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Microgravity Research Opportunities for the 1990s: Chapter 8 Microgravity Research Opportunities for the 1990s PART IIIâPROGRAMMATIC ISSUES 8 Flight Opportunities and Challenges INTRODUCTION The microgravity research program began in the late 1960s, primarily as a microgravity materials processing activity. It was initially an outgrowth of the space race between the Soviet Union and the United States and was concerned not just with scientific endeavors but also with the commercial materials marketplace. Early experiments on Apollo, Skylab, and Apollo-Soyuz provided the foundation for a research field that is now nearly a quarter of a century old. In the 1970s, the credibility of the program suffered somewhat when efforts failed to demonstrate the near-term rewards promised by enthusiasts. Europeans, REPORT MENU meanwhile, focused their program on microgravity fluid and materials science issues using NOTICE drop towers, parabolic flights, sounding rockets, and high-altitude balloons. The Soviets took MEMBERSHIP advantage of a nearly continuous presence in space with flight experiments in microgravity PREFACE environments. During the late 1980s and early 1990s, the United States strengthened its EXECUTIVE SUMMARY programs in microgravity research with numerous shuttle flights, leading to a plan to utilize a PART I space station as a long-term, permanently crewed facility. CHAPTER 1 CHAPTER 2 In developing a program of microgravity research, NASA has attempted to provide a PART II balance of opportunities for its various constituencies: industry, government, and academia. CHAPTER 3 Academia has traditionally focused on fundamental research, industry on commercial CHAPTER 4 CHAPTER 5 applications, and NASA on the mission frequency and ground-based infrastructure to CHAPTER 6 support reasonable research progress. With the reduction of flight opportunities following the CHAPTER 7 Space Shuttle Challenger accident, NASA recognized that a reliable spaceflight PART III infrastructure was essential if U.S. industry and academia were to expand their participation CHAPTER 8 in the program and provide the next generation of space scientists. Providing a reliable APPENDIX A infrastructure, however, has greatly increased the complexity of flight experiments and, APPENDIX B consequently, increased the costs of shuttle missions. Together with the development and utilization of sophisticated technologies in optics, electronics, computers, communication devices, and sensors, dedicated missions in particular have become more costly to develop and operate. In an attempt to keep costs to a minimum, NASA constructed a number of multiuser spaceflight instruments and hardware for several microgravity scientific and technological communities. These instruments, identified through a complex internal evaluation and file:///C|/SSB_old_web/mgoppch8.htm (1 of 14) [6/18/2004 11:18:34 AM]
Microgravity Research Opportunities for the 1990s: Chapter 8 comparison process, have not proved as useful to a wide spectrum of investigations as intended. Because microgravity research is a laboratory science, it often requires real-time interactions between scientists and their experiments. Moreover, a laboratory-based researcher must be able to carry out a large number of experiments over ranges of experimental parameters and demonstrate the reproducibility of results. When necessary, the researcher must be able to modify the apparatus and employ diagnostics to achieve these goals. NASA's microgravity research program must, therefore, be sufficiently broad and robust to encompass requirements for basic research as well as provide the foundation for development of the many processes leading to future space-based technologies. The program of support should be designed to be flexible so that it can rapidly accommodate new areas of investigation or scientific interest as they develop. To achieve greater flexibility in terms of instruments, procedures, and approaches that should be introduced into the program, a number of special administrative points should be considered: Access to the microgravity environment should be increased substantially; this means, for most practical purposes, access to space shuttle missions and ultimately to a long-duration orbiting platform such as the space station. However, while the value and need for human intervention capabilities are noted repeatedly in this report, nothing herein should be interpreted as advocating or opposing any one of several initiatives or programs, such as a space station, planned by NASA in coming years. Hardware versatility, variety, and flight manifests should be made more flexible so that research development builds on flight experience in a systematic and expedient manner. Some microgravity experiments can benefit from an investment in teleoperational capability, allowing scientific