apollo experience report pressure vessels

8/8/2019 Apollo Experience Report Pressure Vessels

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T E C HN I C A L

ab-

APOLLO EXPERIENCE REPORT -PRESSURE VESSELSby Glenn M . EcordManned S'acecrdft CenterHouston, Texas 77058

NASA TN

L

D-6975

'

N A T I O N A L A E R O N A U TI 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. SEPTEMBER 1972


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18. Dist r i bu t ion S tatement

1 Report NoNASA ?Tu' D-0972APOLLOEXPERIENCEREPORTPRESSURE VESSELS

2 Government Accession No 3 Recipient's Catalog No

4 Ti t le and Subt i t le -Reoort Date

National Aeronautics and Space AdministrationWashington, D. C . 20546

7. Au thor(s)Glenn M. Ecord , MSC

9. Performing Organizat ion Name and AddressManned Spacecraft Cen terHouston, Texas 77058

14. Sponsoring Agency Code

l------

8 . Performing Organizat ion Report No.MSC S-327

10. Work Un i t No .914- 13-20-06-72

11. Contract or Grant No.

5. Supoiementary NotesThe MSC Di rec tor waived the use of the International System of U n i t s (SI) fo r thi s ApolloExperience Report because, in h i s judgment, the use of SI units would impai r the usefulnessof the report or result in excessive cost.

2 . Sponsoring Agency Name and Address

6. AbstractThe Apollo spacecraft pre ss ur e vess els, associated problems and resolutions, and related ex-perie nce in evaluating potential problem a re as a re discusse d. Information is provided that canbe used as a guideline i n the establishment of baseline cri te ria f o r the design and use of light-weight pres su re vessels. One of the first prac tica l applications of the use of fract ure- mech anic stechnology to protect against serv ice failures was made on Apollo pre ss ur e vessels. Recommen-dations ar e made, based on Apollo experience, that a r e designed to reduc e the incidence of fail-ur e in pressu re-ves sel operation and service.

13 . Type of Report and Period CoveredTechnical Note

17. Key Words (Suggested by A u t h o r l s ) )* Nondestructive Testing'Vessel Management ' Safety Factors* Pre ssu re Vesse ls' Quality Assurance

'Apollo Pro gra mFra ctu re Mechanics * Vesse l Analysis

19. Securlty Ciassif . (of th is report1 20. Secu rity Classif. this page)None None

2 1 . NO. of Pages 22. Price27 $3.00

' For s a l e b y t h e N a t i o n a l Te ch n ica l I n f o rm a t io n S e rv i ce , Springfield V i rg in ia 22151


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APOLLO EXPERIENCE REPORTPRESS URE VESSELSBy G l e n n M. Ecord

M a n n e d S p ac e cr a ft C e n t e rS U M M A R Y

Apollo spacecraft pressure vessels were designed using safety fac tors as low as1.5 to save weight. The relatively low safety factors, combined with the cri tical ity ofa vessel failure, required t ha t stringent fabrication requir ements and controls be main-tained throughout the ve ss el progr ams to ensure production of quality vessels. Addi-tionally, posffabrication analytical techniques were introduced to confirm and controlsafe ve sse l operation.

N o problems we re experienced that were a direct resul t of reduced safety-factorlevels . Major ve ss el problems involved isolated instances of mater ia ls incompatibilitywith pressurants, of inadequate pr ocess control, and of mate ri al s anomalies. However,it has been shown that reduced safety factor s may increase the susceptibility of mate-rials to s t re ss corrosion or incompatibility problems.The resolution of each problem provided a resultant in crease in the knowledgenecessary to ens ure safe operation as reflected i n seve ra l recommendations concerningdesign, fabrication, use, and management of lightweight pre ssu re vesse ls. Specificidentification and assignment of responsibility for spacecraft pr essu re vessels w a s nec-essary to accomplish effective control of vessel fabrication and use.

INTRODUCTIONThe Apollo spacecraft press ure v essel s comprise a significant pa rt of the dr y

str uct ura l weight of the spacecraft. Seventy-one vessels are used in the commandmodule (C M) , ser vice module (SM), and lunar module (LM); the launch escape syst em(LES) use s three rocket motor cases that become pres suri zed when fired. Each of thevessels constitutes a potential single failure point f or cri tical subsystems and containsstore d energy at operating press ure s that could be catastrophic to the spacecraft shouldrupture occur.Historically, high safety factor s (ratio of design bur st pr essu re to maximum de-sign operating pr es su re ) have been used in the design of commerc ial pres su re ves se lsto compensate for unknowns in pressu re-ves sel fabrication and serv ice . A safety


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fa ctor of 4 . 0 was considered a minimum. In the early 1960's, the U. S. Air Force de-veloped a new technology for fabrication methods and ma te ri als contro l by means ofstringent specification requi rements that minimized ri sk and justified a safety factor of2.0 fo r pres sure vessels used in high-performance aircr aft.Safety fa ctor s of 2 .0 on some pres su re ves sel s used in the Apollo spacecr aft sys-tem s became impracticable because of the associated weight penalty. As a result, sev-

er al Apollo pres sur e ves sels were designed to a minimum safety factor of 1. 5. Thisunprecedented minimum safety factor for vessels in manned systems combined with thecriticality of a vess el failure r equir ed that eve ry known precaution be taken in fabrica-tion and use t o en sure safe ves sel operation. Despite the precautions, a few failu resand related probleiris wer e experienced. The resolution of these problems and the stepstaken to prec lude rec ur rence, including the introduction of additional precautions andthe application of f rac ture-mechanics technology in postfabrication analysi s, are dis-cussed in this report.DES I GN AN D TEST

