IMPLEMENTATION OF LEAK TEST METHODS FOR THE

IMPLEMENTATION OF LEAK TEST METHODS FOR THE INTERNATION SPACESTATION (ISS) ELEMENTS, SYSTEMS AND COMPONENTSSteve Underwood(1), Oleg Lvovsky(2)The Boeing Company 499 Boeing Boulevard Huntsville, AL, USA, Email: steve.d.underwood@boeing.com(2)ARES Corp., 16441 Space Center Blvd., Bldg. A, Houston, TX, USA, Email: olvosvsky@arescorporation.com(1)ABSTRACTThe International Space Station (ISS has Qualificationand Acceptance Environmental Test Requirementsdocument, SSP 41172 that includes manyenvironmental tests such as Thermal vacuum &Cycling, Depress/Repress, Sinusoidal, Random, andAcoustic Vibration, Pyro Shock, rence(EMI)/Electromagnetic Compatibility (EMCO), etc.This document also includes (13) leak test methods forPressure Integrity Verification of the ISS Elements,Systems, and Components. These leak test methods arewell known, however, the test procedure for specificleak test method shall be written and implementedpaying attention to the important proceduralsteps/details that, if omitted or deviated, could impactthe quality of the final product and affect the crewsafety. Such procedural steps/details for differentmethods include, but not limited to:- Sequenceoftesting,forexample,pressurization and submersion steps forMethod I (Immersion);- Stabilization of the mass spectrometer leakdetector outputs for Method II (vacuumChamber or Bell jar);- Proper data processing and taking aconservativeapproachwhilemakingpredictions for on-orbit leakage rate forMethod III(Pressure Change);- Proper Calibration of the mass spectrometerleak detector for all the tracer gas (mostlyHelium) Methods such as Method V (DetectorProbe), Method VI (Hood), Method VII(Tracer Probe), Method VIII(Accumulation);- Usage of visiblility aides for Method I(Immersion), Method IV (Chemical Indicator),Method XII (Foam/Liquid Application), andMethod XIII (Hydrostatic/Visual Inspection);While some methods could be used for the total leakage(either internal-to-external or external-to-internal) raterequirement verification (Vacuum Chamber, PressureDecay, Hood, Accumulation), other methods shall beused only as a pass/fail test for individual joints (e.g.,welds, fittings, and plugs) or for troubleshootingpurposes (Chemical Indicator, Detector Probe, TracerProbe, Local Vacuum Chamber, Foam/LiquidApplication, and Hydrostatic/Visual Inspection). Anyisolation of SSP 41172 requirements have led to eitherretesting of hardware or accepting a risk associatedwith the potential system or component pressureintegrity problem during flight.INTRODUCTIONBoeing in was the prime contractor for building,outfitting, and testing of the U.S. pressurized modulesfor the International Space Station: the Airlock,Laboratory, and Node 1 modules. This work wasperformed at Marshall Space Flight Center inHuntsville, Alabama. Each of these modules has a totalallowable leakage rate of about one tenth of a pound ofair per day, a requirement based on the available resupply gas.Component-level and system-levelrequirements based on hardware capability are morestringent, and are the basis for the lower-level leak teststo ensure good hardware installations.The Node 1 or “Unity” module contains six ports and isused for interconnection of modules in space. The U.S.Laboratory module will house equipment andexperiment racks, and the Airlock module will providecrew members access to space for extravehicularactivity (EVA). Figure 1 shows the Node 1, U.S. Lab,and Airlock modules. Potential leakage paths to spaceinclude the module structure, made of machined andformed aluminum panels welded together, andfeedthrough penetrations in the module structuresthrough which fluids and power pass to different areason the station. Other leakage paths are the CommonBerthing Mechanisms (CBMs) that mate the variousmodules together, the Hatches that close out themodules, and windows that allow crew observations ofexternal activities and space. A summary flow chart ofthe leak testing activities for acceptance of the flighthardware is shown in Figure 2.

Hatch (6 places)Common Berthing Mechanism (CBM)(6 places)Node 1 End ViewU.S. LaboratoryNode 1 Side ViewAirlockFigure 1: Boeing-built International Space Station Pressurized Modules

ModuleStructureFab & AssyModule ProofPressure/Leak Rate TestFeedthroughInstallation &Leak TestFeedthroughFabrication& AssemblyAModuleFunctionalTestingCBMInstallation& Leak TestStandoff/EndconePlumbing Install& Leak TestHose/TubeFabrication& AssemblyHatchInstallation& Leak TestEquipmentRackInstallationAEquip. RackFab & Assy,Plumbing Leak TestTransportModuleto KSCBHatch Fab &Assy, ProofPress/Leak RateBWindow Install& Leak Test(Lab Module)ModuleAcceptanceLeak TestFinal ModuleCloseout andGross Leak TestLaunchFigure 2: International Space Station Module Acceptance Leak Test FlowSTRUCTURAL WELD LEAK TESTINGLeak tests of the structural welds were performedimmediately following a 22.8 psig proof pressure testof the modules in 1996 and 1997. The leak testmethod was a helium detector probe test usinghelium mass spectrometer leak detectors (MSLDs).The modules were pressurized to 12-14.7 psig for an87% helium concentration. Portable counter-flowMSLDs were used in conjunction with 75 foot snifferhoses, and welds were sniffed at a speed of 1foot/minute. Probe heads which conformed to theweld surfaces were installed over the tip of thedetector probes to increase test sensitivity.The requirement for weld leakage was 1E-4 sccshelium per linear inch of weld. Calibrated heliumleaks sized slightly smaller than the requirement wereinstalled on weld test plates and were used tocalibrate the detector probes by passing the probehead over the leak and recording the MSLDresponse. This calibration was performed prior tostart of sniffing, a minimum of every hour thereafter,whenever detector probe operators were changed,and before shutting down the test. The calibrationverified that the operator’s technique and theequipment’s operation were sufficient to detect a leakequal to or less than the requirement. Over 1000 feetof structural welds were tested in this manner.FEEDTHROUGH LEAK RATE TESTINGFeedthroughs are installed in the module structure toroute power, data, and fluids to the ISS modules. TheNode 1 module contains 160 feedthroughs, theLaboratory module contains 100 feedthroughs, andthe Airlock module contains 40 feedthroughs. Thefeedthroughs were leak tested following installationto verify a proper installation. The leak raterequirement for the feedthrough leak test was 5E-6sccs air, and was verified with a helium massspectrometer leak detector (MSLD) hood test. Thetest setup is shown in Figure 3.