investigators direct interaction with their experiments. NASA should encourage and support the development and deployment of such techniques in future microgravity experiments. The cost of flight instruments for research in microgravity should be substantially reduced by lessening bureaucratic overhead, minimizing technical complexity, and eliminating unnecessary requirements. Approaches must be devised to obtain initial feasibility data and proof of concept from simplified, economical devices before any commitment is made to the long-term development of expensive instrument systems. As much as possible, instruments should be shared among divisions, particularly between the Life Sciences and the Microgravity Science and Applications Divisions, and between NASA and the space agencies of other countries, such as the European Space Agency. This could serve to broaden the base of available flight instruments, reduce expense, and encourage a closer cooperation among space scientists. Transient accelerations of spacecraft due to equipment operation and crew movement can affect microgravity experiments. In certain experimental situations, the file:///C|/SSB_old_web/mgoppch8.htm (2 of 14) [6/18/2004 11:18:34 AM]
Microgravity Research Opportunities for the 1990s: Chapter 8 effects of these accelerations and, in particular, the acceptable maximum magnitude are quantitatively well understood. In other cases, however, the effects of these accelerations are only qualitatively known and the maximum acceptable level cannot confidently be prescribed. Research is needed to characterize the effects of such accelerations. Some attention should be given to the moderation of safety requirements for experiments in space. The level of containment currently required is often unnecessary, adds needless complexity and overhead, and appreciably increases costs. In addition, it limits flexibility, slows development rates, and places an encumbrance on instrument development that weighs heavily on the overall program. The same is true of the level of toxicological analysis, the time required to effect minor and otherwise insignificant changes in an experiment, and the excessive amount of documentation required to achieve flight qualification. Although any, or all, of the more stringent requirements might be entirely appropriate in specific instances, they should not be broadly and indiscriminately applied. Another issue relates to the assignment of administrative responsibility in the areas of microgravity effects on cellular and subcellular processes and mechanisms, investigations into microgravity effects on macromolecular assemblies, their responses to physiological stimuli, and the biochemical processes that underlie all of these phenomena. A clear concept and description of the overall scope of the Microgravity Science and Applications Division research effort in the area of biotechnology should be developed. Some mechanism should be found to establish and maintain close cooperation and coordination among investigators in the area and between the Life Sciences Division and the Microgravity Science and Applications Division. OPPORTUNITIES: MICROGRAVITY RESEARCH FACILITIES A variety of facilities are available for research in low-gravity environments. These range from drop towers and aircraft, used to conduct tests that need only short-duration milli- to microgravity conditions, to the space shuttle and space station. The characteristics of each facility in the NASA program are described below and summarized in Table 8.1. Except where a specific document is cited, the technical specifications of the facilities were obtained through interactions with NASA staff. Drop Towers and Drop Tubes The drop tower or drop tube low-gravity facility relies on free-fall of the experimental package to produce a microgravity environment. Efforts are made to eliminate the retarding effect of the atmosphere, either by evacuating the region in which the free-fall takes place, by use of a drag shield, or by gas thrusters. Typically, microgravity conditions last for only a few seconds, and the experimental package must withstand extreme deceleration at the end of a test. 5.18-Second Zero-Gravity Research Facility The 5.18-Second Zero-Gravity Research Facility, which consists of a tube embedded file:///C|/SSB_old_web/mgoppch8.htm (3 of 14) [6/18/2004 11:18:34 AM]
