International Space Station (ISS) - Ongoing Assessments for Life Extension Appear to be Supported
Gao ID: GAO-11-519R April 11, 2011This document is in response to the mandate contained in the National Aeronautics and Space Administration (NASA) Authorization Act of 2010, Pub. L. No. 111-267, Section 503(c)(2), for GAO to provide an evaluation of the accuracy and level of confidence in the findings contained in NASA's assessment of the essential modules, operational systems and components, structural elements, and permanent scientific equipment required to ensure complete, effective, and safe functioning and full scientific utilization of the International Space Station through 2020. We provided to Congress a draft copy of this briefing in meetings with them on April 6 and 7, 2011. We also provided a draft to NASA for comment. NASA agreed with our findings and provided technical comments that we incorporated as appropriate.
GAO-11-519R, International Space Station (ISS) - Ongoing Assessments for Life Extension Appear to be Supported
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GAO-11-519R:
United States Government Accountability Office:
Washington, DC 20548:
April 11, 2011:
The Honorable John D. Rockefeller IV:
Chairman:
The Honorable Kay Bailey Hutchinson:
Ranking Member:
Committee on Commerce, Science, and Transportation:
United States Senate:
The Honorable Ralph Hall:
Chairman:
The Honorable Eddie Bernice Johnson:
Ranking Member:
Committee on Science, Space and Technology:
House of Representatives:
Subject: International Space Station (ISS) - Ongoing Assessments for
Life Extension Appear to be Supported:
This letter formally transmits the attached briefing (see enclosure I)
in response to the mandate contained in the National Aeronautics and
Space Administration (NASA) Authorization Act of 2010, Pub. L. No. 111-
267, Section 503(c)(2), for GAO to provide an evaluation of the
accuracy and level of confidence in the findings contained in NASA's
assessment of the essential modules, operational systems and
components, structural elements, and permanent scientific equipment
required to ensure complete, effective, and safe functioning and full
scientific utilization of the International Space Station through
2020. We provided your staff a draft copy of this briefing in meetings
with them on April 6 and 7, 2011. We also provided a draft to NASA for
comment. NASA agreed with our findings and provided technical comments
that we incorporated as appropriate.
Our objectives were to determine (1) how NASA will ensure that the ISS
is structurally sound and that essential spares and replacement
components are available to support safe functioning and full
scientific utilization through 2020, and (2) the extent to which
NASA's assessment of the essential spares necessary to assure
continued operations of the ISS through 2020 is supported by
sufficient, accurate, and relevant underlying data and appropriate and
reasonable methodologies. NASA is using analytical techniques,
physical tests, and inspections to assess primary structures and
functional systems and determine sparing needed to support safe
functioning and full scientific utilization of the ISS through 2020.
These assessments are ongoing, so all results are not yet available.
Our work indicates that NASA's assessments appear to be supported by
sufficient, accurate and relevant underlying data. We found, however,
that NASA's estimates of ISS sparing needs are sensitive to
assumptions about hardware reliability.
To evaluate NASA's approach, we reviewed relevant technical documents
and data NASA used to support its analysis and interviewed responsible
officials. In a limited number of cases, we replicated NASA's
calculation used to update predicted failure rates for essential
spares. Our work was conducted in accordance with generally accepted
government auditing standards.
As agreed with your staff, given the limited time available to conduct
our analysis, we plan to continue our work to provide a more in-depth
evaluation of the level of confidence and accuracy of NASA's
assessment and provide an additional report to you at a later date.
We are sending copies of this report to the appropriate congressional
committees. We are also sending copies to NASA. This report will also
be available at no charge on the GAO Web site at [hyperlink,
http://www.gao.gov].
Should you or your staff have any questions concerning this report,
please contact me at (202) 512-4841 or chaplainc@gao.gov. Contact
points for our Offices of Congressional Relations and Public Affairs
may be found on the last page of the attached report.
Key contributors to this report include Shelby S. Oakley, Assistant
Director; John Warren, Analyst-in-Charge; Andrea Bivens; Tana Davis;
Jay Tallon; Sonya Vartivarian; and Laura Greifner.