Design data and other pertinent information for Apollo spacecraf t pre ss ur e ves-sels a r e presented in tables I and 11. Many of the larger or more numerous vessel shave safety factors as low as 1 . 5 to achieve a lightweight st ruc tur e. The locations andus es of many of the ves se ls throughout the spacecraft ar e shown in figures 1 to 9. Theassimilation of pr essu re ve ssels in the spacecraf t design and the relationship to space-craf t operation a r e illustrated in those figures.The Apollo spacecraft was designed and built in Block I and Block I1 configurationsfor the command and service modules. Essentially, Block I vehicles did not contain allthe systems necessary fo r docking and lunar flight and were used unmanned to qualifyvar ious Apollo sub sys tems. Block I1 vehicles were fully configured for manned lunarmissions . For the Block I1 configurations, some minor changes were made in thepres sure -vess el designs for the servi ce propulsion syst em (SPS), and vess el additionswer e made to the SM reaction control sy st ems (RCS). Generally, the SPS helium andpropellant ves sel s maintained the s am e overall dimensions and operating st re ss es , butoperating pre ssure s and wall thickness wer e reduced. Materials and proce sses re -mained the sam e. Two vess el s were added to each SM RCS panel to provide additionalRCS capacity f or the Block I1 configuration. The se additional vessel s were of the sa medesign as the CM RCS vessel s.Rigorous qualification tests (vibration, shock, accelerat ion, pr es su re cycling,and burst) wer e perfo rmed on production ve ss el s of each type to demons trate attainmentof design requirements. The tes ts perf ormed on each ves sel are identified in table ID.In some cases, the qualification of Block I vessels sufficed for Block I1 configuration.Each qualification vessel was given a nondestructive te st (NDT) evaluation and was ac-ceptance tested (proof pressurization and leak test) before the qualification test, as allproduction vess el s have been tes ted befor e delivery . Subsequently, the number of pr es -surizations, the fluids, the pr es su re levels, and the temperat ures have been controlledto ensure safe test conditions and adequate flight capability. Thi s control has been ac-complished by means of the regulation of spacecraf t- sy st ems te st s involving vesse lpressurizatio ns through the application of fracture -mech anics cri ter ia.

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T A B L E I. - APO LLO COMMANU AND SER VICE MODULE P-RE VESS ELSPressure "I

wee

Pr a s a ur e . he l iumPropellanl. oxldlzerP r oW l h l . lue lP r e s u r e . ' helwmPr"pellan1, a

oxidtier storagePlc>pPll."l.a 1"Clstorage

C ryu ecn ic . liquidhydrogenPressure. lue lc e l l easems

nilrugen

1L a a l l o n

CM RCSCM RCSCM RC SSM SPSSM SPS

Sht SPS

SM SPSSht SPS

SM SPS

S M RCS

CM ECS"C\ I LC S

SI1 RCS

s h i r o

Shl RC S

S M RC S

S M EPS'

SM EPS

SM EP S

OlP"lltyregulred-2221

1

I

I

2

41

I

3

III

1!I4

i

4

4d

d2

3

v e s s e l dlrnens1ons. h.DLsmeIer

9 . 2 012. 612. 610 . 1745

45

51

51

4 . 9 2

12 012 0

I 3 07 0

12 25I 2 2s

26 28 8

2 112 6

I2 6

12 6

12 . 6

25 5interiord i am et er

2 8 .2 4

6. 0

19 . 90117. 32_ _

153 . 05

153 . 05

152 30

15 2 38

9 16

l i I12 I

16R2221 R27 6

22 i

19. 907

17 . 32

Thlcheaa

0 .102, 0 2 2022366

041 to , 0 2 5

, 0 4 7

054 lo ,028

054 l o .02R

135

1 351 3 5

033 u n iR

050050

20 9

102I 0602 2

022

022

022

060 411

. 0 4 5

099

Dea1gn presure, PSIL l rn l t-

m360360

368522 5

22 5

22 5

22 5

2900

45004500

10201210

4040fin

17252247l i o na n24 8

24 8

24 8

1020

285

15W

606752552 5

1910300

300

300

300

5000

598559RT

131616nn

60

en90

245030001 8 5 5

36 0

360

48 0

48 0

1356

IW

3000

50 051054 0854033 7 5

337 5

33 7 5

337 5

i5wIWOinon

1530!ROO

7272

1503370J i 4 02550

37 2

372

54 0

54 0

1530

45 0

5180_ _

-ormalperatlngreallurePe l1150

29 129 1

3600186

__

I6 6

186

18 6

2550

41504400

90 0900

323240

16001 4 6 01330

18 5

I 85

18 5

185

90 0

!2 5 and 21

1500

1. 51 51 51 51 5

I 5

1 5

1 5

2 5

I 51 55

1 5I 5

i nI R2 51 95I 651 s c1 5

1 5

1 5

1 5

1 5

1 5

2 0

I50 and 8900385 an d 1013185 and 1043

RIM41 3

41 3

41 3

41 3

820 2n d i n onp8000mo o

140 an d 2RRO76 7 an d 3 0 7 8

I 2 @120

- 1 5 03 7 2 6

w i n an d 4088?Y2'> an d Z ' i 4 1 1

56 7

60 3

885 and 1043

885 an d 1043

1873 amblenl

77 5 and 81 2P m b l en lPk00 an d I O OW

3


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4


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I 1 1urgetank7

Sy s te m B helium tank

Figure 1. - Command module reactioncontrol system vessels.

cnidizer tank cnidizer tank fuel tank tank

Figure 3. - Typical service modulereaction control system quadvessels- lock 11.

Rapid repressurization tanks

Figure 2 . - Command module environ-mental control system oxygen supplyvessels.r Fuel cells

- -.- hydrogen tank installedin opposite bay forApollo 14 and subsequentspacecraft

Figure 4 . - Service module electricalpower system vessels.

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Fuel storagetank sector 6f

xidizer sumptank sector 2- /

Figure 5 . - Service module servicepropulsion system vessels-Block 11.

Gaseous oxygen(env i ronmenta lcontrol system)7

Oxidizer

L F u e l a nkHelium lank l reactioncontrol cy5leini

d x i d i i e r ank (reactioncontrol 5yrteiri l

Pitch control motor clower jettison motor case

,Launch escape motor casey//

Figure 6. - Launch escape systemvessels.

System Atankage

H e l i um p r es s u r eregu lating package,System B tankage

Figure 8. - Lunar module reactioncontrol system vessels .Figure 7. - Lunar module ascent st agevessels.

