W..e_: NASA Technical Memorandum 4628

Recommended Techniques for EffectiveMaintainability

A Continuous Improvement Initiative of the NASA Reliability andMaintainability Steering Committee

December 1994

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(NASA-TM-4628) RECOMMENOEOTECHNIQUES FOR EFFECTIVEMAINTAINABILITY. A CONTINUOUSIMPROVEMENT INITIATIVE OF THE NASARELIABILITY AND MAINTAINABILITY

STEERING COMMITTEE (NASA) 105 p

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PREFACE

Current and future NASA programs face the challenge of achieving a high degree of missionsuccess with a minimum degree of technical risk. Although technical risk has severalelements, such as safety, reliability, and performance, a proven track record of overall systemeffectiveness ultimately will be the NASA benchmark. This will foster the accomplishment ofmission objectives within cost and schedule expectations without compromising safety orprogram risk. A key CharaCteristic of systems effeci_veness is the impiementation ofappropriate levels of maintainability throughout the program life cycle.

Maintainability is a process for assuring the ease by which a system can be restored tooperation following a failure. It is an essential consideration for any program requiring groundand/or on-orbit maintenance. TheiOffice of S_._ty"and Mission Assurance (OSMA) hasundertaken a continuous improvement initiative to develop a technical roadmap that willprovide a path toward achieving the desired degree of maintainability while realizing cost andschedule benefits. Although early life cycle costs are a characteristic of any assuranceprogram, operational cost savings and improved system availability almost always result froma properlY administered maintainability assurance program. Past experience on NASAprograms has demonstrated the value of an effective maintainability program initiated early inthe program life cycle.

This memorandum provides guidance towards continuous improvement of the life cycledevelopment process within NASA. It has been developed from NASA, Department ofDefense, and industry experience. The degree to which these proven techniques should beimposed resides with the project/program, and will require an objective evaluation of theapplicability of each technique. However, each applicable suggestion not implemented mayrepresent an increase in program risk. Also, the information presented is consistent withOSMA policy, which advocates an Integrated Product Team (IPT) approach for NASAsystems acquisition. Therefore, this memorandum should be used to communicate technicalknowledge that will promote proven maintainability design and implementation methodsresulting in the highest possible degree of mission success while balancing cost effectivenessand programmatic risk.

Frederick D. GregoryAssociate Administrator forSafety and Mission Assurance

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DEVELOPING ACTIVITY

The development of this technical memorandum has been overseen by the NASA Reliability andMaintainability (R&M) Steering Committee, which consists of senior technical representativesfrom NASA Headquarters and participating NASA field installations. This Committee exists toprovide recommendations for the continuous improvement of the R&M discipline within theNASA community, and this manual represents the best technical "advice" on designing andoperating maintainable systems from the participating Centers and the Committee. Eachtechnique presented in this memorandum has been reviewed and approved by the Committee.

CENTER CONTACTS

Appreciation is expressed for the dedication, time, and technical contributions of the followingindividuals in the preparation of thismanual. Without the support of their individual Centers,and their enthusiastic personal support and willingness to serve on the NASA R&M Steeringc-bmmittee, the capture oftlie m_inffinab_tytechniques _ofi/a_ned in this manual would not bepossible. -"

All of the NASA Centers are invited to participate in this activity and contribute to this manual.The Committee members listed below may be contacted for more information pertaining to thesemaintainability techniques.

Mr. Donald BushGeorge C. Marshall Space Flight CenterCR85 Bldg 4203Marshall Space Flight Center, Alabama 35812

Mr. Vincent LalliLewis Research CenterMS 501-4 Code 015221000 Brookpark RoadCleveland, Ohio 44135

Mr. Malcolm HimelLyndon B. Johnson Space CenterBldg. 45 RM 618A, Code NB2Houston, Texas 77058

Mr. Leon MigdalskiJohn F. Kennedy Space CenterRT-SRD-2 KSC HQS 3548Kennedy Space Center, Florida 32899

Mr. Ronald LiskNASA Headquarters Code QS200 E Street, SWWashington, DC 20546

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TABLE OF CONTENTS

SECTION PAGENUMBER

PREFACE ................................................................ iDEVELOPING ACTIVITY .................................................. iiCENTER CONTACTS ...................................................... ii

I. INTRODUCTION ...................................................... v

II.

A. Purpose .......................................................... vB. Control/Contributions ............................................... v

C. Maintainability Technique Format Summary ........................... vi

RECOMMENDED TECHNIQUES FOR EFFECTIVE MAINTAINABILITY

Program Management

Technique PM-I : The Benefits of Implementing Maqntainability on NASA Programs .... PM-2Technique PM-2 : Maintainability Program Management Considerations ............. PM-8Technique PM-3: Maintenance Concept for Space Systems ....................... PM-14

Design Factors and Engineering

Technique DFE-I : Selection of Robotically Compatible Fasteners andHandling Mechanisms .................................... DFE-2

Technique DFE-2: False Alarm Mitigation ................................... DFE-8

Analysis and Test

Technique A T-l: Neutral Buoyancy Simulation of On-Orbit Maintenance ............ AT-2Technique A T-2: Mean Time To Repair Predictions ............................. AT-7Technique A T-3: Availability Prediction and Analysis ........................... AT-12Technique A 7-4: Availability, Cost, and Resource Allocation (ACARA) Model

to Support Maintenance Requirements ......................... AT- 17Technique AT-5: Rocket Engine Failure Detection Using an Average Signal

Power Technique ......................................... AT-21

Operations and Operational Design Considerations

Technique OPS-I : SRB Maintainability and Refurbishment Practices ............... OPS-2Technique OPS-2: Electrical Connector Protection ............................. OPS-9Technique OPS-3: Robotic Removal and Application of SRB Thermal Systems ....... OPS- 11Technique OPS-4: GHe Purging of H 2 Systems ............................... OPS- 17Technique OPS-5: Programmable Logic Controller ........................... OPS-20

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TABLE OF CONTENTS (CONT.)

SECTION: PAGE NUMBER

Operations and Operational Design Considerations (cont.)

Technique OPS-6: DC Drive - Solid State Controls ............................ OPS-24Technique OPS-7: AC- Variable Frequency Drive Systems ..................... OPS-28Technique OPS-8: Fiber Optic Systems ..................................... OPS-32Technique OPS-9: Pneumatic Systems-Dome Loaded Pressure Regulator Loading .... OPS-36Technique OPS-IO: Modular Automatic Power Source Switching Device OPS-39Technique OPS-11: Pneumatic System Contamination Protection ................. OPS-42

m. APPENDIX A: CANDIDATE TECHNIQUES FORFUTURE DEVELOPMENT ............................................ A-1

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I. INTRODUCTION

A. PURPOSE

Maintainability is a process for assuring the ease by which a system can be restored tooperation following a failure. Designing and operating cost effective, maintainable systems(both on-orbit and on the ground) has become a necessity within NASA. In addition, NASAcannot afford to lose public support by designing less than successful projects. In this era ofshrinking budgets, the temptation to reduce up front costs rather than consider total programlife cycle costs should be avoided. In the past, relaxation of R&M requirements to reduce upfront costs has resulted in end-items that did not perform as advertised and could not beproperly maintained in a cost effective manner. Additional costs result when attempts aremade late in the design phase to correct for the early relaxation of requirements.

The purpose of this manual is to present a series of recommended techniques that canincrease overall operational effectiveness of both flight- and ground-based NASA systems.Although each technique contains useful information, none should be interpreted as arequirement. The objective is to provide a set of tools to minimize the risk associated with:

Restoring failed functions (both ground and flight based) Conducting complex and highly visible maintenance operations Sustaining a technical capability to support the NASA mission utilizing aging equipment

or facilities.