Signed by:
Cristina Chaplain:
Director:
Acquisition and Sourcing Management:
Enclosures:
[End of section]
Report to Congressional Committees:
April 2011:
International Space Station: Ongoing Assessments for Life Extension
Appear to be Supported:
For more information, contact chaplainc@gao.cov:
Contents:
* Objectives;
* Background;
* Scope and Methodology;
* Finding 1: Ongoing ISS Life Extension Assessments:
- Primary Structures;
- Functional Systems;
- Safety and Mission Assurance;
- Transportation Plans;
- Essential Spares;
* Finding 2: Evaluation of NASA Analytical Techniques for Assessing
Essential Spares Needs:
- Data Underlying NASA's Assessment;
- Bayesian Update Process;
- Functional Availability Assessment;
* Agency Comments.
Objectives:
1. How will NASA ensure that the International Space Station (ISS) is
structurally sound and that essential spares and replacement
components are available to support safe functioning and full
scientific utilization through 2020?
2. To what extent is NASA's assessment of the essential spares
necessary to assure continued operations of the ISS through 2020
supported by sufficient, accurate, and relevant underlying data and
appropriate and reasonable methodologies?
Background:
The ISS is a research and development test bed that is, in itself, an
experiment in design, development, and assembly of an orbital space
facility.
The ISS is composed of about 1,000,000 pounds of hardware brought to
orbit over the course of more than a decade.
The ISS includes (1) primary structures, i.e., the external trusses
which serve as the backbone of the station and the pressurized modules
that are occupied by the ISS crew, and (2) functional systems composed
of orbital replacement units (ORUs), i.e., system components
modularized to support simple on-orbit replacement.
Figure: ISS Current Assembly:
[Refer to PDF for image: illustration]
Source: NASA.
[End of figure]
* Supporting remote operations is difficult and costly.
* NASA's logistics approach is in some ways similar to those used by
other government agencies with remote-location programs:
- National Science Foundation (NSF) Antarctic research station at the
South Pole;
- National Oceanic and Atmospheric Administration (NOAA) Aquarius
underwater research station in the Florida Keys;
* All the agencies prioritize spares based on safety, mission, and
comfort, in that order.
* The Space Shuttle's retirement eliminated NASA's ability to bring
large external ORUs back to Earth and significantly reduced other ORU
return capability for repair and eventual return to the ISS.
* As a result, NASA has adopted a logistics approach that differs from
other agencies approaches. NASA's current logistics approach uses a
statistical methodology and modeling to forecast ISS sparing needs.
Table:
Sparing priority #1:
NASA: Safety;
NOAA: Safety;
NSF: Safety.
Sparing priority #2:
NASA: Mission critical;
NOAA: Mission critical;
NSF: Mission critical.
Sparing priority #3:
NASA: Items not impacting safety or mission;
NOAA: Items not impacting safety or mission;
NSF: Items not impacting safety or mission.
Methodology for deciding sparing needs:
NASA: Statistical forecasting and modeling;
NOAA: Log book and experience.
NSF: Procure all manufacturer-recommended spares.
Prepositioning spares:
NASA: Key;
NOAA: Not as important;
NSF: Key.
Transportation:
NASA: Complex;
NOAA: Straight-forward;
NSF: Complex.
Maintenance:
NASA: ISS on-station crew;
NOAA: Dedicated staff onboard;
NSF: Antarctic on-station crew.
Source: Agency interviews and documents.
[End of table]
Until 2010, NASA was not authorized to continue participation in the
ISS program beyond 2015.
The National Aeronautics and Space Administration Authorization Act of
2010, Pub. L. No. 111-267, Sec. 503 required:
* NASA to take "all actions necessary to ensure the safe and effective
operation, maintenance, and maximum utilization of the United States
segment of the ISS through at least September 30, 2020," to conduct an
assessment of the sustainability and continued operations of the ISS
through September 30, 2020, and to submit a report on the assessment.
* GAO to report on NASA's assessment of the sustainability and
continued operations and report back to the Senate Committee on
Commerce, Science, and Transportation and House Committee on Science.
In response to past GAO recommendations aimed at increasing scientific
utilization of the ISS, NASA is in the process of creating an
independent, nonprofit organization to manage and oversee ISS National
Laboratory research by U.S. organizations other than NASA.
(International Space Station: Significant Challenges May Limit Onboard
Research (Washington, D.C.: GA0-10-9, Nov. 25, 2009)).
