Apollo Experience Report Lunar Module Electrical Power Subsystem

8/8/2019 Apollo Experience Report Lunar Module Electrical Power Subsystem

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N A S A T E C H N I C A L N O T E N A SA TN D-6977

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APOLLOLUNAR

EXPERIENCE REPORT -vXODULE ELECTRICAL

POWER SUBSYSTEMby Arturo B. CamposM a n n e d Spacecra? 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 P A C E 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. SEPTEMBER 1972


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_.1. Report No.NASA TN D-69774. Title and SubtitleAPOLLOEXPERIENCEREPORTLUNAR MODULE ELECTRICAL POWER SUBSYSTEM

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

5 Remrt DateSeptember 1972+-. Performing Organization Code7. Author(s)Arturo B. Campos, MSC9. Performing Organization Name and Address

8. Performing Organization Report No.MSC S-33710 . Work Unit No.

914- 13-00-00-72Manned Spacecraft CenterHouston, Texas 7705812. Sponsoring Agency Name and AddressNational Aeronaut ics and Space AdministrationWashington, D. C.. 20546

11, Contract or Grant No.

13. Type of Report and Period CoveredTechnical Note

14. Sponsoring Agency Code

~~~ ~ ~~19. Security Classif. (of this report) 20 . Security Classif. (o f this page) 21. No. of PagesNone None 16

~~~15. Supplementary NotesThe MSC Di rector has waived the us e of the International System of Units (SI) fo r this.ApolloExper ience Report, because, in his judgment, the use of SI units would impair the use fulnessof the report o r resu lt in excessive cost.

22. Price$3.00

16. AbstractThe design and development of the elec tric al power subsystem for the lunar module a re d iscussed.The initial requirement s, the concepts used to design the subsystem, and the testing program a r eexplained. Specific problems and the modifications or compromises (o r both) imposed for resolu -tion a re detailed. The flight performance of the subsystem is described, and recommendationspertaining to power specifications for future space applications a re made.

17. Key Words [Suggested by Author ls) )* Apollo ProgramElectrical Power Subsystem* Electromagnetic Interference' Functional RedundancyPower Transfer/Distribution/Conversion

* Power Specifications'Lunar Module

'Protect ive Device

18. Distribution Statement

* Fo r s a l e b y th e N at i ona l T ec hn i c a l In fo rmat ion Serv ice , Springfield, V i rg i n i a 22151


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APOLLO EXPERIENCE REPORT

LUNAR M ODULE ELECTRICAL POWER SU BS YS TE MBy A r t u r o B . C am pos

M a n n e d S p a c ec r af t C e n t e rS U M M A R Y

The lunar module e lec t r i ca l power subsys tem cont rol led, condit ioned, and dis t r ib-all e l ec t r i c a l power i n the lunar module . The subsy s tem w a s requi red to rece ivet -cur rent power f ro m bat te r ie s and dis t r ibute i t to equipment in both di rec t - andat ing -cur rent fo rm s. The subsys tem was developed to inc lude equipment fo r in-rsio n of pow er, c i rc ui t protect ion, dist r ib uted networks, con trols , an d mon itoringions . In thi s repo r t , the sys te m des ign, the t e st ing program , and the prob lemse red th roughout t he pro gram a r e desc r ibed .

INTRODUCTIONThe co nt rac t fo r the lunar module ( L M ) was awarded in 1963. The e le c t r ic a l

qu i rem ents w ere def ined to provide approximate ly 65 ki lowat t-hours a t a r a t e4000 wat ts to sa t i s fy a 35-hour lunar s tayt ime. The subsy s tem w a s designed to fail-within the weight const ra ints ; th i s requ i remen t necess i ta ted the use of redundantes and isolat ion equipment . Inversion equipment wa s provided to mee t the requi re -the equipment pow ered by al tern at ing cu rre nt (ac). Conversion equipment w a si ded to sa t i s fy t he requi rem ent s fo r levels of d irec t-cu rre nt (dc) pow er other than28 V dc obta ined f ro m the sou rce . Ci rcui t protec t ion w a s provided bya l c i rcu i t b rea ke rs , fuse s , and e l ec tron i c c i rcu it ry . The subsys t em des ign , de-, and f light hi s tory a r e di scussed , with emphasis on the pro blem s encountered


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S U B S Y S T E M D E S C R I P T I O NOrigina l ly , the e lec t r ica l power subs ys tem (EPS) wa s des igned to accomm odate afue l -ce l l source as depicted in f igur e 1 . The in i t ia l EPS specificat ion included the fol-lowing major components.1 . Th ree fue l ce l l s (2 8 V dc, 1 1 0 0 wat ts each)2 . Three e l ec t r i ca l supe rv i sory pane ls ( t o ensure cont ro l and pro t ec t ion f o r eacfue l ce l l)3 . Tw o or t h r e e i n v e r t e r s ( 3 5 0 vol t -amperes , 1 1 5 V ac , 400 he r t z , s i ng l e phase4. One l ight ing-control assembly (LCA) (for power conversion and variable vol t -ag e con trol fo r l ight ing equipment)5. Peaking ba t te ry and ba t te ry c ha rge r (The ba t tery was needed to sa t i s fy the4000-watt power requirement . ) i

