AIAA 99-2832 Delta IV COPV Ris.k Mitigation Henry Babel and Lorie Grimes-Ledesma The Boeing ‘Company Huntington Beach, Califfornia

35th AIAA/ASME/SAEdASEE Joint Propulsion Conference & Exhibit

‘20-24 June 1999 Los Angeles, California

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AIAA-99-2832

DELTA IV COPV RISK MITIGATION

Henry W. Babel and Lorie Grimes-Ledesma The Boeing Company

Huntington Beach, California

ABSTRACT

This paper describes the steps taken by the Delta IV program to ensure the continuation of the record estab- lished by Delta II in its 40 years of operation. Delta II has never had a launch abort or failure caused by a pressure vessel. The pressure vessel heritage of Delta II and Delta III are described. For Delta IV, the function, shape, size, and location are defined for the 20 to 29 helium- and nitrogen-containing metal-lined, graphite-yarn compos- ite overwrapped pressure vessels (COPVs) used on each vehicle. The COPVs are operated at high pressures rang- ing from 3,000 psig (20.7 MPa) to 9,700 psig (66.9 MPa) with seamless aluminum and welded Inconel7 18 liners. The risk mitigation topics that could lead to a launch abort or a failure of a COPV before or during flight that have been analyzed and reviewed are (1) stress-rupture failme under a sustained pressure, (2) undetected impact damage, (3) gross liner leaks, and (4) stress overload because of the dynamic environment. Item 4 was not reviewed and analyzed to the same degree of technical depth as the other topics. The current level of under- standing and the risk associated with each of these fail- ure characteristics are reviewed. The results of the analysis are discussed as related to each of the four COPVs analyzed. It was concluded that the COPVs selected for Delta IV will provide the same failure-free record provided by Delta II and Delta III.

DELTA PRESSURE VESSEL HERITAGE AND USAGE

‘Small pressure vessels are viewed by Range Safety in the United States as potentially one of the most haz- ardous structures at the launch site. An analysis of the Delta III and Delta IV helium COPVs showed that the stored energy in one helium COPV is approximately the same as 7.8 lb (3.5 kg) of TNT. This COPV has a volume of 5,920 in3 (0.097 m3) and is operated at 4,500 psig (31 MPa). Since the bottles are mounted between the cryogenic fuel tanks, as shown in Figure 1, a catastrophic failure of one of the bottles could lead to a catastrophic failure of the vehicle. Most specialists accept the view that if a COPV fails catastrophically, Copyright01999. The American Institute of Aeronautics and Astronautics Inc. AU rights reserved.

01293REU9

Figure 1. Location of Delta IV upper-stage helium bottles.

the relatively thin membrane of the fuel or oxidizer tank domes, approximately 0.080-in. (2.032~mm) thick, would be penetrated, although the analysis to verify the penetration of the dome was not available at the time of this writing. Penetration of the fuel tanks could result in the loss of the vehicle, serious damage to the launch site, delays in subsequent launches, and impair- ment of the image and reputation of the rocket pro- vider. The aerospace industry, supported by the COPV suppliers, has developed design and test methodology that has precluded catastrophic failures on the launch pad. However, failures have occurred during test and there is one recent case of a noncatastrophic failure when a leak on the launch pad resulted in a launch abort. One well known catastrophic failure occurred during preparation for a static fire test of the Saturn SIV-B when a titanium pressure vessel failed. These two incidences are described below.

In 1967, a Ti-6Al-4V, spherical pressure vessel on the Saturn S-IVB program failed at a sustained pres- sure of one third of its design burst pressure at -423°F (20.3 K). The stage had been loaded with liquid hydro- gen and oxygen and was completing preparation for a static fire test. The Ti-6Al-4V pressure vessels were located on the inside of the hydrogen tank immersed in the liquid hydrogen. The failure of the titanium pres- sure vessel resulted in ignition of the hydrogen and resulted in serious damage to the test stand and destruction of the S-IVB stage. The problem was asso- ciated with the use of a commercially pure, titanium weld wire rather than the specified Ti-6Al-4V wire. The hydrogen in the titanium migrated to the weld and formed an embrittling hydride at the weld interface

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‘between the commercially pure titanium weld nugget and the Ti-6Al-4V base material. Once the failure was analyzed, the problem became well understood, and the authors are not aware of another catastrophic pressure vessel failure caused by the formation of a hydride layer. Although the existence of hydrides was known at the time, the technical community had not recognized the possibility of such hydrides forming during weld- ,ing of dissimilar titanium alloys. Such learning experi- ences have contributed to the knowledge base available ,today and have directly contributed to the excellent record of all-metal pressure vessels.

A leak and subsequent reduction in pressure resulted in a launch abort of a satellite on a Proton launcher on 28 March 1996. Pressure decay had revealed the pres- ence of the leak. The launch was aborted, the launcher destacked to provide the necessary accessibility, and the CQPV replaced. From a safety viewpoint, this might be acceptable, but not for those who are the launch provid- ers. For launch aborts, the cost of delaying the flight and destacking is very large, often cited in the United States as $l,OOO,OOO a day. For the provider of the launch ser- vices, any event that leads to a loss of a mission, or sig- nificantly adds to the launch costs, must be prevented. A failure or leak of a COPV can result in either of these events.

The reason for the failure cited above was exten- sively studied and the root cause of the problem identi- fied and rectified. A chemical residue on the interior of the tank was found to react with water, resulting in pit- ting corrosion of the 6061-T6 aluminum liner. Such les- sons learned contribute to the reliability of the COPVs that are provided today to the aerospace industries.