[End of Background section]
Scope and Methodology:
We interviewed and obtained briefings and relevant documents from
knowledgeable NASA officials regarding the content and decision-making
approach used to prepare the agency's ISS assessment. We reviewed the
scope, methodology, and ground rules and assumptions NASA used in the
ISS assessment.
We analyzed launch schedules and manifests to determine the viability
of NASA's findings regarding transportability for and supportability
of the ISS.
We compared NASA's approach to ISS logistics to the approaches used by
other organizations to support remote operations.
We conducted our work at NASA and the National Science Foundation
Office of Polar Programs headquarters in Washington, D.C. and NASA's
Johnson Space Flight Center, in Houston, Texas. We also interviewed
University of North Carolina-Wilmington personnel responsible for
managing the National Oceanic and Atmospheric Administration's
Aquarius Undersea Laboratory via telephone.
For purposes of determining whether NASA's findings and conclusions
are supported by sufficient, accurate and relevant underlying data as
well as appropriate and reasonable methodologies, we focused our
efforts on the statistical techniques NASA used to calculate
operational mean-time between failure rates and the modeling
techniques NASA uses to assess functional availability. For a limited
number of orbital replacement units, we recalculated the values NASA
obtained for the operational mean-time between failure based on its
statistical methodology. We also conducted limited tests of the data
NASA used to support the functional availability model.
We limited the scope of our assessment to the scope of NASA's report,
i.e., the sparing necessary to support critical functions as modeled
in the ISS functional availability assessment. We did not examine the
sparing needs of the international partners or the sparing needs of
functions not included in the ISS functional availability assessment.
We conducted this performance audit from February 2011 to April 2011
in accordance with generally accepted government auditing standards.
Those standards require that we plan and perform the audit to obtain
sufficient, appropriate evidence to provide a reasonable basis for our
findings and conclusions based on our audit objectives. We believe
that the evidence obtained provides a reasonable basis for our
findings based on our audit objectives.
Bottom Line:
Finding 1:
NASA is using analytical techniques, physical tests, and inspections
to assess primary structures and functional systems, to the extent
possible, and determine sparing needed to support safe functioning and
full scientific utilization of the ISS through 2020. NASA is confident
that it can execute necessary functioning and utilization; however,
the supporting assessments for primary structures and functional
systems are ongoing and all results are not yet available.
Finding 2:
NASA's assessment of the essential spares necessary to assure
continued operations through 2020 appears to be supported by
sufficient, accurate, and relevant underlying data. We found, however,
that estimates of essential spares are sensitive to NASA's assumptions
about ORU reliability.
[End of Scope and Methodology section]
Finding 1: Ongoing ISS Life Extension Assessments:
NASA is using a combination of on-orbit data, analytical techniques,
hardware tests, and limited visual inspections to assess the
feasibility of extending ISS life through 2020. These life extension
assessments are under way, but results will not be available until
assessments are completed in 2015. Assessments include examinations of
the following:
* Primary structures;
* Functional systems;
* Safety & Mission Assurance (S&MA) documentation;
* Transportation plans;
* Essential spares.
ISS Life Extension Assessments: Primary Structures:
NASA continues to assess the structural health of the ISS primary
structures, i.e., the external trusses which serve as the backbone of
the station and the pressurized modules that are occupied by the ISS
crew.
NASA has extended its assessments to examine the ISS structural health
through 2028 as a conservative approach to assessing continued
operations through 2020. This approach is being used to minimize
analysis resources and to identify indicators of structural
limitations that could impact operations beyond 2020.
Structural health assessments involve:
* analytical assessments of the U.S. primary structure;
* hardware tests on the ground test article for the US-funded, Russian-
built Functional Cargo Block (FGB)”-the first segment of the ISS
placed in orbit; and;
* limited visual inspection of the assembled ISS in orbit.
Planned assessments of primary structures are ongoing and will
continue through 2015. Additional assessments beyond 2015 could be
required for specific critical areas depending on the results of the
assessments.
Figure: ISS Life Extension Assessments: Primary Structures:
[Refer to PDF for image: illustration]
Primary structures are depicted and indicated as being any of the
following:
NASA elements:
Russian elements:
Canadian elements:
Japanese elements:
European elements:
Source: NASA.