Microgravity Research Opportunities for the 1990s: Chapter 8 in the ground that can be evacuated to approximately 10-2 torr, permits 5.18 s of microgravity conditions of <10-5 g. The height of the vacuum chamber is 145 m and allows a free-fall of 132 m. One to two tests can be run per day. This facility is located at the NASA- Lewis Research Center. 2.2-Second Drop Tower The 2.2-Second Drop Tower provides 24 m of free-fall. Tests are run in air with a drag shield, which leads to a gravitational acceleration of <10-5 g during free-fall. Up to 12 tests per day can be run at the facility, which also is located at NASA-Lewis. 105-Meter Drop Tube The 105-Meter Drop Tube can be evacuated to a pressure of 10-6 torr, which allows 4.6 s of free-fall with accelerations that may be as low as 10-6 g (currently not measured). The drop tube is designed such that a sample can be melted and then allowed to solidify during the free-fall to study containerless processing. This facility is at the NASA Marshall Space Flight Center. Up to 45 drops per day are possible. Parabolic-Flight Aircraft Microgravity research aircraft obtain weightlessness by flying a parabolic trajectory. Although large forces of acceleration and deceleration are produced during portions of the flight, in the short period of parabolic flight, conditions of less than 1% of Earth gravity are obtained. Such periods of weightlessness may be useful for conducting low-gravity experiments requiring somewhat longer times than are available in the drop towers and for those cases in which the quality of the microgravity environment is not critical. Parabolic flights are useful for testing experimental packages intended for later flight on the space shuttle or space station. KC-135 Aircraft When the KC-135 flies through the top of a parabolic flight path, some 15 to 25 s of 10-2-g low-gravity conditions occur, with 5 to 15 s of this at the level of 10-3 g for experiment packages that are in free-fall within the cabin. Some 40 trajectories can be flown during a single flight. The KC-135 operates from the Johnson Space Center in Texas and the Lewis Research Center in Ohio. Learjet Low-gravity conditions are maintained for 18 to 22 s in the Learjet. Typically the acceleration is of the order of 10-1 to 10-2 g during this period, but the trajectory can be altered to produce intermediate accelerations. Six trajectories can be flown during a single flight. NASA recently acquired a DC-9 that is expected to replace the Learjet sometime in 1995. The capabilities of the DC-9 are similar to those of the KC-135. file:///C|/SSB_old_web/mgoppch8.htm (4 of 14) [6/18/2004 11:18:34 AM]
Microgravity Research Opportunities for the 1990s: Chapter 8 Sounding Rockets High-altitude suborbital rockets (sounding rockets) have been used extensively for space science research since the end of World War II. In recent years, they have been used for microgravity research. At the top of the arc of the trajectory, a gravitational environment of < 10-5 g exists for up to 15 min. Operational launch vehicles permit payloads up to 420 kg. Recovery systems allow soft impact retrieval of payloads. High data-rate telemetry, real-time ground up-link command, and down-link video data are routinely available. Space Shuttle The operating parameters on the shuttle have been measured by the Space Acceleration Measurement System (SAMS) and the Orbital Acceleration Research Experiment (OARE) to be 10-6 to 10-5 g for quasi-steady (i.e., low-frequency) accelerations. Disturbances of the order of 10-3 g during on-orbit maneuvers or crew operations are controllable to some extent. Flights are scheduled to last a number of days, but uninterrupted quiet times are brief. Figure 8.1 shows the capabilities of the accelerometer systems, SAMS and OARE, along with typical shuttle acceleration spectra. The standard spacelab double rack available to experimenters is limited to about 634 kg, with 1.75 m3 in volume available for a payload unit. The possibility exists, however, of connecting several units for a given experiment. FIGURE 8.1 Accelerometer systems (SAMS, OARE) and typical disturbances on the shuttle. file:///C|/SSB_old_web/mgoppch8.htm (5 of 14) [6/18/2004 11:18:34 AM]
Microgravity Research Opportunities for the 1990s: Chapter 8 The times required for experiments in some of the subdisciplines of microgravity research exceed those available on the typical spacelab mission. Spacelab missions with extended-duration capability might be better able to serve the research communities of those subdisciplines. For others such as biotechnology and materials science, longer flight times would provide significant benefits in terms of the quality of scientific yields. Even for those microgravity experiments that do not require long duration in orbit, the limited number of spacelab flights severely limits reproducibility. These limitations, inherent to the spacelab system, can be obviated only by the longer flight durations available on a space station or free-flyer. Space Station The following describes the requirements for microgravity research aboard a space station. While the information given here was taken from "Space Station Freedom (SSF) Program Definition and Requirements Document," Revision L (1992), the microgravity requirements remain essentially unchanged in the more recent "Space Station Concept of Operations and Utilization," Vol. 1, Appendix C, Revision A (March 23, 1994). Although the