NASA GRC support for 40" Orbital Maneuvering System COPV burst test @ WSTF (Credit: NASA).

Since the beginning of the Space Shuttle Program, Kevlar Composite Overwrapped Pressure Vessels (COPV) have been storing inert gases like helium and nitrogen under varying degrees of pressure onboard the orbiter. Concerns regarding the catastrophic nature of COPV failure have prompted NASA’s Engineering Safety Center (NESC) to perform an independent assessment of COPV safety and their flight worthiness since they have been in operation for over 25 years.

Since pressure vessels are necessary throughout the spaceflight program, the NESC is supporting multiple NASA centers to analyze their safety. Glenn Research Center is collaborating with Kennedy Space Center and Johnson Space Center as well as the Jet Propulsion Lab on this effort. They have also teamed with White Sands Test Facility (WSTF), Boeing, and United Space Alliance.

Location of fluids and gas tanks on Orbiter (Credit: NASA).

The orbiter has 24 COPVs to store pressurant gases in various sub-systems onboard the spacecraft. The stored energy of the pressurized gas is used to transport fuel into the orbiter's main propulsion and maneuvering system engines. A typical vessel consists of a titanium liner overwrapped with a Kevlar®/epoxy composite system.

The primary purpose of the liner is to prevent leakage of the gases, although they are strong enough to carry a portion of the loading or pressure. However, Kevlar fibers can fail in static fatigue under sustained loading. This event is known as stress rupture failure and has catastrophic consequences for the shuttle. The reliable operation of the COPV is extremely vital because a stress rupture failure of the overwrap will result in the loss of the crew and vehicle.

Glenn began an in-depth safety issue analysis of the COPV from the materials and structures point of view in March 2004. Several members of the original NESC COPV Independent Technical Assessment team are now collaborating with NASA’s Constellation Program to develop a stress rupture test bed for carbon fiber/epoxy overwrapped pressure vessels. Their goals are to develop a representative flight vessel design, a test facility, and advanced state-of-the-art test methods for composite overwrapped pressure vessels.

OV-102 (Columbia) MPS/OMS COPV: Outside and In (Credit: NASA).

Another significant activity is the non-destructive evaluation of stress on the Kevlar fibers surrounding the pressure vessel. This is especially challenging since the only way to fully test the endurance of the fibers is to apply stress until they rupture, which destroys the pressure vessel. The COPV team is using old pressure vessels taken from one of the shuttles to develop a long-term stress rupture test to determine how long the oldest vessels can last before rupturing. This test will help ensure the safety of current pressure vessels in use on the entire shuttle fleet.

Concerns about aging pressure vessels are investigated by comparing COPVs from Columbia (OV-102) and new COPVs. Recent results from these studies indicate that there are insignificant differences in the Kevlar/epoxy overwrap. Furthermore, titanium liner materials have undergone several thousand fatigue cycles demonstrating significant COPV fatigue life.

In March 2007, NASA formed the COPV Working Group, which includes eight members from across the agency (two members are from Glenn). This working group’s expertise in pressure vessels is used to proactively address safety concerns within the program such as stress rupture. Aside from addressing safety issues, the team performs other tasks including data collection on old pressure vessel systems and studying mechanics and life prediction models for composite pressure vessels.

Glenn’s continued efforts to improve the safety and reliability of space vehicles will benefit missions and crews beyond the shuttle program. Lessons learned about high pressure storage of gases can be applied to other NASA programs including the Ares crew launch vehicle and the Orion crew exploration vehicle. Also, Glenn’s COPV research could improve the safety of carbon fiber (used instead of Kevlar) storage vessels on the International Space Station.