[End of figure]
U.S. Primary Structures:
NASA initially certified ISS structural elements for 15 years of life
after being placed in orbit: 5 years to allow for ISS assembly and 10
additional years for operation. Certifications for the initial
structural elements begin expiring in 2013.
To achieve a 15-year certification, NASA required analytical
demonstration of sufficient design margin to maintain structural
integrity for at least 60 years (i.e., 4 times the 15-year planned
service life) based on:
* anticipated mechanical, pressure and thermal loads, and,
* information in the national material properties database.
NASA is examining the U.S. primary structures in phases to determine
if the original certifications are still valid.
* Phase I (under way): Hardware exceeding original certification
limits starting in 2013.
* Phase II (starts in 2012): Hardware with original certifications
expiring in 2017.
* Phase III (starts in 2012): Hardware with original certifications
extending beyond 2020.
U.S. Primary Structures:
* NASA is using the Structural Health Assessment Program (SHAP) to
annually assess the validity of the ISS primary structures' original
service life certifications.
* SHAP uses reconstructed mechanical, thermal, and pressure loads
created from actual data gathered from sensors onboard the ISS to
update predicted service life.
* Officials indicated that the current SHAP addresses about 40
percent, by weight, of the ISS”assembled through November 2002.
- On-orbit time is necessary to conduct a meaningful SHAP analysis.
* NASA is not assessing the international partner (IP) owned
structures (this is an IP responsibility); however, NASA will assess
structures built by IP's but owned by NASA.
Figure: ISS Assembly as of November 2002.
[Refer to PDF for image: illustration]
Source: NASA.
[End of figure]
* NASA considered over 10,000 design locations in preparing the
current SHAP assessment.
* 829, about 7 percent, of these locations are examined in detail by
the SHAP because they have less than 90 years of service life (i.e., 6
times the 15-year planned service life) or safety margins of less than
10 percent.
* 196 of the 829 locations are using life faster than originally
predicted.
* 19 of the 196 locations are at less than 60 years of service life
(i.e., 4 times the 15-year planned service life).
- 15 of the 19 locations are on the P6 truss”-left exposed to
continuous thermal cycles for an extended period after the Columbia
accident (see images).
- Assessing mitigation options for these”-considering thermal
blankets.
[Figure: Refer to PDF for image: 2 illustrations of P6 Truss]
Source: NASA.
[End of figure.
Functional Cargo Block (FGB):
* FGB was the first ISS component launched, in 1998:
- Provided initial propulsion and power;
- FGB initial certification expires in 2013.
* Initial Russian certification was achieved by conducting accelerated
15-year life-cycle testing to a full-scale FGB test article.
* In 2010, NASA conducted accelerated testing of the FGB test article
hull structure and docking hardware to recertify to 2028.
* Final fracture analysis of testing to 2028, including analysis of
weld seams, to be completed in Spring 2011.
[Figure: Refer to PDF for image: illustration of FGB Docked to Unity
Node 1998]
Source: NASA.
[End of figure]
Assembled ISS:
* NASA has limited capability to, assess the overall structure of the
entire station.
- Extensive internal structures prevent x-ray or sonographic
inspection of all weld seams from inside the ISS.
- External television cameras support limited visual inspection of the
ISS on orbit.
[Figure: Refer to PDF for image: illustration of ISS Current Assembly]
Source: NASA.
ISS Life Extension Assessments: Functional Systems:
NASA is conducting four types of analytical assessments of ISS
functional systems to identify technical issues that could impact
continued operations through 2020.
* Critical operating hardware assessments, ongoing through 2011.
* Nonreplaceable hardware assessments, ongoing through 2011.
* Primary utilization facility hardware assessments, ongoing through
2015 (estimated).
* FGB nonreplaceable and critical hardware assessments, ongoing
through 2014 (estimated).
Critical Operating Hardware Assessments:
* Assessment of all hardware and ORUs with catastrophic failure modes,
to determine cycle or time limitations, as well as technical issues
specific to continued operation beyond original certification.
* Catastrophic failure modes are those which could cause loss of crew
or loss of station.
- Example: pressure vessels can rupture and cause loss of crew or
station.
Nonreplaceable Hardware Assessments:
* Assessment of hardware not intended for on-orbit removal and
replacement due to location, design, installation, or function.
