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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
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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
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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.
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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
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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
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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,
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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
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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
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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.
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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
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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.
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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.
Representative terms from entire chapter:
station program
