Apollo Experience Report Electrical Wiring Subsystem

8/8/2019 Apollo Experience Report Electrical Wiring Subsystem

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N A S A T E C H N IC A L N O TE NASA TN D-7885

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r 8 . I L ECOPY

APOLLO EXPERIENCE REPORT -ELECTRICAL WIRING SUBSYSTEMLyle D. WbiteLyndon B. Johnson Space CenterHouston, Texas 77058N A T I O N A L A E R O N A U T I C S A N D S PA CE A D M I N I S T R A T I O N W A S H I N G T O N , D . C. M A R C H 1975


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1 . Repor t No.NASA TN D-7885APOLLO EXPERIENCE REPORTEL EC TRIC A L WIRING SUBSYSTEM

2. Government Accession No, 3. Recipient's Catalog No.

March 19756. Performing Organization Code

4. Title and Subti t le

~ JSC-07618

5. ReDort Date

7. A u th o r ( s )Lyle D. Whi te

2. Sponsoring Agency Name and AddressNat iona l Aeron au t ic s and Space Admin is t ra t ionWashington, D.C . 20546

8 . Performing Organization Report No.1 JSC S-413

13. Type of Report and Period CoveredTechnical Note14 . Sponsoring Agency Code

10. Work Un i t No.914- 11-00- 00- 72. Performing Organization Name and Address

U n c l a s s i f i e d

Lyndon B. Johnson Space CenterHouston, Texas 77058

Uncla ssified 14 $3.25

11 . Contract o r Grant No

5. Supplementary Notes

6. Abstract .Th e genera l r equ i rem en ts of the e le c t r ica l wi r ing subsys te ms and the p rob lem a re as and solu tion!tha t occur red du r ing the ma jo r pa r t o f the Apol lo P rogr am a r e de ta i led in th i s repo r t . The con-cep ts and de f in i t ions of spec i f ic requ i rem en ts fo r e lec t r ica l wi r ing ; wi re -connec t ing dev ices ; andwi re -h a rn ess fab r ica tion , checkou t , and ins ta lla t ion techniques are d iscu ssed . The des ign anddeve lopmen t of e l ec t r ica l wi r ing and wi re-connect ing dev ices a r e desc r ib ed . Miss ion pe r fo rm-a n c e is d iscus sed , and conc lus ions and recommenda tions fo r fu tu re p rogra ms a r e p resen ted .

7. Key Words (Suggested by A u t h o r ( s ) ) I 18. Distr ibu t ion Sta tementWiringC o n n e c t o r sInsula t ionH a r n e s s e sSTAR Subject Category:12 (Astronautics , General)

19. Security Classif. (of this r e p o r t ) I 20. Security Classif. (o f this page) I 21 . NO. of Pages I 22 . Price' For s a le by th e N at i ona l T ec h n ic a l I n fo rm a tion S e rv i ce , S p r i n g f i e ld , Virginia 22151


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APOLLO EXPERIENCE REPORTEDITORIAL COMMITTEE

The mater ial submitted fo r the Apollo Experience Reports(a s e r i es of NASA Technical Notes) wa s reviewed and ap-proved by a NASA Editorial Review Board at the Lyndon B.Johnson Space Cen ter cons isti ng of th e following m em be rs :Scott H . Simpkinson (Cha irman ), Richard R. Baldwin,Ja me s R. Bates, William M . Bland, J r . , Aleck C. Bond,Robert P. Burt, Chr is C. Critzo s, John M. Eggleston,E. M. Fields, Donald T. Gregory, Edward B. Hamblett, J r . ,Kenneth F. Hecht, David N . Holman (Editor/Secretary),and Car l R. Huss. The pri me reviewer for this reportwas William M. Bland, J r .