* Assessment to determine if sufficient work-around plans exist to
ensure continued operation of a system if nonreplaceable segments fail
or whether more plans are needed.
- Example: assess capability to repair wire insulation jacket vs.
spending resources to analyze wiring life.
Primary Utilization Facility Hardware Assessments:
* Analytical assessment of key laboratory science facilities/hardware
that may have catastrophic failure effects or that may support
critical functionality related to scientific utilization.
- Example: the Material Science Research Rack and General Laboratory
Active Cryogenic ISS Experiment Refrigerator.
FGB Nonreplaceable and Critical Hardware Assessments:
* Analytical assessment of FGB propulsion, electrical, and ventilation
systems.
* NASA in negotiations with Russian authorities to perform the
assessments.
ISS Life Extension Assessments: Safety and Mission Assurance:
The ISS Safety and Mission Assurance (S&MA) office will examine life
extension from an ISS system-level perspective (in contrast to
individual hardware assessments of critical operating hardware and
ORUs).
* These dual methodologies (system-level and individual hardware
perspectives) are intended to serve as a check and balance to insure
critical areas and functionality are addressed.
- S&MA will review existing documentation to determine if any waivers,
closed risks, or corrective actions are impacted by extending service
life or increasing cycles of use.
- S&MA will update risk assessments on a case-by-case basis, e.g.,
increased risk from changes in the orbital debris environment.
* S&MA findings and conclusions will be coordinated with hardware
teams and brought to the ISS program management for discussion.
* S&MA reviews are ongoing and will continue into 2015.
ISS Life Extension Assessments: Transportation Plans:
NASA's sparing plan for ISS life extension through 2020 relies on
development of new launch and transport vehicles to support ISS
operations:
* NASA plans to use transportation systems developed commercially by
Space Exploration Technologies (SpaceX) and Orbital Sciences
Corporation (Orbital) to supply spares beginning in 2011.
- Development of both systems are moving more slowly than planned.
* 38 percent of the flights planned to support ISS operations through
2020 are dependent on vehicles not yet in development.
* According to ISS program officials, the European Automated Transfer
Vehicle (ATV) and Japanese H-II Transfer Vehicle (HTV) production
facilities are not equipped to accelerate production rates and
procuring additional Russian Progress resupply vehicles and Soyuz crew
transport vehicles is problematic.
* Reliant on new Commercial Crew System for crew transport in 2016.
Figure: Refer to PDF for image: pie-chart]
22 Total Progress, ATV, and HTV Flights: 32%;
20 Total SpaceX and Orbital Flights: 29%;
26 Total Future Vehicle Flights: 38%.
Source: GAO analysis of NASA data.
[End of figure]
ISS Life Extension Assessments: Transportation Plans:
Shuttle retirement complicates ISS supportability.
* Shuttle capability allowed NASA to return failed ORUs to the Earth
for analysis, repair, and eventual return to the ISS.
* Without the Shuttle, NASA lacks the ability to return large external
ORU's to conduct on-ground failure analysis to pinpoint causes of
failures.
- Example: Return capability helped NASA diagnose problems with ISS
control moment gyroscopes.
* One ORU, the heat rejection system radiator, is too large for
transport on anything but the Shuttle.
- One spare is prepositioned.
- According to ISS officials, Micro-Meteoroids and Orbital Debris
models indicate that portions of the heat rejection system radiator
are likely to be damaged by strikes every 3 years. Although the heat
rejection system radiator has been struck, it has never been disabled
by a micrometeoroid and according to officials the radiators have
unused capacity that provides redundancy.
* According to the ISS program manager, NASA has prepositioned spares
in anticipation of Shuttle retirement. If commercial vehicles are not
available as expected, however, NASA will need STS-135 (the additional
authorized Space Shuttle flight) to preposition additional spares or
its ability to conduct science efforts will be limited in 2012.
ISS Life Extension Assessments: Essential Spares:
NASA uses two analytical techniques to assess the quantities and types
of essential spares, i.e., ORUs needed to support ISS functionality,
such as the control moment gyro within the guidance and navigation
control system.
* A statistical methodology, called the Bayesian update process, to
calculate an operational Mean Time Between Failure (MTBF) for an ORU
using a manufacturer's estimated MTBF in conjunction with actual on-
orbit operation and failure data.
