Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
3 Issues Related to Utilization and Operations Requirements for the Space Station The overall Space Station utilization and operations requirements stem from the need to satisfy a multiplicity of currently planned as well as potential evolutionary purposes. The Phase 1 Space Station is planned to be a facility supporting such activities as microgravity materials research, biomedical studies, and human habituation studies. It also will accommodate externally attached scientific payloads to a limited extent. The evolution of the Space Station beyond Phase 1 could move in any of a number of directions, including development into a transportation node for manned lunar and/or planetary missions, an assembly facility for the construction of large spacecraft or other in-space infrastructure, a depot for servicing space assets, or perhaps even a dedicated life sciences or materials science laboratory. The Space Station design is intended to allow flexibility in utilization and operation as it evolves. At present, the Space Station is being designed principally to serve users from the major space science and technology disciplines. On the manned base facility, the key resources are the external attachment points with associated power and data link capabilities, and the internal pressurized volume--first in the two pressurized U.S. modules and later in the Japanese and European Space Agency modules. The major users of the pressurized module volume will be the materials science and life science communities. Earth observation requirements are to be met mainly by the Polar Orbiting Platform. Space Station Freedom is intended to be an international facility--in planning, management, and operation. The international space science community, as represented by the 19
20 International Forum on the Scientific Uses of the Space Station has strongly supported a shared international facility with commonality of interfaces and sharing of major facilities, independent of agency boundaries. To the extent that the Space Station Memoranda of Understanding and the management structure support this concept, meaningful international cooperation can take place. The development of an integrated design with well-defined interfaces across the U.S.- and non-U.S.-provided facilities will require continued attention. The overall Station layout and the detailed planning for utilization of the transverse boom and the lab modules are generally consistent with the emphasis on two new contributions of the Station: long durations and sufficient pressurized volume for significant microgravity research. Layout and planning are also consistent with the intended attributes for the Station summarized in Table 1. However, several attributes listed in Table 1 may themselves be inconsistent or even partially competitive. For example, some crew operations may have to be restricted to avoid disturbing the low levels of microgravity (10-6 g) required for classes of proposed experiments. Thus, while some previous studies have indicated that the same laboratory can be used for both materials and biological research, adequate attention must be paid to the impacts of such cooperative or simultaneous operations--for example, the need for vibration and contaminant isolation. The committee believes that the general issue of the compatibility of all the planned/potential users of the Space Station is very important and needs to receive more attention. Moreover, even though requirements in a particular discipline may not be completely defined, the ultimate possible extent of the requirements may be crucial inputs to architectural decisions being made now (e.g., compatibility of animal experimentation with planned Space Station environmental and decontamination provisions). With respect to the evolution of the Space Station, its potential utilization as a base for future manned missions does not present requirements at this time that are specific enough to influence the detailed design of the Phase 1 Space Station. Only when and if requirements for the assembly and launch of planetary or lunar base missions are defined can the appropriate Space Station evolutionary path to support them also be defined. However, continued care must be taken during the Phase 1 Space Station design process so as not to preclude
21 such use by arrangement of the Phase I geometry or other characteristics. The following sections identify specific issues raised by the workshop committee regarding Space Station utilization and operations requirements. Issues include those associated with Design Reference Missions, which illuminate the implications of user and operator requirements for Station design, assembly phase requirements, mature utilization and operations requirements, and evolutionary phase requirements. DESIGN REFERENCE MISSIONS In order to guide the requirements generation process and to check the adequacy of the current Space Station design, some reasonable operational scenarios along with representative experiments to be conducted are required. For the second purpose a set of Design Reference Missions is being developed and used to identify the strengths and weaknesses of the Space Station design. These Design Reference Missions are intended to be both representative and comprehensive. However, it is by no means clear that the missions chosen are really design drivers that illuminate the most difficult features and/or incompatibilities in the design. Clearly, the nature of the Design Reference Missions used is an important determinant of the success of the paper evaluation of the design. The weak points will only be understood by returning to each scientific discipline's list of approved missions and deliberately selecting mission combinations that stress the Station's resources, especially data capacities, power, and crew time. The committee is concerned that the current exercises, which show the Station's resources to be more or less in balance, with the first limitation coming from power, may not allow an accurate assessment of such user and operator needs as: removal of bottlenecks in multiple video links, provision of separate work stations, and additional crew accessibility. Development of Design Reference Missions should be carried out with the intention of probing the ciesign's weaknesses rather than demonstrating its strengths. For example, the committee questions the claim that the volume available--either inside the pressurized modules and nodes or on the external boom--is sufficient to accommodate the users who could benefit from the currently planned power level, data capacities, or crew time.
