NASA decide against COPV change for Atlantis ahead of STS-117

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Shuttle managers have opted against offloading the amount of helium stored in Atlantis’ Composite Overwrap Pressure Vessels (COPVs), which was initially seen as a way of potentially increasing the reliability of the tanks inside the orbiter’s MPS (Main Propulsion System).

The decision not to offload the amount of helium was taken after evaluations showed it didn’t improve reliability “significantly” – while it would have increased the possibility of incurring an engine-out scenario during ascent.

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In all, orbiters have 24 of these tanks, which are kevlar-wrapped storage tanks for helium (He) and nitrogen (N2) gas. These tanks – in various sizes and used at various pressures – are located around the orbiter, mainly in the MPS, Reaction Control System (RCS) and Orbital Maneuvering System (OMS).

These tanks are around 50 percent lighter than their metal versions, which ultimately aided how much payload the orbiters could carry when they were designed. They were initially certified for 10 years, before having their lifetime extended.

A review, which was conducted by a team led by Dr. Jim Sutter (Glenn Research Center) and Dr. Brian Jensen (Langley Research Center) in 2004, found that there was an increased risk of a COPV failing. At the time it was deemed that if a tank ruptured, it would lead to the loss of the orbiter.

‘A 2004 NESC review of Orbiter COPV stress rupture certification identified ‘stress rupture failure’ as an area of increased risk,’ noted NASA documentation this month. ‘Stress rupture is a sudden catastrophic failure mode of Kevlar-wrapped COPVs that can occur at normal operating pressures (individual Kevlar fibers can weaken over time).

‘WSTF test of a 40” COPV in 2006 measured a volume growth of 325 cu in, vice a design spec of 139 cu in. This concern prompted measurements of the five 40” tanks on OV-104 (Atlantis), and two MPS tanks, s/n 6 & 7, measured ~340 cu in volume growth. The other 40” COPV tanks ranged from 178-286 cu in volume growth.’

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It is understood that those tests at WSTF showed that a tank failure would not lead to the loss of the orbiter during a failure event, although the evaluations that have followed have been solely aimed at ensuring such tank failure scenario was not likely to happen.

‘We’ve been working on the science of COPVs for several months now. We did discover that we have a risk factor in these older tanks that were probably over-stressed in their initial tests way back 25 years ago,’ said shuttle manager Wayne Hale recently.

‘We have a pair of tanks that we have sitting in reserve that we took to the White Sands Test Facility (WSTF) – and we’ve been expecting potential replacements for the fleet. We’re talking with other folks that have used COPVs in other applications.’

The COPVs of most concern are the 40” OMS/MPS tanks, as these are the highest stressed COPVs and the largest contributor to stress rupture risk. However, NASA are confident the tanks are safe to fly.

‘We believe that we have an adequate level of safety for the upcoming flights, and we have a long program of tests to look specifically at what to do to mitigate these problems,’ added Hale.

‘We built all the tanks that we thought we’d need, and the vendor has frankly gone out of the business of building those tanks some number of years ago, so the question is what source of operational things could we do with lower pressures and so forth, in order to mitigate or reduce to the lowest possible factor the risk that we have a problem with one of those tanks.’

Ahead of the upcoming STS-117, and after Hale’s earlier comments, the all-powerful Program Control Requirement Board (PRCB) looked at one area of mitigation – the offloading of some of the helium – which is vital for safe engine shutdown – contained in some of the COPVs.

The PRCB decided against the move, following evaluations that showed the increase in reliability was small, and offset by the increased possibility of a unscheduled engine-out situation, which would be more than undesirable.

‘(An) action was issued by the PRCB to determine if some amount of MPS Helium offload could help improve the reliability for the COPV tanks,’ noted NASA information. ‘It was determined analytically that even a 200 psi Helium offload did indeed prove reliability but not significantly.

‘MOD (Management Operations Directorate) recommended against any MPS Helium offload, mainly due to shortened guaranteed engine run time – which could be significant for an Engine-out RTLS (Return To Landing Site) abort case.

‘The PRCB recognized that the small increase in COPV reliability from an MPS Helium offload was more than offset by the increased risk to Engine-out cases during ascent and concurred with the no offload recommendation.’

With the latest data showing that there is an adequate level of safety with the COPVs, STS-117 will proceed as planned. In fact, opening PRCB documentation seen at the start of the year showed the risk factors were extremely small.

The only additional related issue that was evaluated on Atlantis was observed rippling in the COPV’s titanium liners on one of her COPVs. This tank will be replaced on Atlantis after STS-117, while two more tanks will be replaced on Discovery, ahead of STS-120.

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