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OPTIONS FOR EXTRAVEHICULAR ACTIVITY (EVA): NOW AND FUTURE Richard K. Fullerton Richard D. Vann, Ph.D. INTRODUCTION The goals of current EVA research are applicable to a diversity of near-term and long-term exploration applications. As always, safety remains an overriding requirement, but considerable potential exists to also reduce the EVA-induced burdens which limit ISS crew time, mass/volume resupply, comfort and costs. To enable productive operations in the unique environments beyond low earth orbit, a suite of newer technologies are being sought. These technologies will ideally utilize an open architecture that is readily adaptable and common to both planetary and deep space destinations. With adequate sustained levels of research and development resources, the technology “collection” of functional and affordable solutions is intended to provide future decision makers options for science and commercial applications beyond low earth orbit.
EVA Technology Issues Like the unresolved physiological issues, overall advanced EVA priorities and challenges are quite diverse. • .Integrated Concept Definition and Requirements (suit, airlock, robotics) • CO2 system • Mass/Volume reduction and system definition (SSA and LSS) • O2 system • Environmental Protection (thermal, puncture, radiation, dust, mobility) • Thermal Control System • Test Personnel and Facilities • Analysis Tools • Power supply system • Instrumentation and info technology (wireless, sensors, automation, crew/vehicle controls and displays)
EVA and Decompression Sickness From Gemini to the present (about 35 years), there have been 101 EVAs in the US space program for an average of about three per year. The nominal EVA protocol used on most Space Shuttle flights involved a staged decompression from 14.7 psia on air to 10.2 psia on 26.5% oxygen. (The oxygen percentage was elevated to avoid hypoxia.) During the 10.2 psia stage, nitrogen from an astronaut’s tissues was eliminated slowly and harmlessly through the lungs. After a mean period of 38 hrs (range 19-145 hrs) at 10.2 psia, the oxygen level in the Space Suit was raised to 100%, and the suit pressure reduced to 4.3 psia for EVA. Staged decompression was entirely successful during Shuttle operations (a Tier 4 countermeasure in the Critical Path Roadmap terminology), with no reported cases of decompression sickness (DCS). While it might seem logical to use staged decompression for EVA from the International Space Station (ISS), the circumstances are quite different for the ISS. EVAs from the ISS will not proceed at the limited pace of three per year. In the estimated 10-year life of the ISS, 484 EVAs will be needed for assembly and maintenance or about 48 EVAs per year. Moreover, staged decompression cannot easily be conducted since the Space Station will be maintained at 14.7 psia. Staged decompression is only possible in the ISS airlock, and while a 12-hr airlock stage has been proposed, this would reduce the number of EVAs as well as introduce numerous operational constraints.
From NASA Johnson Space Center, EVA Project Office, Houston, TX (R. Fullerton) and Duke University Medical Center, Hall Center for Hypobaric and Hyperbaric Medicine, Durham, NC (R. Vann).
SUMMARY OF PRESENTATIONS Physiological Effects of a Mechanical Counter Pressure Suit Honeywell International, Dietmar Tourbier The physiological effects of an MCP glove are being investigated as a proof of feasibility and medical safety. This is a prelude to more serious engineering design and development effort to overcome numerous other issues. The current effort quantifies skin blood perfusion, finger girth dimensional changes, blood pressure, heart rate and skin temperature at over and under pressure exposures between –200 and +200 torr. Initial preliminary results indicate that skin blood perfusion is increased at negative pressures and slightly decreased at positive pressures. The effect of pressure on the other parameters is less prominent. Assuming continuing success, this effort will feed design criteria for a complete MCP suit. If engineering challenges such as materials, evaporative thermal management and adequate pressure on concave body areas can be overcome, a revolutionary enabling technology could be available for a wide range of destinations and applications.
