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Operations, Maintenance, and Reliability
A major purpose of the ISS is to provide a platform for
long-duration microgravity experiments in the life sciences
and physical sciences. ISS operational principles, mainte-
nance requirements, the reliability of hardware and software,
and the ability of onboard crews to work efficiently will
directly affect the achievement of this goal. This chapter
provides suggestions to NASA for ensuring the success of
long-term ISS operations.
DAILY FLIGHT CREW SCHEDULING
ISS crews will have full daily schedules on board the
station: they will support the day-to-day operation of the
station; they will perform maintenance of ISS systems and
laboratory equipment, make repairs to ISS systems and labo-
ratory equipment, and conduct scientific research and
manage experiments; and they will need time for regular
exercise and periods of rest. Careful planning will be neces-
sary to ensure a balance so that one set of priorities, such as
maintenance and repair, does not conflict with others, such
as scientific research or experiment management. NASA's
traditional methodology for scheduling the day-to-day
activities of the flight crew was appropriate for short-
duration space flights. However, experience with long-
duration space flights on Skylab and Mir suggests that a dif-
ferent approach would ease the pressure on the daily timeline
and ensure that the crew has time for scientific research.
To capitalize on the full potential of the new environment
provided by the ISS (i.e., the ability to operate in space for
extended periods of time), NASA should consider transfer-
ring some control of day-to-day scheduling details to the
crew on board the ISS. The idea is to capitalize on the crew' s
unique experience with the on-orbit laboratory and to permit
the crew to organize their tasks in the most time-efficient
manner. For example, the mission control center could pro-
vide the crew with objectives for the upcoming week,
including a summary of the science and a proposed timeline.
The crew could then make adjustments to the timeline, send
13
them to the ground, and the ground controllers would follow
the crew's suggestions without requiring justifications for
each change. This would rely on the crew' s extensive knowl-
edge of their space laboratory to optimize the order in which
activities were performed and adjust the time required to
perform specific tasks. The detailed crew-developed
timelines would be subject to periodic progress reviews with
the ground controllers to ensure that the overall objectives
for that crew' s increment aboard the ISS were being met (an
"increment" is the duration of a particular crew' s stay aboard
the ISS, the period between crew rotations). Ideally, this
arrangement would leave more time for research by allow-
ing the crew to optimize their daily schedule and recover
time otherwise committed to standard blocks of time for
maintenance and repair tasks and station operations.
Recommendation. The National Aeronautics and Space
Administration should allow the International Space Station
(ISS) crew on orbit to contribute to the development and
optimization of the daily timeline. The time saved would
allow the crew to devote more time to scientific research.
Oversight of the accomplishment of crew tasks ahoard the
ISS should be maintained by mission control through peri-
odic flight crew/ground controller progress reviews.
ONBOARD MAINTENANCE
In briefings to the committee, the committee learned that
the crew would be trained both to work on experiments and
to perform "routine" maintenance on the station. The crew
would also be able to do some troubleshooting of hardware
and make some minor repairs. Tasks that require extra-
vehicular activity (EVA) will be performed by Space Shuttle
crews as much as possible (Harbaugh and Poulos, 1999~.
Plans call for ISS crews to spend some time conducting
research on behalf of the principal investigators (PIs) and
some time performing routine maintenance. However, the
committee believes, based on Phase 1 Mir experience, that
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14
ENGINEERING CHALLENGES TO THE LONG-TERM OPERATION OF THE INTERNATIONAL SPACE STATION
the crew may also have enough time to accomplish some of
the day-to-day mission operations and thus reduce the
aggregate requirement for ground personnel. A reassess-
ment of the crew timelines might reveal that they could per-
form some of the work now planned for mission controllers
on the ground. To that end, NASA should prepare a long-
term "design reference mission" showing projected clusters
of crew activities against a timeline. It would be useful, for
example, to show a typical 30-day timeline with the Space
Shuttle docked either at the beginning or the end of the
30-day period. This would help determine if measures to
increase onboard crew efficiency and conserve the crew's
working time will be necessary. NASA did provide the
committee with copies of the Concept of Operation and
Utilization Mission Scenarios and Mission Profiles, which
documents this type of timeline planning for Space Station
Alpha (NASA, 1994~. That document, however, has not
been updated since 1994 and the timeline scenarios are not
being continued in the ISS Program.