space station concept continues to be redefined, the desired microgravity characteristics are likely to be the same as indicated herein. In specifying allowable accelerations, quasi-steady state means accelerations with frequencies <0.01 Hz. In this frequency regime, an upper limit of 10-6 g is imposed on the magnitude of the instantaneous acceleration, and an upper limit of 0.2 x 10-6 g on the magnitude of the component of the instantaneous acceleration perpendicular to the direction of the average orbital acceleration. (The critical frequency 0.01 Hz was chosen as being one order of magnitude below a reasonable estimate of the fundamental structural vibration frequency.) Figure 8.2 shows the allowable accelerations above 0.01 Hz. Considered individually, the total root-mean-square contribution to the acceleration spectrum from each work package and from each international partner must not exceed the limits indicated in Figure 8.2. Furthermore, the vibrations induced by the combined payloads must not exceed the root-mean-square acceleration magnitude limits of Figure 8.2. These limits are applicable in any one-third octave band from 0.01 through 300 Hz, over any 100-s interval. Crew interfaces with the space station structure, such as treadmill, keyboards, and drawers, are covered by the vibration budget seen in Figure 8.2. Currently, however, astronaut motion is not subject to the vibration restrictions. Clearly, there is little point in designing the structure and instrumentation of a space station to provide a microgravity environment if the movement of the astronauts introduces unacceptable disturbances. The question of astronaut motion is still under consideration. file:///C|/SSB_old_web/mgoppch8.htm (6 of 14) [6/18/2004 11:18:34 AM]
Microgravity Research Opportunities for the 1990s: Chapter 8 FIGURE 8.2 Space Station Freedom combined sources microgravity allocations. Graph shows allocation limits for each work package, each international partner, and the combined payload complement, at module-to-rack interface. Limits applicable in any one-third octave band from 0.01 to 300 Hz, over any 100-s interval. * Individual allocations for three work packages, three international partners, and payloads. ** RMS total of all seven individual allocations. RMS, root mean square. The requirements of Figure 8.2 refer to monochromatic loading with time- independent amplitudes. Specifications have been written for transient effects. Each individual transient is limited to 10-3 g along each of three mutually perpendicular axes. In any 10-second window, the time-integrated acceleration must be less than 10 g-s along each of the three directions. This criterion is applied to each disturbance source separately. In the case of multiple sources, if these are monochromatic with the same frequency the accelerations are summed (with phase relations taken into account) and the result is compared with Figure 8.2. Procedures have been formulated to take into account broad- band disturbances. An example of the analysis of the disturbance to the microgravity environment to be expected from a particular facility is given in Figure 8.3 A,B. Both the narrow-band (A) and the broad-band (B) requirements for the centrifuge facility (CFP) file:///C|/SSB_old_web/mgoppch8.htm (7 of 14) [6/18/2004 11:18:34 AM]
Microgravity Research Opportunities for the 1990s: Chapter 8 previously planned for Space Station Freedom are shown. Also shown are the "allocations" to the CFP, 35% of the force required to produce a 10-6-g acceleration. The frequency dependence of these curves reflects the frequency response of the structure of the SSF to disturbances. This example is presented for the purposes of illustration only, since more recent space station plans do not yet specify allocations for the centrifuge facility. FIGURE 8.3A Approximate Space Station Freedom (SSF) imposed narrow-band force requirement on the centrifuge facility (CFP). RMS, root mean square. file:///C|/SSB_old_web/mgoppch8.htm (8 of 14) [6/18/2004 11:18:34 AM]
Microgravity Research Opportunities for the 1990s: Chapter 8 FIGURE 8.3B Approximate Space Station Freedom (SSF) imposed broad-band requirement on the centrifuge facility (CFP). RMS, Root mean square. Design and operational plans should be such that the microgravity environment is of sufficient quality (including full consideration of transients, multiple sources, and resonant frequencies) and time duration to meet the requirements of the scientific experiments. Free-Flyers Free-flyer data were taken from an Announcement of Opportunity for EURECA-1 experiments. The data sheet specified a microgravity environment of 10-5 g for frequencies of <1 Hz and 10-3 g for frequencies >100 Hz. The EURECA environment has been verified by on-orbit measurements. Total payload mass was approximately 1000 kg. The mission was launched in July 1992 and flew for about 10 months. Lunar Base Experiments conducted on the Moon's surface would take advantage of an order-of- magnitude reduction in the gravitational acceleration to 0.16 g. CHALLENGES TO MICROGRAVITY file:///C|/SSB_old_web/mgoppch8.htm (9 of 14) [6/18/2004 11:18:34 AM]