Key:ESP -Electr ical supervisory panelFC A - Fuel-cell assemblyGS E - Ground-suppor t equ ipment I T04Fl I

F i gu r e 1.- Alterna ting- and di re c t - cu rre nt d i s tr ibut ion conceptua l des ign us ing fue l ce l l2


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Because of the fuel-c el l complexi ty, the development co sts , and the schedulethe pr i m e cont rac tor wa s d i rec t ed to conve r t t o an a ll -ba tt e ry sys t em.requi rem ent s f o r t he addit iona l power necessa ry fo r t he i nc reased lunar s t a y -3 5 to 72 hou rs) resul ted in an added descent -s tage ba t te ry and a bat te ry-re lay ass em bly. The f ina l subsys tem des ign consis ted of the dc , ac , andIn the dc sys t em , f i ve descen t - st age ba t t e r i e s , rated at 400 ampere -hours each ,batteries, r a t e d at 3 0 0 ampere -hours each , w ere used . Twoassemb ly (ECA) uni t s were placed in the descen t s t ag e fo r t he con-ol and protec t ion of all descent -s tage ba t te r ies . One ECA wa s used for each ascent -ge ba tt e ry . T o t r a n s f e r pow er f r om the bat te r ies to the dc bus, two feeder s y s t e m sf redundant power w ire s w ere used. The relay junction box contained anc t r ica l contac t to disconnect (deadface) the descent -s tage feed er f r om the ascent -l ead ing to the lunar module pi lot (LMP) us . The deadface re lay box wasd i sconnec t the descent -stage feed er f rom the ascent -s tage feed er leading to theman der (CDR) bus. One LM P bus and one CDR bus we re located in the displayscont rols panels . The ba t te ry-cont rol re lay assembly was ins ta lled to swi tch theremen t informat ion f rom one ba t te ry to another ba t te ry of the th re e descent -s tagethat s ha r e d t e l e m e t r y c hanne ls .In the a c s y s t e m , tw o inv e r t e r s ( ea ch rated a t 3 5 0 vol t -amperes , 400 he r t z ,15 V ac , s i ng l e phase ) were used . The ac bus was located in the LM P power panel .In the l ighting sy stem , one LCA was used to provid e varia ble ac and dc power fo rand ex t e r io r l igh ting .

S U B S Y S T E M T E S TI N G P R O G R A MTo e ns ure adequate sy s tem and component opera tion, a t es t ing p r og r a m w a sthat included the following te sts .1. Design-feasibil ity te s t s w ere perform ed on breadbo ards to establish confi-e i n des ign pa ram e te r s and to aid in selectio n of p ar ts .2 . Design-ver i f ica t ion tes t s were performed on working models in varying en-enta l condi t ions t o asc e r t a in ha rdware pe r formance be fore a commi tment w a sthe f light hardw are .3 . Qual if ica tion te s t s genera l ly we re performed on a t l e as t two f light- type uni t s .tested t o des ign l imi t s ; the othe r , t o endurance l imi t s . These t e s t s we regned to prov e that the hard wa re would s a t i s fy spec if ica tion requi re m ents .4. Acceptance tests were performed on each fl ight uni t to check such condit ionss vibra t ion, t em pe ra tu re , and e lec t r ica l func tions to ens ure the capabi li ty of the uni tthe t e chnica l r equ i rement s .

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5. Preins ta l la t ion te s ts w ere pe r form ed on each f l ight uni t to determine whetherthe unit had been damaged after the acceptance tests we re per fo rm ed by the vendor .6. Subsystem-development te s ts we re per form ed on product ion hardware a s s e mbled into a working su bsys tem to ensu re t h a t subsys tem chara c te r i s t i c s (vo ltage , r ipp le tc . ), perform ance, contro l , and protect ion sa t is f ie d the sys tem requiremen ts .