This document provides (1) program management considerations - key elements of an effectivemaintainability effort; (2) design and development considerations; (3) analysis and testconsiderations - quantitative and qualitative analysis processes and testing techniques; and (4)operations and operational design considerations that address NASA field experience. Updateswill include a section applicable to on-orbit maintenance with practical experience from NASAEVA maintenance operations (including ground and on-orbit operations and ground-basedsimulations). This document is a valuable resource for continuous improvement ideas inexecuting the systems development process in accordance with the NASA "better, faster,smaller, and cheaper" goal without compromising mission safety.

B. CONTROL/CONTRIBU_ONS

This document will be revised periodically to add-new techniques or revisions to the existingtechniques as additional technical data becomes available. Contributions from aerospacecontractors and NASA Field Installations are encofir_/ged. Any technique based onproject/program experience that appears appropriate for inclusionin this manual should besubmitted for review. Submissions should be fo _n-nattedid_entical!y to the techniques in thismemorandum (Figure 1) and sent to the address below for consideration.

National Aeronautics and Space AdministrationCode QS300 E Street S.W.Washington, DC 20546

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Organizations submitting techniques that are selected for inclusion in this manual will berecognized on the lower portion of the first page of the published item. Contacts listed earlier inthis document should be used for assistance. If additional information on any technique isdesired, the contacts listed earlier in this document can be utilized for assistance.

C. MAINTAINABILITY TECHNIQUE FORMAT SUMMARY

The maintainability techniques included in this manual are Center-specific descriptions ofprocesses that contribute to maintainability design, test, analysis and/or operations. Eachtechnique follows a specific format so users can easily extract necessary information. The firstpage of each technique is a summary of the information contained, and the rest of the techniquecontains the specific detail of the process. Figure 1 shows the baseline format that has been usedto develop each technique.

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Technique Title, page numbeJTechnique XXX-._ *_

TECHNIQUE FORMATTechniaue: A brief statement defining the design technique and how it is used.

Benefits: A concise statement of the technical improvement andor impact on resource expenditurerealized from implementing the technique.

Key Words: Any term that captures the theme of the technique or provides insight into the scope.Utilized for document search purposes.

Application Experience: Identifiable programs or projects that have applied the technique within NASAand/or industry.

Technical Rationale: A brief technical justification for the use of the technique.

Contact Center: Source of additional information, usually sponsoring NASA Center.

Techniaue Description: A technical discussion that is intended to give the details of the process. Theinformation should be sufficient to understand how the technique should be implemented.

References: Publications that contain additional information about the technique.

'* Each technique within a section is identified using one of the following acronyms specific to that section'ollowed by the associated sequential technique number.

PM: Program Management DFE: Design Factors and Engineering AT: Analysis and Test OPS: Operations and Operational Design Considerations

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Figure 1: Technique Format Definitionsw

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Program Management

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A fundamental key to program and mission success is the development of systems that are reliable

and affordable to operate and maintain with today's limited resources. Early definition of bothhardware and software requirements that provide the capability for rapid restoration when failures

occur is essential. While incorporation of a maintainability program may require some additional

early investment, the resulting benefits will include operational cost savings and improved system

availability. The techniques included in this section are intended to provide management personnel

with an understanding of all information necessary to develop, foster, and integrate a successfulmaintainability program that will enhance mission success and lower overall costs. Each technique

provides high-level information on a specific subject, and can be tailored or expanded to achieveoptimum application.

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The Benefits of Impleraenting Maintainability on NASA Programs, Page ITechnique PM-I

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Technique Programmatic provisions for ease of maintenance greatly enhancehardware and software system operational effectiveness for both in-space and ground support systems.

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The Benefits of Implementing Maintainability on NASA Programs, Page 2Technique PM-1

Be_fits of Implementing Maintainabilityon NASA ProgramsTechniqm _ PM-1

environment under which maintenance is:

performed. Applying maintainability principleswill enhance the systems readiness/availabilitythrough factors such as visibility, accessibility,testability, simplicity, and interchangeability of

Over the years, NASA has =successfully the Systems being maintained. Usinglaunched manned spacecraft to the moon, sent maintainability prediction techniques and otherunmanned probes into the outer reaches_f - = qu_titativ e m_n_tainability analyses can greatlythe sc' system, and developed reusable enhance the confidence in operationalspace zems for earth orbitable missions.NASA alS _performs v_uable atmos.pheric -research and development of ground systems,all of which contain complex hardware and: ,ftware that must be maintained during all

:_ases of operafior[s an-d-in multipleenvironments. However, in this age ofshrinking budgets, doing more with less isbecoming the overall programmatic theme.NASA space flight programs are being driventowards more automated, compact designs inwhich fewer support resources will beavailable than in past programs. Thistechnique will outline _e _nefits of ......implementing well-defined and user-friendlyprinciples of maintainability on all NASAprograms, regardless of the operationalscenario. Emphasis is placed on how andwhy a maintainability program can enhancethe effectiveness of a system and its overalloperation. It must be noted, however, thatmaintainability of unmanned deep spacesystems provides a different set of challenges.

Mal tainabilitY is defined inNASAHandbook53(_).4(1E), "Maintainability ProgramRequirements for Space Systems," as: "Ameasure of the e a_ and rapiditY with which asystem or equipment can be restored tooperational status following a failure," and isconsistent with NHB 7120.5, "Managementof Major Systems and Projects." It is acharacteristic of equipment and installation,

capabilities of a design. These predictions canalso ai___d!n des!gn dec!sions_an_d [fade studieswhe_sex, erfil-design options are being _considered. Also, cost savings and fewerschedule impacts in- tl3e oPerational phase of theprogram will result due to decreasedmaintenance time, minimization of supportequipment, and increased system availability.Another benefit is a decrease in managementoverhead later on in the life cycle as a result ofincluding maintainability planning as a fullpar-trier in early maintenance/logistics conceptplanning and development.

PROGRAMMA TIC BENEFITS

Maintainability Program ImplementationProject management is responsible forimplementing maintainability on a program viadevelopment of specific requirements for costeffective system maintenance in the early phasesof the life cycle. Trade studies of the impacts ofmaintainability design on life cycle costs areused to evaluate the balance between cost of

designing to minim!ze maintenance times andthe associated increase m system availabilityresulting from the decrease in maintenancetimes. Usually, the up-front cost of designing- =in maintainability is much less than the costsavings realized over the operational portion ofthe life cycle.

Several programs have opted to accept the

personnel availability inthe_qu_ed s_.kiH .... short-term cost savings by deleting = _levels, adequacy of maintenance procedures maintainability requirements in the designand test equipment, and the physical phase, but the associated increase in

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The Benefits of Implementing Maintainability on NASA Programs, Page 3Technique PM-1

maintenance and support costs incurredduring operations would have beensignificant. An example of this is the SpaceStation Program, which had deletedrequirements for on-orbit automated faultdetection, isolation and recovery (FDIR),saving the program up-front money.However, the alternative concept was toincrease the mission control center manpowerduring operations for ground based FDIR, butthis presented a significant cost increase whenaveraged over the life cycle. Another positiveexample is the Hubble Space TelescopeProgram. Maintainability concepts wereincluded early in the life cycle, wheremaintenance planning and optimum ORUusage in design saved the program significantcosts when on-orbit repairs becamenecessary. Figure 1 accentuates the costtradeoffs between introducing maintainabilityconcepts into a program and the time atwhich they are introduced. These tradeoffscan mean the difference between a successful

maintainability program and a costly, lesseffective one.

Figure 1: Effect of ImplementingMaintainability Program vs. Phase

The NASA systems engineering processshould require that the system be designed forease of maintenance within it's specifiedoperating environment(s), and should ensurethat the proper personnel (design andoperations maintainability experts) and fundsare committed to development of the processto achieve maximum program benefit.Program schedule will be affected by lack ofsystem maintainability because necessaryground support will increase, maintenancetimes will be higher, necessary maintenanceactions will increase, EVA will be at a

premium, and system availability will belower. Table 1 highlights key programbenefits.