Development and Testing of a Mechanical Counterpressure Space Suit Glove University of Maryland, Dr. David Akin Unlike pneumatically pressurized EVA garments, this research focuses on using mechanical pressure applied to body tissues for vacuum environment protection. Building upon 1970 era studies by Dr. Paul Webb, this study has assembled conceptual gloves using commercially available materials such as lycra spandex. Improved materials such as custom shaped triaxial weaves are being sought to improve elongation/elasticity performance and minimize the need for multiple pressure layers. To aid these glove assessments, instrumented hand molds and mechanical grasp simulators have been created to measure glove applied pressures and bend angles. To maintain pressure in the concave areas of the body (e.g. palm), gas pressurized bladders so far yield good results. To further lower metacarpal joint bending fatigue, a mechanical assist concept may also be applicable. Future work remains to find and use improved materials and fit geometries in improved prototype gloves. If used with a traditional pneumatically pressurized arm such as the 4.3 psi EMU, a pressure transition seal is yet another challenge to address. The impacts of thermal and MM/OD protection layers are acknowledged to be another challenge of the ultimate goal. This end goal is a glove which is easy and quick to don/doff with lower fatigue and discomfort than current pneumatic designs. If successful, this technology may be expandable to the entire suited body. The benefits to crew performance and vehicle logistics are very promising. The added leverage of cooperative efforts between University of Maryland and Honeywell’s team should be worth considering.
Self Sealing Bladder for Use in a Pressurized Inflatable Structure ILC Dover, Jody Ware Pressurized inflatable structures of all kinds will benefit from thin and lightweight self-sealing materials which can eliminate leakage following damage. It is also imperative that this material is flexible and gas retaining. As proven successfully by bench testing, the material developed to date eliminates leakage after a 2mm diameter puncture (while pressurized to 4.3psi). Prototype structures similar to the EMU lower arm have been fabricated from this material and will be pressurized and punctured while under vacuum at 10-5 torr. Future goals would push the puncture limit up to possibly 4mm diameter. An additional goal is to someday assess sealing properties when impacted by hypervelocity projectiles. To enable practical implementations another major objective is to minimize mass and thickness constraints while maximizing sealing capability. At the present time, a 30 mil thickness is roughly 4 times the mass of a conventional neoprene pressure bladder. If applied to the present EMU, it would add up 5% or 15lb to the integrated EMU system weight. Fortunately, it is foreseeable that more research could cut this weight impact in half. Targeting material usage to suit areas most prone to damage could further reduce the suit mass impact. If successful, the safety of inflatable space suits, habitats, airlocks can be substantially improved. Commercial shipping and chemical containment vessels would similarly benefit as spin-off applications.
Model of Space Suit Joint Mobility with Applications to EVA Operations MIT, Dr. Dava Newman and Patricia Schmidt An instrumented robot has been used inside one size of the current U.S. EVA suit to quantitatively assess and model suit joint angles and torques. While the official NASA definition of upper body work envelope (documented in NSTS-07700) is written to the full range of crew anthropometries and suit sizes, this particular research does point out the conservative nature of the official definition. From this research, it appears that the baseline work envelope could be expanded and shifted lower by several inches for the tested suit geometry. A study of the full range of suit sizes should help refine the generic work envelope. Such a revision would allow further optimization of vehicle worksite designs and task technique selection. This non-subjective means of assessing suit performance could also be applicable to assessments of future suit design options. Rapid quantitative performance measurements using both robot and human test subjects are needed for such design trades.
Physiological Approaches for the Smart Design of a Multicompartment Cooling/Warming Garment for Routine and Emergency EVA University of Minnesota, Dr. Victor Koscheyev This research is targeted to develop a more efficient EVA suit undergarment for liquid cooling and heating. Because different body segments exhibit varying thermodynamic responses, a garment with multiple zones can optimize the body’s thermal comfort. Eight such zones have been identified with site-specific rates of heat absorption and extraction. By individually or in combination regulating liquid flows to these zones, comfort can be optimized. The fingers also play a role and provide the most informative data for tracing the body’s thermal dynamics under uniform and non-uniform cooling and warming conditions.