22 Further, it was evident to the committee that Shuttle and Spacelab precedents were used in developing procedures for user experimentation on the Space Station. Precedents that are troublesome include the Space Shuttle and Spacelab procedures related to integration and verification of hardware and software for user experiments, which require very long lead times. More innovative processes are needed to improve responsiveness to user needs consistent with safe and effective Station operations. ASSEMBLY PHASE REQUIREMENTS AssemblY Sequence The assembly sequence for the Phase I Space Station is constrained in several important ways. Among these are existing post-Challenger Shuttle performance limitations that have an impact on Space Station assembly by limiting the allowable payload mass and/or volume and maximum extravehicular activity time by the crew per Shuttle flight. Another set of constraints are those imposed by selected congressional directives. The directives deal with launch of the Flight Telerobotic Servicer (FTS) (a robotic arm assembly) by the second assembly flight, early utilization of the U.S. laboratory module for man-tended materials processing prior to the Space Stationts permanently manned phase, and the location of life science facilities on the Station. All of these constraints have increased the design and operational complexity of the Space Station. The committee is concerned that the Space Stationts design, assembly' and operation may have become unduly constrained and believes it is important that the justification for each constraint be firmly established and its impact on the program clearly understood. Premature Flight Telerobotic Servicer Launch The objective of advancing automation and robotics to improve scientific utilization of the Space Station as well as to increase the efficiency of operations is laudable and should, in the long run, provide significant productivity benefits. However, a premature demonstration flight of the FTS concept and early manifesting of the FTS in the Space Station assembly sequence unfortunately is likely to have just the
23 contrary effect. The premature demonstration and manifesting requirements mean that only older and relatively inflexible technologies for robotics and automation will be used, and any potential gains for assembly, repair, and operation will be much less than if a flight demonstration were delayed until clear-cut applications were defined and some of the newer developments could be used. The planned early manifesting of the FTS is not currently of any real use since it cannot be relied on for assembly and will not (and should not) be on the assembly phase critical path. Its possible use for Space Station inspection purposes is of marginal benefit and does not justify its early position in the assembly phase launch sequence. Assemble Phase Utilization The main activity during the assembly phase will be the deployment and checkout of the Space Station and its systems. As noted earlier, the committee believes that the assembly sequence is over constrained, leading to a long-term penalty in terms of laboratory outfitting, utilization, and Station operations. For example, the life science experiments in basic gravitational biology and in preparation for extended manned missions require extensive use of several large facilities on the Space Station. These major facilities have been strongly recommended by a series of NRC and NASA advisory committees, and include specimen-holding facilities, bioisolation work stations, and a centrifuge. The current plan, which does not accommodate the animal/plant centrifuge in the U.S. laboratory module, and which proposes to assemble and check out the life sciences facilities on orbit, does not appear to meet the requirements established by the committees. The removal of the centrifuge from the U.S. laboratory to a node, which may or may not be able to properly accommodate it, is of concern: The separation of the centrifuge from the specimens, cage cleaner, work station, and general laboratory tools creates operational problems, and does not appear to be warranted by concerns over vibrational disturbances in excess of those caused by crew motions. In addition, the installation and checkout of the centrifuge on orbit is likely to prove difficult to accomplish. Finally, the committee believes that, to the extent that the overall Space Station assembly and checkout activities allow, the early, man-tended configuration of the Space Station could be used for meaningful life science and materials science
24 microgravity experiments. In particular, potential life science activities might range from short-duration verification of specimen holders to longer term biological investigations of the influence of weightlessness on cells and microorganisms. However, the committee notes that the proposed launch of the U.S. laboratory module with only materials sciences facilities should be reconsidered, as it would preclude the above-mentioned life science research activities. MATURE UTILIZATION AND OPERATIONS REQUIREMENTS Once the assembly phase has been completed, the Phase 1 Space Station will enter its period of mature utilization and operations activities. The committee identified a number of issues pertaining to the requirements associated with these activities. Micro~ravitv Environment The Space Station design reflects the emphasis on providing a volume within the U.S. laboratory module, and later within the Japanese Experiment Module (IEM) and Columbus modules, in which experiment racks experience a very low (on the order of 10-6 g) steady-state acceleration. A particularly sensitive issue to both the Space Station life science and materials science users is the production, transmission, and absorption of vibrations at frequencies that could damage the microgravity processes under study. Until the Space Station program tackles the vibration transmission issue squarely (including provision of structural damping within and between modules), crew-induced disturbances, which are inherent to any manned spacecraft, as well as the additional vibrations from fans, pumps, valves and centrifuges, will have to be treated empirically--with a resulting awkwardness in layout and operations. The committee believes that, as part of resolving the vibration transmission issue, the additional analysis should be done to determine whether responsibility for providing vibration isolation at the rack level of certain experiments should rest with the Space Station program or with the indiviclual experimenter. In general, the provision of a very low steady-state acceleration environment will require careful, continuous tracking and control of the center of gravity. However,