Recommendation. The National Aeronautics and Space
Administration should reassess the crew's activities against
a more realistic timeline based on the Phase 1 Mir experi-
ence, as well as experience gained during assembly of the
International Space Station. If the crew could take on more
of the day-to-day mission operations, the aggregate require-
ments for ground crew personnel would be reduced.
Recommendation. Maintenance onboard the International
Space Station (ISS) should be scheduled during resupply
missions as much as possible. Resupply missions are also
the preferred times for conducting extravehicular activities
because the microgravity environment will already have
been disturbed by the resupply vehicle docking with the ISS.
If the Space Shuttle is the resupply vehicle, additional per-
sonnel will also be available to help with maintenance tasks
during docked operations. Cooperative tasks between the
arriving crew and the departing crew will also facilitate the
passing of detailed information to the arriving crew (the so-
called "crew hangover".
STAFFING OF THE OPERATIONS CONTROL CENTER
Current flight and ground control operations are under-
standably focused on the ISS assembly processes, planning
for EVAs, training, testing flight hardware, and planning
logistics. NASA personnel who briefed the committee
expressed the concern that this intense activity could lead to
burnout of ground support personnel. The Phase 1 Mir
experience shows that work/rest cycles are important for
ground personnel and for the productivity of the long-
duration mission. Chronic fatigue from working long hours
and extended work weeks result in an increase in incidence
of illness, loss of performance, and team member attrition
(NASA, 1998a). In support of Mir operations, the Russian
practice has been to maintain only a small team in the mis-
sion control center and have experts on call to support them
whenever necessary. New information technologies could
enable NASA to go one step further by establishing remote
workstations, which would allow people to focus on solving
problems and rely on support software to identify problems.
Although problems can occur at any time, experts "on duty"
at their homes or offices could have on-line access to the
data and analysis tools necessary to enable them to work
with the smaller mission control center staff to prioritize and
resolve problems with the help of support software and on-
line displays.
NASA' s current plans for flight and ground control opera-
tions are concentrated on the ISS assembly flights and are
based on the fully staffed, "all-up" mission control center
philosophy that has been used by the U.S. space flight pro-
gram since the days of Mercury, Gemini, and Apollo and has
been continued for the Space Shuttle program. Almost none
of these operations, however, lasted more than two weeks.
NASA has acknowledged that the new operational environ-
ment of the ISS (i.e., continuous operations in support of a
very long-duration mission) and the intense continuous
activity might eventually cause burnout of ground support
personnel (Harbaugh and Poulos, 1999~. The safe reduction
in on-site mission control staff is also essential to the long-
term fiscal soundness of the ISS program. NASA has not
yet addressed the challenges associated with optimizing the
long-term operation of the ISS.
Recommendation. The National Aeronautics and Space
Administration (NASA) should develop a new concept of
operations for the long-term operation of the International
Space Station that includes the integration of new informa-
tion technologies into mission control center processes.
NASA should consider adopting the Russian operational
practice used for Mir (i.e., maintaining a small team in the
mission control center and relying on experts on call with
remote access to the data and personnel in the mission con-
trol center).
