of fire-related issues that are at the level of critical impact on technology needed for human space exploration. f   For example, NASA-STD-6001 Test 1 looks at upward flame propagation. Other tests include the cone calorimeter (ASTM E 1254) for materials that fail the upward flame propagation test, ASTM D 93 for the flashpoint of liquids, and a special test for electrical wire flammability, containers, and metal flammability, which uses a version of the upward flame propagation test in pure oxygen.

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Review of NASA Plans for the International Space Station combustion ignition and propagation, transport of the combustion products, and the mechanism of detector response in zero gravity are not fully understood, especially with respect to design parameters for fire detection systems. NASA’s combustion research program has laid a good foundation for understanding spacecraft fire issues. However, it is not clear to the panel that NASA has fully addressed the fire research knowledge gaps whose filling in is crucial to reducing risks on long-term exploration missions. Fire safety is grounded in the fundamentals of combustion, and gaps in understanding of the fundamentals of microgravity combustion must be assessed in terms of the additional risks they pose to crew and vehicles in the exploration missions program. Finding: The history of spacecraft fire incidents suggests that the risk of a fire incident on a long-term mission such as a trip to Mars is high. Recommendation: NASA should develop fire safety design performance criteria for long-term exploration missions as soon as possible. These criteria should be used to drive an analysis of additional research or testing necessary to ensure fire safety at the design level, an endeavor for which the ISS is the only viable research facility. Finding: If risk mitigation and technology requirements studies indicate that there are fire safety gaps for exploration missions, the ISS may well prove an essential facility for further studies. In particular, drop tower duration is too short for viable fire safety studies, and computational prowess is unlikely to be sufficient to solve these problems via modeling in any relevant time frame. Recommendation: NASA should convene a panel of internal and external experts to conduct a complete review of the potential risks associated with fire safety issues in the exploration missions. The panel should be asked to comment specifically and technically on adequate and appropriate research programs needed to mitigate the risks associated with fire safety for exploration missions. The panel should also be asked to comment on risk that would be added to the total risk picture for exploration missions by not updating current NASA fire safety tests. MULTIPHASE FLOW AND HEAT TRANSFER ISSUES Multiphase flow and heat transfer (MFH) processes involve a fluid of two or more phases (typically twoliquid and vapor). Such processes rely on the latent heat of liquid-vapor phase change and are highly efficient in transferring large amounts of heat. Systems that demand high efficiency for heat transfer and require high power- to-weight ratios operate under multiphase flow conditions. In the past, NASA almost exclusively employed single-phase systems in space exploration, but it has now given research in MFH a high priority.g For achieving the space exploration goals envisioned in ESAS,h issues and concerns related to MFH processes in microgravity environments are not understood adequately to mitigate risks.10 The ISS may be the only testbed suitable for achieving this understanding. g   E. Trinh, NASA Headquarters, “International Space Station Research Plan: Implementing the Vision for Space Exploration,” presentation to the Review of NASA Strategic Roadmaps: Space Station Panel, October 3, 2005, National Research Council, Washington, D.C. h   John Connolly, NASA, “ESAS: Exploration Systems Architecture Study. ESAS Study Summary,” presentation to the Review of NASA Strategic Roadmaps: Space Station Panel, October 3, 2005, National Research Council, Washington, D.C.

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Review of NASA Plans for the International Space Station An example of the potential criticality of MFH is in environmental management, where the chemical composition of the atmosphere and properties such as relative humidity and temperature must be rapidly and accurately measured and controlled. Such control has a high likelihood of being achieved by employing MFH processes. Since capillary forces become strong in the absence of gravity, thermocapillarity, together with the use of suitable geometries of the solid heated (for humidification) or cooled (for dehumidification) surfaces, can be used to facilitate a flow of liquid, whether in the form of drops or thin films. NASA’s current ISS research plansi do not appear to include an examination of the combined effect of solid surface geometries and thermocapillarity in long-term microgravity. If an examination of technology needs for long-term exploration missions such as a mission to Mars were to identify MFH processes as strong candidates for meeting those needs, studies aboard the ISS of the flow and stability of drops and thin films on variable wetting surfaces in the presence of phase changes might be the only way to develop design parameters for a reliable optimized (in terms of mass and energy consumption) system. Another example of the potential need for greater understanding of MFH processes is thermal control. An active thermal control mechanism based on MFH might consist of a heat pump/refrigerant unit with the attendant evaporation and condensation components. Passive systems for thermal control include the use of heat pipes—conventional and loop systems—where thermocapillarity provides the driving force needed for liquid motion in microgravity. Operating either of these multiphase systems in an exploration mission would require understanding the phase separation processes well enough to design systems that can operate reliably for long periods of time in microgravity. Again, the ISS may prove to be the only facility that can be used to improve understanding of microgravity thermocapillarity to the levels needed to provide design parameters that will enable risk mitigation. Related to thermal control, the panel notes that two pool boiling experiments have been proposed for the ISS.j It has been demonstrated with pool boiling, but not yet with flow boiling, that heat transfer rates are greater in microgravity than on Earth. Although the critical heat flux with pool boiling is known to be reduced in microgravity, the critical heat flux with flow boiling in microgravity remains undetermined. The panel believes that flow boiling may be more relevant to the thermal control needs for exploration missions. As such, studies involving heat fluxes, subcooling parameters, and a range of Reynolds numbers would be important to enable system optimization. Previous NRC reports, including one in 2003,11 have pointed out that in the past, because of risk and reliability considerations, NASA has chosen not to use active, high-power-density systems that involve heat transfer by phase change (e.g., condensation and boiling). This panel was presented with material that current and future technology developments of space power are in the hundreds of kilowatts (electric) range and thus may require a nuclear reactor as a high-temperature heat source12-14 whatever the thermal-to-electric conversion system may be. The high efficiency and high power-to-weight ratio of closed-cycle multiphase systems, based on the use of the latent heat of phase change (i.e., condensation and evaporation) to transfer energy, will thus be significantly more attractive to NASA in these future systems. (For example, alkali metal heat pipes have been considered as an efficient way of supplying high-temperature heat energy to a power conversion engine at its hot end.15,16) This is another critical example in which use of multiphase systems in an exploration mission scenario would require understanding the phase change processes well enough to design systems that could operate reliably for long periods of time in microgravity. i   E. Trinh, NASA Headquarters, “International Space Station Research Plan: Implementing the Vision for Space Exploration”; NASA, “Pre-Brief Materials for the NRC Review of NASA Strategic Roadmaps: ISS Panel. Current Working Manifest for ISS/Shuttle: Payload Descriptions,” presentation dated September 19, 2005. j   NASA, “Pre-Brief Materials for the NRC Review of NASA Strategic Roadmaps: ISS Panel. Current Working Manifest for ISS/Shuttle: Payload Descriptions,” presentation dated September 19, 2005.