Membrane Based CO2 Removal from Spacesuits Hamilton Sunstrand, Karen Murdoch A membrane device that separates CO2 from breathing gas can effectively control suit concentration levels by venting the CO2 to the vacuum of space without the need for regeneration. The goal is to develop a system sized as a drop in replacement for the existing EMU LiOH canister. This research tested immobilized liquid membranes in which the liquid is tailored to transport CO2 while inhibiting the loss of O2. Current results demonstrated a selectivity of CO2 over O2 greater than 3000. This translates to an O2 loss of less than 3% of metabolic usage. Current accomplishments are 1 order of magnitude away from the needed permeability levels to achieve the overall sizing goals.
Membrane Based Selective CO2 Removal Honeywell International, Dietmar Tourbier The removal of CO2 from the space suit can be greatly simplified and made regenerative by using selective membranes that pass CO2 but block the flow of O2. For realistic EVA applications, such a membrane needs to have a selectivity of 1000-3000 for CO2 over O2. In the first preliminary stage of this research we have identified membranes made of polyethylene-amine that can have selectivities in excess of 100,000 when used at thicknesses of 250 microns. Thinner membranes have been manufactured onto a support of porous polypropylene to increase permeability. The challenge remains to improve permeability by 2 orders of magnitude while retaining favorable selectivity levels. With successful new investment, the weight and volume of the envisioned end product could be reduced relative to current EMU LiOH and metal oxide technologies.
Biomimetic Strategy for Carbon Dioxide Capture Sapient’s Institute, Michael C. Trachtenberg We have developed an enzyme-based contained liquid polymer membrane bioreactor to selectively extract CO2 from mixed gas streams including those from the crew chamber and the plant chamber. The current technology, stage 1 development has a permeance of 2*10-4 cm3 cm-2 s-1 cmHg-1. This value is 1/3 the value
needed for EVA application and above that needed for cabin use and for redistribution of CO2 between the crew and plant chambers. At the same time the selectivity of CO2:N2 is 2500:1 and for CO2:O2 is 1200:1. Thus a membrane of about 12 m2 is needed to support one astronaut. This would fit into a volume smaller than currently used and should be on the order of 8kg. Technology stage 2, when funded, will increase performance by 10-100-fold. Importantly this will significantly reduce O2 permeance. It will also result in a smaller volume and lower mass. Further development will capitalize on the space vacuum to make the device smaller yet and reduce energy requirements dramatically.
Miniature RF Mobility Analyzer as a Gas Chromatographic Detector for Oxygen Containing Volatile Organic Compounds New Mexico State University, Charles Stark Draper Lab, US Agriculture Research Station, USDA, G. A. Eiceman Novel ion mobility analysis technology has been developed for volatile organic compound detection. This particular research features a high electric field radio frequency analyzer to perform gas chromatograph separations of alcohols, aldehydes, esters, ethers, pheromones and insect chemical attractants. The limit of detection achieved averaged from 9.4 x 10-11 grams. Space applications for this compact device include human habitats and plant growth enclosures. Vann et al. (5) ISS operations require an EVA protocol of not longer than about 2 hrs if the desired construction schedule is to be met. A protocol that satisfied this requirement was developed during NASA-JSC sponsored multi-center trials in the altitude chambers of Hermann Hospital (University of Texas), Defence and Civil Institute of Environmental Medicine (working in conjunction with the Canadian Space Agency), and Duke University Medical Center. The protocol used exercise and 2 hrs of oxygen breathing at 14.7 psia to eliminate tissue nitrogen prior to simulated EVA at 4.3 psia. It was tested in 45 human trials without decompression sickness. This was unprecedented in ground-based EVA studies for which a DCS incidence of about 60% would have anticipated for a 2-hr oxygen prebreathe. Webb et al. (6) The success of the 2 hr protocol may reflect an