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L elium tank

Significant performance data on Apollovesse ls have been generated during livepropulsion-system tests at the NASA WhiteSands Test Facility (WSTF). Generally, aspacecraft ves sel re ceives 10 to 1 2 pressurecycles during its lifetime; however, manyof the flight-type vessel s at the WSTF havecompleted, without incident, more than350 cycles to or above operating pres su re .This high degree of su cc es s is attributedlargely to the close cont rol of ves se l fabri-cation and use that has been exercised onthe Apollo Pr og ra m and on the use offracture-mechanics technology to predictsafe vessel operation.

Figure 9. - Lunar module descent stagevessels.TABLE 111. - SPACECRAFT PRESSURE-VESSEL QUALIFICATION

I Test performedIVibrationank functionService propulsion system

Pressurizat ion Block I

Oxidizer Block I

Fuel Block I

Sump Block I1Gaseous nitrogen valveactuation

Shock Burst

XXXXXXXXXXXXXX

7


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TABLE III. - SPACECRAFT PRESSURE-VESSEL QUALIFICATION - Continued

Tank function

CM reaction control systemPressurization

Oxidizer

FuelSM reaction control system

Pressurization

Oxidizer

Fuel

Secondary vessels Block I1Oxidizera

bFuel

VibrationTest performed

aSame as CM reaction control system oxidizer.bSarne as CM reaction control system fuel.

