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2 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. 15
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16 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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17 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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18 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.