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Nondestructive Evaluation Program

Pressure Vessel Inspection Technologies for NASA and Commercial Spacecraft

Active Technology Project

Project Introduction

Composite overwrapped pressure vessels (COPVs) are heavily used in NASA and commercial spacecraft for propulsion and life support systems and are becoming commonplace in industrial applications including natural gas fueled buses. The rapid response needed to meet the demanding schedule of NASA and its spaceflight partners highlights the importance the Agency places on inspection capability readiness. In response to a launch pad failure, had the prototype advanced scanning system not been developed, the Agency would have been unable to resupply the International Space Station in a timely manner. Projects selected for this portfolio, funded by the NASA Office of Safety and Mission Assurance Nondestructive Evaluation Program, target the top inspection and monitoring needs identified by COPV community leaders and push inspection methods to higher states of readiness. These efforts include (1) statistically verifying surface breaking crack detection limits for a COPV scanning system and (2) testing a flight-qualified COPV orbital debris impact detection system. Ground testing capabilities sought to enhance the understanding of COPV material mechanics include (3) investigating an electromagnetic testing system with subsurface material property measurement capabilities for COPV ground testing and (4) developing methods for internally instrumenting COPV liners with fiber optic strain sensors with high special resolution.

Introduction

Composite overwrapped pressure vessels (COPVs) were heavily used on Shuttle orbiters and are also being used on Orion, the European Service Module, commercial spacecraft, the Space Launch System, the International Space Station (ISS) and virtually all spacecraft including those of commercial spaceflight partners. Commercially, they are used on natural gas fueled buses, water filtration systems, satellites, and commercial spacecraft including the SpaceX Dragon and Virgin Galactic SpaceShipTwo. Although other materials are occasionally used, most COPVs are thin-walled metallic liners overwrapped with high-performance carbon fibers for strength. This results in higher performing pressure vessels with superior capacity-to-weight performance over metallic pressure vessels. Higher performance parts often come at the expense of safety margins. Hence, these parts require additional diligence during design, manufacturing, testing, usage, and inspection. To date, three fatalities due to COPV failure have occurred in the U.S. transportation sector. Fortunately no fatalities have occurred in the aerospace sector, although COPV failures resulting in injury have occurred. All of these incidents were preventable.

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Developing the Multi-purpose Pressure Vessel Scanner

In 2014, two commercial spacecraft launches were scrubbed due to COPV leaks through the internal liner. Although the contributing factors will not be discussed here, to prevent reoccurrence enhanced nondestructive inspection and evaluation (NDE) were required beyond current industry capabilities. Two years prior to these incidents, the NASA Office of Safety and Mission Assurance (OSMA) had successfully demonstrated a Multi-purpose Pressure Vessel Scanner (MPVS) at a manufacturing center supporting the development of Nitrogen and Oxygen Recharge System (NORS) COPVs destined to resupply ISS. NASA rapidly responded to the support request that followed by enhancing the inspection capabilities of the MPVS in a joint effort by the NASA Office of Safety and Mission Assurance (OSMA) under its Nondestructive Evaluation Program and the NASA Engineering and Safety Center (NESC).

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Using specialized eddy current probes, the resulting MPVS is now capable of inspecting cylindrical and domed sections of metallic COPV liners for cracks smaller than traditional fluorescent dye penetrant methods can reliably detect. Surface-breaking flaws can now be identified, mapped, and measured from either the pressure vessel interior or exterior. Metallic liner wall thicknesses are also accurately measured via eddy currents, as are the pressure vessel 3D profile, inside and out. The system was highly successful at detecting cracks below the critical initial flaw size sought by the manufacturer and stands ready for implementation for safer and more reliable COPVs.

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Although the MPVS was effectively demonstrated at White Sands Test Facility (WSTF), the next phase under way is statistically verifying the surface breaking crack detection limits and detection range in a Full Probability of Detection (POD) study. NASA pioneered POD for NDE techniques in the 1970’s and now requires this statistical check to qualify inspectors, equipment, and techniques for fracture critical metallic hardware (NASA-STD-5009).

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Developing COPV Monitoring and Ground Test Equipment

The rapid response needed to meet the demanding schedule of NASA and its spaceflight partners highlights the importance the Agency places on NDE capability readiness. Had the prototype MPVS not been developed, the Agency would have been unable to react in a timely manner. In cases where spacecraft are tasked with resupplying the International Space Station with food and vital supplies, delays of months or even years cannot be tolerated while physicists and engineers develop leading-edge inspection equipment to meet unanticipated needs of missions and programs.

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Distributed Impact Detection System

A Distributed Impact Detection System (DIDS) was initially developed by NASA’s Small Business Innovative Research (SBIR) Program to detect impacts to space critical hardware. These small, low-power, and lightweight acoustic emission (AE) detection systems are highly capable of detecting impacts and classifying event types. Much larger industrial AE systems monitor bridges for cracks, requalify pressure vessels for use, monitor for bearing failure in mechanical equipment, and detect leaks in pressure systems. Recognizing that orbital debris impacts are a leading threat to spacecraft, DIDS arrays have been modified and installed on the International Space Station to detect and locate leaks caused by impacts. Although structural health monitoring is one of the long-term goals for NASA, there are no requirements for monitoring COPVs during missions. The interim cryogenic propulsion stage (ICPS) just aft of the Orion crew module on the Space Launch System (SLS) designed to carry humankind into deep space will fly COPVs unshielded for orbital debris impact, one of the top risks to spacecraft. Appropriately designed debris shields can protect spacecraft components from the millions of particles as large as 1 cm in diameter orbiting the earth that routinely impact spacecraft at speeds roughly ten times that of a rifle bullet. In anticipation of future needs, Langley Research Center (LaRC) is hypervelocity impact testing COPVs instrumented with DIDS at White Sands Test Facility (WSTF) for the OSMA Nondestructive Program as part of an assessment determining the likelihood and severity of this risk.