8

~~~

Creep Cyclic Burst


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TABLE 111. - SPACECRAFT PRESSURE-VESSEL QUALIFICATION - Continued

Tank function

CM fuel cellCryogenic oxygen

Cryogenic hydrogen

Gaseous nitrogenpressurization

Oxygen surge

LM reaction control systenPressurizationOxidizerFuel

LM ascent propulsionsystemPressurization

Propellant

VibrationTest performed

Shock Creep Cyclic Burst

XXXXXXXXXXXXXXXXX--XXXXXX

XXX

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TABLE III. - SPACECRAFT PRESSURE-VESSEL QUALIFICATION - Concluded

Tank function

LM environmental controlsystemAscent stage gaseous

oxygenDescent s tage gaseousoxygen

LM descent propulsionsystemPressurizat ion ambient

Supercritica l helium

Propellant

Vibration

XXX--

XXXXX

Tes t performedShock Creep cyclic [ Burst

QUALITY ASSURANCEThe req uirements for the qual ity-assurance programs of NASA contracto rs aredelineated in NASA quality publication NPC 200-2. Applicable portions of this documenwere imposed by the NASA Manned Spacecraft Center (MSC) on pressure-vessel manu-facturers at the beginning of hardware fabr icat ion to en sure adequate cont rol of processing and fabrication vari ables and to provide fo r the availability of pe rtinent informationand data concerning materials proper ties, deviations fro m requirements, Material Re-view Board (MRB) actions, and test data on each pre ss ure vessel.

Addi t iona l Requi rementsBecause of problems that had been exper ienced f ro m the beginning of v es se l fab-rication to early 1967, a decision w a s made to evaluate each vess el on each Apollospacecraft fo r flight worthiness be fore launch. Since that time, each Apollo pr es su reve ssel has been accompanied at vehicle delivery by a data package or "pedigree" thatcontains a manufacturing and processing h isto ry, including any discrepancies, and apressurization data log. This requ ireme nt is in addition to those contained in the NASAquality publication. In this way, each vess el has been fully characte rized fo r individua

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evaluation of s truc tura l integrity and projected life capability. The following specificinformation is contained in each data package.1. Material and proce ss specifications (submitted one time pe r generic ve sse lunless subsequently revised)2. W e l d para meter s and repair history3. Chemical analysis of ma te rial s (including weld wire)4. Mechanical properties verification for materials5. Certificat ion of acceptance test ing6. Pressu rizat ion history (including fluids and times)7. Fluid exposure history (other than during pressuriza tion)8. Discrepancy rep ort s and reviews9. Nondestructive evaluation requir ements and certification

Inspec t ion and D i sc repancy Report sPre ssu re vessels have been inspected by the contractor at prescribed stag es dur-ing fabrication and jointly inspected by contractor/NASA representatives at mandatoryinspection points incorporated throughout vessel fabrication and during subsequent sys-tem buildup. Discrepancy repo rt s have been written for all anomalies or out-of-specification conditions detected during inspection o r test. These discrepancy reportshave been resolved by means of MRB action by responsible cont ractor and NASA repr e-sentatives and a r e part of each data package. Particularly serious discrepanc ies suchas anomalies requiring detailed stress analysis or generation of empirical data for ma-terials evaluation have been resolved at higher levels but ar e reflected in MRBdocumentation.Discrepancies originating at the prime contractor facilities can be classi fied gen-erally as scrat ches , dents, o r fit-up irregularit ies. Discrepancies originating at thesubcontractor (v ess el manufacturer) faciliti es can be classified generally as machininger ro rs , scr atches, dents, weld mismatch, weld porosity, dimensional anomalies, and,in one instance , a heat-treating err or . The discrepancies have been random; no trends

have been observed.In the first 13 command and se rv ic e modules and nine lunar modules, 176 and123 ve ssel discrepanci es, respectively, were found. The number of di screpancies,which is sma ll considering the total number of vessels (926) involved, includes manythat were minor, such as superficia l scratches. The number of vess el discrepanciesper spacecraft is shown in figu res 10 and 11.

11


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- ccurring at prime 0 ccurring at vessel vendorcontractor24

Spacecraft

= ccurring at prime 0 ccurring atcontractor vessel vendor

111 112 113 114Spacecraft

Figure 10. - Command and se rvi cemodule pressure-vesse 1discrepancies.Figure 11. - Lunar module pre ssure-vesse l discrepancies.

Nondestruct ve Test ngThe major NDT methods that have been used on Apollo ve ss el s are ultrasonic in-spection of vessel details after the machining phases of fabrication, X-ray inspection,dye-penetrant inspection of welds, and dye-penetrant inspection of certain ve ss el mem-brane surfaces. In addition, magnetic-particle inspection has been used on s teel ves-

sels such as the lunar module descent s tage gaseous oxygen ves se l fabricated fr omDGAC steel. However, NDT has had some uncertainties that have introduced limita-tions in the use of these methods. For example, X-ray inspection can reveal surfaceand subsurface flaws, but flaw orientation can mask detection; whereas, penet rant in-spection detects only flaws that are open to the surface. In addition, routine inspectionmethods are dependent on the technique and training of the inspector and the nature ofthe f l a w . For this reason, a precis e l imit could not be assigned to the minimum sizeof the f law or to the population that would be detected by the inspection methods.In the case of some metallurgical flaws in the material , such as massive embrit-tled alpha in titanium or titanium-hydride formation at welds, no satisfactory methodof NDT is known. Such flaws are rare but have caused failure in pressure vessels. A

neutron radiography technique has met with some success but requ ires development.Because of t h e uncertainties involved, NDT techniques have not been used aloneto guarantee the integrity of pressure vesse ls. However, NDT techniques have beenexcellent means of monitoring fabrication control and ensuring production consistencyand compliance with specification requi rement s. In th is respect, the application ofNDT has facilitated the delivery of basically sound pr es su re vesse ls that have been con-trolled during use according to fracture-mechanics cri ter ia to ensure safe operation.

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A P P L l C A T I O N OF FRACTURE-MECHAN ICS TECHNOLOGYThe heat treatment of many pressu re-vesse l alloys to high strength levels and useat high stresses (low safety facto rs) result s in a potentially inc reased sensitivi ty tosmall f l a w s and various environments. Limitations in present nondestructive inspec-