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CONTENTS

Section Page

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .UMMARY 1INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. . . . . . . . .ONCEPT AND DEFINITION OF SPECIFIC REQUIREMENTS 2

Electrical Wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Wire-Connecting Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. . . . . . . .arn ess Fa brication, Checkout, and Installation Techniques 2

DESIGN AND DEVELOPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . 3Electrical Wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Wire-Connecting Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Har nes s Fabrication , Checkout, and Installation . . . . . . . . . . . . . . . 8

MISSION PERFORMANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Elect rical Wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Wire-Connecting Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

CONCLUDING REMARKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Electrical Wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Wire-Connecting Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

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FIGURES

Figure1234567a

Command module wir e construc tion . . . . . . . . . . . . . . . . . .Lunar module wir e cons truction . . . . . . . . . . . . . . . . . . . .Lunar module c rim p spl ice (cutaway)Lunar module sold er splice (cutaway)Crim p connector contact (cutaway) . . . . . . . . . . . . . . . . . . .Lunar module connector and sp lic es (sho rtes t path to ground)Lunar module solder splice . . . . . . . . . . . . . . . . . . . . . .Typical modular terminal board . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . .

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APOLLO EXPERIENCE REPORTELECTRICAL W IR I NG SUBSYS TEM

B y L yle D. W h i t eLyndon B. Johnson S p ac e C e n t e rS U M M A R Y

The gen era l requir eme nts of the el ectr ica l wiring sub syste ms and the problemareas and solutions that occu rred during the major par t of the Apollo Prog ram a r edetailed in th is re po rt . Thick-wall Teflon wiring insulation was used initially in theBlock I command and s ervi ce modules but was changed to thin-wall Teflon fo r Block 11.This change resu lted in subst antia l weight savings, although the thinner insulation wasmo re susceptible to handling damage. These and othe r related proble ms wer e consid-era bly reduced by the implementation of specia l train ing pro gra ms. Thin-wall polyi-mide tape -wr ap (H-film) wiring insulation was used in the lunar modules, and a 26-gageminimum wire si ze was used to achieve an overall weight savings. Initial handlingpro blems subjected luna r module wiring to considerab le damage, but implementationof specia l tra inin g pr og ra ms and incorporation of high-strength all oys fo r the 26-gagewire-conductor mat eri al reduced these and other related probl ems to an acceptablelevel. Exper ience gained durin g the Apollo Pro gra m indica tes that the use ofpolyimide-film insulation, nickel-plated wire, and high-strength alloys fo r small-gagewire will sa tisfy the requi remen ts of most spac ecraft environments. Additional reli-ability and weight savings are expected in future pr og ra ms through investigation,re se ar ch , and trade-off studies in the ar ea s of flat conductor cables and lightweight,high- temper ature insulation and conductor materials . Th is subsystem performed welldurin g the many and various spac ecraf t missions.

I TRODUCTI O NConsiderable experience was gained with the elec tri cal wiring subsy stems duringthe Apollo Pr og ra m in the proc es s of estab lishing requ iremen ts, procuring components,

implementing proce dures fo r fabrication and installation, and re solving problem areas.The purpose of thi s re po rt is to re cor d thes e experi ences fo r the possible benefit offu tu re spac e prog ra ms . Becau se of the la rge amount of wiring used and its impact onweight, volume, and the function of other sub sys tems, t he impor tance of maintainingelectrical wiring and connecting devices a s a subsystem cannot be overemphasized.Specifically, the command and ser vi ce module (CSM) contains approximate ly3 3 700 me te rs (110 500 fee t) of wir ing that weighs approximately 600 kilog ram s(1330 pounds). The CSM wiring comp ri se s approximately 30 000 wire segments and


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60 000wire connections.22 900 meters (75 000 feet) of wiring that weighs approximately 200 kilograms(460 pounds) and c om pr is es approximately 20 000 wire segments and 40 000 wireconnections.

Similarly, the luna r module (LM) contains approximately

A s an aid t o the reader, where nec essa ry the original units of meas ure have beenconverted t o the equivalent value in the Systgme International d'Unit6s (SI). The SIunits are written first, and the o riginal units are written parenthetically thereafter.

CONCEPT AND DEF INIT IO N OF SPE CIFIC REQUIREMENTSThis commentary includes electrical wiring, wir e -connecting devices, and ha r-ne ss fabrication and installation techniques f o r both the L M and the CSM.