The feedthrough leak test procedure is as follows: Abell jar is placed over the external side of thefeedthrough and evacuated to 1E-2 torr with anMSLD. A helium containment hood is installed overthe internal side of the feedthrough. Helium isinjected into the hood and the MSLD output isrecorded until the feedthrough seals are fullypermeated and a leak rate is obtained.In addition to verifying the total leak rate of thefeedthrough seals, the ISS also has a requirement toverify that the seal installation meets its redundancyrequirement. Each ISS sealed joint is required toHeliumContainment Hoodhave a minimum two seals, and must be tested on theground prior to launch. ISS seals larger than sixinches in diameter have leak check ports whichprovide access between the two seals for leak testingeach seal individually. The feedthrough seals,however, have no leak check ports, so theredundancy is verified through analysis of thefeedthrough permeation curves. (Permeation throughtwo seals is slower and lower in magnitude thanpermeation through one seal.) Many installationproblems were detected and corrected by using thepermeation data to verify seal redundancy.Bell JarFeedthroughUnder TestMass SpectrometerLeak Detector(MSLD)Figure 3: Feedthrough Leak Rate TestingPLUMBING LEAK TESTINGTubing and hoses in the International Space Stationroute fluid and gases throughout the Station. TheU.S. Laboratory Module also has a Vacuum Systemwhich provides experiment racks with a spacevacuum for experimentation and with an exhaustvacuum for venting unneeded gases. Plumbinginternal to the modules is routed through the modulestandoffs and endcones and in the equipment racks.The plumbing was thoroughly leak tested as it wasinstalled to ensure crew safety.Leak testing of the vacuum system plumbing wasperformed on the pre-integrated standoffs prior toinstallation of the standoffs into the module, and thensystem level tests were performed followingcompletion of the flight systems. The test methodwas a helium MSLD hood test and is illustrated inFigure 4 for one manifold of the Vacuum System.The manifolds under test were evacuated with anMSLD. A helium standard leak was connected to therack location farthest from the MSLD; the leak wasvalved onto the manifold in order to determine thesystem sensitivity to a known helium leak. The jointsin the manifold were bagged using plastic and tapeand injected with helium one at a time. The seals inthe vacuum system are metal gaskets, and each joint

had a leak rate requirement of 1E-8 sccs helium.RoughingPumpHeliumStandard LeakQuick Disconnectsat Equipment Rack LocationsTest EquipmentB1B3B5B7Bellows in VacuumSystem ManifoldB2B4B6B8B11B12B13MSLDTestEquipmentFigure 4: U.S. Laboratory Standoff Vacuum System Manifold Leak TestPressurized Plumbing SystemsISS pressurized plumbing systems include theThermal Control System which provides coolingwater to the electronic hardware and Oxygen andNitrogen Systems for air re-supply. Flex hoses andsystem components were acceptance tested by thevendor prior to delivery and installation. Theinstalled plumbing was leak tested by pressurizingthe systems with helium to maximum operatingpressure and performing helium detector probe testson the joints. A major challenge with the leak testingof the plumbing was the teflon-lined flex hoses thatpermeated helium, thus increasing the heliumbackground levels during testing. High quality/purityair was used to purge the helium from the jointsunder test during sniffing. A helium leak sizedslightly lower than the per joint requirement of 1E-4sccs helium was used to verify sensitivity before,during, and after testing of the flight joints.The CBM is used to join modules together in space.Each module port has a CBM bolted to it while onthe ground, and this joint is referred to as the CBMpressurized element, or CBM-PE joint. This jointcontains two elastomeric O-rings, the inner one madeof fluorocarbon and the outer of silicone. The ISSprogram has a requirement that all sealed joints thatcan leak to space must have two seals forredundancy. This requirement applies to joints thatcan be tested on the ground before launch. If aconnection is made in space, then three seals arerequired. This is the case when two modules aremated on-orbit to comprise the CBM-CBM joint,which contains three seal beads. The CBM-CBMseals consist of four sections of silicone seals moldedinto an aluminum substrate. Figure 5 shows a crosssection of a CBM with the CBM-PE and CBM-CBMseals installed.COMMON BERTHING MECHANISM (CBM)LEAK TESTINGCBM-PE Leak Testing