tion techniques could allow some relatively small f l a w s i n the vessels to escape detec-tion. Therefore, attention had to be given to the method by which these f l a w s couldpropagate during ground testing and flight to ensure that such f l a w s would not result infail ure of the pre ssure vessels. In 1967, approximately 3 years after vessel fabrica-tion w a s begun, the ves sel proof te st became a valuable inspection technique using afra ctu re -mechanics approach and provided a baseline fo r postfabrication ves sel analysis.The concepts of linear-elas tic fr ac tu re mechanics (refs. 1 and 2) have been usedto examine the relationship between the maximum f law size in a pressure vessel thatpasses a proof test and the subsequent subcritical crack growth possible in ground-testand flight environments.The fracture-toughness/stress- ntensity approach to fr ac tu re analy sis generallyhas been accepted as the best available means of using fracture-mechanics technologyin practi cal problems. The fracture-toughness parameter describes the maximum f l awsize that a mater ial can tolerate without rapid fr actur e when stressed to a prescribedlevel and is the value obtained fo r the stress-intensity factor that results in f l a w insta-bility (str uctur al failure). This value then is called the critical stress intensity orfracture toughness. Any stress intensity at a flaw that is less than the fract ure-toughness value is subcritical.Fracture - toughness (apparent K IC ) proper tie s have been obtained fo r Apollo

pressure-vessel materials. To investigate the compatibility of a pressure-ves sel mate-rial with the fluid the vessel is intended to contain, str ess ed vesse l-ma teri al specimenshaving cracks introduced of known size and shape have been exposed to the environmentin question. The determination was made that, f or each environment test ed, an appar-ent threshold stress intensity Kth exi st s below which cracks in a given vess el alloy donot grow. The experimental threshold values fo r various Apollo vessel mater ials andenvironments are listed in table IV. Most of the threshold data in table IV have beendeveloped for tim e exposures consistent with Apollo requirements. For missions longerthan those of the Apollo Program, long tim e data must be obtained. A method for de-termining thresholds is described i n reference 3.

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TABLE IV. - THRESHOLD VALUES FOR APOLLO PRESSURE-VESSEL MATERIALS AND SELEC TED ENVIRONMENTS

Alloy

ritaniuni 6AI-4V

AM 35 0Inconel 71 8Titanium 5AI -2 . 5S n

remperature." F

66707 47 88 28 69 09 49 810 210 611 011 411 812 212 613 013 413 814 214 615 08585- 2 9 785

Nitrogentetroxide

828 18 0797 8777674737 17 06867656352605856555351_ __ __ __ _

Mononlrthy Ihydrazine

88828 18 1808 07978-777 67574737 17 06R676564626059_ __ __ _- -

Threshold stress intensity K t h . percent KIC

Aerozine- 5 0

83828 18 18 08 0797877767574737 17 068676564626059- -_ __ _- _

Trichlorolri-I luoroethane-eld

Air. helium.nitrogen. andoxygen

TABLE V . - APOLLO PRESSURE-VESSEL STRUCTURAL FAILURES

Pressu re vesselSM 'RCS propellant

SM/SPS propellantSM/SPS propellantSM/electrical powersystem gaseousnitrogenLES motor caseLM descent stagegaseous oxygenLM descent stagepropellantSM/electrical powersystem cryogenicoxygen

Failures10

a211

2

1

1

a 1

Cause~~ ~~

Nitrogen-tetroxide stresscorrosionMethanol s tre ss corrosionWeld cracks (proof)Crack f rom embrittled weldrepair (leak test)

Weld crac ks plus incompatiblefluid (water) (proof)Material defect plus incompatiblcfluid (water) (acceptance test)Massive alpha defect (proof)

Internal ignition resulting inoverpressurization failure

Distilledwater

Date1965 to 1966

October 1966October 1963April 1967

August 1967

June 1966

March 1965April 1970

Liquidoxygen

1 4

aVesseis were installed in spacecraft.


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1 08

Apparent threshold K, KIC = 0 6 * K t hI= 420

--.--

(a) Log time to failure as a function (b ) Flaw size as a function ofof Ki/%C. membrane stress.Figure 12. - Application of stat ic fra ctur e toughness and subcrit ical f l aw growthto pressure -vessel analysis.

aA

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method of analyzing pr es su re vess el s using the modified Irwin equation fro m refer-ence 2, safe operation could be predicted fr om the ratio of the proof pr es su re to theoperating pressure and fr om the threshold or stress-intensity rat io required for crackgrowth.Although during flight the pre ssu re in Apollo vessels is noncyclic, the effect ofcyclic f l a w growth during leak checks and other pre ssure tests of Apollo ve ss el s als o

had to be considered in vessel analysis. The maximum size flaw scr eened by the prooftes t was assumed to exist, and subcr itical fatigue f l a w growth caused by cyclic stresswas evaluated and incorporated into each analysis. For example, the cyclic flaw-growthrate for 6A1-4V titanium alloy was determined experimentally in te rm s of stress inten-sity as a function of the number of cycles to failu re a s described in re ference 4. Fromthese data, a curve showing f l a w growth rate per cycle at various Ki/KIC levels canbe constructed and used in vessel analysis.To analyze a pre ssu re vessel, three types of basic data a r e needed.1. The f racture toughness KIC of the material2. The constant-stress flaw-growth threshold Kth in the environments to whichthe vessel w i l l be exposed3 . The rate of cyclic growthFracture-mechanics analyses of Apollo pr es su re ve sse ls and determinations ofenvironmental compatibility thresholds were init iated at the MSC in late 1966. The ef-for t resulted in the initiation of pressurizat ion re str ict ions and environmental exposurecontro l of a ll ve ss el s by mid- 1967. The fracture-mechanics cri ter ia and the techniquesfor analyzing and controlling Apollo pre ss ur e vessel s were contractually imposed on theApollo spacecraft contractors. The implementation of these specifications establishedcommon a n d consistent posffabrication evaluation procedures fo r all Apollo spacecraftpres sure vessels. Basic overall considerations and guidelines fo r fr act ure control ofpress ure vessels ar e presented in reference 5 .

P R O B LE M S A S S O C I A T E D W I T H VE S SE L S U S E D I NAPOLLO SPACECRAFT

The pressure-vessel problems experienced by the MSC and associated Apollo con-tracto rs a re summarized in this section. Hardware problems that resulted in struc -tural failures ar e listed in table V. Only th ree fa ilures occurred on ves sel s installedi n spacecraft: two during systems te st s and one during the flight of Apollo 13. Gen-erally, structural f ail ure s resulted fro m unexpected environmental-stress-crackingeffect s on the ves se l materi als o r insufficient cont rol of materi als o r processes . Spe-cific problems and resolutions a r e discussed in the following paragraphs . Potentialproblems that were not actually experienced but that required evaluation and resolutionare discussed in the last three paragraphs in this section.

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S p e c i f ic P r o b l e m sNitrogen tetroxide incompatibility with 6A1- 4V titanium-alloy pr es su re vessels. -In 1965, a 6Al-4V titanium-alloy vessel containing nitrogen tetroxide (N204) nder pres -

su re suffered localized stres s-cor rosio n failure. Because of known reactions betweentitanium and other oxidizers, extensive test programs to establish compatibility betweenN204 nd the vess el alloy had been conducted successfully before 1965. Investigation ofthe cause of the fa ilure resul ted in the unexpected discovery that the alloy would undergogeneral stress corrosion in certain gr ades of N 2 0 4 (refs. 6 and 7). This discovery w a sparti cularly ser iou s for the Apollo Pro gr am because 17 titanium vessel s on each space-cra ft contain N204.

