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Executive Summary
The Space Station Freedom program is the next major U.S.
manned space initiative. It has as its objective the
establishment of a permanently manned facility in low earth
orbit. The facility is intended to be used for a range of
activities and to accommodate a number of alternative
evolutionary growth paths. The Space Station also is
envisioned, by the National Aeronautics and Space
Administration (NASA) and others, as an essential element in
the recovery and maintenance of U.S. leadership in civil space
activities and as an opportunity for increased international
cooperation.
In early 1987, the NASA Associate Administrator for Space
Station indicated an interest in having the National Research
Council (NRC) examine the Space Station program with a view
toward identifying critical engineering issues related to the
design and operation of the station. This activity was
preempted by a 1987 NRC study of the Space Station program
undertaken at the request of the Presidents Assistant for
National Security Affairs, the Director of the Office of
Management and Budget, the Presidents Science Advisor, and the
Administrator of NASA. However, in early 1 98S, the Associate
Administrator for Space Station submitted a formal request for
an NRC workshop to identify and prioritize Space Station
engineering issues. A week-long, intensive workshop was held
on November 7-11, 198S, at the Arnold and Mabel Beckman Center
of the National Academy of Sciences and the National Academy of
Engineering in Irvine, California; this report summarizes the
main findings and recommendations of that workshop, and
reflects the vieurs and opinions of the workshop committee.
The workshop was not a technical audit of the program. The
report thus does not focus on identifying all the good features
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of the Space Station program, although the committee has noted
some of these aspects at various points in the report. In
addition, a number of areas of possible concern, such as the
effect of the space environment on Space Station materials and
structures, were not represented in the briefings to the
committee or by its expertise. Consequently, the issues
identified in this report should not be construed as an
exhaustive set of concerns, but rather those that appeared most
important to the committee based on its necessarily limited
review of the Space Station in November 1988. Finally, the
limited time available to the committee made it impossible to
do a more detailed prioritization of the issues identified in
this report, especially across subsystems.
Major issues identified by the workshop committee were
those related to general design, utilization and operations
requirements, and specific Space Station systems and
subsystems. The committee also raised a number of management
issues that it believes could have an important impact on Space
Station design and operation.
GENERAL DESIGN ISSUES
1. The major issue of assuring the safe return of the
Space Stations crew under emergency conditions has not been
addressed in the Space Station concept presented to the
committee; the committee believes a crew emergency rescue
vehicle is required.
2. The allocation of verification activities and/or
checkout of subsystems and assemblies (including software)
between those activities that are ground-based and those that
will be done on orbit is an important unresolved issue.
3. There is an extensive and perhaps excessive reliance on
modeling of systems and software for verification in lieu of
ground or flight testing.
4. The current schedule for the assembly of the Space
Station on orbit appears incompatible with integration and
verification requirements.
5. The committee believes NASA is to be commended for
recognizing the criticality of software and data management to
the Space Station program and for taking a proactive approach
to addressing software issues through investments in a
program-wide Software Support Environment (SSE) and Technical
and Management Information System (TMIS). However, it is
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concerned that the SSE as mandated may be relatively inflexible
and restrictive for some program purposes, such as real-time
dynamic systems modeling.
6. The committee is concerned that there are inadequate
margins and in some cases actual resource shortfalls (e.g.,
power, existing post-Challenger Shuttle payload weight margins
for assembly flights) at this early stage of the Space Station
program.
7. Preliminary design considerations for system
alternatives to serve as ~insurance" for flight-critical
systems (e.g., electrical power, thermal control, life support)
are not yet being developed.
8. Questions of common measurement standards, commonality
of tools and equipment, and so forth do not appear to have been
resolved in a concrete fashion that is consistent with the
planned long life of the Station.
9. Hardware and software provisions for future expansion
or evolution of the Space Station are under study by the Space
Station program. However, provisions for refurbishment,
repair, and rebuilding over a multiyear time span do not appear
to have received the same attention.
ISSUES RELATED TO UTILIZATION AND OPERATIONS
REQUIREMENTS
1. 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.
2. The current Design Reference Missions, which show the
Station resources to be more or less in balance, may not allow
an accurate assessment of user and operator needs. Development
of Design Reference Missions should be carried out with the
intention of probing the design's weaknesses rather than
demonstrating its strengths.
3. Further, it was evident to the committee that Shuttle
airs Spacelab precedents were used in developing procedures for
user experimentation on the Space Station. More innovative
processes are needed to improve responsiveness to user needs
consistent with safe and effective Station operations.