MaintenanceLogistics Concept DevelopmentDevelopment of the maintenance and logisticsconcepts for a program early in the life cyclemust include the maintainability characteristicsof the design. The maintenance concept is aplan for maintenance and support of end-itemson a program once it is operational. It providesthe basis for design of the operational supportsystem and also defines the logistics supportprogram, which will determine the applicationof spares and tools necessary for maintenance.The use of other logistic resources, such astools and test equipment, facilities and spareparts, will be optimized through includingmaintainability planning as a key operationalelement. Derivation of these plans early on inthe life cycle solidifies many operational aspects

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The Benefits of Implementing Maintainability on NASA Programs, Page 4Technique PM-I

Table 1: Maintainability ProgrammaticBenefits

Enhanced System Readiness/Availability- Reduced Downtime- Supportable Systems- Ease of Troubleshooting and Repair

System Growth Opportunities- Hardware/Software Modifications

- Interchangeability- Modular Designs- Decreased Storage Considerations

Reduced Maintenance Manpower Reduced Operational Costs Compatibility with other Programs Reduced Management Overhead

of the program, thus allowing for integrateddesign and support planning development.

MAINTAINABILITY DESIGN BENEFITS

VisibilityVisibility is an element of maintainabilitydesign that provides the system maintainervisual access to a system component formaintenance action(s). Even short durationtasks such as NASA space shuttle orbitercomponent inspection can increase downtimeif the component is blocked from view.Designing for visibility greatly reducesmaintenance times.

AccessibilityAccessibility is the ease of which an item canbe accessed during maintenance and cangreatly impact maintenance times if notinherent in the design, especially on systemswhere on-orbit maintenance will be required.When accessibility is poor, other failures areoften caused by removal/disconnection andincorrect re-installation of other items that

hamper access, Causing rework. Accessibilityof all replaceable, maintainable items willprovide key time and energy savings to thesystem maintainer.

TestabilityTestability is a measure of the ability to detectsystem faults and to isolate them at the lowestreplaceable component(s). The speed withwhich faults are diagnosed can greatly influencedowntime and maintenance costs. For example,deficiencies in Space Shuttle Orbiter testability

design have caused launch delays, whichtranslate to higher program costs. Astechnology advances continue to increase thecapability and complexity of systems, use ofautomatic diagnostics as a means of FDIRsubstantially reduces the need for highly trainedmaintenance personnel and can decreasemaintenance costs by reducing the erroneousreplacement of non-faulty equipment. FDIRsystems include both internal diagnosticsystems, referred to as built-in-test (BIT) orbuilt-in-test-equipment (BITE), and externaldiagnostic systems,referred to as automatic testequipment (ATE), test sets or off-line testequipment used as part of a reduced groundsupport system, all of which will minimizedown-time and cost over the operational lifecycle.

SimplicitySystem simplicity relates to the number ofsubsystems that are within the system, thenumber of parts in a system, and whether theparts are standard or special purpose. Systemsimplification reduces spares investment,enhances the effectiveness of maintenance

troubleshooting, and reduces the overall cost ofthe system while increasing the reliability. Forexample, the International Space Station Alphaprogram has simplified the design andpotentially increased the on-orbit maintainabilityof the space station, thus avoiding manyoperational problems that might have flownwith the Freedom Programl One example is theCommand and Data Handling Subsystem,which is the data processing backbone for thespace station. Formerly, the system consistedof several different central processing units,

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The Benefits of lmplementing Maintainability on NASA Programs, Page 5Technique PM-1

of several different central processing units,multiple level architecture, and severaldifferent network standards. The new designcomprises only one network standard, onestandard CPU, and a greatly reduced numberof orbital replaceable units (ORU's).Maintainability design criteria were definitefactors in the design changes to this spacestation subsystem.

Reduced training costs can also be a directresult of design simplification. Maintenancerequires skilled personnel in quantities andskill levels commensurate with the complexityof the maintenance characteristics of the

system. An easily maintainable system can bequickly restored to service by the skills ofavailable maintenance personnel, thusincreasing the availability of the system.

InterchangeabilityInterchangeability refers to a component'sability to be replaced with a similarcomponent without a requirement forrecalibration. This flexibility in designreduces the number of maintenance

procedures and consequently reducesmaintenance costs. Interchangeability alsoallows for system growth with minimumassociated costs, due to the use of standard orcommon end-items.

Human Factors

Human factors design requirements alsoshould be applied to ensure proper designconsideration. The human factors disciplineidentifies structure and equipment featuresthat impede task performance by inhibiting orprohibiting maintainer body movement, andalso identifies requirements necessary toprovide an efficient workspace formaintainers. Normally, the system designmust be well specified and represented indrawings or sketches before detailedanthropometric evaluation can be effective.

However, early evaluation during conceptdevelopment can assure early application ofanthrop0metriee0nsiderations. Use of theseevaluations results leads to improved designslargely in the areas of system provisions forequipment access, arrangement, assembly,storage, and maintenance task procedures. Thebenefits of the evaluation include less time to

effect repairs, lower maintenance costs,improved supportability systems, and improvedsafety.

SummaryImplementation of maintainability features in adesign can bring about operational cost savingsfor both manned and unmanned systems. Theprogrammatic benefits of designing systemhardware and software for ease and reduction

of maintenance are numerous, and can save aprogram, as seen with NASA's Hubble SpaceTelescope. Maintenance in a hostile, micro-gravity environment is a difficult andundesirable task for humans. Minimal exposuretime to this environment can be achieved byimplementing maintainability features in thedesign. The most successful NASA programshave been those which included maintainabilityfeatures in all facets of the life cycle. Remotesystem restoration by redundancy managementand contingency planning is particularlyessential to assuring mission success on projectswhere manned intervention is either

undesireable or impractical.

References

1. NASA Handbook 5300.4(1E)"Maintainability Program Requirements forSpace Systems, "March 10, 1987, NASAHeadquarters.

2. NASA Handbook 7120.5, "Management ofMajor Systems and Projects, "November 1993,NASA Headquarters

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The Benefits of Implementing Maintainability on NASA Programs, Page 6Technique PM-1

3. Air Force Design Handbook 1-9"Maintainability (for Ground ElectronicSystems)," Second Edition, Revision 7,Febm_r'y 25, 1988, United States Air FOrceAeronautical Systems Division.

4. "Maintainability Engineering Design andCost of Maintainability," Revision II, January1975, Rome Air Development Center.

5. Reliability, Maintainability, andSupportability (RMS) Guidebook, SecondEdition, 1992, Society of AutomotiveEngineers G- 11 International RMSCommittee.

6. MIL-STD-470B "MaintainabilityProgram for Systems and Equipment," May30, 1989, Department of Defense

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Benefits

KeyWords

ApplicationExperience

TechnicalRationale

Contact Center

Identify program management considerations necessary whenimplementing maintainability principles for NASA spaceflight,atmospheric, or ground support programs.

Early and effective planning and implementation of a maintainabilityprogram can significantly lower the risk of reduced system operationaleffectiveness resulting from maintainability design shortfalls. Thisreduces maintenance time/support, which directly relates to reducedoperating costs and increased system operational time.

Maintainability Management, Maintenance Concept, Logistics Support,Quantitative Requirements, Maintainability Planning

Hubble Space Telescope, SRB's, Shuttle GSE, and Space AccelerationMeasurement System

Decisions by program management to establish maintainabilityrequirements early in the program will provide design impetus towardsa system with higher operational availability at lower operational costsLower downtime and less complicated maintenance actions will beneeded when maintenance is required.

NASA Headquarters

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Maintainabili_F Program Management Considerations, Page 2Technique PM-2

Maintainability Program ManagementConsiderationsTechnique PM-2

This technique outlines managementconsiderations to observe when applying theprinciples of maintainability on a program atNASA. It also provides information on howto realize cost savings and reduced systemdowntime. This information complementsPM- 1, "Benefits of ImplementingMaintainability on NASA Programs," byproviding guidelines for establishing amaintainability program once the benefits havebeen understood.

