Engineers working through final issues ahead of STS-120 countdown

by Chris Bergin

As is usual in a pre-launch flow, engineers have been troubleshooting a couple of issues at launch pad 39A ahead of the start to the STS-120 launch countdown on Saturday.

The main issue relates to a problem observed during Composite Overwrap Pressure Vessels (COPV) pressurization, which has gone to the noon board for closure or waiver. Earlier, the health status of the COPVs underwent a re-evaluation at the PRCB (Program Requirements Control Board) meeting.

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Pad processing is continuing with Discovery, with the target of starting S0007 Operations – otherwise known as the launch countdown – with call to stations at 1:30pm Eastern on Saturday.

The COPVs are kevlar-wrapped storage tanks for helium (He) and nitrogen (N2) gas. These 24 tanks – in various sizes and used at various pressures – are located around the orbiter, mainly in the MPS (Main Propulsion System), Reaction Control System (RCS) and Orbital Maneuvering System (OMS).

The issue with the COPVs was noted during phase 1 of the pressurization process on the tanks, with indications showing some of the vessels did not reach the desired pressure. However, this has been troubleshooted and the pad flow is continuing. It will, however, be reviewed by the noon board today (Friday), who review all IPRs for the purpose of closure or the issuing of a waiver. 

‘COPV pressurization (phase I) in work; pad is closed. Picked up a couple IPR (Interim Problem Reports) from the operation, which will require Noon Board attention. Completing work on these IPRs in hopes of going to Noon Board for review (on Friday),’ noted the latest Shuttle Stand-up/Integration report.

As expected, the issue didn’t stop the completion – via phase 2 of the pressurization – of operations in the pad processing bar chart, keeping Discovery on track for next Tuesday’s launch.

‘S0071 MPS/SSME and OMS/RCS Stage 1 pressurization completed yesterday (Thursday). Stage 2 pressurization to flight mass completed at 23:44L yesterday. S0071 (aft access doors) closeouts to pick up this morning,’ added this morning’s processing report.

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Other minor issues at the pad related to a vent line pressure transducer on the Mobile Launch Platform – which has since been removed, replaced and re-tested over the previous 24 hours, and a number of fuel regulators, which exceeded spec requirement and will be reviewed by NASA Engineering.

‘Expect a waiver, and a deferred IPR will result,’ noted the Stand-up, confirming they will not be a constraint to enter the launch countdown.

Thursday’s PRCB meeting included a separate discussion on the aforementioned COPVs, relating to their reliability numbers for each mission to the end of the Program with a four day hold for each mission compared to the cost, schedule and value of performing additional testing.

‘Provide the reliability numbers to the end of the Program with both a 2 and 4 day hold (not quite 0.999) along with the forward actions: To perform tank health assessments. Evaluate a NDE proposal to determine strain in high delta volume tanks. Determine trade offs for additional He offload. And a dissenting position to perform stress rupture testing,’ noted the PRCB presentation on the subject.

The presentation noted guidelines have been put in place to protect the tanks, in the event of a scrubbed launch attempt which equates into an extended stay at the pad of more than 24 hours (turnaround).

The issue of COPVs came about following 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, that found there was an increased risk of a COPV failing/rupturing.

At the time it was deemed that if a tank ruptured, it would lead to the loss of the orbiter. However, further testing at White Sands showed an orbiter could survive such an event.

Regardless, more tests may be conducted next year – using a method known as Raman Spectroscopy – on three tanks currently in use with the fleet, to ensure safety for the shuttle remaining missions.

Another test, using data is scheduled for November, used to provide a risk trade off ‘of reduced pressure in high delta volume tanks for improved reliability at the cost of reduce performance potentially affecting payload and/or contingency abort scenarios capability.’

Despite the risk numbers showing the COPVs could be left as-is with only a very small – and acceptable – risk of a problem, the shuttle program will continue to evaluate the status of the tanks, via data gained on two COPVs that were removed from Atlantis for testing, Raman Spectroscopy testing and the risk trade assessment of additional He offload of high delta volume tanks.

Engineers believe they have the issue fully understood, with no real testing required on the actual tanks, although a dissenting opinion – something that is now commonplace and even welcomed when required – was noted in the presentation.

‘Dissenting Position: An accelerated stress rupture/cycle test of a full size, Orbiter design COPV should be performed as a validity check of the stress rupture reliability model.

‘Rationale: While the current stress rupture reliability model is made up of the best available data and analysis, no full size Orbiter design vessel has ever been stress rupture tested for comparison to the model predictions.

‘Differences exist between the reliability model data set and Orbiter vessels which introduce potential uncertainty. A stress rupture/cycle test of an Orbiter vessel could provide confidence that the reliability model accurately accounts for these differences and predicts failure at the appropriate time.’

The dissenting party will present a cycle/stress rupture test plan proposal to the PRCB in December.

L2 members: All documentation and quotes – from which the above article has quoted snippets – is available in full in the related L2 sections, updated live.

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