NASA/SP–2011–573

Composite Overwrapped Pressure Vessels, 

A Primer

Pat B. McLaughlan, P.E.

Scott C. Forth, Ph.D.

Johnson Space Center, Houston, Texas

Lorie R. Grimes-Ledesma, Ph.D.

Technical Review

Jet Propulsion Laboratory, Pasadena, Calif.

March 2011

National Aeronautics and

Space Administration

Johnson Space Center

Houston, TX 77058
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NASA/SP–2011–573

Composite Overwrapped Pressure Vessels, 

A Primer

Pat B. McLaughlan, P.E.

Scott C. Forth, PhD

Johnson Space Center, Houston, Texas

Lorie Grimes-Ledesma, Ph.D.

Technical Reviewer

Jet Propulsion Laboratory, Pasadena, Calif.

March 2011

National Aeronautics and

Space Administration

Johnson Space Center

Houston, TX 77058
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iii

Contents

Introduction............................................................................................................................... 1

Overview................................................................................................................................... 2

History of composite overwrapped pressure vessel development ............................................ 3

Current design experience......................................................................................................... 4

Manufacturing........................................................................................................................... 5

Inspection.................................................................................................................................. 5

Performance database ............................................................................................................... 6

Standardization of composite material for multiple composite overwrapped 

pressure vessels ................................................................................................................. 7

COPV Failure Modes................................................................................................................ 8

COPV Stress Rupture Discussion............................................................................................. 8

Recommendations..................................................................................................................... 9

Conclusion ................................................................................................................................ 10

Figures

1 COPV failure modes........................................................................................................ 11

2 Kevlar®-epoxy (K/Ep) COPVs on shuttle ....................................................................... 12

3 Carbon fiber COPV, 6×16 in. .......................................................................................... 13

4 Typical carbon fiber COPVs............................................................................................ 13

5 Fabrication of filament wound pressure vessels with hoop and helical layers................ 14

6 Plastic-lined stress rupture test vessel.............................................................................. 15

7 Stress rupture testing and a hydraulic failure................................................................... 16

8 Hydraulic burst of a COPV.............................................................................................. 17

9 Pneumatic burst of a COPV............................................................................................. 17

10 Automotive application failure in Malaysia .................................................................... 18

11 Carbon strands probability of failure ............................................................................... 19

12 Kevlar® COPVs stress vs. probability of time to failure

(for probability concept illustration only)........................................................................ 20
iv

Acronyms

COPV composite overwrapped pressure vessel

DOT Department of Transportation

ISS International Space Station

LBB leak-before-burst

NDE nondestructive evaluation

NESC NASA Engineering and Safety Center

psi pounds per square inch

RFP request for proposal
1

Introduction

Due to the extensive amount of detailed information that has been published on composite 

overwrapped pressure vessels (COPVs), this document has been written to serve as a primer for 

those who desire an elementary knowledge of COPVs and the factors affecting composite safety. 

In this application, the word “composite” simply refers to a matrix of continuous fibers contained 

within a resin and wrapped over a pressure barrier (as in string over a ball) to form a vessel for 

gas or liquid containment.

COPVs are currently used at NASA to contain high-pressure fluids in propulsion, science 

experiments and life support applications. These COPVs have a significant weight advantage 

over all-metal vessels; but, as compared to all-metal vessels, COPVs require unique design, 

manufacturing, and test requirements. The most significant difference from metal designs is that 

COPVs involve a much more complex mechanical understanding due to the interplay between 

the composite overwrap and the inner liner. A metallic liner is typically used in a COPV as a 

fluid permeation barrier. The liner design concepts and requirements have been borrowed from allmetal vessels. However, the application of metallic vessel design standards to a very thin liner, and 

especially the composite, is not straightforward. Different failure modes exist for COPVs than for 

all-metal vessels, and the understanding of these failure modes is at a much more rudimentary 

level than for metal vessels. Three significant differences exist between composite and metal 

vessels:

1) While composites of carbon, Kevlar®, and glass are subject to a reduction in burst 

strength as a result of surface impact, Kevlar® and glass are more damage tolerant than 

carbon. Metallic and Kevlar® overwrapped vessels, which were used extensively in the 

Space Shuttle program, do not lose significant structural strength due to minor surface 

damage. Handling personnel at launch facilities, COPV designers, and the crew have 

needed to be educated on the risks of damaging a COPV.