COMPO NENT DEVELOPMENTD i r e c t - C u r r e n t S ys te m

The dc sys tem co ns is ted of the power sou rce (ba t ter ies ) , the contro l and protec-tion of the power s ourc e (ECA uni ts ) , the feeder sys tem to rou te the power f ro m thesourc e to the buses , the in ter face contro l ( re lay junction box and deadface re lay box)between the descent- and the asce nt-s tag e feed ers , the two dc buses , and the contro lsfor switching bat te r ies and o ther e lec t r ic a l informat ion (bat tery-co ntro l re layassembly) .Bat ter ies . - To achieve an ov era ll low effective weight, f ive 400-am pere-hou rbat ter ies were located in the descent s tage and two 300-ampere-hour ba t ter ies w erelocated in the asce nt s tage. These bat tery energy ra t ings were for a nominal bus volt-age of 28 V.dc under load.E lec t r i ca l con t ro l a s sembl ies . - Two ECA uni ts we re requ ired fo r the descents tage. Each unit originally controlled two of the four descen t-s tag e ba tter ies . Eachof the two se ct io ns of ea ch ECA unit c on tro lled the output of a battery having a low-voltage tap connection (17 cells) and a high-voltage tap connection (20 cel ls) . The low-voltage tap preven ted overvoltage during ea rly low-load perio ds (e. g. , pre launch) .When the fifth descent-s tage bat tery w a s added, the low-voltage tap connection fro monly one sectio n of eac h desc en t-sta ge ECA w a s used to switch and co ntro l the f i f thbattery (fig. 2). Thus, the need for a third ECA w a s el iminated . This ac t ion resu l tedin the capabil i ty to connect the f ifth battery to e ithe r bus. Each descent-s tage ECApro tec ted each ba tt e ry f rom overcu r ren t by automatically disconnecting the battery f r o

the system when an overload on the bat tery exceeded 160 am pe res . Also , indicat ionswere prov ided when r eve r se cu r re n t exceeded 10 a m p e r e s or when bat tery tempera-tu re s exceeded 145" F.Two ECA uni ts we re re quire d for the asc ent s tage; each unit contro l led the con-nection of an asc en t battery to two fe ed er sys tem s. Only the norm al dis tr ibution feed econnection for the ascent-s tage battery had the automat ic ove rcurre nt pro tect ion . Thebackup connection overcurrent capabil i ty w a s dele ted to e l iminate weight.During the development of the ECA un its, two conditions forc ed design chan ges.First, the h igh v ibra t ion level caused re lay-co ntact cha t ter be cause of the ascent- s tagECA location. T hi s condition w a s disco vere d when the ECA w a s tes ted while mounted

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on the second ary s t ru c tu re s im ula tor to expected f l ight -qual if ica t ion leve ls . Because ofthe high vibra t ion leve ls , the pr ime spacec ra f t con t rac tor re loca t ed the a scen t- s t ageECA fro m the cant i levered cold rails on the aft equipment rack to a position within therack . In this new locat ion, th e lower resu l t ing vibrat ion level did not cause contactchat ter when the ECA was re tes ted wi th the s t ruc ture s imula tor .The second condit ion tha t forced des ign changes occu rred dur ing the su bsys tem -development tests. It w a s discovered t h a t the shor t -c i rcu i t c ur ren t of an L M bat te ry(1700 a m pe r e s ) w a s much higher than the curr ent - inte rru pt capac ity of the 1100-amperecontac tor . When the contac tor wa s tes ted wi th f l ight - type batteries, the contactbutton of the contac tor separa ted . This fa i lur e resul ted f rom the hea t genera ted by the170 0-am pere cu rre nt . The contact but tons and the bridge we re modified by changingthe bonding process between the cadmium al loy and s i l ve r f rom braz ing to d if fus ionbonding and in cre asi ng th e length of th e bridge by 0.045 inch. Th e diffusion bonding ofthe contac t l ay e rs re s u l t ed i n l e s s con tac t r e s i s t ance and the re fo re a lower heat buildup.The lengthened br idge , conta ining the con tac ts , a llowed grea ter hea t rem oval f ro m thecontac ts and maintenance of s t r uc tur a l r igidi ty a t the gr ea te r tempe ra tur e . The e lec-t ronic device fo r t r ipping the contac tor w a s modif ied to obta in fas te r t r ipt im es f or

shor t -c i rcu i t cu r re n t s g rea t e r t han 1100 am pere s , These modif ica ti ons were made onall asce nt -s ta ge and descent-s tage ECA uni t s .F e e de r s y s t e m . - Two American w i r e gage 6 w i r e s w ere used in each pos i t ive legof the red un dan t feeder sys t em . The veh ic le p r ima ry s t ruc ture w a s us e d f o r the r e t u r n .The feede r sys t em provid ing power f r om the ba t t e r i e s t o t he L M P bus w a s routed on theright-hand sid e of the vehicle , and the CDR bus feeder s ys tem w a s routed on the l e f t -hand side. Each feede r sys t em s t a r t ed a t each descen t -s t age and a scen t - s tage ECA(fig. 2). Th e descen t-stage port ion of the fee de r sy ste m w a s connected to the ascen t -stage port ion just beyond the relay junct ion box on the L M P side and just beyond thedeadface relay box on the CDR side.During ground tes t ing of L M t e s t a r t i c l e 8 and jus t after the L M P power pan el hadbeen in s ta l led, the vehic le feeder sys t em on the L M P s ide failed (shor ted to ground) .The feede r w i r e s on the back s id e of th e L M P power panel had been t rapped and dam-aged because of insuff ic ient c leara nce be tween the panel s t ruc tur e and the vehic le s t ru c-t u r e . T he s y s t e m w a s mod ified by the add ition of sp ac er s to the power panel to a llowm or e c learanc e . Again, dur ing tes ting, the w ires w ere found to be damaged af te r re -instal lat ion of the panel in the vehicle . Th is prob lem final ly w a s solved by flat teningthe rounded ha r ne ss and secur ing the ha r nes s with rec t angula r c l am ps .During the ground test in g of L M - 2 , the descent -s tage feeder sy s te m on the L M Ps i de be ga n t o a r c a nd sm oke . T he f e e de r w i r e was found to be dam aged and burned