E l e c tr ic a l W i r i n gThe gener al conductor r equ ire men ts provide that the conductor be plated copperwith a multistrand construction such that ductility, elongation, and br it tl en ess do not

affect flexibility or ease of handling. Other pro visions include a maximum operati ngtem per atu re of 478 K (400" F) or insulation mat eri als , specific conductor resistances,and sizing tolerances.The numerous insulation requi reme nts include general cat ego rie s of flexibility,abrasion and chemical resist ance, dielec tric properties, and flammability. Therequired upper operational tempe ratur e limi t of 478 K (400"F) s maintained by theprop er sizi ng of the circuit-p rotection d evices.

W i e - C o n n e c t i ng D e v i c e sWire splices are either crimp or solder types. The genera l requi rement s includediel ectric properties, flammability, tensi le strength , and voltage -drop chara cte ris tic s.Terminal boards are eithe r the termin al stud o r the modular plug-in type. The general

requi rement s include diele ctric prop erties , flammability, environment al sealing, andvoltage-drop charact eristi cs. Connectors are either the cri mp or solder type. Thegenera l requirements include dielec tric prop erties , flammability, environmental seal-ing, and voltage-drop ch arac ter ist ics .

H a r n e s s F a b r i c a ti o n , C h e c k o u t , a n dI n s t a l l a t i o n T e c h n i q u e s

The general requ iremen ts for har ness fabrication include determination of config-uration, length, location of chafing protection, spacing of ha rn es s ties, and harnessidentification. The general requir ements for ha rn es s installation include maximumspacing between harn ess sup port s, routing fo r chafing protection , separ ation of wiringto preclude electromagnetic inter feren ce, and specific spacing between har ne ss es andfluid lines. The gener al req uir eme nts include el ect ri cal checkout of the ha rn es s befo re2


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and af ter installation to as su re continuity and diel ect ric integrity. Three-dimensionalhar nes s tooling boards and mockups a re essential to as sur e p roper w i r e lengths andharness f i t in the flight vehicle.DES GN AND DEVELOPMENT

Electrical W ir ingConductors. - The conductor requir ements fo r the CSM we re m et by selecting

nickel-plated copper as the basic material. This materia l makes soldering to connectorpins m ore difficult; as a result, most CSM connections are crimped rathe r thansoldered. Twenty-four-gage wir e was designated as the minimum gage wire to be usedfor vehicle hwnesses.The conductor r equire ments for the LM were met by selecting silver-plated

copper as the basic material. Thi s materia l combination is mor e susceptible to oxida-tion than nickel-plated copper but is slightly lower in resistance values. A few casesof se ve re oxidation (red plague) were noted; however, reasonable ca re during ha rn es sfabrication, such as disallowing the use of hygroscopic cleaning sol ven ts and limitingthe use of liquid fluxes, effectively eliminated this problem.

Twenty-six-gage wir e wa s selec ted as the minimum size fo r LM vehicle ha r-nesses. Because of breakage probl ems in handling th is s ize wire, 22-gage wir e wasdesignated as the minimum size for dis play panels, and 26-gage high-strength wire wasprocured for use in other vehicle harnesses . This change in wire si ze requireme ntswa s effective in LM-9 and subsequent vehicles. Because the panels we re removedmany ti me s fo r rework , exc ess ive handling caused cons iderable breakage of the 26-gagewir e, but the breakage w a s reduced to an acceptable le vel by the change to the 22-gagewir e. The 26-gage high-strength wire was selected fo r genera l vehicle usage pri mar ilybecause of weight considerations. This change a lso conside rably reduced the wir ebreakage problem. A s on the CSM, most connections on the LM are crimped ratherthan soldered, although solder connections were used in areas where ambient tempera-tu r es would not weaken the solde r connection. Crimping was selected for the LM pri-mar ily because the pr oc es s can be controlled more reliably than soldering. Becauseof the e mphasi s on weight savings, the use of aluminum condu ctors w a s investigatedea rl y in the Apollo P rogra m. The investigation revealed little development in te rmin a-tion techniques and insufficient airc ra ft or spacecraft usage histo ry; therefor e, anyfu rt he r considerati on of using aluminum was discontinued.