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Review of NASA Plans for the International Space Station A final example of what may be anticipated with exploratory missions such as those to Mars is the management of cryogenic liquids such as oxygen. The importance of technologies to manage cryogenic liquids in microgravity has been recognized in previous NRC studies17 and in some NASA exploration planning studies.k Management of a cryogenic liquid involves its storage and transport via ducts. It may be anticipated that the non-venting mode of storage will be used for the long periods of time with exploratory missions. This may require refrigeration to condense the vapor formed if inward heat leaks are sufficiently large. Proper temperature gradients on the condensing surfaces together with their geometry can provide the motion by thermocapillary effects to move the liquid to the inlet of a suitable pump. The necessary temperature gradients can be provided by the refrigeration device. Successful cryo management under such circumstances will require an understanding of multiphase flow and heat transfer issues, and the ISS could serve as a unique testbed if appropriate engineering studies are planned and carried out. Finding: Multiphase flow and heat transfer systems operating in microgravity environments are profoundly influenced by thermocapillarity effects and may be significant components of exploration missions. Studies aboard the ISS may be the only way to obtain information on temperature and geometry effects on the motion of films and fluid particles at interfaces (with emphasis on thermocapillary effects). MATERIALS RESEARCH One of the primary justifications for construction of the ISS was that it would allow unique materials to be developed and processed in microgravity. To that end, NASA encouraged and supported a significant research effort in microgravity materials research with the intent of better understanding the effects of gravity on materials processing. With the new vision for the ISS, NASA no longer supports microgravity materials research. The panel is concerned that this wholesale elimination of materials science research has also eliminated consideration of materials processes that might be quite important to exploration missions, such as welding, soldering, and brazingprocesses that rely on poorly understood interfacial effects and thermocapillarity. REFERENCES 1. Friedman, R. 1992-1993. Fire safety practices and needs in human-crew spacecraft. J. Appl. Fire Sci. 2(3): 243-250. 2. Zimmerman, R. 2003. Leaving Earth. Joseph Henry Press, Washington, D.C. 3. National Research Council (NRC). 2003. Factors Affecting the Utilization of the International Space Station for Research in the Biological and Physical Sciences. The National Academies Press, Washington, D.C. 4. NRC. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. National Academy Press, Washington, D.C. 5. Ruff, G.A., D.L. Urban, and M.K. King. 2005. A research plan for fire prevention, detection and suppression in crew exploration systems. 43rd AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nev., January 10-13. k   NASA, “Pre-Brief Materials for the NRC Review of NASA Strategic Roadmaps: ISS Panel. Review Context and Background,” presentation dated September 19, 2005, p. 22.

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Review of NASA Plans for the International Space Station 6. NRC. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. 7. NRC. 2003. Assessment of Directions in Microgravity and Physical Sciences Research at NASA. The National Academies Press, Washington, D.C. 8. NRC. 2003. Assessment of Directions in Microgravity and Physical Sciences Research at NASA, pp. 36 and 87. 9. NRC. 2003. Assessment of Directions in Microgravity and Physical Sciences Research at NASA. 10. NRC. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. 11. NRC. 2003. Assessment of Directions in Microgravity and Physical Sciences Research at NASA. 12. Martin, J., O. Mireles, and R. Reid. 2005. Multiple restart testing of a stainless steel sodium heat pipe module. Pp. 150-157 in Space Technology and Applications International Forum - STAIF 2005 (M. El-Genk, ed.). AIP Conference Proceedings 746. American Institute of Physics, Melville, N.Y. 13. Satter, C.M. 2005. JIMO follow-on mission studies. Pp. 249-257 in Space Technology and Applications International Forum - STAIF 2005 (M. El-Genk, ed.). AIP Conference Proceedings 746. American Institute of Physics, Melville, N.Y. 14. Wright, S.A., R.J. Lipinski, T. Pandya, and C. Peters. 2005. Proposed design and operation of a heat pipe reactor using the Sandia National Laboratories annular core test facility and existing UZrH fuel pins. Pp. 449-460 in Space Technology and Applications International Forum - STAIF 2005 (M. El-Genk, ed.). AIP Conference Proceedings 746. American Institute of Physics, Melville, N.Y. 15. Martin, J., O. Mireles, and R. Reid. 2005. Multiple restart testing of a stainless steel sodium heat pipe module. 16. Wright, S.A., R.J. Lipinski, T. Pandya, and C. Peters. 2005. Proposed design and operation of a heat pipe reactor using the Sandia National Laboratories annular core test facility and existing UZrH fuel pins. 17. NRC. 2000. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies.