39th AIAA PROPULSION CONFERENCE AIAA-2003-4606HUNTSVILLE, ALABAMAJULY 21, 2003

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DESIGN AND MANUFACTURE OF ALIGHTWEIGHT FUEL TANK ASSEMBLY

Paul S. Griffin, Ian A. BallingerPressure Systems, Inc.

and

Donald E. Jaekle Jr.PMD Technology

and

Arthur C. Jackson IIIJackson Consulting

ABSTRACT

A fuel tank assembly is required for acommercial spacecraft. This tank must belight weight and provide gas-free expulsion ofhydrazine propellant in a low gravityenvironment. An internally mounted surfacetension propellant management device (PMD)is utilized to achieve this goal. The vanetrough/trap type PMD, constructed of titanium,was custom designed for the spacecraftmission.

The tank shell is partially overwrapped withcarbon fiber. To minimize cost, an existingtank shell liner was selected as baselinedesign and slightly modified for the mission.1No modification was done to the tankmembrane. The propellant tank shell isconstructed of solution treated and aged (STA)6AL-4V titanium alloy. This material providesexcellent strength to weight characteristics andis widely used in the aerospace industry for itsexcellent material properties andmanufacturability. The overwrap and integralmounting skirt are fabricated from T-1000carbon fiber for high strength and low weight.The PMD is constructed of annealed 6AL-4Vtitanium and commercially pure titaniummaterial.

Stress and fracture mechanics analyses wereperformed to validate the tank shell for thespacecraft mission. PMD performance

analysis was conducted to design the PMD.Qualification testing was conducted includingpressure cycles, random and sine vibrationand burst pressure testing.

INTRODUCTION

Pressure Systems Inc. (PSI) was contracted toprovide propulsion system tanks for acommercial geosynchronous communicationssatellite. The spacecraft utilizes a dual modepropulsion system, and new hydrazine fuel andnitrogen tetroxide oxidizer tanks weredeveloped.

The new hydrazine fuel tank is designed tostore hydrazine propellant and gaseous heliumpressurant, and provides this propellant to acomplement of bipropellant thrusters viapropellant manifolds during the three-axisstabilized velocity increment from geotransferto geosynchronous orbit. It also provideshydrazine propellant to monopropellantthrusters for station keeping activities. Thetank operates under a pressure regulatedmode. A custom-designed, multi-element,internally-mounted surface tension propellantmanagement device (PMD) ensures gas-freeexpulsion of propellant upon demand in thislow gravity environment. A sketch of this tankis shown in Figure 1. One fuel tank is requiredper spacecraft. The propellant tankspecification requirements are listed below inTable 1.

Copyright 2003 by Pressure Systems, Inc. Publishedby American Institute of Aeronautics & Astronauticswith permission.

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Figure 1: Fuel Tank Assembly

Table 1: Propellant Tank Assembly Design Requirements

Parameters Requirements

Operating Pressure MEOP is 300 psia @ 50°C (122°F), 50 cycles

Proof Pressure 375 psia @ 50°C (122°F), 12 cycles

Burst Pressure 450 psia minimum @ 50°C (122°F)

Materials of Construction Membrane: 6Al-4V titanium, solution treated and aged

Overwrap: T1000 carbon fiber, amine resin

Inlet/outlet ports: 3Al-2.5V titanium

PMD: CP and 6Al-4V titanium

Expulsion Efficiency 99.75%

Propellant Weight 1980 lbm (898 kg) maximum hydrazine

Propellant fill fraction 65% minimum, 95% maximum

Tank Capacity 57,215 in3 minimum

Internal dimension 35.25” ID x 69.2” long

Overall Length 75.1”

Tank Weight 70.0 lbm maximum

Propellant Hydrazine

Fluid Compatibility N2H4, GAr, GHe, GN2, D.I. water, Isopropyl alcohol

Shell Leakage <1x10-6 std cc/sec He @ 300 psia

Natural Frequency > 30 Hz in lateral direction

> 70 Hz in thrust direction

Failure Mode Leak Before Burst

On-Orbit Temperatures 50 to 122 °F (10 to 50 °C)

Shelf Life 3 years minimum

On Orbit Life 15 years minimum

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To minimize the overall program cost, anexisting, flight qualified tank shell liner wasselected as the baseline design, and modifiedto meet the specification requirements. This isa popular design approach widely used by PSIto maximize value and minimize cost. Thecost reduction is the result of (1) eliminatingthe need to conduct a weld development andqualification program, (2) minimizing thenumber of new engineering drawings, shoptravelers, and NC programs, and (3)maximizing the use of existing tooling. Thisdesign approach also provided scheduleassurance to the customer. However,complete stress and fracture mechanicsanalyses were conducted to validate the tankshell in the new operating environment. AQualification tank was constructed and testingwas performed.

All completed flight tanks require acceptancetesting prior to delivery. This testing includes astatic load test as a validation of theworkmanship in constructing and bonding thecomposite skirt to the tank shell.

TANK ANALYSES

The tank analyses included stress analysis andfracture mechanic analysis for the tank shell,stress analysis for the PMD, and the PMDperformance analysis. Since the PMD iscompletely enclosed within the tank shell, bydefinition a fracture mechanics analysis is notrequired for the PMD. All analyses usedassumptions, computer tools, test data andexperimental data utilized on a majority of thepressure vessels and PMD’s successfullydesigned, fabricated, tested and qualifiedduring the past three decades. Conservatismwas used throughout the analysis process, andthe worst case scenarios were analyzed.

TANK SHELL STRESS ANALYSIS

A stress analysis was performed to establishthat the tank meets the hydrazine tankspecification requirements. The analysis tookinto consideration the requirements such as:

• Temperature environment;• Material properties, STA titanium;• Material properties, annealed titanium;• Material Properties; carbon composite;• Volumetric requirements;

• Mass properties of tank shell material;• Mass properties of tank fluids;• Fluids used in the tank;• Tank pressurization history;• External loads;• Girth weld offset and weld suck-in;• Size of girth weld bead;• Resonant frequency;• Tank boundary conditions;• Residual stress in girth weld;• Load reaction points; and• Design safety factors.

This stress analysis validated the use of theexisting tank shell liner design, new carbonmounting skirt and overwrap, for the newmission requirements. The analysis dynamicmodel provided predictions on the firstresonant frequencies of the tank.

The analysis concluded that there are positivemargins of safety for all design parameters, assummarized in Table 2.

Table 2: Propellant Tank Safety Margins

Characteristics M.S.

Pressurant sphere, proof, yield +0.13

Pressurant sphere, burst, ultimate +0.01

Propellant sphere, proof, yield +0.06

Propellant sphere, burst, ultimate +0.11

Cylinder- liner, proof, yield +0.43

Cylinder- liner, burst, ultimate +0.28

Cylinder- composite, burst, ultimate +1.90

Weld, proof, yield +0.16

Weld, burst, ultimate +0.05

Composite skirt, burst, bondline +0.49

Outlet/inlet tube, yield +28.41

Outlet/inlet tube, ultimate +12.24

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TANK SHELL FRACTURE MECHANICSANALYSIS

A fracture mechanics analysis was performedto establish whether the growth of an initial flawin the anticipated cyclic and sustainedpressure environment as well as the dynamicenvironment might cause a failure in the tankshell. The analysis was performed usingexternal and internal stresses from the stressanalysis, and using NASA/FLAGRO withminimum thicknesses as parameters. Specialfracture critical dye-penetrant and radiographicinspections are required to detect flaws. Theminimum flaw size that can be detected bysuch special fracture critical inspections wasused as initial flaw size for this fracturemechanics crack propagation analysis. Theanalysis was performed at:

• Girth welds and heat affected zones;• Maximum pressure stress location in the

hemisphere;• Maximum stress location in the cylinder;• Maximum stress location in the

hemisphere/cylinder transition;• Intersection between the hemisphere and

the pressurant boss;• Intersection between the hemisphere and

the propellant boss; and• Maximum external load stress in the

hemisphere near the pressurant and thepropellant bosses.

The fracture mechanics analysis establishedthe leak-before-burst (LBB) characteristics ofthe propellant tank. This analysis concludedthat the existing tank shell meets all thefracture mechanics requirements. The specialNDE requirement established by this fracturemechanics analysis include:

• Special fracture critical dye-penetrant onall surfaces; and

• Special fracture critical radiograph ofwelds.

These requirements were instituted as part ofthe tank fabrication requirements.

PMD STRESS ANALYSIS

A PMD stress analysis was also performed tovalidate the structural integrity of the PMDdesign. The analysis took into considerationdesign requirements such as materialproperties, fluid properties, vibration loads, anddesign safety factors. The PMD stressanalysis concluded that there are positivemargins of safety for all design parameters, assummarized below:

Table 3: PMD Safety Margins

Characteristics M.S.

Long Vane >0

Trap Sphere .02

Trap Cylinder 2.08

Center Post .32

Outlet Side, Bridge 1.60

Outlet Side, Vertical 2.69

Trap Housing Weld .28

Trap Housing Base .06

PMD PERFORMANCE ANALYSIS ANDPMD DESIGN

A comprehensive PMD performance analysiswas performed to design and validate thePMD. The passive, all titanium, surfacetension propellant management device wasdesigned to provide gas-free hydrazinedelivery throughout the spacecraft mission. Aswith most PMD’s, this PMD was designedspecifically for the spacecraft mission. ThePMD is designed to (a) survive spinningoperations, (b) provide gas-free propellantdelivery throughout mission, including systempriming, LAE ignition, and LAE steady statefiring, and (c) provide gas-free propellant forstation keeping.

The PMD was designed to be installed into thetank shell outlined in Figure 1, with 17.63-inchradius hemispherical heads and a 33.62-inchlong cylindrical center section. It was designedfor use with hydrazine propellant. Additionalfeatures were incorporated into the design toprovide optimal service. First, because thePMD is a passive device with no moving parts,the design is inherently reliable. Second, thedesign is constructed entirely of titanium. Thus

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the PMD is lightweight and offers exceptionalcompatibility, long life, and reliability. Finally,the PMD was designed not only to providepropellant during steady flow conditions butalso to allow some operation in some offdesign conditions, thus providing additionaloperational safety.

A sketch of the PMD is provided in Figure 2.The key components of this PMD are:

1) The center post and vanes,

2) The ring baffle,

3) The trough/trap housing,

4) The trough/trap entrance window,

5) The spin baffle,

6) The fins,

7) The liner, and

8) The perforated sheet windows.

All PMD components are located over the tankoutlet.

The PMD is designed for use with hydrazine(N2H4). The design incorporates a centerposted vane device2 and a trough/trap3 withintegral pickup assembly. The PMD isconstructed entirely of titanium. The centerpost and trough/trap assembly is welded intothe propellant end of the tank and is restrainedin the X and Y axes at the pressurant end ofthe tank. The pressurant hemisphere vanesare welded into the pressurant hemisphere.

The Vanes and Centerpost: The four vanesin each hemisphere are positioned on the ±Xand ±Y axes and provide a flow path from theside of the tank to the tank outlet orpressurant end of the tank. The center postconsists of a cruciform with 6 radial pieces ofsheet metal and runs from the pressurant endof the tank to the trough/trap entrance. Thecenter post provides a direct flow path fromthe pressurant end of the tank to thetrough/trap entrance window. The vanes andcenter post are designed for on orbitoperations (the center post also provides fuelduring LAE ignition).

The Trough/Trap: The trough/trap is locatedover the tank outlet. The only entrance intothe trough/trap is through a window at the topof the trough/trap, immediately below thecenter post. Just outboard in this window is a

ring baffle designed to limit radial flow near thewindow. The trough/trap entrance windowuses perforated sheet as a capillary barrier.Inside the trough/trap is a spin baffle, fins, anda perforated sheet liner. The spin baffle is asolid plate just below the trough/trap entrancewindow. The spin baffle contains openings atthe outboard edge of the trough/trap housing.Several fins are mounted over each linerperforated sheet window and run up to thespin baffle. The liner is a solid sheet metalhousing located just above the bottom of thetrough/trap. The perforated sheet window isto allow flow into the outlet. Directly over theoutlet is a last perforated sheet designed tominimize residuals. The trough/trap isdesigned to survive 60 rpm Z axis spin andprovide propellant during all subsequentspinning operations.

The Perforated Sheet: The perforated sheetis located over the tank outlet, inside of thetrough. This is the only location of a porouscapillary barrier in the PMD design. This PMDcontains no perforated sheet. The perforatedsheet was chosen to have low flow losseswhile maintaining a bubble point in excess oftwice the loads applied. The perforated sheetis fabricated from titanium sheet with electronbeam drilled holes.

PMD OPERATIONSThe fuel tank PMD is designed to provide gas-free propellant to the tank outlet throughoutthe mission. During ground operations, thePMD has been designed to enable tank filling,tank handling, and tank draining. Duringlaunch, the PMD does not function and hasbeen designed to maintain propellant over thetrough/trap and not be adversely affected bythe launch conditions encountered. Duringthe final stages of ascent, the spacecraft isspun up to 60 rpm during which a large solidrocket upper stage motor is fired (AKM Firing).Subsequently, the spacecraft is despun from60 rpm to <5 rpm without using any hydrazine.The PMD is been designed to provide gas-free hydrazine to despin the vehicle from 5rpm to 0.1 rpm maximum about the Z axis.The PMD is also been designed to providegas-free hydrazine during LAE firing andduring lateral on orbit hydrazine thruster firing.

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FLOW PATH THROUGH THE PMDThe flow path through the PMD is illustrated inFigure 3. The propellant flows up the vanes toboth ends of the tank. The propellant flowingtoward the pressurant end of the tank(opposite the outlet) will flow down the centerpost to the trough/trap entrance window. Thepropellant flowing toward the outlet of the tankwill flow up the trough/trap housing andthrough the radial baffle to the trough/trap

entrance window. After entering thetrough/trap, the propellant flows around thespin baffle. Once in the main trough/trapcompartment, propellant flows along the finsto the liner perforated sheet windows, throughthe windows and into the liner. The propellantthen flows to the center of liner, through theoutlet perforated sheet, and out the outlet.

Figure 2: PMD Configuration

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Figure 3: Flow Path Within the PMD

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GROUND OPERATIONS

The ground operations can be divided intothree parts; filling, handling, and draining.These are important not only from a flightstandpoint but also from a testing standpoint.One must be able to fill the tank in areasonable time when following a standardprocedure. Similarly, handling and grounddraining must be accomplished withoutexcessive effort. Figure 4 shows theseground operations.

Filling: Filling occurs with the tank upright inthe outlet down position. The tank is atatmospheric pressure when propellant isintroduced into the tank through the propellantoutlet line. During the filling process, a smallquantity of gas may be trapped in the linerunder the perforated sheet. Nominally, no gasshould be trapped but if one assumes worst-case fill conditions, a small bubble may exist.This gas is compressed significantly duringpressurization and is likely to be dissolved intothe unsaturated propellant. In any case, thegas quantity is too small to be a concern. Thefilling process is straightforward and shouldintroduce no difficulties either to the technicianor to the PMD.

Handling: Typical handling occurs with thetank in the outlet down position. The tank canbe tilted significantly before gas will come intocontact with the trough/trap inlet perforatedsheet. Gas will not enter the trough/trap or theoutlet during handling. The slosh amplituderequired to compromise the PMD functionaldesign is so large that it is unlikely gas willcome in contact with the perforated sheet (thecenter post will act as a baffle preventing gasfrom reaching the perforated sheet). Theintegrity of the PMD's functionality is assured.

Draining: Ground draining may have to beaccomplished with hydrazine and certainly willoccur with test fluids. The liquid remaining inthe tank at the end of ground draining willhave to be evaporated from the tank. Theground drain residuals are relatively large. Asillustrated in the operational sequence a liquidpool outside the trough/trap and belowentrance window will be left behind. Since theentrance window is elevated, this volume isrelatively large. It was decided that PMDsimplicity and functional reliability were moreimportant than minimizing ground residuals.The tank will be drained in the outlet down

position. Ground draining is not seen as adifficulty.

Figure 4: Ground Operations

Ground FillGround Handling

Launch

Ground Drain

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ASCENT OPERATIONS

Ascent operations can be divided into fivestages: launch, 60 rpm spinning, AKM firing,despin to 5 rpm and 5 rpm spinning. ThePMD has been designed to withstand thestructural loads during these stages of ascentas well as provide gas-free propellant to thetank outlet during system priming, despin to0.1 rpm, and LAE firings.

Launch: The PMD is designed to belaunched in the outlet down position. Similarto upright ground handling, there is noperceived danger of ingesting gas into thetrough/trap and/or outlet. Slosh is notforeseen as a force substantial enough todrive gas to the perforated sheet. In addition,the positioning of the center post above thetrough/trap entrance window provides a fluidstagnation region where fluid velocities willalways be small. Even if gas were drivendown toward the trough/trap entrance windowby slosh, the center post and the perforatedsheet itself will prevent the gas frompenetrating into the trough/trap. Launch isillustrated in the operational sequence (withfilling and ground handling).

Simple Spin: After launch, the vehicle isspun up to 60 rpm about a spin axis parallel tothe tank centerline. Fuel is not used to spinup or spin down the vehicle from 60 rpm.During spinning, the propellant is positionedoutboard by the centripetal forces. Thepropellant interface is cylindrical and thetrough/trap entrance is exposed to gas. Thetrough/trap entrance perforated sheet bubblepoint may not prevent gas from penetratinginto the trough/trap. Any gas entering thetrough/trap will be limited in volume by thespin baffle, which is solid near the tankcenterline and open only outboard. Theconfiguration of the trough/trap with a singleentrance located on the tank centerline (spinaxis) allows the device to act as a trough;holding liquid as a cup holds liquid in one g.The PMD has limited nutation angle capabilityat 60 rpm due to this spin baffle configuration(no nutation is required). Spinning at theminimum fill fraction is illustrated in Figure 5.

AKM Firing: While the vehicle is spinning at60 rpm, AKM firing occurs producing a largeaxial acceleration. The propellant reorients inthe tank with the surface a paraboloid ofrevolution. Propellant access is not required

during AKM firing. The propellant motioncreated by the high g environment causeshigh loads on the PMD center post. Thecenter post has been designed toaccommodate these loads. AKM firing is notshown in the operational sequence.

Despin from 60 rpm: After AKM firing, thevehicle is despun to between 60 and 5 rpmvery rapidly. The tangential accelerationcreated by the rapid despin is large comparedto the centripetal acceleration and thepropellant will move circumferentially inresponse to the acceleration. No hydrazine isrequired and the PMD is unaffected by despin.Despin from 60 rpm is not illustrated in theoperational sequence.

5 rpm Spin: Following despin (and afterseparation from some launch vehicles), thespacecraft is spinning about the Z axis at upto 5 rpm. At 5 rpm, the propellant location isnearly identical to 60 rpm spinning. Within thetrough/trap, the fins above the spin bafflecause propellant to rise and wet the perforatedsheet. With the perforated sheet wetted, it willprevent any additional gas ingestion. Thetrough/trap is now acting as a trap, with aporous element preventing gas ingestion (thusthe name trough/trap). It is possible that thespacecraft could enter a flat spin where thespin axis lies in the XY plane. The transitionto flat spin and flat spin itself will not causeadditional gas ingestion into the trough/trap.

Figure 5: Ascent Operations

60 rpm Z Axis Spinning

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ORBITAL OPERATIONS

System Priming: System priming of the fuellines occurs while a) spinning at up to 5 rpmabout the Z axis, or b) spinning at up to 5 rpmabout an axis in the XY plane, or c) in zero g.The propellant position during the three casesof system priming is illustrated in figure 6.The trough/trap entrance window may or maynot be submerged with propellant. In worst-case, the trough/trap entrance window isexposed and gas is ingested into thetrough/trap during system priming. The gas iskept away from the perforated sheet windowsby the fins over the windows. The trough/trapis designed to retain this gas throughoutmission.

Despin: Following system priming, thehydrazine propulsion system is active and thevehicle is despun from 5 rpm to 0.1 rpm aboutthe Z axis using hydrazine. Again in theworst-case, the trough/trap entrance isexposed to gas and gas is drawn into thetrough/trap during despin. Gas-free propellantis supplied to the outlet by the perforatedsheet windows in the liner. The fins keep thewindows submerged in propellant. Thetrough/trap has been sized to retain this gasthroughout mission. Despin from 5 rpm to 0.1rpm is illustrated in the operational sequence.

While spinning about the Z axis at 0.1 rpm,the propellant reorients into its zero gconfiguration. The low spin rate coupled witha small spin radius and the relatively highsurface tension of hydrazine makes the 0.1rpm Z axis spin negligible. With a Fill Fractionof 95%, the ullage bubble is very close to asphere but is distorted by the center post. Thebubble is pushed toward the top of the tank bythe taper in the center post. At 47.7% FF, theullage bubble can be in one of two equilibriumpositions. In one position, the bubble isaxisymmetric with each end of the bubbleclose to hemispherical but slightly distorted bythe center post. The bubble will be pushed tothe top of the tank by the tapered center post.In the second position, the bubble isasymmetric, not quite wrapping all the wayaround the center post. Both of thesepositions, as well as the 95% fill fraction fluidposition, are illustrated in the operationalsequence as early coast.

LAE Ignition and Firing: The initial andmajor use of propellant is for LAE ignition and

firing. The LAE produces a settlingacceleration reorienting all the propellant intoa pool over the tank outlet. During the initialreorientation, propellant falls down the outertank walls until it is forced upward at the tankcenterline. A geyser results. Propellant in thecenter post will fall against this upward movingpropellant; limiting the geyser height. Thecenter post will provide gas-free propellantduring LAE ignition. After the geyser isdamped out, the propellant will be settled overthe trough/trap entrance window and gas-freepropellant is easily supplied. LAE ignition isillustrated in the operational sequence.

Station Keeping: After LAE firing, thespacecraft is on orbit where maneuversprimarily consist of lateral hydrazine thrusterfirings. The PMD vane system becomesoperational and is designed to deliver gas-freepropellant during the lateral accelerationsproduced by thruster operation. Smallaccelerations and low flow rates make gas-free propellant delivery possible using asimple vane device. These firings produce arelatively low lateral acceleration and requirepropellant delivery at low flow rates.

During thruster firings, the propellant willreorient to the cylinder sidewall as a result ofthe acceleration. However, the surfacetension forces are sufficiently high thatpropellant surface will be highly curved. Thepropellant easily climbs up the vanes in a filletformed in the tank wall/vane corner. Early inmission, the bulk propellant surface curvaturewill reach and submerged the entranceperforated sheet. Within the trough/trap, thefins will keep the perforated sheet submerged.Gas-free propellant delivery for unlimitedduration lateral firings is assured. Early inmission lateral firing is illustrated in theoperational sequence.

Zero g coast: During most of the spacecrafton orbit life, zero g coast is encountered.During zero g coast, the propellant will occupya position minimizing the gas-liquid interfacialenergy. This occurs when the sum of thereciprocal of the principal radii of curvature areidentical. Most of the propellant will reside onthe center post or around the center post andtrough/trap tank wall intersection. In addition,a fillet of propellant will be adhere to thevanes. Within the trough/trap, the gas bubbleis positioned inboard and upward by the fins.

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As propellant is consumed, a pool no longerforms on the tank cylinder wall during lateralfirings. All the propellant is retained by thevane system. Propellant flows along thepressurant hemisphere vanes to the centerpost and down the center post to trough/trapentrance window. Alternately, the propellantflows along the propellant hemisphere vanes,up the clips that run up the trough/traphousing, and across the radial baffle to thetrough/trap entrance window. The vastmajority of propellant can be found on centerpost. A late lateral steady firing is illustrated inthe operational sequence.

Depletion: Finally, depletion is illustrated inthe operational sequence. During a steadylong duration burn, the highest residualsoccur. This is because a) the acceleration isapplied for a sufficient period to reorient thepropellant from its distributed location to theends of the vanes and b) the flow lossescause a radius of curvature reduction near the

trough/trap entrance window. The depletiontransient is multistaged: as propellant isconsumed, the radius of curvature along thevane decreases until the flow area on thevane near the center post can no longersupply the steady demand. At this point, thepropellant on the center post will begin to beconsumed to complement the inadequate flowup the vane. As the burn continues, thecenter post will no longer supply the propellantnecessary to meet the demand and thepropellant in the trough/trap will begin to beconsumed. Eventually, as the liner perforatedsheet flow area decreases, gas will be pulledthrough the liner perforated sheet, the linerpropellant will be consumed and gas will bepulled through the outlet perforated sheet andinto the outlet line. The ingestion of gas intothe outlet line indicates depletion. The PMDhas been designed to provide gas-freepropellant to the tank outlet during therequired conditions until the fill fraction of thetank falls below 0.5% maximum.

FIGURE 6: ORBITAL OPERATIONS

60 rpm Z Axis Spinning System Priming During 5 rpm Z Axis Spin

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System Priming During Flat Spin System Priming During Zero G

Despin From 5 rpm to 0.1 rpm

Early Coast

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LAE Ignition LAE Firing

Early Lateral Steady Firing Late Coast

Late Lateral Steady Firing Depletion

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TANK SHELL DESIGN HERITAGE

This fuel tank is a derivative of an existing,flight qualified tank. It belongs to PSI’s familyof 35.2-inch diameter carbon hoop-wrappedpropellant tank product line. The titanium lineris the same as the heritage tanks. The carboncomposite overwrap, carbon compositemounting skirt and the internal PMD weredesigned specifically for this tank. The PMD inthis tank is more sophisticated than the PMD’sin its predecessor tanks and offers significantlymore capabilities. The composite skirt wasdesigned to reinforce the girth weld in the lineras well as mount the tank to the spacecraft.The composite overwrap reinforces thecylindrical section of the liner.

TANK FABRICATION

The fuel tank shell consists of twohemispherical heads and a cylindrical centersection. The two hemispherical shellcomponents are machined from 6AL-4Vtitanium alloy forgings. The cylinder isfabricated from 6AL-4V sheet metal. All rawforgings have annealed properties at time ofreceipt. Each forging is rough machined,solution treated and aged, and finish machinedto the tank shell thickness as required by thestress analysis. The solution heat treatprocess increases the strength of the titaniumalloy, thus minimizing the weight of the tankshell. The excellent strength to weightproperty, coupled with its manufacturability,make this titanium alloy the material of choicefor aerospace application. Figure 7 shows amachined hemispherical head.

Figure 7: A Machined Head

Vanes for the PMD are installed in thepressurant hemisphere. The cylinder is thenwelded to the pressurant hemisphere. Figure8 shows the vanes inside this inlet assembly.

Figure 8: Inlet Head/Cylinder Assembly

The componenets of the trap/trough arewelded into a PMD sub-assembly and bubblepoint tested. The centerpost assembly iswelded to the trap/trough to create the PMDassembly. The PMD assembly is shown inFigure 9.

Figure 9: PMD Assembly

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The PMD is installed in the propellanthemisphere over the propellant outlet to createan expulsion assembly. The expulsionassembly is shown in Figure 10.

Figure 10: Expulsion Assembly

Two girth welds are required to assemble thetank. The first weld joins the propellanthemisphere and the cylindrical center section.This is part of the inlet assembly shown inFigure 8. The second girth weld joins togetherthe hemisphere/cylinder assembly to thepressurant hemisphere (with the PMDinstalled) to complete the tank liner. Both girthwelds are subjected to radiographic and dyepenetrant inspections. After closure the linerassembly is stress relieved in a vacuumfurnace to remove residual stress from theweld operations. See Figure 11.

The composite skirt is manufacturedseparately and attached to the liner with apaste adhesive and the cylindrical section ofthe liner is filament wrapped with carbon fiber.The skirt and filament wound tank are shownin figures 12 and 13.

After winding, the pressurant port shear plateis machined and installed. The completed tankis acceptance tested, precision cleaned and

shipped to the customer. The completed tankis shown in figure 14.

Figure 11: Tank Liner Assembly

Figure 12: Composite Skirt

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Figure 13: Filament Wound Tank

Figure 14: A Completed Fuel TankAssembly

QUALIFICATION TEST PROGRAM

Qualification testing consists of the followingtests:

- Preliminary examination- Pre-proof volumetric capacity- Ambient proof pressure test- Post-proof volumetric capacity- PMD bubble point test- Vibration test- External leakage test- Proof pressure cycle test- MEOP pressure cycle test- Differential pressure test- PMD bubble point test- External leakage test- Penetrant inspection of exposed welds- Radiographic inspection of exposed welds- Mass measurement- Final visual examination- Burst pressure test

Conservatism is exercised throughout the testprogram, and all pressure testing istemperature adjusted for the worst caseoperating temperature (50°C). Pass/Failcriteria consist of acceptance type externalleakage and PMD bubble point testsconducted at intervals throughout the testprogram.

Volumetric Capacity Examination: Thecapacity of the fuel tank is measured utilizingthe weight of water method, using filtereddeionized water as the test medium. This testis conducted before and after the proofpressure test to verify that the proof pressuretest does not significantly alter the tankcapacity. A successful validation indicates thatthe tank shell is manufactured properly andthat the tank can operate in the pressureenvironment under which it was designed for.Typically, the volumetric growth after proofpressure test is zero.

The post-proof test capacity examination alsoserves to verify that the tank meets thedesigned volume requirement.

Proof Pressure Test: The proof pressure testis typically the first pressurization cycle appliedto the tank after fabrication. It is intended tovalidate the workmanship by verifying thestrength and integrity of the tank shell. Thetest must be conducted in a “safe” environmentto minimize hazards to test technicians. Thetest is conducted hydrostatically at proofpressure (375 psia, normalized for testtemperature) for a pressure hold period of 1minute minimum.

PMD Bubble Point Test: The PMD bubblepoint is measured at various times during thetest procedure to ensure proper function of theporous elements in the PMD. Testing isconducted using isopropyl alcohol.

Qualification Vibration Test: The vibrationtest is designed to verify the structural designof the PMD and the integrity of the tank shell.There are two phases of the vibration testing:wet and dry random and wet and dry sine. Thethree principal axes are tested at each phase.The vibration spectrum is listed below inTables 4 through 7. For both wet random andwet sine vibration testing, the qualification tankis loaded with 1980 lbm of deionized water.The tank is pressurized to MEOP for allvibration testing.

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The vibration test fixture is designed tosimulate the tank-to-spacecraft installationinterface. The fixture is also sufficiently stiff tobe considered rigid for the test frequencies.The vibration test set-up is shown in figure 15.

Figure 15: Lateral Vibration Test Set-up

Control accelerometers are placed on thevibration test fixture at four locations by thecomposite skirt and near the attachment bossto control energy input. Tri-axial response

accelerometers are used to monitor the tankresponses.

External Leakage Test: The tank shell istested at various times during the testprocedure for leakage of the shell bypressurizing the tank to MEOP with helium andmeasuring the leak rate in a vacuum chamberusing a mass spectrometer. The tank isplaced in a vacuum chamber, evacuated tounder 0.2 microns of mercury, and heliumpressurized to 300 psia for 30 minutes. Thehelium leak rate cannot exceed 1 x 10-6 std ccper second after a 10-minute stabilizationperiod.

Proof and MEOP Pressure Cycling: Proofand MEOP pressure cycling is conductedsimilar to the Proof Pressure Test. A total of12 Proof cycles and 50 MEOP cycles to showthat the tank is capable of surviving fourexpected life cycles.

Differential Pressure Test: The fuel tankmust meet the pressure drop requirement ofnot-to-exceed 5.0 psid at a maximum flow rateof 0.1 lbm/sec. The test is conducted bymeasuring the pressure differential betweenthe ullage and the tank outlet whilepressurizing the tank (and the test fluid)through the pressurant port. The measureddifferential pressure was 4.0 psid.

Weld Inspection: All exposed welds (ontubes and outlet fittings) are inspected usingradiographic and fluorescent penetranttechniques. Tank shell axial and girth weldsare not accessible due to the compositeoverwrap.