Transition From All-Metal Pressure Vessels to CQPVS

Although metal pressure vessels are extremely reli- able, the aerospace industry is always striving for lower cost and more weight-efficient pressure vessels. This has led to the evaluation of COPVs. During the past three decades, a growth and improvement in metal-lined COPVs has occurred, resulting in a transition from all- metal pressure vessels to COPVs, particularly for high- pressure applications where the beneficial features tend to be the highest. For high-pressure applications, the COPVs offer lighter weight, reduced costs, and fre- quently, a shorter delivery schedule than an all-metal pressure vessel. The Space Shuttle Orbiter already had made a transition from metal to KevlarTM overwrapped COPVs when it was designed in the mid-1970s. The benefits of COPVs compared to all-metal pressure ves- sels continued to improve as high-strength graphite yarns became available. By the early 199Os, the aero- space industry began to find applications for metal-lined,

graphite-yarn overwrapped COPVs in launch vehicles and satellites.

Delta II. Delta III. and Delta lV Pressure Vessel History

The evolution of pressure vessel technology is reflected in the design decisions made by the Delta III and Delta IV program that selected COPVs for the high- pressure helium-containing pressure vessels. These were selected with full knowledge of the perfect, no-failure record of the Delta II vehicle in its 40-year life that uses small titanium (Ti-6Al-4V), spherical pressure vessels.

Delta II

The small pressure vessels, which contain helium and nitrogen for the Delta II, are detailed in Table 1.

The feasibility of substituting COPVs for Ti-6Al-4V pressure vessels has been studied by the Delta II program. As a result of a weight change, the dynamic response of the vehicle would be altered and would require a system- level dynamics qualification test. It was determined that the costs of the system as weighted against the number of vehicles to be produced for the project did not warrant the change. In hindsight, had it been known that the Delta II program would bc a continuing program, the change would probably have been made.

Table 1. The ii-6AI-4V spherical pressure vessels used on Delta II.

Size Upper-stage spheres

24-in. dia (610 mm)

17-in. dia (432 mm)

Maximum expected operating pressure (MEOP) Number of pressure vessels Gas

4,400 psig 4,400 psig (30.3 MPa) (30.3MPa)

2 2

Helium Helium I I

Booster - 17-in. (432-mm) spheres MEOP

I MEOP = -3,800 psig

(-24.8 MPa) I Number of pressure vessels Gas

3

Helium MEOP MEOP= -3,600psig

(-23.5 MPa) Number of pressure vessels

1 I

IGas Nitrogen

Delta III

Trade studies conducted by Delta III showed that there were weight and potential cost benefits using graphite-yarn COPVs rather than all-titanium pressure vessels. Compared to an equivalent (same volume and pressure) all-metal titanium pressure vessel, a 4,550

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psig (31.3 MPa) COPV is lighter, less expensive, and in most cases, it can be procured on a much shorter sched- ule than all-metal bottles made from two forgings. By 1995, when Delta III needed to make a procurement decision for helium storage, there were already several graphite-yarn bottles that had been qualified and flown on European and American launchers. Eurostar 2000+ was using a well-engineered graphite-yarn COPV, which was adopted for use on the new upper stage for Delta III.’ It is made with a welded Inconel 718 liner and Toray T-1000 yarn. This selection allowed the pro- gram to avoid some of the costs of a complete qualifica- tion program. This helium bottle is shown in Table 2, along with the Ti-6Al-4V booster all-metal pressure vessels, which are identical to Delta II.

Table 2. Helium COPVs used on the upper stage and all-metal helium and nitrogen pressure vessels

used on the booster of Delta Ill.

Upper-stage COPVs 16.5in. (419-mm) dia by 3%in. (889-mm)-long cylinders

MEOP MEOP = 4,550 psig (31.3MPaI

I

Number of pressure vessels 5 Gas Helium

Booster Ti-6AI-4V - 17-in. (432-mm) spheres

MEOP MEOP = -3,800 psig (-24.8 MPa)

Number of pressure vessels 3 Gas Helium MEOP MEOP = -3,600 psig

(-23.5 MPa) Number of pressure vessels 1 Gas Nitrogen

Delta IV

The Delta IV program also was interested in reducing qualification costs associated with the selection of new COPVs. Thus, the program adopted the same COPV design for purging the (1) engine liquid oxygen propel- lant seal, (2) liquid hydrogen bellows feedline, (3) liquid hydrogen vent valve, and (4) POGO pressurization, as used on Delta III with some minor changes. In addition to reducing qualification costs, this selection will help ensure the continued problem-free record of the Delta II

and Delta III pressure vessels for the Delta IV. However, selecting prequalified pressure vessels for the different Delta IV applications was not possible and three of the four shown in Table 3 are new designs. Table 3 is a sum- mary of the helium and nitrogen pressure vessel charac- teristics used on Delta IV

There are other pressure vessels used on Delta IV for the hydraulic actuation system, which are low-pressure, all-metal pressure vessel systems and one hybrid high- strength steel cylinder, hoop-wrapped with Kevlar that is operated at high pressure. These vessels are not dis- cussed in this paper.

The design burst pressure selected was dependent in part on whether personnel would be working around the COPV after it is pressurized. If so, the COPVs are designed with a burst factor of 4.0 to meet Boeing and U.S. regulatory agency requirements. At U.S. govem- ment launch sites, personnel under controlled and restricted conditions may work around COPVs designed to a 1.5 burst factor after they have been pressurized, but not during the fill operation. COPVs containing helium are pressurized at the launch site, allowing the use of burst factors of 1.5 and 2.0. The selection of a burst fac- tor of 1.5 or 2.0 is an engineering decision.

REVIEW OF SELECTED RISKS FOR COPVS

Risk assessment includes analysis and/or test on the consequence of the failure of a COPV. There are two basic types of failures that can lead to loss or delay of a mission. One is the catastrophic failure of the COPV and the damage that can be caused by overpressure and the flying debris of yarn and metal fragments. The other is the loss of gas either from a catastrophic failure or a leak of sufficient magnitude that precludes performing the mission function. Steps must be taken in the design, manufacture, test, installation, transport, checkout, and use of the COPV that precludes the possibility of either failure.

Catastrophic Failure Assuming an appropriate design and the absence of

manufacturing errors, there are two conditions that can lead to a catastrophic failure of a COPV that deserve special attention: stress-rupture or sustained-load fail- ures of the composite yarns and handling or impact damage of the composite yarns. Cyclic loading is also a

Table 3. COPVs on the Delta IV program.

Function Gas Design burst

Burst factor psig (MPa) Graphite yarn Number Purge Helium 1.5 6,825 (47.1) T-l 000 20-28 Separation Nitrogen 4.0 12,000 (82.7) T-1000 2 Hydraulics Helium 2.0 19,400 (133.75) T-700 2 Hydraulic pressurization Nitrogen 4.0 16,400 (113.1) T-l 000 1

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consideration if it is coupled with impact damage or sus- tained loading, but it is usually considered more impor- tant relative to ensuring safe life of the liner.

The results of stress-rupture studies show that vessels burst after being held at a constant load over a certain period of time. The applied load is at a percentage of the short-time burst strength. The time to failure depends primarily on the type of yarn and the stress level applied. Graphite yarn exhibits longer times to failure than Kevlar and S-glass at the same percent of the ulti- mate load.2-6 The degradation mechanisms have been extensively discussed, but there are no critical tests that clearly demonstrate the controlling mechanisms for Kevlar and graphite yarn. For glass yarn, limited data indicate that the moisture in the environment attacks glass, as COPVs tested inside a vacuum chamber did not exhibit a reduction in strength with time.4 As the time to failure is repeatable for a given material, it appears that the stress rupture is a material characteristic or property.

COPVs impacted so that the damage is barely visible to a trained inspector can exhibit a reduction in burst strength. The amount the burst strength is reduced depends on the design of the vessel, the geometry, and the energy of the impactor. The burst strengths after impact have ranged from no reduction to a 50% loss in strength. The latter is a special case with more typical values ranging around 20%.

Stress RuWure Stress rupture is a concern for launch vehicles even

for the relatively short duration COPVs are pressurized. In the mid-1980s it was reported that the long-term load carrying capability of graphite yarn was better than that of glass and Kevlar.’ Today, the use of graphite has become the standard in the industry based on its light- weight and exceptionally high resistance to stress-mp- hue failures.

Statistical techniques have been successful in pre- dicting the stress rupture of COPVs. It has been found that failure of glass, Kevlar, and graphite yarns follows a two-parameter Weibull function.B7 This methodology has since been applied to COPVS.~ Equation 1 is used to predict the stress-rupture life of COPVs.

Equation 9

In the equation, t is the time at pressure, p is the Weibull scale parameter, and a is the scale parameter. The parameters have been determined for yarns and lim- ited pressure vessel data for each class of material.

Equation 1 can be used to develop design curves that can be used to predict the degradation of a COPV with time for the time interval of interest. These design

curves are based on a predetermined probability of sur- vival (or probability of failure) from which a lifetime can be predicted based on a specific stress level.7 To make an assessment, a number of decisions have to be made that can vastly change the results. The factors to be considered are as follows:

I Probability of Survival. Select an appropriate value for the specified application, e.g., 0.99, 0.999, 0.9999, etc. A probability of survival of 0.99 means that on the average, 1 in 100 will fail.

n Weibull Shape Factor. Select a value to represent the Weibull shape factor, often represented by a (alpha). Alpha is not a constant, but is usually treated as such. The smallest value of a is normally selected for analy- sis, as this results in the most conservative prediction. The minimum value of a is valid only at high stress lev- els above approximately 80% of the ultimate strength. Because it requires such a long test period, the data needed to define a at lower applied stress levels are only available for some types of yarns.

n Life Factor. Decide what factor of safety should be used for stress rupture. A factor of 1 has commonly been used, but some have applied a factor of 4 to the time the part is under load.

n Time Under Load. Decide what constitutes time under load. The time the COPV is pressurized at or above 50% of the ultimate composite strength is consid- ered acceptable for analysis purposes by many in the technical community.

Survival Probability

The curves for a probability of survival P(s) of 0.995, 0.9995, and 0.99996, based on calculations from Refer- ence 7, are shown in Figure 2. For a P(s) = 0.99996, one could not have a design with a burst factor less than around 2.0. Such a COPV, operated at 50% of the aver- age burst strength, has a predicted life from Figure 2 of 1,000 hr. For the upper curve with P(s) = 0.995, a design with a burst factor of 1.5 operated at 67% of its burst strength would have a life of around 100,000 hr.

GRAPHITE 01295REU9.1

a = 0.2, jj = 1.1 x 1051 x 1 o-O.515 W W

1 10 100 1,000 10,000 100,000 1 ,ooo,ooo LOG TIME (HR)

Figure 2. Stress-rupture design curves for three probabilities of survival.

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There is no consensus in the technical community on what survival probability should be used. One group believes that because of safety reasons, a high probability should be used for all COPVs up and through launch, but after launch, it is the responsibility of the user. Others believe that the requirements should be the same for all conditions. At this time, the selection is the prerogative of the user.

The results of stress-rupture testing for graphite yarns have been primarily performed with IM-6 and IM-7. Because of the lack of data for Toray T-700 and T-1000, the results from the IM yarns are assumed to be applica- ble to the Toray yarns. This has yet to be demonstrated.

Weibull ShaDe Parameter Alpha

Alpha is sensitive to the scatter of the times to failure within a stress level and can significantly change the design curves. Figure 3 shows the sensitivity of the resulting design curves with changes in cc.

GRAPHITE 01294REU9.1

COMPARISON OF ALPHA VALUES FOR P(s) = 0.9995

90 a = 0.2, p = 1 .l x 1051 x 1 o-O.515 W W

80

!?I 70

d 60 2 8 5o

40 30

1 10 100 1,000 10,000 100,000 1,000,000 LOG TIME (HR)

Figure 3. A comparison of design curves for a = 0.20 and 0.36.

Based on report data, a minimum a of 0.2 was calcu- lated, while an average a value of 0.36 was found.3 The data were derived from COPVs loaded to 97% of their average ultimate strength. The difference in the two design curves is shown in Figure 3. Many in the techni- cal community believe that 0.2 is too conservative. The shape factor reported for yarn data averaged 0.18 rang- ing from 0.15 to 0.27.r1 Clearly, further work is needed to establish an appropriate value of a for the stress lev- els applied for graphite COPVs, particularly at the stress levels at which they are normally operated.

Life Factor

Whether one uses a factor of safety on life of 1 or 4 has only a minor affect on the results unless it is for extremely long time periods. In Figure 3, for a = 0.20 curve, a COPV designed for a fiber stress of 67% of the ultimate, the curve shows a reduction in the fiber strength from 67% to 62% at 1,000 hr. At 100,000 hr, the reduc- tion is to around 57%. This is viewed as a second-order

effect. Because of the conservatism associated with the design curves, the technical community seems to favor using a safety factor of 1 rather than 4.

Time Under Load

The methodology used to calculate the time at pres- sure is not viewed as a critical factor. Differences in the life only affect the strength reduction a small amount. Therefore, using the concept mentioned earlier, adding together the time the COPV is at or above 50% of the ultimate composite strength, is considered acceptable.

The design approach commonly used is to conduct a conservative analysis to assess whether the COPV will meet the life requirements. The COPVs used for the Space Shuttle Orbiter meet P(s) = 0.9995, which is a good starting value to be used for the analysis. Simi- larly, a value of a = 0.20 is a starting point. If the design does not meet the requirements, then one needs to deter- mine whether less conservative assumptions are accept- able. Of course, testing is always an option to validate the design, but normally one cannot afford to wait for the results of long-duration stress-rupture tests.

Impact Damae The damage tolerance of a COPV depends on the

particular design, whether it is pressurized and to what level, and the geometry and energy of the impacting device. The many variables make it difficult to general- ize the results from several test programs that have been reported.*-lo The reductions in strengths reported have ranged from 0 to 50%, depending on the specific conditions. These results are based almost entirely on the results for Toray T-1000 yarn and generalized with- out the benefit of test results for other yarns. This situa- tion results in several challenges for the user to establish the design approach that should be used.

There are several approaches that can and have been used. These include the following:

w Minimum weight design with strict control proce- dures implemented to prevent handling or impact dam- age. This is the most common approach used by satellites and interplanetary unmanned missions.

w Designs that ensure ruggedness. Such COPVs usu- ally have handling precautions and may also have pro- tective covers. Launch vehicles and vehicles such as the Space Shuttle Orbiter have used this approach.

The remainder of this discussion on impact damage focuses on the requirements and approach for launch vehicles. The key concerns to launch providers involve undetected damage and the consequences of such dam- age. Impact studies on COPVs have shown that deflec- tions can occur that result in broken fibers below, but not on the surface. For higher stiffness composite/liner com- binations the damage tends to be on the outer surface

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layers, but also may have subsurface broken yarns. It is assumed that a COPV detected with visible damage shall be removed and dispositioned by the actions of a Materi- als Review Board.

To define the different design options, one has to develop an understanding of the factors that relate to damage tolerance. Some of these follow:

w Increased stiffness and thickness seems to increase the damage resistance, although a few exceptions exist for impacts of COPVs pressurized to MEOP.

n Thicker overwrap. Hoop wraps seems to be more prone to damage on a cylinder than the helical wraps.

n Stiffer liner, i.e., thicker, or use of a higher modu- lus material. Stainless- and Inconel-lined COPVs exhib- ited a higher damage resistance than aluminum-lined COPVS.

n Pressurization below MEOP seems to reduce the amount of damage compared to an unpressurized vessel. Impacts on several COPVs pressurized to MEOP have shown increases in the amount of damage that has occurred.

n Spherical vessels are more damage resistant than cylindrical COPVs and this may directly relate to the increased stiffness of a spherical COPV.

The design .options that can be considered include, but are not restricted to, those listed below:

1. Use a tough fiber rather than graphite like the organic fiber polybenzoazole (PBO)

n PBO has approximately 90% of the strength of Toray T-1000 and approximately the same modulus.

n PBO has the same stress-rupture characteris- tics of Kevlar, necessitating using a burst fac- tor of 2.0 rather than 1.5.

n PBO does not have a well established data- base.

2. Demonstrate damage tolerance by test n Demonstrate that damage visible to a trained

inspector will not reduce the burst factor of a COPV below 1.5. - This may require the COPV to be designed

with a burst factor close to 2.0. H For COPVs with higher design burst factors,

the COPV AIAA committee preparing SO-81 has proposed that the minimum burst with damage shall be 1.2 x proof pressure for impacts at the visibile damage threshold (VDT).

3. Increased burst factor, that is, increased wall thickness

n Even with nonvisible damage, a COPV designed to a burst factor of 4.0 and with suf- ficient wall thickness is not considered at risk due to impact damage. Because of the greater

thickness, a high-pressure COPV with a burst factor of 4.0 is expected to have a higher dam- age tolerance than a high-pressure COPV designed to a burst factor of 1.5 or 2.0 for most designs. This approach is restricted to high pressures as low-pressure COPVs may have a very thin composite layer that will show major reductions in strength even when the impact damage is not visible, including those with a design burst factor of 4.0

4. Enhance visible damage detection threshold w COPVs used in industries other than aero-

space are wrapped with several layers of glass yarn on the exterior to provide robustness to the COPVs. The glass carries only a small amount of load because of its lower modulus compared to graphite. Impact damage is visi- ble at lower impact energies than on COPVs made entirely with graphite yarn. - Provides protection for the graphite yarn

from normal handling damage and may improve the impact resistance.