Insulations. - Off-the-shelf 254- t o 381-m icrom eter (10 to 15 mil) wall thickness(thick-wall) Teflon insulation w as used in the CSM Block I vehicles to satisfy insulationrequir ements (fig. 1). Various insulation materials wer e te ste d and evaluated t o finda l ighter weight materia l. A s a result , a thin-wall insulation thickness of 1 7 7 micro-meters (7 mils) of Teflon and 13-mi crome ter (0 .5 mil ) polyimide coating was impl e-mented for all Block I1 vehicles. The Teflon insulation ha s good temper atu re,die lec tric , abrasion -res istanc e, and chemical-resistance prop erti es. The polyimidecoating w as added to provide additional abras ion re si st ance and to inhibit the tendencyof Teflon to cold flow. This par ticu lar construction is avail able under RockwellInternat ional /Space Division Specification MBOl50-035.3


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Conductor: nickel-coated copper strands Some pro blem s assoc iated with thi swiri ng wer e breakage, insulation damage,and r adial cracking of insulation. Breakageand insulation damage wer e caus ed by mis -handling; however, se ve ra l handling proce-du re s were effectively implemented t o red uce

Polyimide dispersion coat the occurrence of these problems. Thes eeflon extrusionpro cedu res included fabrication of speci alhandling fix tures and special cover s for ha r-ne ss ends, specific empha sis on careful

Figure 1. - Command module wir econstruction.handling, .and promotion of quality awarenessamong the harn ess fabricators. Radial

crack ing of the insulation resulte d fro m incomplete cur ing at elevated tempe ratu re dur -ing application of the insulation to the conductor. When th is problem .was first detected,a bend test was conducted to determine whether the insulation had been adequately cured.This test consisted of wrapping w i r e around a mand rel that had a diameter four t imesthat of the wire . An improperly cure d wi re would develop cr ac ks in the outer su rfa ceof th e polyimide coating. The bend te st wa s later found to be inadequate, and a"crazing" o r solvent tes t was implemented th at consisted of wrapping the wir e arounda tapered mandrel and immersi ng it in a solution of no rm al methylpyrro lidone. Anundercu red coating will appear cra zed , wh ere as the properl y cured coating will not.The solvent test wa s adopted as a standard test in the Apollo Prog ram f or th is typeinsulation.

Throughout the LM program, a tape-wrap insulation was used that co nsisted oftwo oppositely wound layers, each with a 50-percent overlap. Each wrap was a bonded,laminated tape made fr om l ay er s of Teflon and a polyimide film. After application ofthe o uter tape-w rap on the conductor, a dispersion coating of Teflon was applied overthe tape to provide an environmental seal and a chemical ly res is tant bar r ie r for thepolyimide film (fig. 2). This particular construction is available under specifica-tion MIL-W-81381.

Conductor: silver -coated copper strands

Wrap no. 1 (ri ght hand): helica lly laid, laminatedFEP'lpolyimide tape (H-film)

' Jrap no. 2 left hand): helica lly laid,

laminated FEPlpolyimide tape (H-film)

Dispersion coat: Teflon (FEP)'FEP - Fluorinated ethylene propylene

Figure 2. - Lunar module wireconstruction.

Chemical resistance was an importantfa ctor in both the CSM and LM wiri ng beca useof the pres enc e of engine fuel s, oxides, andwat er glycol. Teflon is impervious to thesechemi cals, but the polyimide readily degr adeswhen exposed to the engine fuel s. Becau se thepolyimide coating on CSM wire w as added p ri -maril y for abrasion resistance, degradation ofthe oute r coatin g did not significantly affect thetotal insulation charac teris t ics. The Tefloncoating over the LM tape- wrap insulation wasvery criticalbecause it protected the polyimidefi lm in the tape, which would readily deg radeand per haps unravel sufficiently to expose theconducto r. However, in both the LM andCSM, elabo rate precautio ns wer e taken toguard against spil ls or leakage, and adequatecleaning procedures were established in ca sea problem occ urre d during ground serv icing.

