Troubleshoot required for countdown fault

by Chris Bergin

It wasn’t just the clouds over the Kennedy Space Center that proved to be a problem during the final minutes of Shuttle Discovery’s scrubbed STS-116 launch attempt on Thursday night.

The “highest ever” air intrusion was recorded on the LH2 side of the TSM (Tail Service Mast) – which could have caused a scrub, had Discovery proceeded with the countdown out of the five minute hold she was in due to the weather.

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A similar – yet smaller – intrusion was noted early in the STS-121 countdown back in July – but that cleared and allowed Discovery to launch on the Independence Day lift-off.

This intrusion appears to be far more serious, requiring engineers to evaluate a course of troubleshooting during the 48 hour stand-down prior to the next launch attempt on Saturday night.

‘At T-0 (location) LH2 side on top of TSM, O2 levels were up to 4 percent with erratic readings. Possibly air intrusion in the area but this is the highest seen,’ noted a memo acquired by this site.

‘Argon presence indicated air intrusion in the area of the T-0 cavity on the LH2 side. Haz gas reports the O2 reading of 3.9 percent, reading erratically higher than 2 percent, which is the highest that they have ever seen.

‘Planning on performing troubleshooting on the interface during the 48 hour scrub.’

UPDATE: This has now been cleared. Full details in the “attempt 2 preview” in the morning.

On the plus side, the infamous ECO (Engine Cut Off) sensors behaved throughout the launch countdown. Even though there is a LCC (Launch Commit Criteria) waiver to allow one ECO LH2 sensor to fail ‘wet’ – all four ECOs worked as required.

On recent post Columbia missions the External Tank has been one of the main cause of problems. However, External Tank sources noted that this countdown was the most ‘uneventful’ they have ever worked.

During tanking, all four ECOs correctly turned to ‘wet’ – indicating the propellant that was being pumped into the External Tank. A test was carried out on the health of the sensors, called the SIM test, which commanded all four sensors to indicate ‘dry’ – before heading back to ‘wet’. The SIM test was successful.

Weather for Saturday’s launch only has a 30 percent chance of becoming a problem at present.

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