- Adds weight. - A precise correlation has not been estab-

lished between the amount of crazing of the glass and reduction in burst strength.

5. Use a high modulus liner material n Damage tolerance tests have shown that stain-

less- and Inconel-lined COPVs are able to take an impact at a higher energy than alumi- num liners before there is a reduction in the burst strength.

Liner Leaks Safe life is the goal for any COPV so that a through-

the-thickness crack will not develop during the service life. Because this failure mode is not a safety issue for nonhazardous fluids, it is not addressed in the require- ments for COPVs by Range Safety and it is the contrac- tor’s choice on how it should be addressed. A gross liner leak before launch is expected to result in an abort to replace the COPV, but could result in loss of mission if this occurs during flight. Undetected small leaks may be acceptable, but that depends on the crack growth rate. There is no known data on the stability or increase of the leak rate with time for different liner materials at MEOP pressures.

The magnitude of a leak dictates whether it will influ- ence the success of the mission. The normal measure for the detection of gross leaks is pressure decay. The detec- tion of a gross leak is expected to result in a launch abort. However, if the leak were small, it would not be detected by pressure decay. If a through-the-thickness crack exists, the issue is whether the crack extension rate is small so

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the leak would not jeopardize the mission if it goes unde- tected. For the Delta IV booster, the flight time is less than 3 min so that a nondetected leak at launch would probably not grow to a sufficient size that would result in loss of support for the mission. On the other hand, upper stages frequently go through a coast phase that may last from a few hours to several weeks. For Delta IV, coast times greater than 6 hr are not expected. The Delta IV upper stage has determined that one pound of helium can be lost from 5 to 8 COPVs manifolded together with no effect on the mission. A leak rate of 10m4 standard ccisec would not be a problem, but higher leak rates may create a problem.

There are several test approaches that can be used to help guarantee that the safe life of the liner is achieved:

n Subject the COPV with its inherent undetectable flaws to pressure cycling equivalent to four lifetimes.

n Introduce a flaw in the liner before wrapping, wrap the COPV, and then subject it to pressure cycling equiv- alent to four lifetimes. Usually multiple flaws are intro- duced. Tensile coupons can also be used, but to duplicate the load conditions, one may have to bond a composite to the aluminum.

- One approach is to use electrical-discharge- machined (EDM) notches.

- Another approach is to EDM a notch on a thick- ened liner. The liner is cycled without over- wrapping until the desired crack size has been grown. The EDM notch is then machined away.’ The overwrap is applied and the COPV pressure cycled equivalent to four lifetimes.

Generating fatigue cracks in the liner is clearly the most severe approach, but also the most expensive. For this reason, some specifications allow the flaw to be in the form of an EDM notch.

The design options that relate to increasing the fatigue life are well known and include, but are not restricted to, the following:

n Use a material with good fatigue and slow crack growth characteristics, e.g., Inconel is known to be bet- ter than precipitation-hardened stainless steel. Alumi- num also has good ductility and crack resistance.

n Increase the material thickness. n Use “special” inspection techniques that guarantee

the detection of smaller flaws than those detectable by conventional inspection techniques.

w Inspect the liner. One approach is to dye-penetrant inspect the thin sidewall of the liner prior to spinning the domes. The liner, upon completion, can be ultrasoni- cally inspected. The ability to detect cracks is enhanced if the liner, prior to overwrapping, is pressurized close or slightly above the yield strength. Any cracks will tend to yawn, stay open, and be more readily detectable.

n Select a design that permits dye-penetrant inspec- tion of the interior surfaces. This approach requires a

weld closeout. Penetrant inspection of the interior of a seamless liner is impractical.

There are tivo well known approaches for the making of liners: (1) welded titanium, Inconel, stainless steel, and some aluminmn liners and (2) seamless aluminum liners. Seamless aluminum liners offer the lowest cost approach and are the standard for commercial COPVs. The issue is whether designs using seamless aluminum liners for aerospace applications are acceptable or pose an added risk to a program such as Delta IV A study was initiated to assess this question.

Aluminum Liners

Common to all aluminum liners, independent of how they are made, is whether they operate in the elastic or plastic range, and the approach for establishing mini- mum thickness.

Aluminum liners can operate in the elastic or plastic range depending on the design burst factor. Aluminum- lined COPVs with burst factors of 1.5 and 2.0 are designed to plastically yield every time they are pressur- ized, but can operate in the elastic range if they are designed with a burst factor of 4.0. The extra weight for a design with a burst factor 4.0 makes this approach unattractive, and normally is used only when personnel have to work around the pressurized COPV. There are fracture mechanics analysis techniques available when a material is plastically strained, but are not as widely used as those involving linear elastic fracture mechanics.r2

Liner thickness should be chosen so that a preexist- ing crack below the detectable inspection threshold will not propagate through the thickness in four lifetimes. For Delta IV, this is four cycles to proof and 120 cycles to MEOl? The presence of surface irregularities on seamless liners needs to be considered in establishing the thickness.

Welded Aluminum Liners. The arguments that favor the welded approach for aluminum liners are described below. Although there are several different ways that a liner can be assembled, the one selected for reference purposes is a cylindrical liner made from two halves with integral domes and bosses.

n The surface finish of the interior can be manufac- tured to a specified level and will not have orange-peel or other surface irregularities on the interior that could lead to a crack formation.

n Excellent thickness and hence weight control can be achieved for a minimum weight COPV.

w The interior can be dye-penetrant inspected. H The boss can be tailored to an optimum configura-

tion as machining can be performed on both the inside and outside diameter.