2) Composites are subjected to an effect that is referred to as stress rupture or static fatigue 

in which, depending on multiple factors, a composite may fail as a function of time while 

it is at operating pressure. The likelihood of such a failure is remote if the failure 

potential is properly addressed in design and test.

3) Quantitative nondestructive testing methods used for flaw screening of thick-walled 

metallic structure are generally not applicable to COPV designs.

Pressure vessels have historically been classified as failing due to either burst or leak-beforeburst (LBB). These designations have been misleading in the past, suggesting that a vessel that is 

designated as LBB does not require further monitoring or vigilance. However, it is not recognized

that the pressure vessel failure modes must be addressed during all phases (design, qualification, 

and use) to ensure safety. Because of the additional composite failure modes for COPVs, determination of appropriate ways to address failure modes is usually more complex than for metallic vessels.

The ability to, with certainty, predict a specific failure mode is difficult due to the interaction

between the liner and the composite. However, COPVs are safely used in a wide range of 

applications, from natural gas vehicles to military rockets. Generally, it is only in ultra-
2

lightweight designs for space flight, where there is little margin on strength, that all of the COPV 

failure modes are credible and must be clearly addressed in design and operation.

Overview

A composite, as defined in this COPV application, is a combination of structural fibers and a 

resin that forms the overwrapped structure for a COPV. Continuous fibers provide tensile strength

for structural integrity while the resin carries shear loads in the composite and maintains the fiber 

position. As the fiber/resin composite is generally not considered pressure tight, the composite is 

applied over a fluid-retention barrier that serves as an interior liner for the composite. These fluidretention barriers may be a rubber, plastic, or thin ductile metal liner. These liners serve to maintain 

acceptable leak rates and fluid purity but add little, if any, structural integrity. Development is 

currently under way for the use of no liners or of extremely thin liners in future COPVs. On 

most ISS and Shuttle COPVs, the vessel liner is substantial and serves as a partial structural 

element by carrying a portion of the pressure load. The rigid metal liner also serves as a mandrel 

on which the fiber/wet resin is wrapped to form the composite structure. A vessel that contains a 

structurally significant liner in which the liner and the composite share in resisting the internal 

pressure load is said to have a load-sharing liner.

Both metal vessels and COPVs offer unique advantages. An assessment based on their 

performance needs and application is required to define the optimal design approach. Both types

of construction range from vessels of large burst pressure safety factors for industrial and

commercial applications to high-efficiency (efficiency is the ratio of product capacity to vessel 

weight) vessels for rocket and spacecraft applications. For lightweight, high-efficiency 

applications, the COPV will offer a significant weight advantage, approximately one-half the 

weight of a comparable metal tank. The all-metal vessel, while possibly heavier, usually offers 

the advantage of lower manufacturing cost, verification of fracture control by analysis, and simply 

a more straightforward design. Demonstrating fracture control requirements for a COPV, such as 

safe life and/or LBB of the thin metallic liner and damage tolerance of the composite overwrap, 

is usually verified by costly qualification tests. Well-established nondestructive evaluation 

(NDE) methods are in use for thick metallic structures that are used to quantitatively establish a 

critical flaw size that is screened for during acceptance testing. The nondestructive inspection is 

used in the safe-life and LBB analyses as a starting point for predicting life or vessel failure.

These NDE methods have not been quantitatively demonstrated for thin metallic liners or 

composite materials. Work in this area is ongoing and significant progress has been made in 

composite NDE technology as of this writing. Proof testing is an excellent screening tool for 

manufacturing anomalies, but may not effectively screen a vessel that is susceptible to fatigue 

cracking in the liner or a premature stress rupture failure in the composite.

High-efficiency COPVs have been very effectively and safely used in single-use, crewless 

launch vehicles for space applications. It is always important for a designer to remember that 

each application of a pressure vessel should have a mass, performance, cost and risk trade.
3

History of composite overwrapped pressure vessel development

In the early 1970s, the primary author was assigned to a technology transfer task that focused on 

improving respiratory protection for municipal firefighters – a task that eventually led to the 

widespread commercialization of COPVs. The NASA Firefighter’s Breathing System Program

that arose from this assignment quickly evolved into a significant technology effort involving 

cooperation between NASA and multiple fire service organizations across the United States, 

including those in Boston, Los Angeles, and New York City.