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In the ear ly st ag es of the progra m, separatio n of the la ye rs was a problembeca use the exposed polyimide fi lm was susceptible t o the LM engine fuels. Insufficientcuring time during the fabrication pro ces s again caused the problem. Other problemsincluded breakage , insulation damage, and low surface- insulati on res ist anc e. Break-age and insulation damage were caused by mishandling, and handling pro ced ure s si mi la rto those implemented f or the CSM effectively reduced the oc curr ence of those prob lems.Breakage o r insulation damage (o r both) were checked before and after h ar ne ss in stal -lation by a continuity check and a 100-megohm insulati on/resis tance check. Har nes sesfailing to meet the specified requ ire men ts were replace d.

The problem of low surface-insulat ion resi stan ce o ccu rred when the black colo r-ing added to the disp ers ion coating wa s found to contain an exc es s of carbon, whichmade the coating less re si st iv e than the minimum specification value of 5 megohms.Th is condition wa s not a problem in the 28-V dc o r 115-V ac power circu its, and anextensive cir cui t analy sis indicated th at this can become a potential problem only in thecaution and warning sensing ci rcu its by shunting the ir sign als through a 4-megohm (orless) resi stan ce path to ground. A resistanc e grea ter than 4 megohms does not affectthese signals. Th is path to ground can only occur when the conductive coating co mesinto contact with the wire conductor and then mak es contact with ground through a resis-tanc e path of 4 meg ohms (o r less). A s shown in the cutaway view s in figu res 3, 4, and 5,

Wire,-Possible point of contact7uter metallic sleeve

LDispersion coat' L -film insulationInner metallic barrelFigure 3. - Lunar module cr imp spli ce

(cutaway).

"Wicked" solder (i n contact with dispersion coat)

Dispersion coatH-film insulation Solder band

the coating may make contact with the con-ductor through the metalli c porti ons of cr im pand solder splices. If the wire conductorstrands are "birdcaged" by a compressiveforc e applied axially along the wire , thecoating may al so make contact with connec-tor contacts. Figure 6 shows that, with theheat - shrinkable -type tubing ins tall ed ove rall the splices, the shorte st possible dis-tanc e to ground fro m the conductor throughthe conductive coating is 1.27 centimeters(0.5 inch). If an adjacent wire is atground potent ial, the path to ground is evengreater . The case is si mi la r with LM con-nectors (fig. 6 ) , al l of which a r e potted.Much of th is wire w as re turn ed to the ven-d o r ; however, some wire was already in-stalled in s eve ral vehicles. Because theresi stan ce value of th is "shortes t d istanceto ground" wa s found to be gr ea te r than the4-megohm suscept ibil ity lim it , no vehiclewiring had to be changed. Acceptance test-ing did not originally include surface-insulation resistance; however, a changewas implemented to include th is test ing forthe LM program. Th is problem did notoccur in the CSM program.Figure 4 . - Lunar module sold er spli ce

(cutaway).

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7Wire7ontact crimp barrel

/ Black dispersion coat

. 4 l- P s l e e v i n g r1.27 cm (0.5 in.) SpliceL ontact I onnectorPottingmH-film insulation

Figure 5.- Cr im p connector contact(cutaway).Contacts

Wire-Connecting Devices 1.27 cm (0.5 in.)Maximum:5 . 56 cm (2.19 in.)Nnrninal.._.. .W i r e splices. - Crimp splices rather 2.54 cm (1 n. )than sold er splices were sel ected for theCSM to achieve better compatibility withnickel-plated wi re . Approximately sp li ce s (s ho rt es t path to ground).250 splices were used on the CS M; thebalance of "wire splicing" was accom -plished by modular plug-in-type ter min al boa rds . Becau se of empha sis on proceduresthat included sev er al specific inspection points, sam ple pull t es ts , and daily calibr ationof crimping handtools, relatively few proble ms have occurr ed with t h x e splice s. Pullte st s detect such problems as defective c rimp tools, incorr ect wire and ferr ule s ize s,and some operator er ro rs . Probl ems early in the progra m included inadequate crimp-ing , which allowed the wi re to pull out, and ove rcr imp ing , which crus he d the wi restr and s and made them brittle and more susceptible to breakage.