Program management is responsible forestablishing proper integration ofmaintainability early in program developmentand ensuring adequate control of theapplication of the maintainability disciplinethroughout the development program. Figure1 provides flow diagram for an effectiveMaintainability program beginning withdevelopment of its goals and objectives,followed by development of the program/systemmaintenance concept and the Mainta[nabilityProgram Plan, and establishment of programcontrol and evaluation during design, production(manufacturing) and operations. The order ofthese program development elements isimportant, as each affects the next step in theprocess.

PROGRAM

PROGRAMCONTROL

Figure 1: Maintainability ProgramDevelopment

reflect the function (mission) of thesystem/subsystem and the impact onoperational objectives of the program if thesystem isnon'operational for any length oftime. System availability (the ability of thesystem to operate whenever called upon to doso) is very important, _and maximumavailability should be a goal of the program.Program maintainability goals and objectivesmust be developed with cost and schedule in

....... mindi hqwever, careful considerationo must(1) ESTABLISH MAINTAINABILITY AS also be given to the technical-and operationalPART OF THE OVERALL SYSTEMSENGINEERING AND OPERA TIONPLANNING PROCESS.

Set Goals and ObjectivesOne of the missions of the maintainabilityprogram is to measure the ability of an item tobe retained or restored to a specified conditionwhen maintenance is performed. The degree ofmaintainability designed into a system should

goals of the program. These qualitative goalsand objectives are developed by analyzing thesystem oPerating cycle, the physical andmaintenance support environments, and otherequipment characteristics consistent withmission and cost objectives.

Attention must also be given to existingsupport programs to avoid needlessduplication during development of new

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Maintainability Program Management Considerations, Page 3Technique PM-2

support systems. Development of themaintainability goals and objectives will lead toderivation of the maintenance concept,maintainability plan, and definition ofmaintainability requirements discussed in thefollowing paragraphs.

Establish Interfaces with Other EngineeringDisciplinesMaintainability engineering is a systemengineering discipline that combines systemanalysis and equipment design with a knowledgeof safety, reliability, human factors, and life-cyclecosting to optimize the maintenancecharacteristics of system design and to providean awareness of interface problems. Its goal isto optimize the combination of design features,

repair policies, and maintenance resources tothe desired level of maintainability atacceptable life-cycle costs. The manyinterfaces and feedback paths betweenmaintainability engineering and other productdevelopment and operational disciplines areshown in Figure 2.

While maintainability personnel must beintimately involved in the productdevelopment process and provide inputs todesign through design techniques andanalysis, it is program management'sresponsibility to develop and support therelationship between maintainability and therest of the system engineering disciplines.This support is key to establishment of a

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E REQU|REMENTS DEF|N|'_ON I SYSTEM DESIGN AC'R_PI'r_ES ITEST AND EVALUATION

Characteristic:

OperationalSuitabilityAnalysis

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DesignFeedback

SYSTEMAVAILABILITY

PERFORMANCE

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Figure 2: System Reliability, Maintainability and Support Relationships (typical)

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Maintainability Program Management Considerations, Page 4Technique PM-2

concurrent engineering process. Theserelationships must be mirrored in theMaintainability Program Plan.

(2) DEVELOP MAINTENANCE ANDLOGISTICS CONCEPTS EARL Y IN THE

CONCEPTUAL PHASE OF THEPROGRAM..

The program maintenance concept provides thebasis for establishing overall maintainabilitydesign = quirements on the program, andcontains detailed planning on maintenancepolicy.

It defines overall repair policy, organizationaland depot maintenance, system availability,repair vs. replacement policy, level ofreplacement, skill level requirements, sparingphilosophy, diagnostic/testing principles andconcepts, contractor maintenanceresponsibilities, payload maintenanceresponsibilities, and crew time allocations formaintenance (PM-3 provides details on each ofthese elements). Development of themaintenance concept is based on initialmaintainability analysis and program inputs suchas mission profile, system availability andreliability requirements, system mass propertiesconstraints, and personnel considerationsl Ttiemaintenance concept may be developed from theground up, or may come from a similarsuccessful program, tailored to meet the needs ofthe new program. New technology may alsodictate the maintenance concept, :e.g.maintainable items may be scrapped instead ofrepaired because the cost of repair outweighs thereplacement cost.

Definition of logistics and support concepts is afunction &the maintenance concept. Theoperational environment of the system, the levelof support personnel defined by the maintenanceconcept, and cost and schedule are importantdrivers for the logistics/support programs.

These elements are also important _contributors to system maintainability in thatlogistics planning can define how muchsystem down time is required duringmaintenance operations.

For example, downtime can be held to aminimum if spares are co-located with thesystem during operations. It is important thatProgram management closely monitor alllogistics development to ensure inclusion ofmaintenance and logistics concepts early inthe program. Both concepts drive thedevelopment of lower-level requirements.

Assess Existing ResourcesAnother important aspect of planning for anew program is assessment of the existinglogistic and support infrastructure. As anexample, the infrastructure of the NSTS=:system at KSC comprises the launch pad,numerous assembly and support buildings,and support personnel and equipment. Theseare important factors to consider whenplanning for new programs that will use KSCas the central operations base. If some of theexisting structures and equipment can be usedby the new program, then the developmentaland operational costs of the program will bereduced. During early planning stages,management should also look at how the newprogram can adapt to the existing supportinfrastructure, and what equipment andpersonnel may be used to eliminateunnecessary costs.

Establish a Maintainability Program PlanThe maintainability program plan is themaster planning and control document for themaintainability program. It provides detailedactivities and resources necessary-to attain thegoals and objectives of the maintainabilityprogram. It must be developed with the _program contractor(s) if they exist, or if theprogram is in-house, all developmental and

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operational disciplines must be represented. Theplan must be consistent with the type andcomplexity of the system or equipment and mustbe integrated with the systems engineeringprocess. It identifies how thecontractor/program office will tailor themaintainability program to meet requirementsthroughout the three major program phases Development, Production, and Operations/Support. Typically it contains the followingelements shown in Table 1"

Table 1. Elements of the MaintainabilityProgram Plan

Duties of each organizational dementinvolved in the accomplishment of themaintainability tasks cited in the productspecification or statement of work.

Interfaces between maintainability andother project organizations, such as designengineering, software, reliability, safety,maintenance, and logistics.

Identification of each maintainability task,narrative task descriptions, schedules, andsupporting documentation of plans fortask execution and management

Description of the nature and extent thatthe maintainability function participates informal and informal design reviews, andauthority of maintainability personnel inapproval cycle for drawing release.

(3) PROVIDE UNIFORM QUALITATIVEAND QUANTITATIVE MAINTAINABILITYREQUIREMENTS.

Maintainability design requirements areestablished from the Maintainability ProgramPlan and the derived maintenance concept.

These requirements are intended as rulessystem designers follow to meet overallprogram goals and objectives. They includemission, operational environment, and systemconcepts. They must be baselined early andnot changed unless absolutely necessary.

The requirements can include bothquantitative and qualitative values ofmaintainability parameters. Quantitativemaintainability requirements are usually theresult of maintainability allocations based onsystem availability and operational timingrequirements, with allocations made at eachlevel down to the replaceable module,assembly or component level as needed.Examples of quantitative requirements areshown in Table 2:

Table 2. Examples of QuantitativeRequirements

Maintenance manhours per operatinghour (MMH]OI-I)

Mean-Time-To-Repair (MTTR) Mean-Time-To-Restore-System

(M'rrgS) Fault detection and isolation of sub-

systems task times End item change out time Unit removal/installation times Availability

They may be established at any, or all, levelsof maintenance and can help definemaintenance criticalities and reduction of

necessary system components. Qualitativerequirements are used to accomplish twopurposes. First, they address maintainabilitydesign features which are vital in achievingthe maintainability goals, but cannot bemeasured. For example, elimination ofsafetywire/lockwire, standardization of

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Maintainability Program Management Considerations, Page 6Technique PM-2

fasteners, use of captive fasteners, and color-coding of electrical wiring are some basicqualitative maintainability requirements used onorbital programs. Second, qualitativerequirements are used to meet customer/program requirements and enhance themaintainability:characteristics of the system.Examples include specification of commonhandtools only for organizational andintermediate levels of maintenance, anddesigning so that only one skill level is requiredfor all organizational level maintenancepersonnel.