COMMUNICATION WITH PRINCIPAL INVESTIGATORS
For the long-term operation of the ISS, NASA has an
opportunity to reinstate some of the procedures that were
used for Spacelab, when the crew was allowed to communi-
cate directly with the PIs instead of having to coordinate
responses through the ground support team. NASA's
traditional practices worked well for short-duration space
missions, but with new communication technologies and
long-duration space flight, NASA could consider allowing
crew members to communicate directly with PIs on ques-
tions of science, experiment protocol and experiment status,
and other subjects that do not affect the operation or safety
of the ISS. Direct communication would be more efficient,
would result in better science, would lead to significant time
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OPERATIONS, MAINTENANCE, AND RELIABILITY
savings for both the crew and the ground support team, and
would ease the pressure on the crew's daily timeline, giving
them more time to perform research.
During Phase 1 operations on Mir, the Russians were able
to take advantage of direct communications between PIs and
crew members. This approach saved the crew time and pro-
vided them with answers directly from the experts. If a crew
member had further questions, the PI could respond immedi-
ately. For the long-term operation of the ISS, direct commu-
nication would be efficient and would greatly enhance the
crew' s ability to support experiment protocols and scientific
objectives.
Recommendation. The National Aeronautics and Space
Administration should adopt the practice demonstrated dur-
ing the Mir program of direct communications between the
crew and principal investigators (PIs). Crew members and
PIs should be able to exchange data and instructions to enable
the crew to carry out experiments in the way that best fulfills
the goals of the experiment. Computer links should be devel-
oped and communications systems upgraded to provide real-
time assessments of the data and the capability of responding
to change.
Recommendation. The National Aeronautics and Space
Administration and the scientific community should explore
ways to enable principal investigators to access their experi-
mental data directly from ground locations at universities
and research facilities to increase the direct involvement of
the science community in ISS experiments.
PAYLOAD SPECIALIST
NASA's projected requirements for the ISS crew do not
include a payload specialist. The payload specialist program
worked very well on Spacelab missions on the Space Shuttle.
The payload specialist was in a unique position to make
significant contributions to the quality of science and the
success of experiments.
Because some very precise experimentation will be
performed on the ISS, having a scientific expert onboard
would increase the quality of support available to the PI.
The payload specialist could make in situ adjustments to
experiments that might otherwise require time on the ground
to analyze results and, perhaps, require reflight of the experi-
ment. The payload specialist would also be able to provide
early warning of the need to change or upgrade experimental
equipment.
Recommendation. The National Aeronautics and Space
Administration should reassess its crew requirements for the
International Space Station and consider including a pay-
load specialist in the seven-person crew.
15
CREW HANDOVER
NASA has not developed a plan for on-orbit handover to
ensure that efficiency is maintained and no errors are intro-
duced that would result in costly reworking or increase
operational risk. Every effort should be made to capitalize
on the knowledge of the departing crew members. The value
of a smooth, efficient handover is well documented by the
Mir experience (NASA, 1998a).
Mission planning could allow Space Shuttle crew mem-
bers to perform many joint tasks during the handover period.
Ideally, crew members would have all of the docked time for
the handover. Although this may not always be possible, it
should be established as a goal to help mission planners
establish priorities. During the handover period, the new
flight crew can become familiar with the protocols of
ongoing experiments through actual operations under the
guidance of the departing crew.
Recommendation. The National Aeronautics and Space
Administration (NASA) should develop a plan and process
to ensure that crew members on board the International
Space Station have as much time as possible for their on-
orbit handover during docked operations. The handover
should be formal and should include essential hard copy and/
or laptop computer files of historical records to ensure that
arriving crew members have a complete understanding of
activities in progress before returning crew members depart.
In developing the handover plan and process, NASA should
assess the desirability of performing critical maintenance
tasks during Shuttle handover periods to take advantage of
the availability of additional crew members.