unde r a cable c lamp . The problem, caused by an unders ized c lamp tha t damaged theH-f i lm pr im ar y insula t ion of the fee de r w i re , was cor rec ted by the ins ta lla t ion of heat-shr inka ble tubing a long the ent i re length of the feeder s ys te m and the u s e of prope r lys i zed cable c l a m ps .

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I I I I I III1III Ln t e r r u p t e r C o n ' n e c t0

C r o s s t i ebalance load!

WO f f l r e s e t H V Offlreset LV I deadfaceI CD R bus(28 V d cl

Figure 2. - Elec t r i ca l power subsy s t em b lock- l l -ba t t e ry dc sys t e m .Relay junction box and deadface relay box. - The relay junct ion box containeddiodes and junc tions for the var iou s ba t te ry con t rols f ro m the automat ic checkout equip-ment, the L M cabin, and the command module ( CM) . Addit ional ly, the rela y junct ionbox and the deadface r elay box iso lated the descent-stage port ion of the f e e de r s y s t e mjus t before stagin g of the L M vehicle .The dc buses . - Two main dc bu ses we re provided for redundancy. The followingload groupings were provided.1. Redundant loads, one on each bus2. Nonredundant c r i t i ca l loads , powered f ro m both bu ses w i th diode i sola tion3 . Noncri t ica l nonredundant loads , powered f ro m e i ther bus

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Fo r c as es in which two loads could perfo rm the sam e funct ion in a di fferent man-ner , one load was tied t o t he first bus and the other load w a s t i ed t o the second bus , apr oc e du r e called load grouping for fun ct ional redundancy,Redundant loads (su ch as t he tw o i nve r t e r s , t he s y s t e m A a nd B r e a c t i on - c on t r o lquad hea t e r s wi th co nt ro l c i rcu i t ry , and the two s et s of ul t ra-high-freq uenc y and ve ry -

h igh- frequency t ran sce ive rs ) were p l aced on sepa ra t e buses . Nonredundant c r i t i ca lloads (such as bat te ry c on t rols ) w er e put on both bu ses (with diode isolat ion). Func-t iona l redundancy load s (such as e lec t rolum inescent (E L) lighting, incandescent f lood-l i gh t s , p r im ary gu idance , and abor t gu idance) were p l aced on se pa ra t e buses t o p reventlo ss of th at function if a bus were los t .After the fire in Apollo CM 012, f i re -re ta rd an t m ate r ia l s were added t o c o v e r thef l ammable po tt ing ma te r i a l , t he c i rcu i t b re ake rs , and the buses . Dur ing the acceptancetest ing of the power pane l s , t he i nsu l a ti on re s i s t ance w as found to va r y f r om 1 to10 mego hms depending on humidity . The minimum requ i rem ent w a s 100 megohms.T h e f i r e - r e t a r d a n t m a t e r i a l u se d by t he pr im e cont rac tor was found to be nonhygro-scopic , bu t the Beta-c loth bootees that cove red the c i rcu i t b re ak e rs w ere hygroscopic .Because test re s u l t s p roved tha t the insula t ion-res i s tance reading s tabi l ized at va luesg r e a t e r t han 1 megohm, the spec i f ica t ion w a s changed to 1 megohm. Leakage cu r re n tof a fe w m i c r o a m p e r e s w a s not a problem, because a 30- m i c r oam pe r e l o s s f o r the m i s -s ion dura t ion amounted to approximate ly 5 mi l l iamp ere-ho urs of a to ta l ba t te ry capac i tyof 2675 am pe re- ho ur s.Ba t t e ry-cont ro l r e l ay a s sem bly . - The addition of the f i fth ba t te ry on the descen ts t age d i c t a ted the r e qu i r e m e n t f o r the ba t t e ry-cont ro l r e l ay a s sem bly (a re lay box).This assem bly could be used to swi tch t he ba t te ry i n forma t ion in to ex i st ing m easu re -ment channels . Thus , fu r th er compl ica t ion of the ins t rum enta t ion sub sys te m and in-c r e a s i ng the vehic le weight by the ad ditio n of another ECA we re avoided.R e l a ys a nd c i r c u i t b r e a ke r s . - The or igina l L M re lay spec i f ica t ion s t ipula ted tha tas many as 32 po les should be inc luded to sav e weight in the re lay-box des ign s . Theor ig ina l r e l a ys , des igned and made to t hese requi rement s , failed to satisfy t he v ibra -t ion req ui rem ent s . Consequently , the off-the-shelf, prev iou sly qualified relays w e r ese l ec t ed and p l aced in re l ay boxes t ha t had been redes igned to accommod a te a l a r g e rnum ber of sm a l l e r s i zed re l ay s with fewer contact s .E a r l y i n the f li ght p rog ram , these re l ay s ac tua t ed in t e rmi tt en t ly . The prob lemwas cau sed by m eta l l ic and nonmeta l l ic contamination tha t prevented the a r m a t u r e f r o mopera t ing ful ly . A scre enin g pr oc es s (high-vibrat ion tes t ing des igned to exc i te thes m a l l p a r t i c le s that caused the contaminat ion w h i l e t he re l ay w a s t e s t ed e l ec t r i ca l l y )w a s i m pos e d on all r e l a y s u s e d in the EPS. Early vehic les were re t rof i t t ed with se-l e c ted s c re en ed re l a ys , depending on c r i ti ca l it y .H e r m e t i c a l l y sealed c i rcu i t b r eak e rs were spec if i ed ; however , t he manufac ture rwas un able to quali fy this type of breaker because of asse mb ly pro blem s. One as se m -b ly p r ob l e m w a s the c r i t i ca l p r oc ess of f i t ti ng the ma jor c i rc u i t -b reak e r a s sem bly(containing the pushbutton, the bridge co ntac ts , and the bim eta l l ic e lement ) into thehe r m e t i c s e a l i ng c an . Because the can contained the contac ts that mated with thebr i dge c on t a c ts , a spe c i f ic contac t p re ss u re was di ff icult to obta in when the two pa r t sw e r e a s s e m b l e d an d sealed with s o l de r . F a i l u r e s r e s u l ti ng f r om the lower contac t