After a requirements definition phase, several trade studies were conducted to address the 

major concerns of firefighters regarding respiratory protection. Drawing on results of these 

trade studies, researchers determined that a reduced weight and bulk compressed-air breathing 

system was optimum. In this type of system, the greatest contributor to weight and bulk is the

compressed-air pressure vessel. The breathing system that was commonly employed in the 1970s 

used compressed-air vessels of conventional steel or aluminum design. While these vessels met 

appropriate Department of Transportation (DOT) requirements (operating at 2,200 pounds per 

square inch [psi] pressure), these requirements were not optimum for a reduction in physical

dimensions. To achieve a reduction in the bulk of the improved system, air storage pressure was 

increased to 4,500 psi. Other performance parameters, such as a weight reduction, were also subsequently defined, and a request for proposal (RFP) was released by NASA to industry to design,

test, and manufacture a lightweight and higher-pressure compressed air vessel.

The primary objective of this RFP was to reduce pressure vessel weight while maintaining

required firefighter breathing air quantity. Secondary objectives included: durability, low 

production costs, minimum maintenance, and a potential for future high-volume commercial 

production. Various high-strength alloy steel vessels were proposed by current suppliers in

addition to two proposals for glass composite overwrapped vessels. Both composite vessel proposals projected rugged vessels of one-half the weight of the existing metal vessels and, most 

importantly, a desire to enter commercial production to satisfy future firefighter demand.

As part of the NASA Firefighter’s Breathing System Program, additional improvements 

were made in system pressure regulators, face masks, end-of-duration warning, and human 

factors. The resulting NASA Firefighter’s Breathing System concept was accepted by the fire 

service, which led to a demand for commercial production of the lighter COPVs. The composite 

vessels received certification from the DOT in 1975, and commercial production was initiated.

Although the DOT-certified vessels are designed to a greater factor of safety on burst pressure than 

is currently used in some COPV military and aerospace application, a 50% pressure vessel weight 

reduction over the metal design was nevertheless achieved.

As production of the COPVs has continued, fiber technology has evolved from glass to 

Kevlar® to carbon composites. This evolution of higher-strength fibers has led to further 

weight reductions. Hundreds of thousands of composite vessels for commercial applications

are now produced per year, and these are used worldwide. The safety record of COPVs in 

commercial service has proven exceptional.

To some extent, the success in the early 1970s of these firefighter COPVs may have bolstered

the use of Kevlar® COPVs for storage of the large quantities of high-pressure helium and nitrogen
4

on the space shuttle orbiter. At this time, metal vessel design and test standards dominated,

rather than new standards, to support COPV performance characteristics. During the development of the orbiter COPVs, the primary design emphasis was on a load-sharing metal liner 

with less concentration on the overwrap, which was the primary structural element. The orbiter 

COPV used cylindrical or spherical metal liners that served to share load with the composite, 

retain the gaseous pressurant, and provide a mandrel or form on which to wrap the resin-coated 

Kevlar® fiber. Analysis and test emphasis of the orbiter vessels appeared to be on the performance of the metal liner while the wrap was considered only a highly reliable agent with which 

to supplement metal liner performance.

Owing to our lack of composite experience in the late 1970s, the Space Shuttle Orbiter COPVs 

were produced and tested to a procurement and certification standard better suited for all-metal 

vessels. As the responsible technical team had extensive experience in metal vessels, team member emphasis was on the metal liner rather than a system-approach integrating liner and composite 

performance. The factor-of-safety margins, development testing, burst test articles, manufacturing 

inspections, and acceptance procedures did not fully address the unique, and generally unknown, 

composite characteristics. For example, determination of fiber stress at operating pressure and 

burst pressure may have been clouded by a limited understanding of the load-sharing relationship

between the metal liner and the overwrap.

The expected range of time to stress rupture failure dominated all subsequent assessments of 

the space shuttle orbiter COPV performance. Stress rupture was the dominant concern because all 

other COPV failure modes had been adequately addressed during development and qualification 

testing. A stress rupture failure would, in all probability, be catastrophic – potentially resulting 

in loss of a processing facility, a space flight vehicle, and life. This, therefore, is a serious consideration, particularly in light of the fact that no method existed for predicting an impending 

failure. Fortunately, recent analysis and test has indicated acceptable reliability.