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Seamless Aluminum Liners. The arguments favor- ing a seamless liner are as follows:

l Lowest cost. n Good ductility, resulting in inherent leak-before-

burst performance. n Absence of welds that historically have caused

intermittent problems in aerospace. Possible exception is variable polarity, pulsed-arc welds, but these have not been commonly used for COPV liners.

n Dye-penetrant, X-ray, and ultrasonic inspection can ensure the absence of critical flaws on both the inte- rior and exterior of the liner.

n Testimonial from the commercial sector that suc- cessfully uses seamless liners for applications such as natural gas containment of vehicles, firemen backpacks, and aircraft inflation and oxygen systems.

n Interior surface irregularities such as orange peel and folds are notches, not cracks, so there will be an incubation period during fatigue cycling leading to crack initiation.

The interior surface irregularities are viewed as the highest risk associated with the development of a leak in a seamless aluminum liner. Catastrophic failure is not a likely failure mode because a crack is present or devel- ops in the liner. There are known cases where a through- the-thickness crack was detected after the autofrettage process and also after the proof test. Although such fail- ures are undesirable, their detection helps ensure that a defective COPV would not be installed on flight hard- ware. Such testimonials suggest that cracks developed either during the spinning operation or during the plastic straining of the liner.

The experiences above indicate that for aerospace applications, nondestructive examination (NDE) of the liner should be performed. There are several dif- ferent approaches that can be used. For a seamless liner, penetrant inspection of the inside surface of the cylindrical surface prior to spinning the domes is worthy of consideration. After the domes are spun, the internal and external surface of the liner can be

inspected by ultrasonics or X-ray. Penetrant can be used for the exterior surface. Since cracks tend to yawn open after loading, pressurizing the liner prior to overwrapping and then inspecting would provide the greatest probability of detecting a crack. Special- ists in the field have stated that the surface finish quality has improved significantly on the interior of seamless spun liners as compared to 10 years ago. One therefore needs to assess the conditions as they exist today and be cautious in using experiences that occurred many years ago. Analysis, evaluation, and test are continuing on this risk area.

Both approaches for making aluminum liners, seam- less or welded halves, will provide a reliable COPV with very low risk to the user. This is the author’s view and is not universally shared in the technical community. Launch vehicles do not have the weight criticality of a satellite or interplanetary vehicle, which allows greater margins, i.e., increased liner thickness, to be used.

Overload Caused by the Dynamic Environment This topic has not been analyzed and reviewed to the

degree of the other topics. Two schools of thought were identified as how the requirements should be specified. The traditional or most frequently used approach has been to specify a random environment spectrum. The random vibration tests are considered to be more severe than flight conditions and consequently provide good confidence that this condition will not create a failure during flight. The other view states that the COPV has to be considerably over designed to meet the require- ments of the random vibration test and that hardware should not have to bc designed to pass a test. The selec- tion of the prcfcrrcd option is still under review.

COPV RISK MITIGATION FOR DELTA IV

The single largest use of COPVs on Delta IV is for helium containment, as shown in Table 3. The current baseline for Delta IV helium bottles includes both a seamless 6061-T6 aluminum and welded Inconel 718 liner, both with Toray T- 1000 yarn. These COPVs have

Table 4. Delta IV COPV MEOP, shape, size, volume and yarn type.

Approximate composite Burst MEOP Size Vol. thickness

Gas factor psig (MPa) Shape in. (mm) in.3 (m3) in. (mm) He 1.5 4,550 Cylinder 16.5 dia x 35 long 5,920 0.262

(31.3) (419 x 889) (0.097) (6.7) N2 4.0 4,000 Cylinder 15 dia x 20.2 long 2,120 0.400

(27.6) (381 x 513) (0.0347) (10.2) He 2.0 9,700 Cylinder 11.5 dia x 22.4 1,300 0.540

(66.8) (292’0xns968.9)

(0.0213) (10.2)

N2 4.0 3,000 Cylinder 5.5 dia x 19.2 long 324 0.109 (20.7) (139.7 x 487.7) (0.0053) (2.77)

8 American Institute of Aeronautics and Astronautics

.-. . , -

design burst factors of 2.0 and 1.5, respectively, because they are first pressurized on the launch pad. Because of the low burst factors, these COPVs are considered to have the highest risk to the program. An assessment was made to show that these COPVs do not present a signif- icant risk to the Delta IV program. The two nitrogen- containing COPVs are briefly discussed, but as both have design burst factors of 4.0, less risk is associated with these COPVs.

The risk factors previously identified include (1) stress rupture, (2) undetected impact or handling dam- age, (3) liner leaks, and (4) overload caused by the dynamic environment. Stress-rupture data for graphite COPVs are limited, but sufficient to make a reasonable engineering assessment. Boeing has included undetec- ted impact damage in its design requirements prior to the issuance of a formal requirement to this effect. It is recognized that safe life is best demonstrated by cyclic loading with purposely flawed specimens. Boeing is also considering making this part of the requirements for COPVs.

Boeing and its subcontractors elected to use different options for different applications as shown in Table 4 and described below.

Nitrogen COPVs Both of the COPVs containing nitrogen require a

checkout at the factory and thus have burst factors of 4.0. One vessel is designed to operate the hydraulic system at 4,000 psig (27.6 MPa), and the other is a part of the booster separation system, which operates at 3,000 psig (20.7 MPa). They are both made with a seamless, alumi- num liner.

Damage Tolerance

Although initially it was thought that damage impact assessment tests should be conducted, these authors believe that this approach is no longer necessary for these designs because of the high burst factor.