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pressure inc luded contac t cha t te r , h igh contac t res i s tance in dry c i rc ui t t es t ing, andhigh vol tage drop . The con tract w a s modified t o include the pu rc ha se of qualifiedcomm and-se rv i ce module (CSM) i r cu i t breakers f r o m the sam e manufac ture r . Thesebreakers w e r e sealed environmental ly rather than hermet ica l ly . The problems en-countered dur ing t h e development of the c i rcu i t -b reake r panel and the eventual solut ionsto these problems are discussed in the following paragr aph s ,Dur ing c i rcu i t-b reake r -pane l a s sem bly , the pr i m e c on t r a c t o r baked the panels toac cel era te the curing of the sea l ing compound use d on the c i rcu i t -b reake r t e rmina l s .During panel-acceptance test ing, i t was found that this baking had caused a deformat ionof the breake r case and in t e rna l s t ruc ture , r e su l t ing in a shift i n the ca l i b ra ti on cu rveof the breake r by as much as 50 percen t . To solve the problem, the c i rcu i t -b reake rmanu fac turer changed the i n t e rna l s t ru c tu re m a te r i a l f ro m me lamine to d ia l ly l ph tha l-ate, which had better t he rm a l s t ab i li ty .After the f i r e i n Apollo C M 012, t he pr im e cont rac tor cove red the c i rcu i t b reake rswith Beta-cloth bootees to stop the prop agat ion of a fire. Subsequent ly , c racks werefound in the ca se a round the base of the threaded mounting bushing of s o m e ho t-w ire-

type break e rs . Two fac tor s cont r ibuted to this condi t ion. Fi rs t ly , the c i rcu i t -b reake rload-bearing mounting surface w a s subjec ted to uneven stresses bec aus e of crum pledBeta cloth between the mounting surf ace and the panel. Secondly, the l oad-bea r ing a reaof the brea ker th read ed bushing had been reduce d by a product - improvement change .The prime contractor modificat ion to resolve this problem was twofold. TheBeta-cloth-bootee design w a s changed fr o m the so-cal led tobacco pouch design, inwhich a s t r i ng w a s draw n to clo se the pouch aro und the m ounting bushing, to one thatincorpora ted a piece of flat plas t ic betweenthe c ircui t -breake r mounting surfa ce andthe panel (fig. 3) . In addit ion, the torq ueon the threaded-bushing mounting nut w a s

reduced f rom 30 to 20 inch-pounds. Theeffect iveness of a good mount at the lowertorque va lues w a s proven by means oftes t ing .Dotted lin e shows cha nge 0 020 f 0 010 i n

A-ZE - 0 080 inProduct improvement change o circuit breakeicase that created high er stress paintDetail A

When the pr im e c ont rac tor ins t itutedt r i p -cha rac t e r i s t i c t e s t ing in the fall of1968, it was found that som e bimeta ll icc i rcu i t b reake rs did not trip. Both Apollos pa c e c r a f t p r i m e c on t r a c t o r s assisted thebreaker manufac turer in performing e lec-t r i c a l t r i p t es t ing on approximate ly100 c i rcui t br eak ers . Based on the t e s tres ul t s , i t was s t ipula ted tha t the e lec t r i -c a l t r i p life should be l imi ted to 20 t r ipcyc l e s , an NASA-approved specificationchange. Fa i lu re ana lysis of circ ui t break-ers indicated that , with increasing t r ip-t ime, the bimeta l l ic pla te se em ed to bindwith the la tching bracket as shown inf i gu r e 4.