Figure 6.- Lunar module connector and

Splices were selected for the LM as the pri ma ry m ea ns of joining two or morewires to a common point. Ear ly vehic les had appro ximat ely 2500 splices, but in latervehicles t his number w a s reduced to approximately 1300. Early vehicles also usedmore solder splices than crim p splices, but the lat er vehicles reve rte d to approxi-mately 900 crimp and 4 0 0 solder splices.L M silver-plated wir e. (See fig. 7 for typical solde r splice. ) A major problem withsolder splicing occurre d early in the program after a considerable number of spliceshad already been installed in s ev era l vehi-cles. A lar ge number of the se sp li ce sfailed during the solde r- splice qualificationtesting because the so lder joint was ei therunderheated o r overheated. In some cases,this resu lted in a weak joint that would notpa ss preesta blished values for pull testing.An extensive requalification program andimplementation of more comprehensiveinspection and training pro ced ure s even-tually resolved this problem f or L M - 4 andsubsequent vehicles. Those spl ice s alreadyinstalled and accessible were reinspected,and sev era l splices wer e removed and pull

Eit her type of s pli ce is compatible with the

Figure 7 . - Lunar module solder splice.

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test ed to deter mine their validity. Lunar module 1 wa s to be used fo r an unmannedtest flight and LM-2 became a ground test vehicle. The LM-3 vehi cle wa s scheduledfor an Earth-orbital test flight, within easy res cue by the command module; there fore ,crew safety would not be jeopardized. Lunar modules 1 and 3 we re flown succ ess full yafter completion of only minor rework.Late r in the program , an electronically controlled, presettab le soldering gun wasused in an attempt to fabrica te a more consistent solder splice, thus reducing fabrica-tion ti me and producing a high er quality splice. However, t hi s att empt was not suc-ces sful because of th e larg e number of vari able s such as the amount of s ol de r i n a

splice, the number of wir es, the wire size, and the co lo r of w ir e insulation.Terminal boards. - Modularized plug-in-type ter min al bo ar ds were s elected for

the CSM as the pr im ar y method of joining two o r more wires at a common point (fig. 8) .Typica l problem s included i mpro per seating of pins into the board and inte rmittent orno contact between the p ins and the bus bar. The latter and most seri ous problem w ascaused by component dimensional to ler an ces being too great. Changes were made bythe vendor to cor re ct thes e tolera nces , but the potential problem wa s not flagged bythe vendor and a number of bo ards susceptible to this problem wer e install ed on vehi-cles up to the spac ecra ft fo r Apollo 16. Because of ci rcu it crit ical itie s, a few ter mi-nal boa rds we re repl aced with the newer boards that had the design modification toeliminate the susceptibility to this problem. Stri ct control ove r vendor changes mus trwire

Environmentalseal (silicone)

Modulehousing

SocketBus bar

Gasket( s i cone)

ModuleModule cross section base(pi n and retention socket)

8-pin modular terminal board4-pinR-pin bus configuration

Figure 8. - Typical modular terminalboard.

be maintained to preclude th is type problem.These boards were also X-rayed after thewiring was installed to help verify prop erseating of the plug-in pins.

Connectors. - The connectors selectedfor the CSM had c rimp-typ e, removabl e con-ta cts with a rear environmental seal thatdid not re qu ir e potting. The connect orsselected fo r the LM had cri mp-type contact swith a rear environmental seal and addedpotting fo r additional sealing. The pottingwas added pri mar ily because of the conc ernthat the environmental seal would not ade-quately se al around the LM thin-wall, tape-wrap wir e insulation. Subsequent tesi ingproved that the environme ntal seal aroundthe tape-wrap insulation was adequate; how-ever, the potting w a s still used as an addedprecaution.