Current design experience

The COPV design requires an analysis of the liner, the fiber overwrap, and the interaction 

between the two. COPV liners may be of ductile materials, such as soft aluminum, with only 

minimal load-sharing capabilities. Higher-strength steel, Inconel®, titanium, or cryogenically 

formed stainless steel may alternatively be used, resulting in a load-sharing liner. Recent 

projects have begun using plastic-lined COPVs to minimize weight. The fiber is generally 

applied as a ribbon of multiple fibers that passes through a bath for resin application. This

ribbon of fiber and resin is wound on the liner as one would wind a ball of string. The pressure 

vessel liner and the dispensing head for the fibers ribbon move in relation to one another in such 

a way as to wrap the fiber on the liner in a desired pattern. If the vessel is cylindrical rather than 

spherical, fiber winding is normally applied in both a longitudinal (helical) and a circumferential

(hoop) wrap. This winding process consists of multiple critical steps, such as resin content, fiber 

configuration, winding tension, and the pattern of the wrap in relation to the axis of the liner. The 

resin is then allowed to cure (dry and harden) at an elevated temperature.

Following post-cure inspections, the vessel may be sized, or autofretaged, to improve structural

performance. Autofretage is a process whereby a COPV is pressurized to above the liner yield 
5

strength, causing the liner to plastically deform or expand. The liner is now permanently expanded

so that, at the time of depressurization, a tensile strain (or stress) remains in the fiber composite due 

to the enlarged liner and a compressive strain is induced in the liner. A measurement of the expansion in internal volume is desirable to quantify the amount of residual strain in both the liner 

and the overwrap. The compressive residual strain in the liner results in an improved cycle life 

performance of the liner. The sizing (or autofretage) of the wrapped liner may serve as a proof test, 

or a second proof (at a lower pressure) may be accomplished.

Manufacturing

Manufacturing process certification is key to reliable composite performance. Process control 

and consistency are mandatory when minimizing performance variability. This is because limited

testing will not fully define the range of performance variability expected in burst pressure or the 

time to stress rupture failure. Composite variability may, therefore, present serious safety issues. Of

course, any structure has some level of variability that is compensated for by design margins; 

but, for composites, the extreme performance variability that has been experienced, coupled 

with the potentially catastrophic nature of failure, demands consistency in manufacturing.

Fibers may experience variability in ultimate strength and time to stress rupture between 

production batches. This inconsistency in fiber performance may be rooted in the variability 

in chemical composition, processing, and fiber diameter. Effective sampling and verification of

fiber characteristics at the time of acceptance is mandatory. Performance consistency of the completed COPV requires mandatory consistency in the materials of composite fabrication (e.g., the 

fiber spools and resin). It is thus important that COPV materials manufacturing and inspection 

practices be controlled to limit performance variability and the possibility of unexpected 

catastrophic failure.

Prior COPV testing has indicated significant difference in stress rupture performance as a 

function of the different spools of fiber delivered by some fiber manufacturers. Therefore, the 

burst and time to stress rupture variability issue may not rest entirely with the COPV manufacturing

process or design, but with the supplier of the fibers. To formulate an acceptable understanding 

of final product performance characteristics, rigorous standards and material acceptance testing 

must be in place from raw materials through the finished product. The production of COPVs also 

involves fabrication of a cylindrical or spherical metal liner . This metal liner serves multiple 

functions: as a pressure-tight fluid seal, a form, or a mandrel on which to wrap the fiber/resin 

composite and, possibly, as a load-carrying structural element. As an integral part of the COPV, the 

liner must satisfy appropriate NDE inspections prior to application of the overwrap.

Inspection

Although several inspection techniques are currently in place for COPVs, unfortunately none 

of them provides the level of confidence that metal vessel inspection offers. Metal vessels may 

be subject to well-established tests to:

1) Determine material properties prior to fabrication.
6

2) Provide NDE inspection of the completed vessels screen for cracks, porosity, partial 

thickness welds, and other defects.

3) Determine localized stresses by strain gauges or other measurement systems.

4) Achieve structural verification of the completed product through over-pressurization tests.

For COPVs at this time, NDE, as an indicator of future performance, is not a viable inspection 

method to use when effectively verifying the composite. In fact, composite inspection lacks the 

definitive results that we rely on in metal inspection. Proof testing, as previously mentioned, may 

be an effective screen for defects; but a satisfactory proof test is not considered a predictor of 

future performance since it does not address stress rupture.