The chemical makeup of the N 2 0 4 , primar ily the nitric oxide (NO) content, deter-mined whether stress corrosion would occur. Previous tests had been conducted usingN204 hat had a relatively high NO content. Subsequent investigations showed thats t re ss cor ros ion would not occur with NO contents grea ter than 0.5 percent. The actuallower limit has not been defined. A t the MSC, a specification w a s generated for N 2 0 4requiring an NO content of 0.8 f 0 .2 percen t to prevent recurrence of the problem. Inaddition, each lot of N 0 used fo r Apollo flights has been tes ted using precracked6A1-4V titanium-alloy specimens. These data have been analyzed using fracture-mechanics principles to verify t h a t the stress-corrosion threshold w a s sufficientlyhigh to preclude environmental cra ck growth i n flight. The application of fr ac tu remechanics to Apollo ve ssels is discussed in a separate section.

2 .4

A s a precautionary measure, each lot of fuel (Aerozine-50 and monomethyl hydra-zine) used fo r Apollo flights has been test ed using precracked specimens to verify thatthe stress-corrosion thresholds are acceptable because all possible combinations andlevels of impu rit ies permitted by specification may not have existed in ea rl ie r success-ful compatibility tests .Methanol incompatibility with 6Al-4V titanium-alloy pre ssu re vesse ls. - Substi-tute fluids that s imulate the specific gravity and flow character is tics of the propellantshave been used to avoid the hazards associa ted with hypergolics when tes ting propulsionsys tems installed in spacecraft. Methanol was used to simulate Aerozine- 50 duringflow tests of the SM propulsion system at operating pressures.The first of two fa ilures with methanol occur red on October 1, 1966, when a

Block II SM vessel developed three sep ara te leaks near the bottom girth weld while atnormal operating pres sure . Originally, the cause of the fai lure appeared to be localizedstress corros ion caused by an undetected materials anomaly. Twenty-four days la ter ,a Block I ve sse l containing methanol failed while pres suri zed to maximum operatingpr essu re . The failur e was explosive, rupturing an adjacent vess el and destroying theSM. Because the ve ss el w a s not filled to capacity, the ullage contributed to the highlevel of damage.Investigation of the fai lures showed that pure anhydrous methanol is incompatiblewith the 6Al-4V titanium-alloy v essel materia l and result s in sev ere stre ss- cor ros ion

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attack (re f. 8). An SPS vessel that had successfully undergone tes ts with methanol wassubjected to a proof pressure test and failed at 90 percent of the design proof level,showing that damage to the vesse l had occu rred during methanol exposure although fail-ure had not occurred. Therefore , methanol was eliminated fro m use in Apollo pres su revessels.Subsequently, with initiation of f rac ture-mechanics evaluation techniques, ;?11

other fluids used in Apollo ves sel s were tested using precracked specimens. A s a re -sult, trichloromonofluoromethane was res tric ted fro m use because of a relatively lowstre ss-corrosion threshold although hardware failure had not occurred.Fai lure of a serv ice propulsion~~~ system main propellant vess el during proof test. -In 1963, during an acceptance proof te st , a failure of a Block I vessel occurred that w a sattributable to the development of an axial fracture in the heat-affected zone of a dome-to-cylinder weld. The fracture had transgranular cleavage of a type observed inconjunction with contamination of the 6Al-4V titanium alloy. The source of the contam-ination was not definitely established. However, the contamination probably occu rredduring welding o r preparat ion for welding. The procedures and techniques were re-viewed and modified in the areas that would protec t against contamination during thewelding operation. Handling with clean-gloved hands was made a requirement, and avapor-blast cleaning procedure was initiated shortly before welding to ensure cleansurfaces. No subsequent occurrences of contamination were detected in SPS propellant-vess el welds.Acceptance test fai lure of a lunar module descent gaseous oxygen ves sel . - An L Mdescent stage gaseous oxygen pr es su re ves se l made of D6AC s teel failed during accept-ance testing. Failure analysis showed that the origin of the failu re was a preexistingcrack in the radius of the mounting boss. During acceptance testing, which consistedof p ress ur e cycling the vesse l in a water bath, the cra ck grew. The D6AC st ee l has avery low flaw-growth threshold in water. The tank manufacturer had been dir ected bythe L M contractor not to use wate r as a pressurizing medium, Unfortunately, no men-tion was made of the medium in which the ves se l was im mer sed during testing, and theus e of water in thi s application was not di scovered before the fail ure. Another contrib-uting discrepancy was omission of inspection. The final machined boss ar eas receivedno detailed nondestructive testing o r inspection afte r machining. The si ze of the pre-existing crack in the failed vessel was of s uch magnitude that the ve ssel would have beenrejected.Subsequently, water w a s replaced by a more compatible fluid, trichlorotrifluoro-ethane. All boss a re as have been dye checked and magnaflux inspected fo r flaws; radio-graphic inspection was instituted on all domes before welding. Subsequent rigorousenvironmental tes ts of sample vesse ls have completely qualified the design and use of

the vess el s on Apollo spacecraft .Fai lure of launch escape system motor cases during proof tests. - In 1967, twoLES motor cases made of 4335V ste el failed during proof testing. These ca se s, andothers in the same lot, had been welded with a fill er wi re having a higher carbon con-tent than had been used on previous lots, including the qualification ca se s. This unau-thorized substitution made the ca ses more susceptible to cracking in the weld areas andmore susceptible to attack by water (the pres suri zing fluid) w h i l e under stress. As aresult, all cases in the affected lot were res tri ct ed fr om flight use. In subsequent lots ,

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oi l w a s substituted for water as a pressurizin g fluid. Quality-assurance requirementswere revised to ensure that all fabrication steps and tes ts w ere accomplished in a sat-isfac tory manner . Additional case s have been fabricated and tes ted under the new re-quirements with no recurrence of the problem.M a s s i v e alpha inclusions in 6A1-4V titanium alloy. - An L M descent propulsionsystem (DPS) essel failed during a hydrostatic acceptance proof test at 267 psig, whichwas 74 percent of the required 360-psig proof pressure. A metallurgical investigationshowed that the failure w a s caused by a localized micros tructure abnormality consistingof "massive" alpha-phase str uc ture in the upper dome. A second instance of massivealpha inclusion was detected in an L M ascent propulsion system (APS) vessel dome dur-ing a hand-blending phase of fabrication because of the comparative hardness. Thedome w a s rejected and therefore never subjected to a proof test.Alpha inclusions of this sor t a re rare but, if they are present, cannot be detectedby the standard NDT techniques used. Reliance on the proof te st to scr ee n a gro ss con-dition in a finished pre ssu re ves sel has been the only practi cable approach. Becausethe condition cannot be detected at the time of acceptance of the ve sse l forgings, a con-