Bent pins, rec ess ed contacts, and sealdamage were the most common problemsand were typ ical of both the CSM and LMprog rams . Bent pins resulted mostly fro mmishandling, such as contact with structureor mi salinemen t of connecto rs during mating.Correcti ve action for t his problem included

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plastic co ver s fo r demated connector s, solid plastic in se rt s with predri lled hole pat-te rn s to fit over male conta cts while demated, and prealinem ent ma rking s on the con-nector shells to aid in more adequate mating. The plastic ins ert s were als o an aid indetermining that pins we re not bent befo re mating and could, in some ca se s, str aig hte nbent pins. To reduc e rewor k o r replacem ent of connec tors due to bent pins, testi ngwas performed t o deter min e the numbe r of t im es and the extent that pins could bestraightened without pin degradation. A bent-pin log was establ ished f o r eac h vehicleto tra ck the history of each bent pin to ensu re that the esta blished cr it er ia were main-tained. If the cr it er ia wer e exceeded, the pin was replaced.

Rece ssed contac ts wer e caused mainly by technici ans' failure to ens ure that con-tac ts were properly seated after insertion, although some ca se s resul ted from brokenspring retention clips.

Seal damag e falls into two catego ries, the facia l seal and the rear grommet wireenvironmental seal. The facial seal ens ure s an environmental sea l at the receptacle/connector inte rface . Damage to thi s seal can be caused by the connector pin s if theconnector is not properly alined before mating. Rear seal damage is gener ally causedby imprope r use of, o r damaged, contact insertion tools. Eit her the damaged tool orit s improper use can cut the seal such that its environmental integrity is destroyed.This seal can also be damaged i f the wire s are routed too sha rpl y to one side and pulledtoo tightly, thus elongating the ci rc ul ar seal to the extent that no environmental seal ispossible.

The occurre nce of t hese pr oblem s wa s greatl y reduced by the use of pictorialaids, by additional train ing, and by sp eci al empha sis on quality and pride inworkman ship.

H a r n e s s F a b r ic a t io n , C h e c k o u t , a n d I s t a l l a t i o nFabrication. - Ear ly fabrication problems included w ir es that were too short o rtoo long, sh ar p edg es on tooling boa rds and mockups, lack of p rotec tive co ve rs durin gfabrication, lack of support of h ar ne ss during trans porta tion to vehicle, and inadequate

personnel training. To reduce these probl ems, the tooling boards we re improved toeliminate potential damage areas, and the termination points we re mor e accuratelylocated to ensur e proper wire lengths. Some tooling board s incorporated extensivethree-dimensional fixtures for corr ectio n of t hes e problem areas. Protective cover swe re added during fabrication fo r completed port ions of the ha rn es s, and impr oveme ntswe re made in the tooling aid s to prevent damage during trans porta tion to the vehicle.Personnel training w a s improved to cr eat e an a war ene ss of the technical details re-quired to c reate cons istent quality during fabri catio n. Imple menta tion of m or e frequentinspection points al so grea tly improved the fin al product.

Checkout. - Completed harn es se s on the Apollo sp ace cra ft we re checked f or con-tinuity, conductor res ist anc e, insulation re si st an ce , and die lec tri c stren gth. Thesecheck s were performed on the ha rn es se s both befo re and after installation on the vehicleand aided substantially in locating wiring e r r o r s and damag e to wiring insulation.

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These check s can be performed at vari ous voltage le vels but mus t always be limited to75 percent of the voltage rat ing of the lowest-rated connector on the vehicle ha rn es s topreclude d amage to connector components.

Installation. - Installation pro blems included da mage fr om sh ar p edges, damagefr om technicians working on adjacent harn ess es, connector damage, and wire breakagecaused by handling duri ng installation and rework. These problems were substantiallyreduced by the addition of chafe guards , p ermanent ha rn es s tray co ver s, tem por aryhar nes s covers , and connector protective covers. Increased personnel training andemp has is on awa rene ss of quality workmanship al so helped redu ce installatio n prob lems.