Liner Leaks

Safe-life analysis or testing is required using start- ing flaw sizes established by NDE to demonstrate life requirements. The results of these tests were not avail- able at the time this paper was prepared. Leaks at joints must be less than that which can be detected by bubble soap. The 4,000 psig COPV is at approximately 3,200 psig when filled approximately 24 hr before launch and pressurized to 4,000 psig several minutes before launch. A bubble soap test at the time of assembly is adequate to ensure that the leak rate is sufficiently small (less than 10v2 standard cc3/sec) so that an adequate quantity of gas will be available.

Stress Rupture

Stress-rupture failures do not occur when the operat- ing stress. is at one quarter of the design burst strength. For all practical purposes, these COPVs have infinite stress-rupture life.

Overload Caused bv tbe Dvnamic Environment

Both nitrogen COPVs specify a random vibration qualification environment and acceleration loads.

Helium COPVs There are two types of helium COPVs: one for the

solid rocket motor (SBM) hydraulic system and the other for the launch vehicle to purge several pieces of hardware.

SRM Helium COPV

The MEOP of this small 1,300-in3 (0.021-m3) COPV is 9,700 psig (66.9 MPa). The responsible Boeing sub- contractor elected to use a seamless, aluminum-lined, cylindrical COPV with a specified burst factor of 2.0. It is made from Toray T-700 yarn. At the launch site, special carts provide the small amount of 9,700-psig helium gas required.

Damage Tolerance. The approximately 0.54-in.- thick composite provided the required robustness along with an outer glass layer added to a portion of the cylin- drical section. The burst strength was determined after impacts at VDT in multiple locations. This was a quali- fication test. Large impact forces were required, in the neighborhood of 220 ft-lb (298 .I), to obtain VDT. Addi- tional impacts were made above VDT to 280 ft-lb (379 .I). The vessel burst 12.5% less than the nominal burst and 2.1% below the minimum requirement. It burst at 18,990 psig. This was well within the new requirement not yet implemented that the burst strength should be equal to or greater than 1.2 x proof pressure or 1.2 x 1.5 = 1.8 or 17,600 psig. As the maximum expected impact energy is around 35 ft-lb, there is very high confidence associated with the ruggedness of this COPV

Liner Leaks. These COPVs are normally filled 3 days before launch, but are required to maintain a minimum specified pressure if the launch is delayed up to 21 days. The qualification test program demonstrates that the design meets the specified leak tightness of 10e5 standard cc3/sec. In addition, this is one of the only COPVs that can be vented on the launch pad and replaced if a gross leak ever occurred.

Stress Ruuture. This vessel is designed to hold pressure for 21 consecutive days with a total useful life of 1 year. For a P(s) of 0.9995, a burst factor of 1.7 could be used, but the factor of 2.0 allows for a more conservative design (see Figure 2). There is no stress- rupture issue for the selected design.

9 American Institute of Aeronautics and Astronautics

Overload Caused by the Dvnamic Environment. Random vibration qualification environments and accel- eration loads are specified.

Helium Purge COPV

There are multiple helium COPVs manifolded together on the booster and separately manifolded together for the upper stage. This COPV has an Inconel 718 liner. The COPVs are cylindrical shaped and oper- ated at 4,500 psig (30.9 MPa) with MEOP specified as 4.,550 psig (3 1.2 MPa).

DamaPe Tolerance. The COPV survived several pressure cycles including one to design burst after receiving 30 ft-lb (41 J) impact with a 0.5-in. (12.7-mm) diameter impactor. (For reference, a spherical, stainless steel COPV made by the same manufacturer was impact tested; energies below 48 ft-lb [65 J] did not result in a reduction in the burst strength.“)

Liner Leaks. Because the COPVs are manifolded together, a gross leak in one bottle would cause the rapid loss of gas from the other COPVs. Check valves on each bottle could be used, but was not considered necessary because the risk of such an occurrence is negligible. The Inconel7 18 liner operates in the linear elastic range and was analyzed by linear elastic fracture mechanics meth- ods with high confidence. A leak-before-burst demon- stration was made. A test with a through-the-thickness crack showed that it was stable and the leak rate was low. It was concluded that for the short duration of flight that the leak rate was sufficiently small not to jeopardize the mission. The robustness of the COPV was further demonstrated by a safe-life demonstration test. An EDM notch was cut into the liner and the COPV pressure cycled. It was found that the largest nondetectable flaw by standard inspection methods would not result in a crack through the thickness in 120 pressure cycles. Leaks through mechanical joints are not an issue as they passed the required acceptance test and are filled approximately 4 hr before flight.

Stress Rupture. The stress rupture only relates to the characteristics of the matrix composite of yarn and resin. The helium COPVs are filled approximately 4 hr before launch. Because of the program requirement to handle 30 aborts, the time at pressure could be as high as 120 hr. If an abort occurs, the tanks remain pressurized for a worst- case scenario of an additional 4 hr. Hence, the worst-time scenario at pressure on the launch site is 240 hr. If one adds the time of the proof test, the total hours at pressure is no more than 241 hr. Graphite yarn stressed to 67% of the average ultimate strength (not the design stress) would be reduced to a predicted strength of 62% at the end of this time period assuming a 0.9995% probability of survival as used in Figure 3. Because of the small decrease with time, a factor of four lifetimes or 964 hr

would only reduce the predicted burst strength to 61%. The average strength of the COPV demonstrated by test results is a stress of around 57% of the ultimate. The anal- yses show that the requirements to ensure against stress- rupture failures has been met.

Overload Caused bv Dynamic Environment. Only acceleration limit loads are specified, but there are no vibration requirments. It may be noted that Eurostar 2000+ had already conducted the random vibration testing. ’

DISCUSSION

Boeing has had an essentially flawless record relative to pressure vessels for all applications with the one exception on S-IVB. This success is attributed to many factors, but probably most important was and is detailed attention to every aspect of the design, procurement, test, and use of COPVs and all-metal pressure vessels by Boeing and its suppliers. The approach selected by Delta IV is consistent with the other Delta programs.