(4) EXERCISE PROGRAM CONTROL ANDE VAL UA TI ON.

and existence of these examples will enhancethe chance of program success (based onhistorical experience).

References

I. NASA Handbook 5300.4(iE),"Maintainability Program Requirements forSpace Systems, "March 10, 1987, NASAHeadquarters.

2. Air Force Design Handbook 1-9,"Maintainability (for Ground ElectronicSystems)," Second Edition, Revision 7,February 25, 1988, United States Air ForceAeronautical Systems Division.

The maintainability program must be an integralpart of the systems engineering process and alldesign and development activities. Activitiesinclude design reviews, development andimplementation of methods for assessingmaintainability effectiveness, dissemination ofmaintainability data, and proper implementationof program test and evaluation. Subcontractor/supplier control is also a key areas for programevaluation and monitoring.

3. ''Maintainability Engineering Design andCost of Maintainability, "Revision II,January, 1975, Rome Air DevelopmentCenter.

4. Reliability, Maintainability, andSupportability _S) Guidebook, 'SecondEdition, 1992, Society of Automotive

Engineers G- 11 International RMSCommittee.

SummaryProgram management's participation in thedevelopment and implementation of soundmaintainability practices on NASA programs isextremely important. Whether the programcontains ground based systems, or is orbital andbeyond, maintainability plays a key role insystem operations, providing for increasedsystem effectiveness and availability, and lowerlife cycle costs. The steps outlined above areguidelines towards success, and can be tailoreddepending on the type of program. However,the importance of a concurrent engineeringapT.,'oach and the existence of intimatep" _essional relationships betweenma:ntainability personnel and other systemsengineering disciplines can not be overstated,

Related Techniques

Technique PM- 1, "Benefits of ImPlementingMaintainability on NASA Programs"

Technique PM-3: "Maintenance Concept forSpace Systems."

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Benefits

Key Words

ApplicationExperience

TechnicalRationale

Contact Center

Develop a maintenance concept early in the program life cycle toprovide a basis for full maintainability support. It should be used toinfluence systems design to ensure that attributes for ease ofmaintenance, minimization of repair and down time, and logisticssupport will be present in the final design.

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Effective development of a maintenance concept can enhance theeffectiveness of maintenance support planning and aid both logisticsplanning and design of a maintainable system. The maintenance conceptcan also provide assessments of cost savings for maintenance activitiesand resources allowable at each maintenance level.

Maintenance Concept, Spares Requirement, Logistics Support,Maintenance Plan, Maintainability Requirements.

Space Acceleration Measurement System (SAMS), CombustionModule- 1 (CM- 1) Shuttle/Station Experiment.

The need to identify quantity, cost, types of spares, and relatedservicing techniques required to sustain a space system missioncapability is a prime driver in developing maintainability requirementsfor a space system at the onset of its design. A system maintenanceconcept should be developed to define the basis for establishingmaintainability requirements and to support design in the systemconceptual phase. The maintenance concept provides the practical basisfor design, layout, and packaging of the system and its equipment. Thenumber of problems associated with product support and maintenanceof space systems can be reduced, if not eliminated, by applying theprinciples prescribed in the system's maintenance concept.

Lewis Research Center (LeRC)

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Maintenance Concept for Space Systems, Page 2Technique PM-3

Maintenance Concept for Space SystemsTechnique PM-3

orbital space program where on-orbit andground maintenance is planned.

The maintenance concept provides the basis foroverall maintainability design requirements forthe program, and contains detailed planning ofmaintenance policy for the operational system.It establishes the scope of maintenanceresponsibility for each level (echelon) ofmaintenance and the pers0_el resources _ .......(maintenance manning and skill levels) requiredto maintain a space system. Early developmentand application of the maintenance concept instructuring the maintainability plan caneliafinate or reduce occurrence of problems thatm: interrupt system operation.

The maintenance concept for a new systemmust be systematically formulated during theearly conceptual design phase of a program tominimize maintenance problems during theoperational phase. This proactive approach isbeing used on Space Station-based experimentdevelopment programs at LeRC to incorporatecurrent Space Station Program supportprinciples, prescribed Space AccelerationMeasurement System (SAMS) and CombustionModule One (CM-1) operational and repairpolicy, and identified sparing requirements.

Elements

This maintenance concept will aid in logisticsplanning and will guide design by providing thebasis for establishment of maintenance supportrequirements in terms of tasks to be performed,frequency of maintenance, preventive andcorrective maintenance downtime, personnelnumbers and skill levels, test and supportequipment, tools, repair items, and information.Inputs to the maintenance concept shouldinclude: a mission profile, system reliability andavailability requirements, overall size andweight constraints, and crew considerations.The concept should support the followingdesign elements as they apply to a manned

Repair PolicyThe repair policy should consider thesupport to be provided at the maintenanceechelons (levels) summarized in Table 1.

Table 1. Echelons of Maintenance

Where

Performed

SystemMaintainer

Basis

Type of workaccomplished

OrganizationalMaintenance

On-orbit

Flight Crew

Repair and retainequipment

Inspect equipment

Remove andreplace modulesand ORU's

Adjust equipment

DepotMaintenance

ii

NASA Center orContractor

Center Engineersand Technicians

Repair and returnequipment tostock inventory

Repair atmodule, ORU,and componentlevel

Repair andmaintain groundsupportequipment

Calibrateequipment

Organizational MaintenanceOrganizational maintenance is maintenanceperformed by the using organization (e.g.,flight crew) on its own equipment. Thismaintenance consists of functions and repairswithin the capabilities of authorizedpersonnel, skills, tools, and test equipment.Organizational level personnel are generallyoccupied w_th the operation and use of theequipment,: and have minimum time availablefor detailed maintenance or diagnosticcheckout; consequently, the maintenance at

Page PM- 15

wthis level is restricted to periodic checks ofequipment performance. Cleaning ofequipment, front panel adjustments, and theremoval and replacement of certain plug-inmodules and Orbital Replaceable Units (ORUs),referred to as black boxes, are removed andforwarded to the Depot Level.

Depot MaintenanceDepot maintenance is maintenance performed atNASA Centers or contractor facilities for

completely overhauling and rebuilding theequipment as well as to perform highly complexmaintenance actions. The support includestasks to repair faulty equipment to the partlevel, if deemed necessary. This level ofmaintenance provides the necessary standardsfor equipment calibration purposes, and alsoserves as the major supply for spares.

@stem AvailabilityOperational Availability (Ao) is defined as theprobability that at an arbitrary point in time, thesystem is operable, i.e., is "up." It is a functionof the frequency of maintenance, activemaintenance time, waiting time, logistics time,administrative time, and the ready time of thesystem, and is expressed as:

UPTIMEA

o TOTAL TIME (1)

Where:

UPTIME = the total time a system is in anoperable state, and

TOTAL TIME = the combination ofuptimeand downtime, in which downtime is the time inwhich a system spends in an inoperable state.