* Monte Carlo[A] modeling techniques to prepare the Functional
Availability Analysis (FAA), which calculates the probability that
targets for key functions will be met at ISS end of life.
- NASA uses the operational MTBF from the Bayesian update process as
inputs to its Monte Carlo modeling.
- NASA updated the FAA in 2010 to identify ISS sparing needs through
2020.
[A] Monte Carlo modeling is used to approximate the probability
outcomes of multiple trials by generating random numbers.
[End of Finding 1 section]
Finding 2: Evaluation of NASA Analytical Techniques for Assessing
Essential Spare Needs:
NASA's assessment of the essential spares necessary to assure
continued operations through 2020 appears to be supported by
sufficient, accurate, and relevant underlying data. However, we found
that estimates of essential spares are sensitive to NASA's assumptions
about ORU reliability.
We examined the following areas:
* Data underlying NASA's assessment.
* Bayesian update process.
* Functional availability assessments.
Evaluation of Analytical Techniques: Data Underlying NASA's Assessment:
NASA's findings and conclusions on ISS maintenance and continued
operations through 2020 appear to be supported by sufficient,
accurate, and relevant underlying data.
* Bayesian update process is an accepted tool for forecasting failures
with limited data and is used by NASA, the Nuclear Regulatory
Commission, and others to modify original estimates of failure rates
as new information becomes available.
- NASA applies the Bayesian update process to all ORUs that have
experienced random failures.
- Before applying Bayesian updates to an ORU that has not failed,
NASA allows the ORU to operate at least one-half of its original
predicted MTBF.
* For each ORU, reliability of Bayesian update process depends upon
the accuracy of four key pieces of input data: original MTBF; assumed
original MTBF variance; runtime; and actual failure rates”data
maintained in NASA's Modeling and Assessment Data Set database.
- Original MTBF-”GAO's limited tracing of original MTBF in the
Modeling and Assessment database to source documentation provided by
the original equipment manufacturer did not reveal deficiencies in the
accuracy of the data. Our test, however, was for a very small number
of records. Original MTBF estimates generated by original equipment
manufacturers are paper records; according to NASA representatives,
retrieving these records is a cumbersome, labor-intensive, and costly
process.
- Assumptions regarding original MTBF variance-”involves selecting an
appropriate dispersion of failure for the distribution of the MTBF.
- Actual run-time and failure rates”-according to NASA, they maintain
detailed logs on each ORU at each location on the ISS.
Evaluation of Analytical Techniques: Bayesian Update Process:
Bayesian update process may overstate Operational MTBF in instances of
few or no failures.
* Bayesian update process is a statistical methodology that NASA uses
to calculate an operational Mean Time Between Failure (MTBF) for an
ORU using a manufacturer's estimated MTBF in conjunction with actual
on-orbit operation and failure data.
* Bayesian update process is very sensitive to assumptions and initial
failures. A single failure of an ORU can dramatically change the
operational MTBF calculated by this process.
* For example, as shown below, successive annual updates of the MTBF
(in hours) for the ISS Pump Module Assembly increased each year
through 2009. A single failure of the Pump Module Assembly in 2010, if
determined to be random, would result in a significant decrease in
MTBF.
Table: Hours:
Pump Module Assembly:
Original MTBF In Oct. 2006 Data: 39,800;
Updated MTBF Oct. 2007: 52,450;
Updated MTBF Oct. 2008: 86,560;
Current MTBF Oct. 2009: 104,683;
Estimated run-time hours at failure per ISS Risk Team to August 3,
2010: 63,600;
New MTBF Bayesian updated w/one failure Aug. 2010: 58,119.
[End of table]
Simplification of Bayesian mathematics decreases accuracy.
* NASA uses a simplifying assumption to reduce the computational
complexity of the Bayesian calculations.
* Complex numerical integration is reduced to simple algebra at the
cost of some accuracy.
Evaluation of Analytical Techniques: Functional Availability
Assessment:
NASA's determination that 72 percent of ISS functions meet or exceed
functionality targets with minimal risk acceptance to 2020 may be
overstated.