Field-use damage from impact and potentially harmful environmental exposure will also 

affect COPV performance. Damage-tolerance testing of the composite overwrap has

demonstrated that certain tank designs can be very sensitive to impact. Recent COPV designs 

have improved the damage tolerance by including sacrificial outer layers and affording more 

material to the dome-transition region, where early COPV designs were overly sensitive to small 

amounts of damage. Trained inspectors, well-defined protection schemes, and knowledgeable 

engineering support are critical to maintaining the safe use of COPVs.

Performance database

In some ways, composite vessel design and manufacturing may still be considered more an

art than a science. Today’s COPVs – which are used in commercial applications such as firefighter 

breathing system air cylinders, vehicle compressed-gas fuel storage, and commercial aircraft 

applications (e.g., escape slide nitrogen pressurization system and emergency cabin breathing

oxygen) – are very widespread and available worldwide. These COPVs have large safety margins

and are approved by DOT regulations. Other reduced-weight, non-DOT-approved COPVs may 

operate at smaller safety margins; these include gas storage COPVs for space launch vehicles, 

satellites, and spacecraft; military aircraft; and military weapons systems.

Whether a COPV is intended for use commercially or in space flight, it is vital to be able to

establish appropriate confidence in its reliability and safety. As previously mentioned, COPVs

may, in the event of composite failure, experience a catastrophic explosive release of energy due 

to the high pressure of the stored gas. Although the possibility of failure is remote, the potential 

consequences of such a failure, especially in a human space flight application, mandates a high 

level of design reliability. Reliability analysis must therefore be based on a significant composite 

performance database of like composite materials that are in like configurations. This database

should provide performance data that include: ultimate strength, time to stress rupture failure as a 

function of fiber stress, and the effect of temperature on time to stress rupture failure at various 

stress levels. Typical methods for data accumulation include:

1) Ultimate strength of individual fibers

2) Strands, which are a small bundle of fiber and resin formed as a composite longitudinal 

element, that may be loaded in tension to test for ultimate strength or stress rupture

3) A subscale test sphere or cylinder, which may contain a ductile or load-sharing liner

4) Full-size COPVs in the end-use configuration
7

Testing of full-size COPVs provides the ability to assess actual performance of the specific 

fiber wrap configuration without the unknown of interpretation from material coupon data, such as

strands, or subscale units. Some data have indicated a reduced average ultimate strength in larger, 

as compared to smaller, COPVs. Whether this is due to a thicker composite, more material being 

used in the application, winding pattern differences, or longer fibers has not yet been quantified. 

The longer fibers required to circle a larger vessel may relate to the theory that a long rope is not as 

strong as a short rope due to the potential for a greater number of flaws in the longer rope. The size 

effect has to be modeled using the Weibull Strength Theory, which has indicated a reduction in 

the ultimate strength of the thicker composite on larger COPVs. The most important data point 

is to determine whether the stress rupture performance of the full-size article falls within the 

expected database established by the subscale test articles. Due to the very long time required to 

demonstrate stress rupture in normal design conditions, it will be necessary to accelerate the time 

to stress rupture by increasing the stress in the composite through higher pressure and by 

elevating temperatures that may accelerate the time to stress rupture.

Standardization of composite material for multiple composite 

overwrapped pressure vessels

If multiple COPVs of alternate configurations will be used within a given program, it is highly 

desirable to test a single fiber and resin composite to establish a material performance baseline 

from a large quantity of subscale test articles. Commonality in fiber, resin, and processing characteristics will aid in establishing a very early-in-program development material performance 

baseline for full-size articles of multiple configurations and applications. Other composite

performance variables, such as wrap thickness, winding patterns, and vessel size, must be 

specifically evaluated.

Appropriate certification of the flight system reliability of a COPV presents challenges that 

are not normally encountered in conventional all-metal vessels. Metal vessels are constructed 

of well-defined materials designed to mature standards, and are inspected by a means that allows

specific identification of deficiencies in a specific vessel. As compared to metals technology,

composites technology presents challenges in the raw material variability, manufacturing

consistency, and design verification process. Certification testing is a requirement that must 

strictly apply not only to the design, but also to the consistency of materials and manufacturing.