4. The committee is concerned that the Space Station's
design, assembly, and operation may have become unduly
constrained by such things as existing post-Challenger Shuttle
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performance limitations, certain congressional directives, and
some user requirements. It is important that the justification
for each constraint be firmly established and its impact on the
program clearly understood.
5. A premature demonstration flight of the Flight
Telerobotic Servicer (FTS) concept and early manifesting of the
FTS in the Space Station assembly sequence is likely to have a
contrary effect to the intended objective of advancing
automation and robotics.
6. The current program 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 NASA and NRC advisory committees.
7. A particularly sensitive issue to 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, such disturbances will have to be treated
empirically, with a resulting awkwardness in layout and
operations.
S. The protection of the amount of crew time available for
user activities is vulnerable to erosion as maintenance and
operations requirements are better defined.
9. The committee is concerned that the Space Station
program has no provision for unmanned resupply of the U.S.
modules (e.g., facilities for unmanned rendezvous and docking).
SELECTED ISSUES RELATED TO SYSTEM REQUIREMENTS
AND DESIGN
1. The committee die! not find as ready an awareness of the
impact of system interactions as it believes is desirable at
this stage in the Space Station development program.
Software and Data Management
2. No software risk management plan exists, although major
software risk factors exist.
3. The current software schedule emphasizes early
completion of software requirements documents, which then
become baselined and more difficult to modify. The early
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schedule should preferably emphasize execution of risk
management plans and resolution of risk items before software
requirements are "cast in concrete.
4. The coordination of the Space Station Information
System (SSIS) with the other data distribution systems proposed
by NASA user organizations does not appear to be well defined.
Communications and Tracking
5. No end-to-end perspective in communications and
tracking was evident or presented to the committee.
6. The committee is concerned that all communications to
and from the Space Station will flow via the Tracking and Data
Relay Satellite System.
7. The Space Station is dependent on many communication
links for its successful operation; the vulnerability of these
links to deliberate or inadvertent interference or access is a
continuing concern.
S. The relative inaccessibility of the Space Station and
the relatively meager resources that will be available to the
crew heighten the need for special attention to the potential
problem of electromagnetic interference at the Station.
Automation and Robotics
9. More emphasis is needed on advanced automation tools
and capabilities to improve the efficiency of Space Station
user operations.
10. There is no clear vision of the spectrum of
applications to be supported by the FTS, particularly given the
existence of the Canadian Mobile Servicing System.
Electrical Power System
11. Power adequacy for the Phase 1 Space Station is of
concern to the committee.
12. There is need for a management review of all
utilities: electricity, heat, water, and so forth.
Thermal Control System
13. More attention should be given to the analysis of
thermal control system trade-offs.
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Environmental Control and Life SUDDOrt Svstem. Man Systems.
Extravehicular Activity Svstem
14. Microbial and toxin control in the Space Station is a
continuing area of concern.
15. The committee is concerned that sufficient in-space
testing be done on the planned Space Station life support
system.
16. The requirements for medical evacuation and its
associated problems need to be addressed.
17. The committee believes that the issue of Space Station
crew efficiency over time is an important one, with
implications for operational timelines and crew schedules.
Fluid Management Svstem
18. The control and handling of waste liquids and gases,
as well as hazardous materials used in the laboratories, need
to be addressed vigorously.
MANAGEMENT ISSUES RELEVANT TO DESIGN
1. The committee believes that the Space Station
management structure has complicated the Space Station
development task.
2. Budgetary uncertainties, certain congressional
directives, and operational limitations of the existing
post-Challenger Shuttle have produced program instabilities
that may have increased design complexity and reduced design
conservatism.
3. The retention of the polar platform within the Space
Station program has further complicated the programs
management task.
4. The committee is concerned that a disciplined and
contractually binding process for integration is not evident;
one needs to be established early in the program.
5. A Space Station System Specification is needed to
contractually define the system and provide a basis to effect
change discipline and change control.
6. While there is considerable work under way, there does
not appear to be a strong and crisp integration and
verification management plan and architecture in place.
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7. The communications and data systems for the Space
Station pass through elements that are developed by up to four
separate NASA offices, with little evidence that adequate
oversight is being provided by NASA. Thus, the potential
exists for errors to be made (notably those of omission) even
among well-intentioned, skilled organizations.
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Representative terms from entire chapter:
station program