improved understanding of decompression mechanisms developed from work conducted during the preceding 10 years. The new protocol was based on two hypotheses: (a) exercise can be a DCS countermeasure; and (b) gravity may be a DCS risk factor. The theoretical basis for the exercise-countermeasure hypothesis was that increased perfusion due to exercise would accelerate nitrogen elimination by an astronaut who breathed oxygen prior to decompression. This would reduce the DCS risk. Webb et al. (6) presented a series of human studies from the Air Force Research Laboratory (AFRL, Brooks AFB) that supported this hypothesis. They used 10 minutes of strenuous arm and leg exercise to accelerate nitrogen elimination during oxygen prebreathe followed by 50 min of rest before decompression to 4.3 psia. They found that subjects who exercised during prebreathe had 35% less DCS than did subjects who rested. This exercise regimen was adopted as part of the successful ISS EVA protocol (5) and has also been used by pilots of high altitude reconnaissance aircraft to reduce DCS risk. Balldin et al. (1) Hypothesis (b), that gravity is a DCS risk factor, was based on the theory that bubbles causing DCS grow from small “precursor” bubbles known as gaseous micronuclei. The theory holds that micronuclei originate from mechanical stresses induced by exercise or anti-gravity activity. (Joint cracking is an example of such bubble formation.) Micronuclei are dissolved by surface tension and have half-lives on the order of one hour. If an astronaut were active or subject to terrestrial gravity, more micronuclei might form, and the risk of DCS could increase. If an astronaut were inactive or in microgravity on-orbit, more micronuclei might dissolve, and the DCS risk could decrease. This is a putative explanation for the apparent absence of DCS during on-orbit EVA in contrast to the 20-30% DCS incidence during comparable ground-based trials. If the gravity risk factor were correct, the lowest DCS risk would exist in microgravity, the greatest risk in terrestrial gravity, and there would be intermediate risks on the Moon and
Mars. According to the gravity-risk hypothesis, ground-based EVA trials should attempt to simulate the gravity environment of their intended use. The gravity-risk hypothesis was tested by Balldin et al. (1) of AFRL who used the exerciseenhanced oxygen prebreathe described by Webb et al. (6) in which 10 min of strenuous exercise was followed by 50 min of rest before decompression to 4.3 psia. Two groups of subjects were evaluated at 4.3 psia for DCS while performing arm exercise to simulate EVA work. One group stood and walked at 4.3 psia while the other group reclined to relieve the anti-gravity stresses on the legs and simulate microgravity. Microgravity simulation had no effect on the number of subjects who developed DCS symptoms anywhere. Balldin et al. concluded that simulated weightlessness did not affect DCS risk. Further work is needed to resolve the question, however, as the reclining subjects had fewer DCS symptoms or Doppler-detected bubbles in the legs than did the standing subjects. Since gravity affects the weight-bearing legs more than the arms, this was consistent with the gravity-risk hypothesis in which less DCS and bubble formation would be expected in the legs. The beneficial effect of microgravity simulation also may have been masked if the 10 min of strenuous exercise generated micronuclei that did not resolve during the 50 min of rest. Balldin et al. (1) will conduct a further study to resolve this question by using a 4-hour resting prebreathe in which exercise-generated micronuclei could not be a possible confounder. Pilmanis et al. (4) Just as the Shuttle EVA protocol was not suitable for the Space Station, so the Station protocol may not be optimal for a lunar base or Mars expedition. The argon present in the Martian atmosphere, for example, might be used to replace nitrogen as the diluent inert gas in the habitat. This would reduce the nitrogen mass that must be carried from earth. Further mass reductions could be achieved by using lower pressures in vehicles, habitats, and suits. A lower suit pressure would not only improve flexibility but also reduce the weight of the Space Suit an astronaut must carry in Lunar or Martian