Prior to beginning a certification process, an early and extensive development test must be performed to define the characteristics of selected fibers and resins and manufacturing processes. A

very early start of this development testing process is mandatory due to the effect damage tolerance

and stress rupture considerations can have on the vessel design; e.g., exterior helical wraps have 

been shown in some designs to be more damage-tolerant than exterior hoop wraps. If a COPV is 

required to withstand a specific impact energy level to meet damage-tolerance requirements, this 

must be incorporated into the design, usually resulting in a thicker overwrap. Further, to understand

the probability of a stress rupture failure, data must exist for a period of calendar time that 

exceeds the use time for both the specific composite structures material and the vessel 

configuration.
8

COPV Failure Modes

The NASA Johnson Space Center Fracture Control Board Chairman and coauthor, Scott Forth,

Ph.D., has identified the following four COPV failure modes for control in hazard reports:

1) Burst from Over-pressurization. The unexpected burst of a COPV from overpressurization is mitigated by material certification, designing to conservative material 

allowable, and proof-pressurization of the vessel during acceptance testing. The external 

pressurization source must also be controlled to prevent accidental excessive 

pressurization.

2) Fatigue Failure of the Metallic Liner. Fatigue failure of the metallic liner is mitigated by 

inspection and testing. Nondestructive inspection of the liner at manufacture ensures that 

critical flaw sizes are adequately screened. The COPV design may then be pressurizationcycle-tested during qualification to a factor significantly greater than the design life to 

ensure an adequate margin on design cycle life.

3) Burst Resulting from Metallic Liner or Composite Damage. Failure of the composite is 

mitigated by appropriate protection from damage and damage-tolerance testing. Visual 

inspection may be valuable in identifying surface damage, although subsurface damage to 

the composite or liner may not be easily identifiable.

4) Stress Rupture of the Composite Overwrap. Fiber-wrapped composite vessels differ from 

metal vessels in that they experience an effect known as stress rupture, or static fatigue. 

Stress rupture is a situation in which the composite experiences degradation, as a function 

of time. This degradation results in a sudden structural failure of the pressurized vessel’s 

composite overwrap, resulting in the rapid release of the vessel’s contents and the stored 

energy of the pressurized gas – possibly causing serious injury and damage to the 

surroundings.

COPV Stress Rupture Discussion

Dr. Leigh Phoenix of Cornell University provided the following definition of COPV stress 

rupture (failure mode number 4) above:

“Stress rupture is a sudden failure mode for [COPVs] that can occur at normal 

operating pressures and temperatures. This failure mode can occur while at stress 

levels below ultimate strength for [an] extended time. The failure mechanism is complex, not well understood, [and] difficult to accurately predict or detect prior to failure.

The location and mechanism of triggering damage causing sudden failure is highly

localized, but at a random location. This location and extent of local damage has not 

been able to be [reliably] detected by current [NDE] techniques prior to catastrophic 

failure. Pressure, duration of time at pressure, and temperature experienced contribute 

to the degradation of the fiber and/or the fiber-matrix interface, particularly around 

accumulations of fiber breaks, and these increase the probability of COPV stress 

rupture.”

To date, the composite stress rupture failure mode may not be fully understood, although 

conservative design and thorough testing can produce highly reliable COPVs. To date, there 

have been zero stress rupture failures of COPVs in NASA spaceflight applications. However, 

stress rupture failures have been observed in accelerated test programs at NASA JSC for both 
9

Kevlar® and carbon overwrapped COPVs. For Kevlar® COPVs, the manufacturer can compute

the stress rupture reliability using a Weibull model because a large, robust database exists. Unfortunately, as of the writing of this paper, there is not a similar database for carbon. Two NASA 

independent stress rupture test programs are currently under way that should provide additional 

data availability in 2013 for analysis. Until that time, the NESC and JSC Engineering recommend

that the carbon fiber strain remain at or below 50% of the ultimate strength. Based on industrywide experience, the risk of stress rupture at a strain ratio of 50% is minimal for short-duration 

space missions.

Due to the unknowns relating to stress rupture variability in composite performance and a 

lack of well-defined inspection methods, the COPV factors of safety (or design safety margins)

may be addressed differently than those applied to all-metal vessels. In other words, the COPV

factor of safety may be higher to compensate for unknown variability. Factors of safety may also 

be adjusted for usage conditions. The application of COPVs may range from single-pressurization, 

remote military uses to long-term, public applications involving many pressurization cycles. The 

design and factor of safety must be adjusted for the application.