Microgravity Research Opportunities for the 1990s: Chapter 8 RESEARCH EXPERIMENTATION The present microgravity research infrastructure does not readily accommodate the needs of laboratory-type research. Drop tower experiments and precursor experiments on airplanes flying parabolic trajectories are limited to short durations. Robotic materials processing and telescience experiments can be done on free-flyers. Spacelabs and a space station are required for conducting laboratory-type microgravity research. Experience with spacelab for microgravity research, however, indicates that (1) it is likely to take years to develop an experiment that can yield high-quality scientific data and (2) it is an extremely expensive process. At present, the time required from the selection of a new principal investigator to launch of his or her spaceflight experiment is 5 to 6 years for the mid-deck (noninteractive) location and 6 to 8 years in spacelab. Reflights with minor modifications to the equipment require a minimum of 1 year for mid-deck and 2 years for spacelab experiments. Every element of an experiment must be documented in detail and pass safety tests. Figures 8.4 and 8.5 outline the process, including numerous extensive reviews at various stages of the development and integration of the experiment. For example, there are 5 reviews in the experiment definition and development process and 11 reviews (including 4 for safety) in the mission integration process. The experiment is required to be delivered a full year before launch for integration into the spacelab and orbiter. Because the number of spacelab missions is limited, as many experiments as possible are scheduled for each flight. Thus, the number of experimental runs is limited even if all goes as planned on the mission. As a result, compromises in the form of flight rules often have to be made in the acceleration environment due, for example, to the orbiter orientation. These flight rules are often favorable for one experiment but not another. Because of heavy demand, experimenters are guaranteed only one flight and any reflights must compete again. file:///C|/SSB_old_web/mgoppch8.htm (10 of 14) [6/18/2004 11:18:34 AM]
Microgravity Research Opportunities for the 1990s: Chapter 8 FIGURE 8.4 Elements of the flight experiment development process. file:///C|/SSB_old_web/mgoppch8.htm (11 of 14) [6/18/2004 11:18:34 AM]
Microgravity Research Opportunities for the 1990s: Chapter 8 FIGURE 8.5 USML-1 space experiments processes. MSAD, Microgravity Science and Applications Division; NRA, NASA Research Announcement; KSC, Kennedy Space Center; SRD, Science Requirements Document. Surmounting the various hurdles to which a flight experiment is subject requires interactions with three different NASA centers: Johnson Space Flight Center, which is in control of the orbiter; Marshall Space Flight Center, which controls the spacelab; and Kennedy Space Center, which is in charge of integration and launch. The conditions imposed on the experiment and the principal investigator by the various NASA centers are not always consistent. In addition, there are many people involved that have sharply delineated areas of responsibility, including systems, safety, quality control, time line, and orbital mechanics. All of this complexity is a result of the concern for the safety of the crew and spacecraft and results in long times for experiment development. It is suggested that (1) the interactions among centers and headquarters be simplified and unified and (2) the principal investigators be continually involved in all aspects of these procedures. Every procedure and requirement should be examined to determine if it is relevant and essential. TABLE 8.1 Research Facilities for Microgravity Gravity Duration Facility Environment of Environment Comments file:///C|/SSB_old_web/mgoppch8.htm (12 of 14) [6/18/2004 11:18:34 AM]
Microgravity Research Opportunities for the 1990s: Chapter 8 5.18-s Zero-Gravity <10-5 g 5.18 s Operational early 1993 Research Facility 2.2-s drop tube <10-5 g 2.2 s 105-m drop tube 10-6 g 4.6 s High-temperature capacity KC-135 aircraft 10-2 g 15-25 s Free-fall within cabin 10-3 g 5-15 s Learjet 10-1-10-2 g 18-22 s Sounding rockets <10-5 g Up to 15 min Space shuttle 10-6-10-5 g Variable (quiescent period) 10-3 g (active period) Space station 30 days continuous, 180 Additional specifications for <10-6 g, f 0.1 Hz days/year complex disturbances <10-3 g, f 100 Hz f x 10-5 g, 0.1 f 100 Hz Free-flyers <10 -5 g, f< 1 Hz Several months <10-3 g, f> 100 Hz f x 10-5 g, 1 < f< 100 Hz Lunar base 0.16 g NOTE: f = frequency. file:///C|/SSB_old_web/mgoppch8.htm (13 of 14) [6/18/2004 11:18:34 AM]