25 apparently little attention has been given to the possibility of continuous reboost' using the resisto,~et thrusters, in order to overcome the effect of the 3.0 x 10- g average atmospheric drag expected at the planned Station altitude of approximately 220 nautical miles. The drag necessitates a periodic reboost to maintain the Station's orbital altitude. Periodic, as opposed to continuous reboost, could necessitate taking acceleration-sensitive experiments off-line during the reboost interval, which would have a negative impact on some user experiments. However, there are concerns relative to continuous reboost associated with potential contamination of attached payloads, as well as with possible torque disturbances that would have to addressed. Moreover, the atmospheric drag at the nominal 220 nautical mile altitude is not steady, but is strongly affected by, among other things, the diurnal bulge, seasonal variations, and solar storms. Over the span of a few orbits, the drag variations can exceed a factor of ten, with limited predictability. While the committee did not have time to examine in detail the trade-offs involved with continuous reboost, it feels that the concept has sufficient merit to warrant evaluation by the Space Station program. Crew Utilization On-orbit crew time will be a scarce resource, given all the conflicting demands made on it. The amount of crew time available to users is estimated to be about 30 percent of total time on orbit. However, this estimate was made prior to detailed studies of system maintenance, operation, and repair requirements on available crew time. In addition, extended duration crew operations (EDCO) activities--medical studies to verify the feasibility of long crew stay times on orbit--are planned to take up 4 percent of the available crew time. Moreover, extravehicular activity will have a significant time overhead. The committee believes it is important that the amount of crew time available for user activities be protected against erosion as maintenance and operations requirements are better defined. One important way that the impact of scarce crew time for user activities can be minimized is through remote monitoring and control of experiments by ground investigators. Thus, the committee believes that two-way data and voice-video communication to the user laboratories, taking advantage of the new telescience developments, must be designed into the
26 communication network. The use of these systems should then be incorporated into the operations plans. One impetus for making maximum use of telescience is that it will not be possible to assemble full payload teams on the ground to serve at the Payload Operations Integration Center for long durations. The Space Station program and the Office of Space Science and Applications need to find the technical (and financial) means to provide remote experiment monitoring and command of experiments. Another means of increasing the effectiveness of crew utilization in support of user activities is to get Space Station scientists assigned to specific user experiment payloads as early as possible. For example, by 18 months prior to flight, a major payload has already gone through preliminary and critical design reviews, and probably through major phases of construction, as well. This is much too late in the payload preparation cycle to involve the primary operators (and repairers). Assignments should be Rekeyed to payload development cycles to ensure that appropriate operations expertise is available during the early stages of design of the hardware and associated operation concepts. Attention and involvement early in the payload development process will reduce wasted time on orbit. Coupled with the need for early crew involvement in experiment payload development is the need for a concerted effort to reduce the integration time for proposed Space Station experiments. If the Space Station is to be a useful 30-year research facility, it must be accessible for experiments and follow-up investigations on a timely basis. Furthermore, the flow of instruments and facilities onto and off of the Station must be carefully planned from a practical point of view. As was mentioned earlier, the committee is concerned that documentation, verification, and integration requirements for user payloads, if not specified and implemented reasonably, can have negative impacts on cost, time, and scientific productivity. Finally, the committee supports NASA's current plan for Space Station crew selection and training. That plan provides for the assignment of Station operators, Station scientists, and payload scientists, with the first two categories being staffed by career astronauts and the last category being filled by researchers who are not career astronauts. Based on the Space Shuttle Spacelab mission experience, it is important that there be a class of crew that is assigned to
27 and primarily responsible to the user organizations, as the introduction of independently assigned payload specialists has provided advocacy for the user community in the crew and operations areas. The committee is concerned that post-Challenger crew selection procedures do not lead to a diminution of opportunities for independently assigned Space Station payload scientists. Furthermore, the committee believes there is merit in having the career Space Station scientists assigned and evaluated by appropriate user organizations, since the scientists' prime duty is the support of payload operations. Commonality The science users of the Space Station all desire standardization, and where possible, commonality of equipment both across disciplines and across modules. The Space Station should not be limited to a set of fixed and inflexible protocols, but should resemble a ground laboratory in which new pieces of equipment are brought in to replace or supplement other pieces as an experiment matures or as new and unexpected data Is produced. Standards for power, video, data, and fluid coupling should permit and encourage the interchange of equipment and minimize the need for spares within the entire Station. The committee believes that the Space Station program should not rely upon the possibility that certain common manufacturers might be called upon to supply all modules with equipment to reach such commonality (the approach briefed to the committee). The consequences of failing to achieve commonality are significant: imagine the frustration of an astronaut who needs a special lens, available only on a video camera in another module, but who finds that the lens connectors are not compatible and that the camera on which the lens does fit is at another video standard than the recorder in the rack where he or she is working! Logistical ResunDlv Logistical support for a 30-year, orbiting research facility will be a complex undertaking, with a range of associated issues. Time limitations did not allow the workshop committee to examine this area in great depth, but it did identify a number of issues that it believes need to be addressed.