siderable dollar loss is experienced with failure of a completed vessel . Although theoccurrence of mass ive alpha is rare, the development of a suitable NDT technique maysave costs and add confidence in the future use of titanium pr es su re vessels .Fai lure of an elect rical power sy stem gaseous nitrogen pressu re vessel. - In 1967,an elec tri cal power system (EPS) nitrogen vessel leaked in the gi rth weld during a pres-su re decay test. Investigation of the fai lure disclosed that a rep aired ar ea of the weldwas contaminated with oxygen, result ing in an embrit tled condition t h a t w a s susceptibleto crack initiation and growth under cyclic conditions. An evaluation of the weld repairtechnique and equipment showed that a high probability of oxygen contamination existedduring weld repa ir . Investigation of other nitrogen tanks that had weld repa irs verifiedthat oxygen contamination was a problem. A s a result, all EPS nitrogen vesse ls that

had weld rep ai rs wer e deleted fro m the Apollo Prog ra m and the repair technique w a smodified. Contamination was not present in unrepaired ves sel s, and no subsequentproblems have ari sen when re pa ir s have been required.Failure of a cryogenic oxygen vessel during the flight of Apollo 13. - InFebruary 1970, the Apollo 13 spacecraf t experienced a rapid los s of p re ss ur e in thenumber 2 liquid oxygen vessel, which is fabricated from Inconel 718 alloy. Essentially,the findings of the investigation of the Apollo 13 anomaly indicated that ignition occurredinside the vessel at a point of degraded Teflon wire insulation. A fire propagated rap-idly inside the vesse l, increasing the temperature and pres su re until the strength capa-bility of the vessel itself w as exceeded and failure occurred.The structural capability of the oxygen vessel had been demonst rated in qualifica-tion tests and on previous Apollo flights. Inconel 718 w a s al so known to be compatiblewith liquid oxygen. The fault existed in internal design where elec tri cal components,Tef lon-coated wi re , and terminals exposed directly to an oxidizing environment re-sulted in a hazard under cer tain adverse conditions.

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Potential P r o b l e m sPotential hydride formation in titanium-alloy-vessel welds made with unalloyedfiller wire. - An explosion of a Saturn S-IVB tage during a test in March 1967 was at-tributed to the fail ure of a 6A1-4V titanium-alloy pr es su re ve ssel containing helium gas.The ves sel had inadvertently been welded using unalloyed filler wire instead of the spec-ified 6Al-4V titanium-alloy fil ler wire . The failed weld had severe titanium-hydride

banding that weakened the weld and ultimately result ed in vessel fa ilu re at operatingst re ss . Investigators of the failure postulated that the hydrides formed over a periodof t ime in the region of the re latively abrupt change in hydrogen solubility between the6Al-4V titanium alloy and the essentially unalloyed titanium weld. It w a s suggested thathydrogen migrated under s tr es s conditions from a region of high solubility (alloy ve ss elmaterial) to a region of low solubility (unalloyed weld) and precipitated as titanium hy-drides upon reaching this zone.The latent occurrence of hydrides in the failed S-IVBvessel s caused concern aboutthe integrity of the SPS, DPS, and APS 6A1-4V titanium-alloy propellant ve ss el s that a r ewelded, by requirement, with unalloyed fill er wire. A comprehensive program w a s un-dertaken to evaluate the Apollo welds, and it was determined that the concern about la-

tent hydride formation in the Apollo welds w a s unwarranted.In summary, the S-IVBhelium-vessel weld w a s different geometrically and thicker(0.452 inch) than the Apollo welds (0..070nd 0.090 inch), which us e unalloyed wire. Inthe Apollo welds (including repair ed welds that have additional unalloyed fil ler wire), ap-parently sufficient alloying exists in the weld nugget to preclude a hydride problem.Conversely, some Apollo titanium vesse ls a r e required by specification to bewelded with alloy fill er wire. These ves sel s a r e relatively thick and more closely ap-proximate the geometry and conditions of the S - N B weld. To ensure that the se weldswere made using alloy wire, an eddy-current technique was used that would distinguishbetween welds made with alloyed o r unalloyed wire . No instances of wrong fil ler -wire

use were detected.Potential alpha-stringer structure problem in 6A1-4V titanium alloy. - The 6A1-4Vtitanium alloy has been used for 4 7 vessel s in the various Apollo spacecraft propulsionsystems. Therefore, a potential problem with this mate ria l was reason for concern be-cause a serious general materi al anomaly would have a severe negative impact on theApollo Program.Briefly, the metallographic struc ture of the alloy normally cons ist s of a beta-phase matrix that has a dispersion of equiaxed alpha-phase "islands. " During test s toverify the properties of a certain lot of forgings f o r LM vessels, comparatively lowelongation values were noted on a number of tensi le test specimens taken from forgingtrim rings. A l l other mechanical proper tie s were within specification limits. Metallo-graphic examination showed that many of the alpha constituents in the s truc ture had anelongated shape and were representat ive of alpha platelets o r "stringers" rather thanof the equiaxed alpha islands. A definite correl ation was established between the lowelongation and the presence of s tr inge rs .A program was conducted to evaluate normal and alpha-stringer st ructure underidentical test conditions. Results of the te st s indicated that no significant differences

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in behavior existed between the two stru ctur es except fo r ductility. The presence ofalpha-st ringer st ructure would not significantly affect the performance capability of the6Al-4V titanium-alloy pr essu re ves sel s as long as elongation values were greater thanthe minimum specification limit of 8 percent.Acicular o r elongated st ruc ture must not be confused with the "massive" o r "lowdensity" alpha- inclusion problem that is a threat to st ructural integrity.Potential fail ure of lunar module descent propulsion syst em vessels during flight. -Cyclic-flaw-growth analysis for the LM DPS vesse ls showed that the maximum flaw thatc&ld exist x he ves sek after a normal proof test could grow during ground pres sur iza -tions to a si ze that would give a s t re s s intensity above propellant threshold values duringflight. To provide added cyclic life, a cryogenic liquid-nitrogen proof test was insti-tuted and added to the existing ambient proof-test requirements. The cryogenic prooftest has benefited by two changes in mater ia l properties that occur in the 6A1-4V t ita-nium alloy with temperature . At -320" F, the alloy strength is increased, providingfor a higher proof-test pr es su re (430 psi), and the fractu re toughness of the alloy isslightly decreased , making the alloy sensitive t o smaller f l a w s during the proof test.Therefore, the cryogenic proof t est screened to a f l a w size much smal ler than the am-

bient proof tes t, providing increased flaw-growth capability during vessel use. Thiscapability ease d tes t rest rict ions and made contingency pressur e cycles available forre te st s of the DPS if required.PRESSURE-VESSEL MANAGEMENT

During the ea rly phases of the Apollo Program (to the end of 1966), each NASAsubsystem manager had sole responsibility for the pre ssu re ves sels in his respectivesubsystem. Because the background and training of the subsystem managers varied,inconsi stencies evolved in press ure-v essel requirements and usage.As the Apollo Progra m progressed, many problems with pres sur e vessels re -quired special ized ski lls and training not possessed by the subsystem managers.included problems rela ting to metal lurgical considerations (welding, heat treat ing,forging, and so forth) and se rvi ce problems involving mate rial s compatibility and safeoperation analysis .