Several other aspects should be considered when using COPVs. One is the combined effect of damage, and sustained and cyclic pressures, damage protection during normal operations, and the differences in rugged- ness of different designs. The current views on these topics are presented in the following paragraphs.

Combined Effects Only limited combined effects data exist that assess

the interaction of sustained pressure loads and/or pres- sure cycling with impact damage. A joint NASA/AF test program studied four configurations of COPVs that were impacted to the minimum visible detectable level. From each configuration, two vessels were tested. One vessel was burst and the other was subjected to a sus- tained pressure at MEOP for 6 months and then burst. A comparison of the burst pressures with damage before and after the 6-month pressure loading did not exhibit any significant change in the burst strength beyond the normal scatter that can be expected.” Ten COPVs were impacted to the visual damage threshold and currently have been under sustained MEOP pressure for 1.5 years without a failure.

Impacts on nonpressurized COPVs at the visible threshold can cause an indentation of the liner, which can locally separate from the composite over-wrap. Pres- surization will return the liner to its original condition, but there is a possibility that the fatigue life will be reduced. The results of a test on one bottle resulted in a premature leak at the impact location as a result of pres- sure cyc1ing.l’

The results of three COPVs impacted to the visual damage threshold, cycled to MEOP (500 or 1,000 cycles) and then burst tested, showed no reduction in

10 American Institute of Aeronautics and Astronautics

burst strength compared to vessels that were damaged but not cycled.”

These data suggest that combined effects below the visual damage threshold can probably be ignored. A bottle with a minimum design burst of 1.5 is usually designed to provide a burst value of 1.75 or higher. At this value, MEOP is around 57% of the burst strength. The actual fiber stress will be higher because of the pre- strain introduced by the autofrettage, but still suffi- ciently low that stress rupture should not be a credible failure.

Damape Control The probability of survival of Delta COPVs is further

increased by providing protection schemes to prevent damage to the COPV A control plan is being proposed that would examine the threats of damage that are insti- tuted during transportation, integration, test, shipment, and preparation for flight at the launch site and institutes steps that prevent damage from occurring. The COPVs are inspected just before closeout when they will no longer be accessible to ensure there is no visible damage. Removable or flight covers are used when accessibility to the COPV is not required. Recently, it was recommended that the covers be able to withstand a 70 ft-lb (94.9 J) impact without damaging the COPV. These precaution- ary measures are a standard way of doing business for Delta III and will become so for Delta IV.

Desim Ruggedness to Preclude Damage The key to providing greater ruggedness is to

increase the stiffness of the metal-liner, composite over- wrap combination (increase the thickness of the com- posite) and providing a tough outer layer for COPVs. This can be done using one, or a combination of the con- cepts identified below:

n Using a higher modulus liner material (Inconel or stainless rather than alrmmmm).

n Using a lower strength yarn so that the composite overwrap becomes thicker (e.g., use Toray T-700 instead of T-1000).

n Using a higher liner thickness for any given metal liner, and select a shape that provides the greatest resis- tance (spherical preferable to cylindrical).

n Using an outer layer of glass for impact protection and easier detection of impact damage.

FUTURE TRENDS

Future trends indicate that the requirement for helium will grow rather than diminish. There are three changes that can accommodate larger stores of helium: increase the volume, increase the pressure, or decrease the temperature. Each of these options has been used at one time.

Increased Pressure: The pressure of a COPV can be doubled up to around 10,000 psig (68.9 MPa) without doimliiig the weight or cost. In fact, COPVs with 40,000 psig (275.8 MPa) and 50,000 psig (344.7 MPa) burst fac- tors have been built and successfully tested even though there is a reduction in the delivered fiber strength as the design becomes a thick-wall rather than a thin-wall anal- ysis. The major problem is not in the design of the COPV, but rather in the infrastructures at most launch sites. The launch sites currently are not capable of han- dling the required helium quantities at pressures above 6,000 psig (41.4 MPa), except for low-use quantities at higher pressures as discussed previously.

Increased Volume: An increase in the number of COPVs is practical as long as there is space to mount them. For a large increase, mounting space for all the COPVs is expected to be a problem.

Lower Temperature: One method of increasing the storage is to decrease the temperature of the gas. At liquid hydrogen temperatures around -423°F (20 K), 14.75 times as much gas can be stored at the same pressure than at ambient temperatures. The Saturn S-IVB tank had the tita- nium spheres located on the inside of the hydrogen tank. The sphere port was attached to the sides of the hydrogen tank so that the gas could be brought outside of the tank without the need for providing tubing inside the hydrogen tank. There was also a heater to raise the temperature of the gas to the temperature required for purging. This approach would use an all-metal pressure vessel and is not relevant to COPVs as they do not have the cold temper- ature capability.

Although the S-IVB program was successful using cold helium storage, this currently is not the favored approach by the advanced design team. Thus, increasing the pressure of a COPV seems to be the most attractive approach recognizing the investment required in the infrastructure to support higher pressure COPVs.

SUMMARY AND CONCLUSIONS

This paper has presented a summary of the pressure vessels used on the Delta vehicles and the transition from all-metal Ti-6Al-4V to COPVs with 6061-T6 seamless almninum and welded Inconel718 liners. The three major design risk factors discussed were stress- rupture, handling or impact damage, and unacceptable leaks. Although there are uncertainties in the analysis for each of these conditions, there are sufficient data and methodology to ensure a safe-life design without a leak and the complete absence of a catastrophic failure assuming no manufacturing or quality problems. It was concluded that with appropriate designs and precau- tions as used for Delta IV, the COPVs with seamless aluminum and Inconel liners will continue the blemish- free record of the Delta II program.

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