28 The Space Station program has no provision for unmanned resupply of the U.S. modules (e.g., facilities for unmanned rendezvous and docking). The 1987 NRC Committee on the Space Station recommencled that "NASA should make plans for eventual logistical support of the Space Station with expendable launch vehicles, as well as with the Shuttles (Report of the Committee on the SDace Station of the National Research Council, p. 7, 1987~. The workshop committee agrees with that recommendation. Furthermore, the flow of instruments and facilities onto and off of the Space Station must be carefully planned from a practical point of view. As was mentioned earlier, the committee is concerned that documentation, verification, and integration requirements for user payloads, if not specified and implemented reasonably, can have negative impacts on cost, time, and scientific productivity. The committee also feels that NASA should continue to examine how much trash will be returned from the Space Station by the Shuttle versus disposal by controlled burn-up of trash canisters during reentry into the atmosphere. Finally, the committee notes that the current design has no life-support provisions for animal and plant specimens during ascent to the holding facilities in the Space Station laboratory or during return to earth. This requirement is complicated by the scientists' requirement for late access to the animals before launch and prompt removal of them on orbit and on their return to earth. The committee is concerned that there appears to be no focal point for aggressively dealing with this item, despite its importance. Contingenev Planning The workshop committee believes that contingency planning has progressed in a very positive sense over the past year. There still is an enormous amount of work to be done in the area, but the results to ciate appear to be the outcome of an established process--one that was not in place at the time of the 1987 NRC Committee on the Space Station's study. As an example of the situations that involve contingency planning, the assembly of each of the Space Station elements will be constrained. The 100 percent contingency time for extravehicular activity during assembly flights appears to provide an adequate margin for now. However, past experience has shown that anomalies can occur that might exceed this budget, such as was demonstrated on the Solar Maxi retrieval
29 mission. The availability of the extended duration orbiter configuration for each of the Shuttles should be considered to enhance the probability of completing the deployment sequence in the event of contingencies. However, the committee recognizes that use of extended duration orbiters imposes a weight penalty on the amount of payload that can be delivered to orbit by each Shuttle. The committee believes further analysis of this subject is needed. EVOLUTIONARY PHASE REQUIREMENTS The Space Station program properly and commendably provides accommodations for future software and hardware (~hooks and sparse) to permit ready modifications and growth as the Station evolves. This is an essential feature of the design that must be retained; not all future evolutionary needs and contingencies can be anticipated. Essential Evolutionary Capabilities It appears inevitable that successful operation of the Space Station will stimulate demands to (1) increase available electrical power, (2) add internal and external laboratory facilities, (3) increase crew size, (4) improve flexibility of operations, (5) incorporate new technologies, and (6) add volume for improved habitability. These potential demands appear to be accommodated by the current design, and must be protected during the development. It also appears that efficient, mature operations near the manned base will require transportation capabilities not available cluring the assembly phase. One option is the space-based orbital maneuvering vehicle. The capability to add such capabilities should be protected. Heroic Evolution Visionaries will likely always imagine more revolutionary futures for the Space Station than can be attained by the evolutionary process discussed in the preceding paragraphs. Among the possible futures are the use of the manned base as a staging facility for human interplanetary flight or the deployment of a major microgravity factory in space.
30 While the above-mentioned examples may be legitimate aspirations, their influence on the design of the current Station must be tempered by the uncertainties of the missions, their affordability, their likely deployment schedule, and other considerations. There is much to be learned about space research and operations in the current Space Station design and its expansion using the planned Hooks and scars." It is not evident, however, that 20 years hence the current design will be the most cost effective or technically appropriate for major new functions. It could well be that a more advanced facility would be needed. Nonetheless, evolutionary capabilities must be retained in the design of the Space Station, and NASA appears to be taking steps to see that this occurs. The committee cautions that the incorporation of heroic evolutionary capabilities should not be permitted to add excessive complexity, increase costs, or extend the deployment schedule of the current Space Station, and supports NASA's continued vigilance in this regard.