ONBOARD FAILURE DETECTION AND
CORRECTIVE ACTION
On-orbit maintenance includes the detection of an
anomaly, the identification of the failed orbital replacement
unit (ORU), and replacement of the ORU or creation of a
work-around. The first two of these activities are very diffi-
cult to plan and train for comprehensively because failures
are often unpredictable. Localization of a failed component
is essential for the efficient functioning of the ISS. The most
successful way to localize failures is for each ORU to be
responsible for its own failure monitoring, with backup
monitoring at the system level. The starting point for the
process is usually the failure modes and effects analysis
(FMEA), which is generated as part of reliability assess-
ments. The FMEA made available to the committee was
focused on safety-critical items but did not address overall
maintenance. The methodology supporting this approach is
referred to as failure detection, isolation, and recovery
(FDIR). In the ISS context, FDIR has two meanings:
(1) software programs for the automated detection of failures
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16
ENGINEERING CHALLENGES TO THE LONG-TERM OPERATION OF THE INTERNATIONAL SPACE STATION
and identification of the failed units and (2) the overall pro-
cess of which the software programs are a part.
Only one individual is presently responsible for manage-
ment oversight of the FDIR software program at the NASA
Johnson Space Center. Overall failure identification is being
investigated by Boeing as part of the Launch Package and
Stage Assistance Program (Wolf, 1999~. Neither of these is
directly connected to the FMEA generated by the reliability
and quality assurance organization, and neither addresses the
need for training, providing spare parts, or other logistics.
The committee could not determine if failures in monitoring
circuits, which might lead to the unnecessary removal of an
ORU, have been considered by the ISS Program.
The stated goal of the FDIR is to achieve 90-percent
identification to a single ORU; 95-percent to two ORUs; and
98-percent to three ORUs. This goal appears to be unrealis-
tic in light of the limited efforts NASA has devoted to FDIR
to date. No plans for verifying these levels of identification
were provided to the committee. Considering the impor-
tance of FDIR for the efficient long-term operation of the
ISS, the lack of attention to this activity is an indication of
NASA's focus on the assembly phase of the ISS to the
exclusion of important operational considerations and con-
tingency planning for its long-term operation.
Recommendation. The National Aeronautics and Space
Administration should greatly expand its focus on failure
detection, isolation, and recovery (FDIR) in conjunction with
the failure modes and effects analysis (FMEA). The follow-
ing issues should be addressed specifically:
· allocation of responsibility to automated/nonautomated
functions
· consistency of the FDIR with known failures
· integration with space and ground crew training and
logistics
SAFETY, RELIABILITY, AND MAINTAINABILITY
PROGRAM
The safety, reliability, and maintainability program
(SR&M) follows policies established by the NASA Head-
quarters Office of Safety and Mission Assurance (Code Q)
and meets the conventional requirements of SR&M pro-
grams. In examples furnished to the committee, areas that
impact safety had been analyzed in depth. Nevertheless, the
committee found no evidence that NASA had taken into
account the aggregate of interfaces in the ISS and the heavy
dependence on software. NASA' s approach is not proactive
in that it has not identified early, small expenditures for im-
provements in SR&M that could avoid the eventual high cost
of failures.
The SR&M program would benefit from more trend
analyses of all data that could have a bearing on the
long-term failure rate, maintenance capabilities, and spare
parts requirements of the ISS. The Hubble Space Telescope
(MST) program, for example, now considers "trending" as
its primary SR&M analysis method.
Trend analysis for the ISS could include the following
elements:
· incoming inspection reports
· in-process test reports
· failure reports from on-orbit segments
· maintenance records for ground and on-orbit
operations
Recommendation. Analyses of the incoming inspection and
in-process testing data should be used to establish a six-
sigma environment in which failures will be extremely rare.
Analyses of failure reports and maintenance records should
be used to improve on-orbit procedures and the quality of
replacement items.
SPARE PARTS PHILOSOPHY
NASA provided the committee with information on plans
to provide spare parts and logistics for the ISS (NASA,
1998b). The committee also reviewed the same information
for the HST, the only other long-duration U.S. space vehicle
that has involved crew servicing. The opportunities for com-
ponent replacement on the ISS and the HST differ four or
five visits per year for the ISS and three or four years between
visits for the HST. In terms of manufacturing lead-time for
producing spare parts, however, the biggest difference is that
significant lead-time is available between repair visits to the
HST to secure replacement parts.