Repair v_ Replacement PolicyNormally, on-orbit repair should not beperformed on any plug-in modules or 0RUs. If

Maintenance Concept for Space Systems, Page 3Technique PM-3

any on-orbit repair actions are planned, theyshould be clearly identified in the concept.At the organizational level, failed itemsshould be either discarded or sent to the

NASA Center or contractor for exchangeand repair in accordance with repair/discardpolicies identified in the systemrequirements. Corrective maintenance,limited to replacement of faulty ORUs andplug-in modules, should be specified to beperformed during the mission period. Primeequipment should be designed to have readyaccess for maintenance. Quick-openingfasteners should also be specified.

Level of ReplacementThe design for proper level of ORUdefinition should consider compatible failurerates for hardware parts within the sameORU. Relative ranking of ORLPs throughreliability and maintainability considerationsand mission criticality analysis can alsocontribute toward the proper level ofreplacement definitions. The required levelof replacement should be specified at theplug-in module and ORU levels.Maintenance and support of a system shouldinvolve two-tier maintenance echelons. The

first level provides for repair of the end-itemon-orbit by replacing select faulty ordefective plug-in modules and ORUsidentified through use of specified diagnosticprocedures. Faulty ORUs should then beevacuated to the second level of the

maintenance echelon (depot level), whichwill be at a NASA Center for repair ifdeemed necessary. The particular NASAcenter/facility should act as the depot forrepair of faulty items.

Skill Level RequirementsHardware should be designed to aid on-orbitand ground maintenance, inspection, andrepair. Special skills should not be requiredto maintain a system. The following design

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Maintenance Concept for Space Systems, Page 4Technique PM-3

features should be incorporated:

Plug-in module and 0RU design to minimizeinstallation/removal time and requirements forhand tools, special tools, and maintenanceskills.

Plug-in modules and ORUs should bedesigned for corrective maintenance by removaland replacement.

Plug-in module and 0RU designs requiringpreventive maintenance should be optimizedwith respect to the access, maintenance hours,and maintenance complexity.

Software and its associated hardware should

be designed so that software revisions/corrections can be easily installed on-orbit withminimum skill level requirements.

Flight crew training for payload flightoperation should identify hands-oncrewmember training, at the NASA centerwhere the system is built, to familiarizecrewmembers with the removal/replacement ofhardware.

Spares PhilosophyTwo basic types of spares should be required tosupport a maintainable system: developmentspares and operational spares. Developmentspares are those that must be identified andacquired to support planned system testactivities, integration, assembly, check-out andproduction. Operational spares are those sparesthat must be acquired to support on-goingoperations on-orbit.

The quantity of development spares requiredfor each system, and the total quantities tosustain the required availability during theplanned test activities, integration, assembly,and check'out test should be determined

according to the following:

Custom-made components/parts Long-lead time items

The quantity of spares required for eachsystem and the total quantities to sustain therequired operational availability on-orbitshould be determined according to thefollowing:

Items that are critical to system operation Items that have high failure rate Items that have limited life

In the initial spares provisioning period andto the maximum extent practical, sparesshould be purchased directly from the actualmanufacturer; i.e., lowest-tier subcontractor,to eliminate the layers of support costs ateach tier. The initial provisioning periodshould cover early test and evaluation, plus ashort period of operation, to gain sufficientoperational experience with the system. Thiswill provide a basis for fully competitiveacquisition of spares.

Spares with limited shelf life should beidentified and should be acquired periodicallyto ensure that adequate quantities of sparesare available when needed. Spares withexpired shelf lives should be removed andreplaced.

Procurement of spares should be initiated insufficient advance of need to account for

procurement lead time (administrative andproduction lead time).

The location of the spares inventory (on-orbit and on-ground) should be a function ofthe on-orbit stowage allocation capabilitiesand requirements. A volume/weight analysisshould be conducted to determine the

quantity and types of spare items necessaryto sustain satisfactoryoperationaiavailability. The volume/weight analysis shall

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assure available or planned payload volume andweight limits, and planned or available on-boardstowage area.

Breakout should be addressed during initialprovisioning and throughout the replenishmentprocess in accordance with NMI 5900.1,Reference 1. Breakout is the sparesprocurement directly from the originalequipment manufacturer, prime contractor, orother source, whichever proves most cost-effective. A spare item requirement list shouldbe maintained by procurement and technicalpersonnel.

Diagnostic/7"esting Principles and ConceptsThe system should meet the following failuredetection requirements as a minimum:

The system should have the capability todetect, isolate and support the display offailures to the plug-in module level. Crewobservations may be used as a method of failuredetection of the following: visual displays,keyboards/buttons, general lighting, speakers.

System design should provide the capabilityfor monitoring, checkout, fault detection, andisolation to the on-orbit repairable level withoutrequiring removal of items.

Manual override and/or inhibit capability forall automatic control functions should be

provided for crew safety and to simplifycheckout and troubleshooting.

All failures of the system should beautomatically detected and enunciated either tothe flight crew or the ground crew.

Accesses and covers should be devoid ofsharp corners/edges and be equipped with graspareas for safe maintenance activities.

Systems/subsystems/items should be designed

Maintenance Concept for Space Systems, Page 5Technique PM-3

to be functionally, mechanically, electrically,and electronically as independent as practicalto facilitate maintenance.

The concept should also describeoperating/testing techniques to identifyproblems and consider the complexity of thevarious types of items in the space systemand associated maintenance personnel skills(for all software, firmware, or hardware).The techniques will identify maintenanceproblems. In all cases of fault simulation, thesafety of personnel and potential damage tosystem/equipment should be evaluated in theconcept. The concept should request that asafety fault tree analysis be the basis fordetermining simulation. Also, a FailureModes, Effects, and Criticality Analysisshould be used to evaluate and determinefault simulation. Some of the fundamentalmaintenance actions to be evaluated,monitored, and recorded are as follows:

Preparation and visual inspection time Functional check-out time

Diagnostic time: fault locate and faultisolate

Repair time: gain access, remove andreplace, adjust, align, calibrate, and closeaccess

Clean, lubricate, service time Functional check-out of the repair action

Responsibilities for ContractorMaintenance

The prime contractor's maintainabilityprogram should provide controls for assuringadequate maintenance of purchasedhardware. Such assurance is achieved

through the following:

Selection of subcontractors from the

standpoint of demonstrated capability toproduce a maintainable product.

Page PM- 18

Developmentof adequate designspecifications and test requirements for thesubcontractor-produced product.

Development of proper maintainabilityrequirements to impose on each subcontractor.

Close technical liaison with the subcontractor(both in design and maintainability areas) tominimize communication problems and tofacilitate early identification and correction ofinterface or interrelation design problems.

Continuous review and assessment to assure

that each subcontractor is implementing hismaintainability program effectively.

Maintenance Concept for Space Systems, Page 6Technique PM-3

ground processing or maintenanceoperations. The rationale for supportingthese recommendations should include

factors such as reduction in groundturnaround time and operational supportCOSTS.

Allocation of Crew Time for MaintenanceAction: _: _Crew time for maintenance should be

identified in accordance with systemcomplexity, reliability, and criticality of theitems to the system and missionrequirements. Analytical methods existwhich can be used to prioritize and allocatecrew time for maintenance actions.

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Responsibilities for Payload MaintenanceDirector of field installations responsible forlaunch preparation, maintenance, or repairactivities should be responsible for maintenanceplanning and for providing the resourcesnecessary to support the efficient identificationof maintenance related problems in accordancewith system requirements. Theseresponsibilities include:

Implementing a system that will identify,track, and status problems related to routinemaintenance activities attributable to the designcharacteristics of flight hardware and sofcware.

Providing information for use in a datacollection system to improve the accuracy ofquantitative maintainability and availabilityestimates. This information can be used toidentify failure trends influencing reliabilitygrowth characteristics during design and tocommunicate "lessons learned" from groundmaintenance experience.

Recommending to the Program Manager,responsible for design and development of flighthardware/software, areas for designimprovement to increase the efficiency in

RefeYencg$ L

1. NASA Management Instruction, SpareParts Acquisition Policy, NMI 5900.1A,NASA Responsible Office: HM/ProcurementSystems Division, Washington, DC,November 6, 1992.