Table 4: Dry Sine Vibration Levels

Axis FrequencyRange (Hz)

Acceleration (g) Sweep Rate

Spacecraft 4 – 11 0.50” DA*

Lateral(X, Y axis) 11 – 100 3.1 2 oct/min

Spacecraft 4-9.5 0.50” DA*

Thrust (Z axis) 9.5 - 100 2.3 2 oct/min

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Table 5: Wet Sine Vibration Levels

Axis FrequencyRange (Hz)

Acceleration (g) Sweep Rate

Spacecraft 4 – 14.4 0.50” DA*

Lateral 14.4 – 50 5.3 2 oct/min

(X & Y axis) 50 - 100 2.25

Spacecraft 10 – 35 0.1597” DA*

Thrust 35 – 50 10.0 2 oct/min

(Z axis) 50 - 100 5.0

* DA: Double Amplitude

Table 6: Dry Random Vibration Levels

Axis Frequency Range(Hz)

PSD** Level(g2/Hz)

X, Y and Z 2020-50

50-600600-2000

2000

0.01+2.1 dB/oct

0.01875–1.6 dB/oct

0.01

Overall 5.4 Grms

Duration: 120 seconds per axis

Table 7: Wet Random Vibration Levels

Axis Frequency Range(Hz)

PSD** Level(g2/Hz)

X, Y and Z 2020-50

50-600600-2000

2000

0.0025+13.4 dB/oct

0.15–10.2 dB/oct

0.0025

Overall 10.9 Grms

Duration: 120 seconds per axis

**PSD: Power Spectral Density

Page 19

Burst Test: The qualification tank burstpressure was validated by testing to therequired burst pressure of 483 psi. The tankactually burst at 557 psi. The burst tank isshown in figure 16.

Figure 16: Burst Tank

ACCEPTANCE TESTING

After the flight tank is assembled, it issubjected to the following acceptance testsprior to delivery:

- Preliminary examination- Pre-proof volumetric capacity- Ambient proof pressure- Post-proof volumetric capacity- Static Load- PMD bubble point test- External leakage test- Penetrant inspection of exposed welds- Radiographic inspection of exposed welds- Mass measurement- Final visual examination- Cleanliness

Acceptance tests are similar to qualificationtests previously described with the followingadditions:

Static Load: The integrity of the bond betweenthe composite skirt ant the titanium liner, andthe workmanship of the composite skirt areverified with a series of static load tests. Thetank is mounted in a fixture similar to thevibratation fixture, and the tank is externallyloaded to simulate flight loads. Successfulcompletion of this test with no damage to thetank is acceptance of the hardware.

Cleanliness Verification: After all testing iscomplete, the interior of each flight tank is

cleaned to the cleanliness level specified belowin Table 8:

Table 8: Tank Cleanliness Level

Particle Size Range(Microns)

Maximum Allowedper 100 ml

Less than 5 Unlimited5 to 10 60011 to 25 8026 to 50 2051 to 100 4

101 and over 0

CONCLUSION

The fuel tank assembly has successfullycompleted qualification testing. Theproduction program is in progress and fiveflight tanks have been delivered.

The fuel tank PMD is specifically designed tomeet the mission requirements. The PMD is arobust design. It has been qualification testedand shows excellent strength, durability, andreliability.

The fuel tank assembly is lightweight, highperformance, and easy to manufacture. Thetank assembly is accomplished using standardmanufacturing processes and procedures.Special materials and processes are notrequired.

ACKNOWLEDGMENT

We wish to thank Mr. Mike Hersh, Mr. GaryKawahara, Mr. Jerry Kuo, Mr. Bill Lay, and Mr.Ben Wada for their significant contribution tothe success of this program.

REFERENCE

1. Tam, W. H., Debreceni, M. J., Hersh, M. S.and Nye, C. D., “Low Cost DerivativeTanks for Spacecraft and Launch Vehicles”,AIAA 99-2831, 1999.

2. Jaekle, D. E., “Propellant ManagementDevice Conceptual Design and Analysis:Vanes”, AIAA-91-2172, 1991.

3. Jaekle, D. E., “Propellant ManagementDevice Conceptual Design and Analysis:Traps and Troughs”, AIAA-95-2531, 1995.

Page 20

ABOUT THE AUTHORS

Mr. Paul Griffin is a Program Manager atPressure Systems, Inc. (PSI), Commerce,California. Mr. Ian Ballinger is the EngineeringManager at PSI.

Mr. Don Jaekle Jr. is PSI’s PMD designer DBAPMD Technology, North Andover,Massachusetts.

Mr. Art Jackson is a composite designer andanalyst DBA Jackson Consulting Services,Fullerton, California.

NOTES