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Genera! Design Issues
A number of issues emerged from the committee's review that
it believes can have an exceptional impact on the design and
ultimate success of the Space Station. These issues either
cross subsystem boundaries or have facets or attributes that
are common to several subsystems. Because of their overarching
nature, they are presented first.
CREW SAFETY
The major issue of assuring the safe return of the Space
Station's crew under emergency conditions has not been
addressed in the Space Station concept presented at the
November 1988 workshop. The committee awaits with interest the
results of the Crew Emergency Rescue Vehicle (CERV) studies
currently under way, but it is reluctant to rely on the
assurances that onboard redundancy will be sufficient to handle
any and all possible life-threatening malfunctions. The
earlier NRC Committee on Space Station believed that NASA
should establish a mandatory requirement for a crew emergency
rescue vehicle and should consider its use, on a man-rated
expendable launch vehicle, as a backup means of manned access
to the Space Stations (Resort of the Committee on the SDace
Station of the National Research Council, p. 7, 1987~. The
workshop committee agrees with that recommendation.
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GROUND VERIFICATION OF ELEMENTS, ASSEMBLIES,
AND OPERATIONS
The allocation of verification 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 another important unresolved issue. Furthermore, the
degree to which flight equipment will be duplicated for
checkout, verification, spares purposes, and so forth, is only
vaguely defined in the current Space Station program plans, as
briefed to the committee. A verification strategy, involving
ground checkout of flight hardware, simulation, dedicated
ground test hardware, and on-orbit verification is sorely
needed and must be specified.
RELIANCE ON MODELS AND MODELING FOR
VERIFICATION
There is an extensive and perhaps excessive reliance on
modeling of systems and software for verification in lieu of
ground or flight testing. In addition, backup plans for
handling the on-orbit problem-resolution activities that may be
needed in the absence of prior ground-based verification are
not in place and do not appear to have been considered in
sufficient depth.
ON-ORBIT ASSEMBLY, INTEGRATION, AND VERIFICATION
The current schedule for the on-orbit assembly of the Space
Station appears incompatible with integration and verification
(I&V) requirements.
The assembly period between achievement of a man-tended
capability and a permanently manned capability (PMC) is
scheduled to last about one year. However, on-orbit I&V
implies considerable testing between assembly stages to
establish operating envelopes.
Each stage of Space Station assembly results in a complete
spacecraft on orbit. The plans for on-orbit I&V likely will
require special coordination to insure that the designers
(e.g., the structural dynamicists and control engineers) who
are most affected have the data needed to verify their models
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in time to improve their analysis of subsequent Space Station
increments.
Establishment of an assembly plateau after achieving a
man-tended capability for thorough on-orbit I&V could enable
incorporation of lessons learned and resulting product
modifications into flight packages that would be deployed
between the man-tended and permanently manned capability.
Another plateau after the permanently manned capability could
enable improved efficiency to be achieved in the full Phase 1
configuration.
A major on-orbit assembly problem is the coordination of
assembly operations between the Shuttle and the protestation
elements. Some of the crucial issues involve joint crew
activities by Shuttle and Space Station astronauts, the nature
of the coupled dynamic behavior during assembly, appropriate
docking/berthing strategies, and assembly sequence resiliency
in an environment of Shuttle mass margin uncertainty.
SOFTWARE ENVIRONMENT
While 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 investment in a
program-wide Software Support Environment and Technical and
Management Information System, it is concerned that the
Software Support Environment as mandated may be relatively
inflexible and restrictive for some program purposes, such as
real-time dynamic systems modeling. Strict adherence to the
Software Support Environment is likely to create an enormous
make-work chore for many activities that should not require
such adherence.
RESOURCE ADEQUACY AND ALLOCATION FOR SPACE
STATION ASSEMBLY AND OPERATIONS
Critical resources for assembly of the Space Station
include existing post-Challenger Shuttle payload weight and
volume margins on individual assembly flights. Extravehicular
activity time margins and Shuttle stay-times for assembly
flights are adclitional resources that must be traded off
against Shuttle payload weights. Electrical power, pressurized
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laboratory volume, external payload attachment points and treat
estates on the truss structure, and data transmission
capacities are among the resources critical to the successful
operation of the Space Station.
The committee is concerned that there are inadequate
margins and in some cases actual resource shortfalls at this
early stage of the Space Station program.
~INSURANCE" ALTERNATIVES FOR FLIGHT-CRITICAL
SYSTEMS
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. The existence of such alternative design
possibilities could reduce program risk or help cover projected
shortfalls. While the Space Station design might not be at the
level of maturity to do this yet, a strategy for developing
"insurances system possibilities is needed to assure that this
activity is not neglected.
STANDARDS AND COMMONALITY
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. This issue will be discussed
further in chapter 3.
DESIGN FOR FUTURE REFURBISHMENT
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. Such considerations are
important for the long-term viability of the Space Station.
Representative terms from entire chapter:
station program