Recommendations

The following are recommendations for development of COPVs in both commercial and space 

flight applications.

1) Design the COPVs to satisfy their intended application. For example, a COPV used in a 

public application with an expected long lifetime and exposed to uncontrolled environments as well as multiple pressure cycles will require more reliability margins, such as 

greater factors of safety and mitigation of environmental degradation, than a COPV that is 

intended for a single-use pressurization in orbital space flight or military applications.

2) Recognize that COPVs have significantly different characteristics than metal vessels and 

require unique standards. Material and manufacturing controls, performance requirements, 

safety margins, useful life, and environmental restrictions all present requirements that differ

from those applied to all-metal vessels. As a COPV is more complex than a metal vessel, 

failure modes unique to COPVs must be addressed. These include composite failure 

modes such as stress rupture and impact damage

3) Develop an extensive performance database for the specific composite system. The 

generalized performance of glass and Kevlar® fibers and subscale COPVs are documented, 

but that of the class of fibers referred to as carbon (or possibly graphite) is not as welldocumented. Carbon fiber characteristics, as applied to the raw material consistency,

manufacturing processes, aging factors, ultimate strength and stress rupture variability, and 

other critical factors ensuring long-term reliability, must be well understood. This requires

that material performance be evaluated in configurations such as individual fibers, composite strands (a linear structural element of fiber and resin), test rings, subscale COPVs, and

ideally full-size vessels of each configuration. A time element exists in the degradation of 

fiber performance, and the full-size COPV configuration may include localized areas of

higher stress sensitive to the effects of time at pressure and environmental conditions. Per-
10

formance correlation from subscale to full-scale testing may not be fully reliable; there

therefore is a need for evaluation by extensive composite instrumentation/inspection methods and possible accelerated (by elevated temperature/pressure) stress rupture testing.

4) Reduce complexity in materials testing by using a single or limited fiber/resin combination

for all vessel configurations in a given program. Some performance differences in the

composite may remain due to size, composite thickness, and liner load share characteristics, 

but an extensive database for a specific fiber and resin combination is desirable.

5) Evaluate the selection of all-metal vessels vs. COPVs for a specific application. An allmetal vessel may offer significant advantages in some applications while a COPV may be

superior in other applications. The use of a COPV requires acceptance of the additional 

complexity of composite usage and treatment of composite failure modes in additional to 

metallic failure modes. While a catastrophic failure of a COPV that results in loss of

property and life cannot be ruled out, the probability of such a failure occurring may 

be reduced to acceptable levels through appropriate design and test.

Conclusion

Undoubtedly, the use of COPVs will be expanding as the demand for efficient storage of 

pressurized gases increases in a variety of commercial, aircraft, spacecraft, and transportation

applications. Metal vessels have evolved from the time, well over a century ago, of exploding 

steam boilers to their mature and reliable technology today. Now COPV technology is continuing 

development to provide an increasing understanding of material characteristics and the resultant 

increasing COPV reliability. As this technology development continues, additional information 

will be available to aid in vessel material selection and design process to satisfy performance

requirements for the intended application.
11

Courtesy of Lorie Grimes-Ledesma, Ph.D., NASA Jet Propulsion Laboratory, Pasadena, Calif.

Figure 1. COPV failure modes.

Failure Mode Failure Result Control Phase Mitigation Method

Shearing of Boss Catastrophic Design /NDE Statistical, NDI

Fatigue Crack Growth in Liner 

under Composite Leakage Design/NDE Fracture Control (Safe-Life)

Crack Growth in Boss Catastrophic Design/NDE Fracture Control (Safe-Life)

Over Pressurization Catastrophic System Design/Operations Thermal Control and System Design

Stress-Rupture Catastrophic Design/Operations Stress-Rupture Data

Corrosion/Stress-Corrosion of 

Liner under Composite Catastrophic Design/Mfg/Operations Control of Chemical Environment

Corrosion/Stress-Corrosion of 

Boss Catastrophic Design/Mfg/Operations Control of Chemical Environment

Embrittlement of Liner Catastrophic Mfg/Operations Metallurgical Control, Control of 