For the HST, solar panels, mechanical relays, and rotat-
ing devices (e.g., wheels, gyros, gimbals, and servos) were
stockpiled, but in MST's nine years on orbit, the electro-
mechanical devices and the purely electronic devices have
had only moderate failure rates (Styczynski, 1999~. Experi-
once with the HST project revealed that NASA could not
afford to stock and maintain the extensive depot facilities
and the large numbers of spare parts required for the HST
based on the "old" measure of statistical mean time between
failures of the hardware (Kelley, 1999~. Another method of
providing spare parts would be to introduce upgrades during
the operational life of the ISS to reduce failure rates and
thereby reduce maintenance requirements (this concept is
discussed further in Chapter 5~.
NASA's current philosophy of providing spare parts for
the ISS requires a very large contingent of personnel and
extensive facilities. Because the ISS is expected to have a
nominal operational lifetime of 15 to 20 years after Assembly
Complete, the project would require repair and maintenance
depots, as well as inventories of spare parts. As the HST
experience has shown, however, predictions of the require-
ments for spare parts are not accurate. The combination of
inaccurate predictions and the possibility of technological
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OPERATIONS, MAINTENANCE, AND RELIABILITY
advances and evolving requirements of ISS operations sug-
gests that NASA could adopt a less conservative approach
except for items essential for life support.
Recommendation. The National Aeronautics and Space
Administration should reassess its current philosophy for
providing spare parts, as well as the depots and associated
personnel required to maintain them for the operational Inter-
national Space Station (ISS). The criticality of hardware,
wear-out factors, and the potential for subsystem upgrades
should be considered in the reassessment. The logistics,
reliability, and mission assurance personnel for the ISS
should establish an ongoing liaison with their counterparts
in the Hubble Space Telescope program to evaluate a new
philosophy for the ISS and the possibility of reducing
associated costs.
REFERENCES
Harbaugh, G., and S. Poulos. 1999. EVA Status. Briefing by G. Harbaugh,
Manager, EVA Projects Office, and S. Poulos, Deputy Manager, EVA
Project Office, to the Committee on the Engineering Challenges to the
17
Long-Term Operation of the International Space Station, NASA
Johnson Space Center, Houston, Texas, February 21, 1999.
Kelley, J. 1999. Personal communication from J. Kelley, Lockheed Missiles
and Space Systems, to B. Bulkin, member of the Committee on the
Engineering Challenges to the Long-Term Operation of the International
Space Station, January 7, 1999.
NASA (National Aeronautics and Space Administration). 1994. Concept of
Operations and Utilization Mission Scenarios and Mission Profiles. SSP
50011-01. Houston, Texas: NASA Johnson Space Center.
NASA. 1998a. Phase 1 Lessons Learned. August 26,1998. Houston, Texas:
NASA Johnson Space Center.
NASA. 1998b. Current Manifest of Assembly Critical Spares: Working
Paper. December 11, 1998. Houston, Texas: NASA Johnson Space
Center.
Styczynski, T. 1999. Personal communication from T. Styczynski,
Lockheed-Martin Hubble Space Telescope Operations, to H. Hecht,
member of the Committee on the Engineering Challenges to the Long-
Term Operation of the International Space Station, January 20, 1999.
Window, K. 1999. Presentation by K. Window, Avionics and Software
Office, International Space Station Program Office, to the Committee
on the Engineering Challenges to the Long-Term Operation of the Inter-
national Space Station, NASA Johnson Space Center, Houston, Texas,
March 24, 1999.
Wolf, K. 1999. Launch Package and Stage Assistance Program. Presenta-
tion by K. Wolf, the Boeing Company, to the Committee on the Engi-
neering Challenges to the Long-Term Operation of the International
Space Station, NASA Johnson Space Center, Houston, Texas, March
25, 1999.
Representative terms from entire chapter:
international space
