Discovery O2 leak concern dimishing

by Chris Bergin

The “huge” oxygen leak recorded by catch bottles in the aft fuselage of Discovery, one minute into ascent during STS-114 may be bad data after all.

An investigation has been on-going for months into the leak data recorded by two catch bottles, recovered after Discovery landed at Edwards Air Force base in August, last year. However, a new analysis machine may have incorrectly recorded the level of O2 in the samples.

The leak, recorded at 100,000 SCIM (Standard Cubic Inches per Minute), was highlighted on a Shuttle Stand-up Report as ‘huge’ by Shuttle manager Wayne Hale, during a PRCB (Program Requirements Control Board) in December. So huge were the recordings, managers initially disregared the information as bad data, which was later to be reversed during the PRCB meeting. – Exclusive article that revealed the information on the leak evaluations.

Noted as an IFA (In Flight Anomaly), several areas of Discovery have been checked for potential sources of the leak, as orbiter engineers checked her systems under the guidance of the leak data being a correct recording.

While areas such as the orbiter’s MPS (Main Propulsion System) checked out as blameless for the leak, the IFA was called due to the lack of collaboration from ascent data – which would have been seen during the ride uphill in performance data.

However, an OMI V1202 test – a MPS/SSME Helium Signature test, used to pinpoint an O2 leak in the SSME system – failed to confirm the SSMEs (Space Shuttle Main Engines) could be ruled out as a source of the leak, leading to two of the engines being removed from Discovery last month.

‘SSME interface leak checks failed again; engines 2 and 3 will be removed today; will inspect interfaces and determine corrective action,’ noted a recent PRCB report. ‘Struggled with interface leak checks last week; changed out interfaces three times; will continue with leak checks this week.’

Yet the latest information from sources now points to the data being incorrect in the first place – which the recordings had been classed as before a decision was taken to class the levels as good data – leading to the investigation.

‘The two highest concentration readings that exceeded flammability limits were most likely due to the calibration files being reversed when the data was analyzed,’ noted a source.

‘There are two calibration files used to analyze the bottle contents using a gas chromatograph (GC) and mass spectrometer (MS), one for low pressure range (high altitude samples) and one for high pressure range (low altitude samples).

‘Apparently, the calibration files were reversed during the initial analysis and the corrected O2 concentrations are about half of what was first reported. Although the high readings are now below the flammability limit, the readings are at the high end of previous experience, so this needs to be understood. Also, the GC and MS used to analyze the samples used to be two different instruments.’

The new analysis machine itself will need to undergo an investigation to help confirm the possibility that it may be the top of the fault tree for the IFA, while engineers ensure all areas of the SSMEs, including their seals and interfaces into the aft of the orbiter, are confirmed as blameless. Technicians will lap, or polish, the engine interfaces to remove any possible microscopic imperfections.

‘As part of return-to-flight upgrades, STS-114 was the first use of a single GC/MS machine to analyze the samples,’ added a source. ‘Although the new machine is certified to meet the specifications for sample analysis, there is some question about the one-to-one comparison of the two machines and any differences between how the old and new instruments would read the same sample.

‘This will be investigated as well. It is not felt that the aft O2 concentrations will be a factor in STS-121 launch date at this time, but the investigation is on-going and all branches of the fault tree will be run to ground.

‘This issue is looking less and less like a real leak issue as the investigation goes on, but no one is jumping to conclusions.’

The SSME removal and testing will not impact on launch processing ahead of a projected May launch on STS-121.

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