Slot added hereBearing area ltypicallBottom view 01 bushing

Product-improvement change that reduced bush ing bearing areaDetail B

r D e t a i l B

Tobacco pouc hde sign New bootee design

Figure 3 . - C i r c u i t - b r e a ke r - c a s eimprovement s .8


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Figure 4. - Lunar module bimetal l ic circuit breakers,Alternating-Current System

The ac system was essent ia l ly a decentra l ized sys tem in which most us e rs of acpower perform ed conversion from dc power internally. Only a few loads required acpower f rom a n external source . In it ia l ly , the environmental cont rol sys tem was tocontain ac mo tors to dr ive pumps and fans; however, brush less dc motors , which we rebeing developed, we re sele cte d. Even though the remaining ac power requiremen t w a slow, the inver ter a l ready had been s ized to accept the to ta l ac motor loads. The acsy ste m cons isted of two in ve rte rs, a f eeder sys tem fo r each i nver t e r , tw o a c b u se s f o rthe high-power side, and one common bus fo r the low-power side.

Two inv ert ers w ere instal led, al though only one inv erte r could be used at a t ime.The o ther inver t e r se rve d as a redundant inverte r but could be energized and left idlingduring critical mission t imes. Each i nver t e r was rated at 350 volt-am peres , 400 her tz ,115 V ac, s ingle phase.By means of the inver te r design, tw o weight-saving innovations w er e introduced:a ramp-type boost regulator (replacing a buck-boost t r an sf or m er type) and si l icon-controlled re ct if ie r (SCR) units for the inversion sections. The ramp-boo st-regulatordesign was sa t i sfactory , but the SCR switching was uneven and random . Consequently,the inver te r design was changed to incorporate s tandard t ran sis to r switching.Before qualification testing was completed, a major design problem was foundthe in ve rte r could not fulfi ll the voltage-modulation spec ifications a t the low freq uenc ieswhen the specified input line ripple (3 volts peak to peak, 0 to 15 kilohe rtz) was intro-duced. The output-voltage modulation was 18 volts but should not have exceeded2. 5 vol ts peak to valley ( ref , 1). In a phase I1 design, the inv erte r w a s modified toinclude a demodulator circuit ( f ig. 5) . With the demodulator circuit, the output-voltagemodulat ion wa s approximately 4 volts. A tes t per formed on L M test a r t i c l e 1, an ea r lyte st vehicle, provided data fro m which it was shown that dc input contained a ripple of

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a few mi l l ivol t s and s ha rp (2 ol t s maximum) spikes . On the basis of this knowledge,the inv er te r input-voltage requ i rem ents w ere changed to spec i fy only 2 vol t s peak topeak f ro m 20 o 250 he r t z and 3 vol t s peak to peak f ro m 250 he r t z t o 15 kilohertz . Aftert h i s test an d the subsequent spec i f ica t ion change , the phase I1 i nve r t e r satisfied thevoltage- modulat ion requ irem ents.

Aux i l ia ry power , 40 V dcKW: IEM1 - Electromagnet ic interference

rm s - Root mean squar eZen ref - Temperature-compensatedzener reference

Aux i l ia ry dcupconverter.semirequlated Output f i l t e r

171 tOvercur rent senseSense-

Closed-loop sense signal

dc upconverter

secondarytapsregulated

I 7-P u s h - p u l lI !I Modulated canceled siqnaI

I-Switch drive I

I II ( t ime- rat o

Battery compensatorT-l- Upconverler cutback signal Rec t i fie r and f i l t e rI

1 IZen ref

squa re-wavesignal- 400 Hr64M) Hr

tv e r c u r r en tc i r c u i tux i l i a r y dcregulator. 15 V7 Yen ref. Tap-chanqer dr ive s igna ls

1Operat ing power for 5 V

CountdownInput f i l t e ra -IM1 f i l terUnregulated28 V dc input - * Tap-changerLogic networkAux i l ia ry dc 5 v dcregulator , 5 - -d r i v e r - am p l i f i e rI L -ap changer holdof f s ignalryn ch on zer osc i l la torexternalFigure 5. - Lunar module gene ra l-purpose i nv e r t e r b lock d i agram.