These

In 1967, a decision was made at the MSC tha t spacecraft press ure v essel s wouldbe treated as a unique subsystem with designated responsibility and that requirementsand us e cr it er ia would be standardized. Definitions of p ressu re-vessel te rm s also werestandardized fo r the Apollo Program, as presented in the appendix. An MSC technicalmonitor, who had the following general responsib ilities , was appointed.1. Ensuring the struct ural integrity of press ure ves sels2. Implementing fra ctur e mechanics to ensure that all flight vesse ls will meetmission requ iremen ts (pr ess ure s, temperatures, and number of cycles ofpressurization)3 . Ensuring that all fluids to which vessels a r e exposed aft er acceptance te stsare compatible with tank materia l '

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4 . Ensuring that required struc tural safety facto rs a r e maintained5 . Examining qualifications tes ts fo r adequacy6. Assessing fabrication procedures and methods7 . Reviewing manufacturing disc repanc ies and resolutions8 . Participating in the resolution of discrepancies where required9 . Establishing allowable pressure/temperature relationships for vessels duringeach Apollo flight

Single points of contact and responsibility for p re ss ur e vessels al so were designated atthe pr ime contractor faci liti es to inte rface with the MSC monitor, thus facilitating work-able pressure-vessel coordination and control. In addition, an MSC quality monitor wasassigned to inte rface with the MSC technical monitor and the designated prime contractorpoint of contact. It w a s the responsibil ity of the quality monitor to ensure contractorcompliance with the quality-assurance data-package requirements .CONCL UDING REM ARKS

During the Apollo Program, pre ssu re vessel s have been criti cal to spacecraftoperation and safety. In addition to stringent mate ria l and fabrication controls, an in-cr ea se in the level of confidence associated with pres sure-vesse l us e w a s achieved bymeans of the application of frac ture-mechanics cr it er ia to provide confidence in estab-lishing f luid/pressure/temperature limitations for pressu re-ves sel operations.Pressure-vessel safety factors as low as 1 . 5 have been shown to be practical,

provided proper mater ials , processes, and usage evaluations a r e made. Stringentmateri al and fabrication control must be implemented to ensu re consistency in metal-lurgical factors that, i f varied, can significantly affect ve ssel performance. Mater ialproper ties may be highly sensitive to variations in composition, manufacturing methods,o r service exposure. Unfortunately, metallurgical analysi s of vessel fai lu res usuallyprovided the first evidence of mate ria l fac tor s adversely affecting performance. Thereason s for the problems and steps to preclude them were generally "after the fact. ''An analys is of the application supplemented by required testing of m ater ia l before de-sign, therefore, is of major importance. Compatibility of pre ssuran ts with the vesselmater ial under use conditions must be established, and subsequent fluid and pre ss ur econtrol consistent with te st data is mandatory. Manufacturing methods and se rv iceenvironments must be evaluated thoroughly. The proper analysis and use of mater ia lsfo r pressure -vesse l serv ice can be achieved only by means of full coordination amongsyste ms engineers, designers, st re ss analysts, and mater ials specialists in meetingsys tems requirements. Responsibility must be delegated to ensu re that all are involved.

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RECQMMENDATIONSBased on Apollo spacecraft experience, the following recommendations a r e madeto reduce ris k and to ensure pressure-v essel integrity on future programs.1 . Apply fracture-mechanics criteria in pressure-vessel design.2 . Evaluate design and materi als selection of components to be used inside pr es-su re vesse ls for potential adverse conditions and effects.3 . By use of the fractu re-mechanics threshold approach, establish the compati-

bility of the vessel mate rials with each fluid which w i l l contact the material whilestressed.4 . Actively protect against the use of improper o r unauthorized mate ria ls duringvessel fabrication.5 . Verify that weld repa ir techniques ar e sufficient to provide repaired ar ea s thathave integrity equal to unrepaired welds.6 . Establish consistent control requirements and cri te ri a for regulation ofpressurizations.7. Establish definite responsibility and authority in pressur e-vessel activity.8. Exercise discre tion when considering the elimination, because of cost savings,of any quality-control documentation requirements o r testing of pr essure vessel s.

Manned Spacecraft CenterNational Aeronautics and Space AdministrationHouston, Texas, March 3, 1972914- 13 - 20 - 06 - 72

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REFERENCES

1. Irwin, G. R. : Analysis of S tres ses and Strains Near the End of a Crack. J. Appl.Mech., vol. 24, 1957, p. 361.2. Tiffany, C. F.; Masters, J. N .; and Pall, F. : Some Fra ctu re Considerations inthe Design and Analysis of Spacecraft Pr es sur e Vessels. Presented at ASMNational Metals Congress (Chicago), Oct. 1966.3. Tiffany, C. F.; and Masters, J. N. : Investigation of the Flaw Growth Charact eris-tics of 6A1-4V Titanium Used in Apollo Spacecraft Pressure Vessels. NASACR-65586, 1967.4. Masters, J. N. : Cyclic and Sustained Load Flaw Growth Characteristics of 6A1-4VTitanium. NASA CR-92231, 1968.5. Tiffany, C. F. : Fractu re Control of Metallic Pressure Vessels. NASA SP-8040,1970.6. Johnson, R. E.; Kappelt, G. F.; and Korb, L. J. : A Case History of TitaniumStress Corros ion in Nitrogen Tetroxide. Am. SOC. for Metals, 1966.7. Kappelt, G. F.; and King, E. J. : Observations on the Stre ss Corrosion of the6A1-4V Titanium Alloy in Nitrogen Tetroxide. Presen ted at AFMC 50th Anniver-sary Corrosion of Military and Aerospace Equipment Technical Conference(Denver) , May 1967.8. Johnston, R. L. et al. : St re ss Corrosion Cracking of Ti-6Al-4V Alloy in Methanol.NASA TN D-3868, 1967.

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APPENDIXPRESS URE-VES SEL DEFl N T I ONS

Pr es su re vessel: Vessel containing a compressed fluid with an energy equal toor exceeding 14 250 ft-lb (0.01 pound trinitrotoluene equivalent) based on the adiabaticexpansion of a perfect gas

Normal operating pres sure: Regulated pressu re during syst em operation or thenominal f i l l value for press urization tanksRegulator lockup pres su re : Back pre ssu re at which the regulator completelystops the flow of pressur izing gasMaximum design operating press ure: Pre ssu re as limited by relief provisionsor maximum expected environmentProof pres sur e: Pr es su re that each vessel must have sustained to be acceptablefo r use in the spacecraftDesign bur st pre ssu re: Maximum pre ssu re that a vessel is designed to sustain.without ruptureSafety factor: The ratio of design burs t pressur e to maximum design operatingpressureSubsystem press ure verification test: A single pre ssu re t est of the subsystem ata specified pre ssu re above maximum design operating pr es su re to verify subsystem

integritySubsystem component installation pre ssu re verification test : A pressure testconducted at maximum design operating pre ssu re o r below to verify joints af te r rep lace-ment of components

apollo experience report pressure vessels

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