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Fiber Optic Strain System

Understanding structural stress and stain is an important part of the design and performance testing of COPVs. Alternatives to this method include costly, but powerful progressive failure analysis software packages and digital image correlation (DIC) measurement systems. Commercial off the shelf DIC systems are available at a few NASA Centers providing high special resolution measurement of exterior surface strain. Well distributed and controlled liner strains are an important factor of COPV design since high localized strains can lead to leaks and failure. Recent advancements in DIC are now permitting interior measurements, but this technology is not commonplace and the Agency does not possess this capability. Armstrong Flight Research Center (AFRC) is leading the NASA COPV liner interior strain measurement effort using a fiber optic uniaxial strain measurement system with one centimeter or better special resolution. This system was first developed by Armstrong to measure wing shapes during aircraft testing. Optical fibers etched with fiber Bragg gratings (FBGs) provide highly repeatable measurements of strain and temperature. These fibers will be installed in liners at the manufacturer before closeout and coated with temperature indicator paints. Temperature indicator paint will support monitoring liner temperature during hot process extrusion to determine the survivability of pre-installed internal FBGs and may later be used to correct readings for temperature variations that occur during pressure testing.

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Magnetic Stress Gage System

Composite overwrapped pressure vessels get their strength from the high performance fibers that restrain the liner. Each fiber ply layer is applied at a specific angle that may vary layer by layer depending on the structural properties desired. This design results in a complex composite structure with anisotropic material properties difficult to model using traditional analysis methods. Fiber pretension may vary at each layer and angle during construction. The autofrettage process expands liners beyond their plastic limit resulting in more consistently performing COPVs, and more complex geometric anisotropy. Because of this anisotropy, a system capable of differentiation between and measuring subsurface material properties is highly desired. In full-vessel scans of COPVs, a capability demonstration by Kennedy Space Center (KSC) suggested printed eddy current arrays (meandering winding magnetometers, commonly referred to as magnetic stress gages) may have this ability for thin-walled structures. By printing application specific arrays, sensors can be tailored to measure conductivity, stress, and other subsurface material properties. Array dimensions vary based on the wavelength required to collect measurements at specified depths. Scans of COPVs in support of ground test efforts seem reasonable. Confirmation of signal correlation with strain has been achieved in previous cyclical pressure tests. Tests scheduled for FY16 expand on these correlations by investigating an apparent signal linkage with fiber breakage.

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OBJECTIVES

Agency leadership in the community identified the following inspection priorities:

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Objective #1: POD-validated and automated COPV inspection system for the identification and sizing of liner cracks and thickness measurements, esp. in domes (ground-based system).

Required for aluminum liners: Up to 0.30-in. thickness measurement and detection of all cracks larger than 0.015-in. x 0.030-in. throughout the dome and barrel sections. Lead center: WSTF (Universal Scanning System POD & ultrasonic probe development); Co-funding: NESC, OSMA

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Objective #2: Validated detection of Orbital Debris and other impacts during flight (Flight

System). Required: Detection of all Orbital Debris impacts and leaks with energies above the ballistic limit that reduces composite strength below the critical limit. Lead center: LaRC (DIDS wireless Acoustic Emission system similar to those used on Shuttle and currently aboard ISS)

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Objective #3: Validated characterization of composite damage (ground based system). Required: Detection of significant damage to the composite overwrap.

Lead centers: AFRC (Fiber-Optic Strain System) & KSC (Jentek MWM Magnetic Stress Gage System)

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CUSTOMERS

This project directly targets the Reliability/Life Assessment/Health Monitoring in the OCT Roadmap TA12, Materials, Structures, Mechanical Systems and Manufacturing Materials, Structures, Mechanical Systems and Manufacturing and is crosscutting to other discipline road maps and benefits ISS, Orion, SLS, and the recent critical needs of commercial space partners.

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PRODUCTS

Test report following the completion of each project.

End-to-end demonstrations of the DIDS system in a relevant environment (ground test).

End-to-end demonstrations of the Fiber-Optic Strain System in a relevant environment (ground test).

End-to-end demonstrations of the Magnetic Stress Gage system in a relevant environment (ground test).

Accept-reject criteria developed for a simulated fill-purge cycling on ISS.

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ACKNOWLEDGEMENTS

This work would not be possible without the support of nondestructive inspection and evaluation experts across the Agency and funding from the Office of Safety and Mission Assurance (OSMA). Some efforts are also synergistically supported by the NASA Engineering and Safety Center (NESC), NASA Centers, and programs that rely on the development of inspection technology. Guidance and direction for this effort was provided by the COPV Working Group, Manned Spaceflight COPV Coordinator, and Agency NDE Tech Fellow.

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PROJECT MANAGERS, by technology area

Integrated Project Management

White Sands Test Facility

Charles T. Nichols

(575) 524-5389

charles.nichols@nasa.gov

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Multipurpose Pressure Vessel Scanner

White Sands Test Facility

Charles T. Nichols

(575) 524-5389

charles.nichols@nasa.gov

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Distributed Impact Detection System

Langley Research Center

Eric I. Madaras

(757) 864-4993

eric.i.madaras@nasa.gov

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Fiber-Optic Strain System

Armstrong Flight Research Center

Larry Hudson

(661) 276-3925

larry.d.hudson@nasa.gov

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Magnetic Stress Gage System

Kennedy Space Center

Miles Skow

(321) 867-2857

miles.skow-1@nasa.gov

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