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After qual ificat ion, an inv er ter in the LM-3 vehic le failed prefl ight . Th e outputof the i nve r t e r wa s i n t ermi t t en t. The problem w a s a t t r ibu ted to sep ara t io n of theosc i l l a to r -capac i to r lead so lde r. When the joint on the qual i f ica t ion uni t w as inspec ted,the s a m e s o l de r s e pa ra t ion as tha t ob served on the failed L M - 3 joint w as noted. Whenthe joints were fu r th e r examined , it was de t e rmined tha t the condi tion wa s a t t r ibu tableto the nonwetting characteristics of the m eta ls used in the lead fabr ica t ion; that i s ,dumet l ead ma t e r i a l on the cap aci t or, copp er flashing of thi s lead, and tin-dipping ofth e lead in 50/50 sold er . Th is 50/50 sold er made the lead noncompatible as explainedin the fol lowing paragraph.The osc i l l a to r -capac i to r lead w a s s o l de r e d t o the gold plat ing on the pr i n t e d c i r -cu i t boa rd wi th 63/37 eutec t ic so ld e r , which has no p l a s t i c s ta t e . The re fo re , dur ingcooling of the joint after the f ina l solder ing, the eutec t ic sold er hardened imm edia tely ,but the 50/50 so lde r remain ed in the plastic s t a t e l onge r and then sepa ra t ed when ithardened. The LM-3 and subsequent i nve r t e r s were rewo rked by unsolder ing the joint,removing the 50/50 s old er , and reso lder in g with eutec t ic so lde r while apply ing flux.

Lighting SystemThe l ighting sys tem con sisted of one LCA, t h ree po t en t iome te rs ( t o con t ro l the20- to 118-V ac annu ncia tor /num eric l ighting power, the 15- to 75-V ac in te gra l l ightingpower , and the 0- to 5-V dc in te gra l l ight ing power), and one rheos ta t ( to cont rol the28-V dc power t o the flood lights).The LCA co ns iste d of the following com ponents.1. A high-power ac voltage reg ula tor m odule (15- to 75-V ac output with remotepoten t iome te r con t ro l ) fo r the LM i n t egra l E L lighting2. A low-power ac voltage regulator module (20- to 118-V ac output with remotepoten t iome te r con t ro l ) fo r the annuncia tor /numeric EL lighting3. Two dc-to-dc converter modules (5.5-V dc output)4. A series dc r e gu l a t o r c i r c u i t (2- t o 5-V dc output wi th rem ote po tent iometerc on t r o l ) f o r t he dc inte gr al l ight ing and the cau t ion and warning lights5. A short-circui t -protect ion module (5-V dc output) for the docking lightsDuring the ground tes t ing of LM- 3 a t the pr im e con t rac tor ' s p lan t , an e l ec tromag-

ne t i c i n t e r fe ren ce prob lem w a s found to cau se no i se i n t he communica tion headset anda shi f t in the f l igh t d i rec to r a t t i tude i nd i ca tor . In the p r o c e s s of t roubleshooting, twoLCA uni t s were damaged when inad verte nt grounding of the LCA c i rcui t through agrounded osc i l loscop e caused f a i l ur e of a zener diode and the rem ote-c on t rol potent iom-eter. Two p r o b l e m s were revea l ed . F i r s t l y , a s tead y-s ta te no ise leve l of 0 .2 t o0.4 volt, 4 t o 10 megahe r t z , was seen on the 2- to 5-vol t c i rc ui t . Secondly, a t r ans i en tpeak vol tage of 20 t o 25 vo l t s at 250 ki loher tz dur ing ini t ia l turnon w as see n on the15-volt output of the dc- to-dc con ver te rs . The energy contents of the t r ans i en t and thenoi se we re cons ide red neg lig ib l e i nsofa r a s caus ing pe rmanent damage to the LCA.

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Circui t an a lys i s and tes t ing by the vendor indica ted tha t the noise o r igina ted inthe 15-volt s ec t i on that p rov ided power t o t he d imm er and cur ren t - l imi t i ng c i rcu i t s andthat decoupl ing was needed be tween the sw i tching e lem ents in the d imm er- l im i te r mod-ules . The decoupling capa c i tors and filter capa ci tors ins ta l led in the var io us m odulesto so lve the problem are shown in figure 6.

C I r c u ibreaKer28 - to 6-Vc o n v e r t e r

-28 v-LM g r o u n d )____( 28- to 6- Vc o n v e r t e rO n t h e d a sh 7 c h a n g e , 0. 1 pF ;o n t h e d a sh 4 and dash 6 c.hanges. loo0 pF

120 pF I1 o v T

I

Key:C i W - C a u t i o n a n d w a r n i n g

N o t eD a s h l i n e s i n d i c a t e t h eaddit ions for the dash 6 changes

- - - - - - - - - - - - -- - - - -I D o c k i n g - li g h t l i m i t e r I1 3 -A m a x . c u r r e n t , 3 . 8 - A l i m i ti n g