* 22 functions assessed by FAA process does not include 10 additional
functions needed for full utilization of station; other sparing
methodologies are used on those 10 functions because most are
government-furnished equipment lacking reliability data necessary for
FAA.
* Statement in NASA's report to Congress suggesting 23 percent of
functions "are within 5 percent" of their goal lacks clarity. Meeting
a functional target at a 94 percent confidence is, in fact, much
different than achieving a functional target at a 99 percent
confidence-”failure to meet a functional target one time in 20 vs. one
time in 100.
* According to NASA representatives, each function is modeled
independently and assumes other functional targets are met.
The functional targets and confidence goals in the FAA were developed
by the ISS team and endorsed by the ISS program manager.
* NASA assumes "graceful degradation" of ISS, which means ISS
operations decline while critical systems continue to work until the
end of ISS life. The functional targets and confidence goals protect
full operations with reduced system redundancy.
* NASA officials indicated the program delays spares procurements as
long as possible to provide for more informed decisionmaking and to
conserve resources.
* The Functional Availability Assessment March 2010 was performed
prior to NASA's authorization to extend ISS operations to 2020. It
indicates that, based on current data, additional spares procurements
may be needed to achieve target functionality beyond the planned 2015
ISS end date.
Evaluation of Analytical Techniques: Functional Availability
Assessment--March 2010:
System: Electrical Power System, Structural/Mechanical Thermal Control;
Function: Usable Power;
Functional Target: 7 of 8 Power Strings;
Confidence Goal: 90%;
Confidence Status: 2015: 95%;
Confidence Status: 2020: 65%;
At-Risk ORUs: [Check].
System: Environmental Control and Life Support System (ECLSS);
Function: Intramodule Ventilation;
Functional Target: 4 Strings;
Confidence Goal: 95%;
Confidence Status: 2015: 95%;
Confidence Status: 2020: 93%;
At-Risk ORUs: [Empty].
System: Environmental Control and Life Support System (ECLSS);
Function: Atmosphere Control System;
Functional Target: All Valves/Sensors;
Confidence Goal: 85%;
Confidence Status: 2015: 84%;
Confidence Status: 2020: 57%;
At-Risk ORUs: [Empty].
System: Environmental Control and Life Support System (ECLSS);
Function: Atmosphere Control System;
Functional Target: At least 2 Pressure Control Panels;
Confidence Goal: 95%;
Confidence Status: 2015: 98%;
Confidence Status: 2020: 83%;
At-Risk ORUs: [Empty].
System: Environmental Control and Life Support System (ECLSS);
Function: Trace Contaminant Control System;
Functional Target: 1 of 2 Strings;
Confidence Goal: 99%;
Confidence Status: 2015: 100%;
Confidence Status: 2020: 100%;
At-Risk ORUs: [Empty].
System: Environmental Control and Life Support System (ECLSS);
Function: Fire Detection System;
Functional Target: All Detectors;
Confidence Goal: 80%;
Confidence Status: 2015: 100%;
Confidence Status: 2020: 99%;
At-Risk ORUs: [Empty].
System: Environmental Control and Life Support System (ECLSS);
Function: Carbon Dioxide Removal;
Functional Target: 1 of 2 Strings;
Confidence Goal: 98%;
Confidence Status: 2015: 99%;
Confidence Status: 2020: 97%;
At-Risk ORUs: [Empty].
System: Environmental Control and Life Support System (ECLSS);
Function: Major Constituent Analyzer;
Functional Target: 1 of 2 Strings;
Confidence Goal: 90%;
Confidence Status: 2015: 98%;
Confidence Status: 2020: 71%;
At-Risk ORUs: [Check].
System: Environmental Control and Life Support System (ECLSS);
Function: Vacuum;
Functional Target: Full Functionality;
Confidence Goal: 90%;
Confidence Status: 2015: 97%;
Confidence Status: 2020: 93%;
At-Risk ORUs: [Empty].
System: Environmental Control and Life Support System (ECLSS);
Function: Sample Distribution;
Functional Target: All Functionality;
Confidence Goal: 90%;
Confidence Status: 2015: 95%;
Confidence Status: 2020: 92%;
At-Risk ORUs: [Empty].
System: Internal Thermal Control System;
Function: Internal Thermal Control;
Functional Target: All Strings;
Confidence Goal: 85%;
Confidence Status: 2015: 97%;
Confidence Status: 2020: 85%;
At-Risk ORUs: [Empty].