M I S S I O N PERFO RM ANC EElectrical Wiring

The wiri ng in both the CSM and LM functioned ve ry well dur ing the va rio us m is -sions. None of the mis sio n anom alies are known to be the r esu lt of a wiring failure.

Wire-Connecti n g DevicesOf the many thousands of CSM and LM wire-connec ting components involved in

the Apollo mis sion s, only the term ina l boards we re the caus e of two mission anomalies:specifica lly, the fail ure of one react ion control system engine cir cui t and the failu reof one oxygen tank hea ter circuit on the spacecr aft for Apollo 11. Both failures wereattributable to the overtolerance boards that resulted in intermittent or open circuits.All su spe ct boards we re eventually phased out of the later vehicles.C O N C L U D I N G REMARKS

Electrical W i ingConductors. - Based on Apollo experience, either silver o r nickel plating is gen-era lly adequate for elect rical wiring. The use of nickel plating re su lt s in a higher

tem per atu re capability and a 5-percen t higher conductor re sis tan ce. In futur e pro-grams, application of aluminum conductors for the large, heavy-gage power feedersshould be investigated f o r poss ible weight savings. Fo r smal l-gage wiring (24 gage orless), the high-strength alloy cond ucto rs should be used. A typic al example is the26-gage copper-chrpmium-cadmium alloy w i r e developed for and used on the lunarmodule; th is wire is equivalent in physical stre ngth to the 22-gage stan dard copperconductor.

Insulation. - Although some probl ems have been expe rienced with the combinationp o l y i m m o n ape-wrap wire insulation, it is still considered to be one of the bestand lig hte st weight insula tions pres entl y available and is recommended fo r future pro-gr am s. Res ear ch and development a r e still continuing in se ar ch of a better, lighterweight, and mo re cost-effective insulation.

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W i r e - C o n n e c t i n g D e vi c esWire splic es and te rminal boards. - Based on pas t and potential prob lem areas,it is recommended that crimp-type wire splices be used in future pr og ra ms instead ofsolder-type o r modular-type plug-in ter mina l boa rds fo r joining two or mor e wir es toa common point. The reliability advantage of the crim p splice is that before-and-aftersam ple s can be made and tested t o ens ure the quality of spl ice s installed in a vehicle.

The crimp splice also has a weight advantage over ter mina l boards. A minimum of5.4 kilograms (12pounds) of te rm ina l boa rds is required to terminate the sa me numberof wi res that can be ter minated by 0.5 kilogram (1 pound) of wire splices. The majoradvantage of the modular te rm in al boa rd is that wire s may be easily connected anddisconnected and, if specific procedures are followed, a reliab le connection will r esu lt.Generally, the main disadvantage of wire splices is that environmental sea ling of twoo r mo re wi re s in one end of the splice is difficult to con trol and mu st be closelyinspected.

Connectors. - For future programs, connectors incorporating a rear environmen-tal seal that do es not req uir e the additional potting pr ocedu re for sealin g purp oses arerecommended. The connector p ins ideally would be cr imped o r welded t o the incomingwir e and would be removable from the rear of the connec tor . Elimination of the pot-ting sav es weight and assembl y time, whe rea s the re ar- rem ova ble pin can potentiallysave retest t ime if pin replacement is neces sary. Thi s featur e precludes the need fo rconnector demating and often obviate s subsequent ret est ing of all affected ci rcu itr y inthe connector.

In conclusion, the prop er se lectio n of m at er ia ls and har dware fo r wiring andconnecting dev ice s can result in weight savings, re duced manpower expend iture s, andincreased re liability.

Lyndon B. Johnson Space CenterNational Aeronau tics and Space Administra tionHouston, Texas, September 5 , 1974914-11-00-00-72

10 N A S A - L a n g l e y , 1975 S-413


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Documents

Apollo Experience Report Electrical Wiring Subsystem