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high vo ltag e of 125 vo lts an d the low voltage of 110 vol ts w ere caused by the inve r terresp ons e t o the puls ing of the gimbal mo tors . When the load was rem oved and the in -ve r te r load dropped below 50 vol t -a mp eres , the inver te r ( in t ry ing to mainta in a r egu-lated voltage) produce d an osc il lat ing output that lasted approxima tely 100 mill is eco nds ,dur ing which t ime the g imbal mo tors were turned on and off se ve ra l t im es each second.This swi tching caused the t r a n s i e n t s o n the inve rter output.Manning of the subsequent spacec raf t resul ted in incr ea sed loading on the ac bus.The add i tional load p reven ted the t r ans ien t f ro m recu r r ing , and the EPS p e r f o r m e dsa t i s f ac to r i ly .On the LM -7 (Apollo 13) flight, th e EPS supplied e lec t r ica l energy to the CM forrech argin g and conserving energy in the CM batter ie s. T ran sfe r of power to the CMwas not a design requireme nt ; however , d ur ing a prem iss ion s imula t ion , the idea oft r a n s f e r r i n g p ow e r t o the CM in ca se of a n em ergency had been proposed. A p r e l i m -inary p rocedure w a s subm it ted and implemented, After the success fu l t r an s fe r ofpower to the Apollo 13 CM , addi t ional chang es in the LM-8 and LM-9 ve hicle s es tab-l i shed (1) he capabi l ity fo r the LM to in i t ia te the power t ra ns fe r , (2) the prote ction of

the LM bus es dur ing power t r ans fe r to the CM by added ci rc ui t protect ion , and (3) thecapab il ity to accomp l i sh power t r an sfe r to the CM after LM staging. Before this finalchange w a s implem ented, stagin g of the LM disrupted the t r a n s f e r l og ic c i r c u i t s .CONCL USIONS AND RECOMMENDATIONS

The lunar module e l e c t r i c a l power subsys tem has fulfilled the miss ion r equ i r e -men ts and no infl ight fa i lu re s have occurred . Af ter the in i t ia l fa i l ure s on developmentalequ ipment, som e con t r ac t s were cance l l ed and off-the-shelf hardw are was used . Theinitiat ion of h ighe r leve ls of accepta nce vibrat ion tes t ing and th er m al test ing g reatlyreduced the num ber of fai lu re s in the vehicle.F o r new p r o g r a m s , it is a d v i sa b l e t o s t a r t with a real i s t ic but conservat ive speci -f i ca tion fo r e l ec t r i ca l power ch arac te r i s t i c s app li cab le to all subsys tems . Such aspeci f icat ion w i l l a llow all eng inee rs to design equipment to wi thstand the s a m e electri-c a l r e q u i r e m e n t s su c h as t ransients, r ipple, modulat ion, and so fo r th . A p roper lywr i t t en spec i f ica t ion a l so w i l l enab le the pr im e con t r ac to r to impose a s t andard seriesof el ec tr i ca l tests on vendo r equipment.On m a j o r a s s e m b l i e s that r e q u i re f u s e s f o r c i r cu i t p ro tec t ion , the f u se s sh o ul d bel ocated fo r easy r ep lacement . P resen t ly , some lunar module equ ipment ( such as the

elect r ic al -co ntrol asse mb ly and l ighting-control as sem bly) has t o be remo ved and tedi -ously re wo rked for re place me nt of b lown fuse s .Impor tan t measurement s such as bus vo l tages and sou rce cu r r en t s shou ld bea s s i g n e d a high sampling rate to detect and analyze real - t im e, unexpected vol tage andc u r r e n t a n o m a l i e s . P r e se n t l y , the lunar module e le ct r i cal power subsy stem has a one-sa m p l e - p e r - s e c o n d r a t e ; the rate should be 10 o r m o r e s a m p l e s p e r s e co n d ( p re f e r a b lymo re) . An incident in Apollo 13 in which the crew repo r ted hear ing a thump and thenseein g "snow flakes" outside the window resulted in a cons iderab le da ta se a rc h to de -t e r m i n e if a cu r re n t sp ike on one sam ple datum w a s real. A higher sampl ing rate

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would have indica ted se ve ra l cur re nt sp ike s wi thin 1 second as obtained through sub-sequent ba t te ry te s t ing when a bat te ry w a s discharg ed through potass ium hydroxide .Redundancy in components , ass em bl ie s , o r sy s t em s should be prov ided in a m a nne rtha t pe rm i ts individual tes t ing of t ha t component , a s sembly , o r sys t em .Manned Spacecraf t C enter

National A erona ut ics and Space Adm inis t ra t ionHouston, Te xa s, Ap ri l 14, 1972914- 13-00-00 -72

REFERENCE1. Anon. : Elec t r i c Pow er , Ai rc ra f t , C ha ra c t e r i s t i c s and Ut i li zat ion of.MIL-STD-704A, Aug. 1966 (N oti ce- 1, Feb. 1968).

14 NASA-Langley, 1972- 1 s - 337

Documents

Apollo Experience Report Lunar Module Electrical Power Subsystem