2. NASA Management Instruction,Maintainability and Maintenance PlanningPolicy, NMI 5350.1A, NASA ResponsibleOffice: Q/Office of Safety and MissionQuality, Washington, DC, September 26,1991.

3. NASA Handbook, MaintainabilityProgram Requirements for Space Systems,NHB 5300.4(1E), Reliability, :_ ........ _=_Maintainability, and Quality AssurancePublication, Washington, DC, March 10,19871

4. Space Acceleration Measurement System(SAMS) Experiment, SAMS-SS ProductAssurance Plan, SAMS-SS-005 (Preliminary),NASA Lewis Research Center, Ohio.

5. Space Acceleration Measurement System

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(SAMS) Experiment, Express PayloadIntegration Agreement, SAMS-SS PIA, NASALewis Research Center, Ohio.

6. Space Station Program, Space StationProgram Definition and Requirements, Sections3 and 4, SSP 30000, NASA Lewis ResearchCenter, Ohio.

7. Combustion Module One (CM-1) Experiment,Product Assurance Plan, NASA Lewis ResearchCenter, Ohio.

8. Blanchard, Benjamin S., Jr. and Lowery, E.Edward of General Dynamics, ElectronicsDivision, Maintainability Principles andPractices, McGraw-Hill Inc., N.Y., 1969.

Maintenance Concept for Space Systems, Page 7Technique PM-3

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|_Design Factors and

Engineering

The objective of the A4aintainabilityfunction is to influence system design such that the end product

can be maintained m a cost effective operational condition with minimum downtime. In order for

the Maintainability discipline to provide maximum influence to a program, design principles to

obtain these objectives must be implemented early in the design phase. Techniques that have proven

to be beneficial on previous programs are presented in this section as design recommendations for

future programs.

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ApplicationExperience

TechnicalRationale

Contact Center

The application of these guidelines to the design process will increasethe effectiveness of dexterous robots by allowing for optimized designof robotics components used during maintenance tasks. In addition,because Extra Vehicular Activity (EVA) tasks performed with robotsmust be simplified to accommodate robotics dexterity (which isintrinsically inferior to that of a human crew member), roboticallycompatible designs will facilitate the simplified (less time consuming)EVA tasks. This equates to less system downtime and higheravailability for both ground and on-orbit systems.

Robotically compatible; maintenance: fasteners; handling fixtures

International Space Station Program

The following selection guidelines enable design engineers to identifythe criteria required for robotics compatibility and to tailor theirspecifications to different robotics systems and environments. Theyprovide general concepts for using robotically compatible fasteners andhandling fixtures that have been applied on the Space Station programand states the advantages of these concepts.

,lohnson Space Center (JSC)

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Selection _f Roborically Comparibh, Fasteners and Handling Mechanisms', Page 2Technique DFE- 1

Selection of Robotically Compatible Fa._'tenersand Handling MechanismsTechnique DFE-I

Before designing an ORU or other componentfor robotics compatibility, the feasibility of suchan effort must first be assessed. Some ;tei-ns

(e.g., thermal blankets), because &theirflexibility, cannot be manipulated by roboticsSystemsl The assessment should show (i) if theORU or component can be manipulated by arobot, (2) if not, whether a major redesign ofthe_tem will be required to make it robotcompatible, and (3) what effect the redesignwill have on weight and cost (a factor that canbe d-etermined by simple ana]_yses),

Reference 1 describes a preliminary analysisthat might be used to determine the feasibilityof designing for robotics compatibility. Once itis determined that the item can be designed tobe manipulated by a robot, it must then bedetermined how the design relates to andaffects the design of(l) other components inthe system, (2) the system's layout, and (3) therobotics system with which it will interface

Provide for alignment. Avoid jamming and binding. Withstand the loads that may be imparted by

the robotics systems. Provide adequate access. Simplify the operation ....... Assist ORUal_gnment and S0_d6ck and

harddock functions. "Softdock" is defined as

the initial temporary attachment between twoor more pieces of equipinen(t_ pre_entinadvertent release prior to permanentattachment.

Reference 2 lists a number of guidelines andreqt!!ren2ents that may be applicable t 9designing for-iobotics Compatibility of SpaceStation hardware. Reference 3 lists a number

of different robotically compatible fastenersand handling fixtures for Space Station use.The purpose of this technique, however, is toassist designers in applying the stated conceptsto their system ORU's and not to listcontractual requirements. The six designobjectives for fastener and handling fixturedesign requirements are addressed in thetbllowing section.

Figure 1, which illustrates the process tbrredesigning for robotics compatibility asdetailed in Reference 1, shows the sequence bywhich the design of items higher in a processflow impact the design of the lower items.Although the sequence may be altered, thealteration may result in increased costs, inschedule delays, and in less flexibility inapplying robotics compatibility. Thebidirectional arrows indicate processes thatshould be performed using an integratedapproach that considers the impacts the ORU,system, and robot design have on each other.Once the above mentioned analysis isperformed and design of the roboticallycompatible fasteners or handling fixtures isbegun, the objectives then must be to:

FASTENER AND HANDLING FIXTUREI)ESIGN REQUIREMENTS

Provide for alignmentAlignment provisions may be implemented as(1) markings, (2) alignment guides, and (3)design of the robotics system and its controlsystem Only the second of these options,alignment guides, is addressed in this section.Markings and robotics system designs aredescribed in References I, 2, and 3.

Fa,_teners

There are more options available for aligningfasteners than there are for handling fixtures.For example, fasteners are captive and are anintegral part of an ORU. Therefore, if theORU contains proper alignment features and is

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properly aligned and inserted, the fasteners willbe properly aligned as well. However, sincehandling fixtures are grappled independent ofthe insertion and alignment of the 0RU, the

incorporation of alignrnent features is confinedto the fixture and end effector. The ORU

alignment feature design, which is discussed inReferences 2 and 3, is an important

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Selection of Robotically Compatible Fasteners and Handling Mechanisms, Page 4Technique DFE-1

consideration, since it can lessen fastenercomplexity. The alignment techniques beingused for Space Station fasteners are describedbelow.

Alignment of Tool to Fastener HeadRobotic testing has shown that, provided thereis proper visual contrast between the fastenerhead and the surrounding structure, a 7/16- inchfastener with a flat head can be easily capturedby the robotics end effector (nut driver).Earlier concepts specified or recornmendedrounded heads because it was believed therounded head would accommodate greatermisalignment tolerances. It was found,however, that a flat-headed fastener providedthe robot with the same misalignmenttolerances as the same fastener with a rounded

top.

Alignment of Fastener to NutThe bolt is aligned to a nut by tapering the end(pilot) of the bolt and by having a cone orcountersink around the nut. For fasteners thatform an assembly or that are, in Space Stationterminology, "attachment mechanisms," thereare housings which contain tapered "fingers."

Handling FLvturesThe two alignment techniques for Space Stationhandling fixtures are described below.

V-slot Insertion

The V-slot insertion technique is used with themicrofixture and H handle, which interface withthe Special Purpose Dextrous Manipulator(SPDM) end effector or the ORU toolchangeout mechanism (OTCM). The OTCMfits as a V into the grooves of the H handlecloses its V-shaped grooves around the cornersof the microinterface (see reference 2 for adetailed description). The positionalmisalignment tolerance allowed for the Hfixture is approximately 0.5 inch with angularmisalignment tolerance of about +2. The

microfixture allows positional misalignments ofabout 0.3 inch and angular misalignments ofabout +3

Cylinder-over-coneThe microconical tool slips over and attachescollets to the microconical interface, which isshaped like a cone. The allowable translationaland angular misalignment tolerances for themicroconical tool are 0.25 inch and +1 ,respectively

A VOID .lAMMING AND BINDING

Fltsteil ers

Once alignment is accomplished and thefastener begins to enter the nut, th_e_ris stillthe possibility of cross-threading Cross, _threading can be avoided by aligning the nutusing the unthreaded portion on the bolt, and itcan also be avoided by using an expandabiethread diameter nut; i.e., a Zipnut. A Zipnutconsists of three separate segments within ahousing that, when assernbled, form theinternal threads of a nut. The segments areheld against the threads of a bolt or screw bysprings that force them to a minimal diameter,and a ramp that allows them to separate orcome together, depending on the direction inwhich the bolt is inserted. When a bolt is

inserted, the segments are allowed to slideback and away, allowing the b01t to slidethrough without obstruction. This type of nutis described in detail in Reference 2.