gravity. Pilmanis et al. (4) of AFRL discussed these possibilities and presented preliminary results of a study designed to explore the decompression properties of argon with a 3.5-psia suit pressure. While the DCS incidence exceeded the 15% upper limit suggested by NASA for EVA, the study represented the first step in defining optimal suit and habitat pressures for an argon breathing mixture. Lambertsen et al. (3) Fire is a significant risk in a space vehicle or habitat, and fire suppression agents must be nontoxic. Gases low in oxygen content are both non-toxic and fire suppressant, but the risk of hypoxia to the astronaut makes them physiologically unsuitable. Lambertsen et al. (3) from the University of Pennsylvania presented data demonstrating that this problem can be corrected by adding 4% carbon dioxide to an hypoxic fire suppressant gas containing 10% oxygen. This strategy prevented hypoxia-induced hypocapnia and increased cerebral blood flow and oxygen delivery with minimal performance decrements. Butler et al. (2) An understanding of the mechanisms of decompression sickness could have important implications for solving the operational problems of EVA. Butler et al. from Hermann Hospital (University of Texas) investigated the responses of a rat model to drugs and to variations in the diurnal cycle of light and dark to evaluate the circadian rhythmicity of DCS (2). Decompression from a relatively mild hyperbaric exposure was found to cause significant increases in the permeability of both the systemic and pulmonary microcirculations. These changes were greatest for large deviations from a 12-hour on, 12-hour off light-dark cycle. Drugs that blocked leukotriene receptors (which mediate endothelial cell injury) were found to have the greatest protective effect.
IMPLICATIONS FOR FUTURE RESEARCH The work described above represents the coordinated efforts of multiple institutions with the goal of moving theoretical concepts from the laboratory to operationally useful countermeasures. The Astronaut Office at Johnson Space Center has provided this coordination under the guidance of Dr. Michael Gernhardt. Continued coordination involving knowledge of both the science and the operations is essential to further developments for the ISS and for missions to the Moon and Mars.
Understanding the optimal use of exercise during oxygen prebreathe, for example, may lead to additional improvements to ISS EVA protocols. The 2-hr EVA protocol has the disadvantages of requiring a cycle ergometer, a 120 ft oxygen hose, extra oxygen, and use of the ISS airlock for suit donning. A more convenient procedure would conduct the entire oxygen prebreathe (including exercise) in the Space Suit. Ground-based trials of such a procedure are planned with the objective of reducing the prebreathe duration to 1.5 hrs (5). Intermittent exercise will be used to maintain high perfusion and rapid nitrogen elimination with the lowest energy expenditure. Looking towards space exploration beyond the ISS, an understanding of the effects of gravity on DCS risk (1) is needed for exploration of the Moon and Mars. This is also true for defining the optimal combinations of inert gas, habitat pressure, and suit pressure (4). The work must be conducted now if the appropriate procedures are to be available when needed in the future.
REFERENCES 1. Balldin UI, Pilmanis AA, Webb JT, Kannan N. Hypobaric decompression sickness and adynamia. 2. Butler BD, Smolensky M, Sothern RB, Little T. Experimental decompression sickness: inflammatory response, circadian rhythmicity, pharmacological intervention. 3. Lambertsen CJ, Gelfand R, Hopkin E. Carbon dioxide-oxygen interactions in extension of tolerance to acute hypoxia. 4. Pilmanis AA, Webb JT, Balldin UI. Staged decompression to a 3.5 psi EVA suit using an argon-oxygen (argox) breathing mixture. 5. Vann RD, Gerth WA, Natoli MJ, Pollock NW, Butler BD, Fife CE, Beltran E, Nishi RE, Sullivan PA, Pilmanis AA, Conkin J, Foster PP, Hamilton D, Acock K, Loftin KC, Waligora JM, Schneider SM, Ross CE, Powell MR, Dervay JP, Feiveson AH, Gernhardt ML. Design, trials, and contingency plans for extravehicular activity from the international space station. 6. Webb JT, Pilmanis AA. Exercise-enhanced preoxygenation for altitude decompression sickness (DCS) protection.