System: Internal Thermal Control System;
Function: ITCS Coldplates;
Functional Target: All Functionality;
Confidence Goal: 95%;
Confidence Status: 2015: 98%;
Confidence Status: 2020: 95%;
At-Risk ORUs: [Empty];
System: Communications and Tracking;
Function: S-Band;
Functional Target: 1 of 2 Strings;
Confidence Goal: 98%;
Confidence Status: 2015: 99%;
Confidence Status: 2020: 94%;
At-Risk ORUs: [Empty].
System: Communications and Tracking;
Function: Ku-Band;
Functional Target: 1 of 2 Strings;
Confidence Goal: 98.5%;
Confidence Status: 2015: 94%;
Confidence Status: 2020: 73%;
At-Risk ORUs: [Check].
System: Communications and Tracking;
Function: Internal Audio;
Functional Target: 12 of 14 Strings;
Confidence Goal: 99.5%;
Confidence Status: 2015: 99.40%;
Confidence Status: 2020: 97%;
At-Risk ORUs: [Empty].
System: Command and Data Handling;
Function: Command and Data Handling;
Functional Target: All Functionality;
Confidence Goal: 95%;
Confidence Status: 2015: 97%;
Confidence Status: 2020: 84%;
At-Risk ORUs: [Empty].
System: Guidance Navigation and Control;
Function: Attitude Determination;
Functional Target: At least 1 String;
Confidence Goal: 95%;
Confidence Status: 2015: 97%;
Confidence Status: 2020: 95%;
At-Risk ORUs: [Empty].
System: Guidance Navigation and Control;
Function: Attitude Translation;
Functional Target: 3 of 4 Strings;
Confidence Goal: 99%;
Confidence Status: 2015: 99%;
Confidence Status: 2020: 92%;
At-Risk ORUs: [Empty].
System: Video;
Function: Internal/External Video;
Functional Target: 3 of 4 ETVCG Strings;
Confidence Goal: 95%;
Confidence Status: 2015: 93%;
Confidence Status: 2020: 70%;
At-Risk ORUs: [Check].
System: Regenerative ECLSS;
Function: Oxygen Generation Assy:
Functional Target: 1 String;
Confidence Goal: 90%;
Confidence Status: 2015: 76%;
Confidence Status: 2020: 22%;
At-Risk ORUs: [Check].
System: Regenerative ECLSS;
Function: Water Processing Assy;
Functional Target: 1 String;
Confidence Goal: 90%;
Confidence Status: 2015: 73%;
Confidence Status: 2020: 5%;
At-Risk ORUs: [Check].
System: Regenerative ECLSS;
Function: Urine Processing Assy;
Functional Target: 1 String;
Confidence Goal: 90%;
Confidence Status: 2015: 90%;
Confidence Status: 2020: 29%;
At-Risk ORUs: [Check].
Source: NASA.
[End of table]
March 2010 Functional Availability: Risk Assessment:
NASA plans to monitor and track At-Risk ORUs annually to reassess risk.
All 2020 functions with no At-Risk ORUs identified indicate that risk
acceptance rationale is in place for ORUs impacting those confidence
targets.
Examples of risk acceptance rationale include:
* Good on-orbit performance to date indicates low risk of failures
through 2020.
* Electrical boxes have demonstrated performance that far exceeds
original manufacturer MTBFs.
* Changes in on-orbit operational usage/design.
[End of Finding 2 section]
Agency Comments:
Agency officials agreed with our overall findings.
* NASA is using analytical techniques, physical tests, and inspections
to assess primary structures and functional systems, to the extent
possible, and determine sparing needed to support safe functioning and
full scientific utilization of the ISS through 2020. NASA is confident
that it can execute necessary functioning and utilization; however,
the supporting assessments for primary structures and functional
systems are ongoing and all results are not yet available.
* NASA's assessment of the essential spares necessary to assure
continued operations through 2020 appears to be supported by
sufficient, accurate, and relevant underlying data. We found, however,
that estimates of essential spares are sensitive to NASA's assumptions
about ORU reliability.
ISS officials provided technical comments that we have incorporated as
appropriate.
[End of Agency Comments section]
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