Handling FixturesWhen using robotically compatible handlingfixtures which apply the slot in the V-grooveconcept as described above (i.e., themicrointerface or X handle), care must betaken that the corners are rounded. This

precaution must be taken to keep the handlefrom binding to the end effector, as happenedin t]ie JSC _?obotlcslabora;cories with the firs(H handle concept which had sharp corners.

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Selection of Rohofically Compatible Fasteners and Handling Mechanisms, Page 5Technique DFE-I

The corners of the H handle (renamed the Xhandle) were rounded, and the binding effectwas thus eliminated.

WITHSTAND LOADS THAT MAY BEIMPARTED BY ROBOTICS" SE._TEMS FORFASTENEIL9 AND HANDLING FIXTURES

SSP 30000, table 3-3, "Factors of Safety,"specifies that for metallic flight structures, thegeneral factor of safety is a yield of 1.25 and anultimate of 2.00.

PRO VIDE ADEQ UA TEA CCESS

Fasten ersAdequate access for fasteners is provided bydesigning a proper layout of the system asdescribed in reference 3. The fastener selection

(or fastening scheme) can be influenced by therobotics access if more than 1 degree offreedom is required by the robot to engage anddisengage the fastener. A lever, for example,requires more than 1 degree of freedom andtherefore requires significantly more accessspace to operate than that required to engage abolt. In addition, the higher the torque value,the larger the end effector (motor), lesseningthe allowable robotics access space. For SpaceStation, no levers will be used by robots.

Handling FLvturesCertain small Space Station ORU's are beingplaced so close to each other that inadequateaccess space is provided for the robot to openits jaws around the interface. The problem wasresolved by using the microconical interfacethat snaps around the interface in a "stabbing"motion. By using a tool that does not requirejaws to open around an interface: i.e, themicroconical tool, the required access space issignificantly reduced.

Simplify the Operation FastenersThe robotics operation can be simplified by the

following methods:

Use Captive FastenersUse of captive fasteners is the best method forsimplifying robotics operation. This eliminatesthe need for the robot to carry and insert thefasteners and thus increases the probability ofmission success.

Reduce Number of OperationsThe type of fastener selected can reduce thenumber of operations required. For example,using the Zipnut eliminates the need forrotation, since the bolt can be slid through thenut and then tightened with a single rotation.

Choose Proper Forms of FasteningForms of fastening that require the robot touse more than 1 degree of freedom should beeliminated. Levers, for example, not only willincrease the access space requirements (asdescribed previously), but may alsonecessitate force moment accommodation andmore complex control software.

A void Fasteners Requiring Excessive TorqueTo engage fasteners that require excessivetorque (ie., 50 foot-pounds or over), the robotmust stabilize itself with one arm, constrictingthe allowable configurations for removing andreplacing the ORU. This necessitatesadditional hardware for robot stabilization. In

general, care must be taken when using roboticsystems for fasteneing due to the reactionforces that will be present.

Reduce Sizes and T.vpes of Fastener HeadsUsing different sizes and types of fastenerheads will reduce the number of tools requiredby the robot.

Handling FbcturesThe grasping of the interface can be simplifiedby allowing the robot to grasp the interfacefiom a number of different orientations. For

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Selection of Robotically Compatible Fastener_" attd Handling Mechanisms, Page 6Technique DFE-1

example, the microinterface and themicroconical interface can be grasped from twodifferent orientations of the OTCM relative tothe handling fixture, while the X handle canonly be grasped from one orientation, Theremay be some instances, however, in which itwould be advisable to limit the allowable

orientations. For example, if the robot cangrasp an ORU from only one orientation, thereis less chance that the ORU will be improperlyinserted in its base plate.

ASSIST ORU ALIGNMENT ANDSOFTDOCK AND HARDDOCKFUNCTIONS

Fasteners

When designing robotically compatible ORU's,the alignment guides and softdock features maybe incorporated as part of the ORU, orfasteners with these features may be designedor selected. Sofldock fasteners are thus more

complex and are called "attachmentmechanisms" in the Space Station Program.Alignment and sofldock functions are describedbelow.

Alignment FunctionsIf alignment features are lacking for the ORU,they can be incorporated via the tapering ofpins, or fingers, located on the housings of theattachment mechanisms.

Softdock FunctionsFor the Space Station Freedom Program,attachment mechanisms achieve sofldock either

through the use ofdetents that are housed onan outer casing of the attachment mechanismsor via the Zipnut method. The Zipnut isramped such that if an attempt is made toseparate the bolt from the nut, the segments arepulled together allowing the bolt to be remo\,edvia rotation only. The Zipnut thereby functionsas an excellent sofidock attachment.

Handling FL,cturesAlignment and sofldock functions aredescribed below.

Alignment FunctionsThe location of the handling fixture cansignificantly impact ORU alignment. Thefurther the handling fixture is from the ORU'scenter of gravity, for example, the mote -difficult it is for the robot to maintain a line of

insertion that will be perpendicular to itsattachment plate.

Other factors to be considered when placinghandling fixtures are the size of the ORU, thelocation and type of alignment guides, and theplacement of fasteners. These items arediscussed in Reference 3 because of theirdependence on ORU features.

So.fidock FunctionSoftdock features may be used to prevent anORU fi'om "floating away" prior to its beingfastened. This may also be achieved byfastening the ORU without releasing thehandling fixture. The three above mentionedhandling fixtures for Space Station have holesin their centers for fasteners, which allows theOTCM to grasp the ORU, insert it, and thendrive the bolt with its nut driver without ever

releasing the ORU handle.

Re[erences

1. t?ohoticx System_ htter/'ace Standards,I ?drone l, Robotics A ccommodationRequirements (Draft), SSP 30550.

2. Rohotic'x ,S);stems Interlace Standard.',}drone 2. Robotics Interface Standards"

(Draft), SSP 30550.

3. 7he [)e.s'i_l Proce.s:sfor AchJevJngRobotics (;ompatibJlity, Contractor ReportNo. .

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False Alarrn Mitigation Techniques, Page ITechnique DFE-2

Technique

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Benefits

Key Words

ApplicationExperience

TechnicalRationale

Contact Center

Minimize the occurrence and effect of Built In Test (BIT) false alarmsby applying principles and techniques that are intended to reduce theprobability of false alarms and increase the reliability of BIT in avionicsand other electronic equipment.

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False Alarm Mitigation Techniques, Page 2Technique DFE-2

False Alarm Mitigation TechniquesTechnique DFE-2

the test data reported while only requiring asingle computer or processor.

In order to mitigate false alarms, a system'sBuilt In Test (BIT) circuitry must be able tocope with a limited amount of anomalousperformance. NASA Handbook 5300.4 (1E)defines a false alarm as "an indicated faultwhere no fault exists." Based on this definition,this technique is concerned only with BIT

_" _nd]cadons0f system mai_ncdoh-ffhi_'-cause

Continuous MonitoringContinuous monitoring with BIT filtering canbe used in place of the voting scheme. Withthis technique, BIT results are based offaintegration of successive measurements of asignal over a period of time instead of a singlecheck of the signal. The monitoring of thesignal does not have to be continuous but onlysampled over the time period. -The-fi-lteH