Thermal and Chemical Environments

Corrosion of Matrix Resin or 

Fiber Catastrophic Mfg/Operations Control of Chemical Environment

Embrittlement of Matrix Resin 

or Fiber Catastrophic Mfg/Operations Control of Cure, Control of Thermal and Chemical Environments

Liner Buckling under 

Composite/fatigue Leakage Mfg/NDE Adhesive Bonding Process Control, Bond-Line NDE

Impact/Mechanical Damage Catastrophic Mfg/NDE/Operations Damage Control

Delamination (of mounting 

interface and bridging) Catastrophic Mfg NDE
12

Figure 2. Kevlar®-epoxy (K/Ep) COPVs on shuttle.

Kevlar® COPV Locations on the Shuttle Orbiter (blue arrows)

• Six 26-in.N2 K/Ep COPVs shown in dark yellow

• Eighteen 19 to 40-in. He K/Ep COPVs shown in lime green
13

Figure 3. Carbon fiber COPV, 6×18 in.

Figure 4. Typical carbon fiber COPVs.
14

Figure 5. Fabrication of filament wound pressure vessels with hoop and helical layers.
15

Figure 6. Plastic-lined stress rupture test vessel.
16

Figure 7. Stress rupture testing and a hydraulic failure.
17

Figure 8. Hydraulic burst of a COPV.

Figure 9. Pneumatic burst of a COPV.
18

Figure 10. Automotive application failure in Malaysia.
19

Figure 11. Carbon strands probability of failure

(for probability concept illustration only).

75% of

ult. strength

70% of

ult. strength

80% of

ult. strength

0.999

0.99

0.90

0.96

0.75

0.50

0.25

0.10

0.05

0.02

0.01

10-4 10-2 100 102 104

80% specimen first failure

75% specimen first failure

70% specimen first failure

Range of failure times, hours

Note:Carbon strand test

terminated at 104

 hours. Majority

of 50 specimensat 70% and 75%

of ult. strength had not failed.

Probability of failure
20

Figure 12. Kevlar® COPVs stress vs. probability of time to failure

(for probability concept illustration only).

Compostie stress as a % of ultimate stress

1

0.95

0.9

0.85

0.8

0.75

0.7

0.65

0.6

0.55

0.5

0.45

0.4

Failure probability time, hours

0.001 0.01 0.1 1 10 100 1,000 10,000 100,000 1,000,000

1 Day

1 Year

10 yrs

25 yrs

Probablility

of Failure

0.5

0.01

0.001

0.0001

0.000001

Test vessels

median failure

Test vessels

range of

time to failure

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March 2011 Special Publication

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Composite Overwrapped Pressure Vessels, A Primer

6. AUTHOR(S)

Pat B. McLaughlan*, Scott C. Forth,* Lorie R. Grimes-Ledesma**

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION 

REPORT NUMBERS

Lyndon B. Johnson Space Center

Houston, Texas 77058

S-1046

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National Aeronautics and Space Administration

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SP-2011-573

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*NASA Lyndon B. Johnson Space Center, **NASA Jet Propulsion Laboratory

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13. ABSTRACT (Maximum 200 words)

Due to the extensive amount of detailed information that has been published on composite overwrapped pressure vessels (COPVs), 

this document has been written to serve as a primer for those who desire an elementary knowledge of COPVs and the factors affecting 

composite safety. In this application, the word “composite” simply refers to a matrix of continuous fibers contained within a resin and 

wrapped over a pressure barrier to form a vessel for gas or liquid containment. COPVs are currently used at NASA to contain highpressure fluids in propulsion, science experiments, and life support applications. They have a significant weight advantage over allmetal vessels but require unique design, manufacturing, and test requirements. COPVs also involve a much more complex mechanical 

understanding due to the interplay between the composite overwrap and the inner liner. A metallic liner is typically used in a COPV as 

a fluid permeation barrier. The liner design concepts and requirements have been borrowed from all-metal vessels. However, 

application of metallic vessel design standards to a very thin liner is not straightforward. Different failure modes exist for COPVs than 

for all-metal vessels, and understanding of these failure modes is at a much more rudimentary level than for metal vessels.

14. SUBJECT TERMS 15. NUMBER OF 

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pressure vessels; propellant tank; pressure vessel design; shells (structural forms); rocket 

linings; lining processes; shielding 30

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