NASA Engineering and Safety Center (NESC) have officially recommended for three of Discovery’s RCC (reinforced carbon-carbon) panels to be replaced, after they were found to have “weakened SiC to carbon substrate adherence,” or ‘debonding’ of the top layer.
NESC’s recommendation is being reviewed by shuttle management, with a decision to be taken during the Flight Readiness Reviews (FRR). Replacement would lead to a rollback of the vehicle.
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The RCC panels in question are on both leading edges of Discovery’s wings, namely panels 9R, 12L, and 13R.
The root cause of the issue is not known, but it is understood to be related to oxidization within the panels that causes the decrease in the adhesive bond that holds the SiC layer in place. However, that has not been confirmed by the resulting scans of the panels.
The issue is impossible to observe with the naked eye, and NASA have been using high tech analysis methods – such as Microscopy – to spot degradation in SiC Coating adherence.
The concern relates to the potential inability of the current on orbit scanning process – via the OBSS (Orbiter Boom Sensor System) – of seeing increased problems with the top coating of the RCC panels.
That becomes a problem due to notes of analytical data showing that a loss of this top coating holds a risk of burn through on all three panels in question, following a sustained period of heating.
As a result, NESC are recommending that the shuttle program should replace panels 9R, 12L, and 13R, which can’t be achieved at the pad due to accessibility issues. It is not clear if the process could be completed in the vertical. Either way, a rollback would delay STS-120 to December at the earliest.
However, given the data appears to point to worst case scenarios, and that flight experience has seen such issues before, shuttle managers may decide that the risk is no greater than they’ve previously flown with, allowing the launch to proceed on track.
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This would appear to be good rationale, as NESC are noting that once the suspect panels have been removed, a variety of tests would be required to back up their claims, and even then the results may not provide conclusive data to justify a decrease in risk – by their own admission.
Managers and engineers at the FRR will continue to pore over the data, and will likely come to a conclusion over the next 24 hours.
That FRR process is more convoluted for this mission, with pre-FRRs and a large Shuttle FRR (taking place this week) ahead of another final top level FRR next week.
Timelines for the FRRs show the Shuttle Program Flight Readiness Review on October 9th and 10th, the MMT (Mission Management Team) Prebriefing on October 12th, the Level 1 SOMD Flight Readiness Review on October 16th and L-2 Prelaunch MMT on October 21st.
Pad processing continues to proceed well, with only minor troubleshooting issues, which are commonplace for such flows.
A few hours of contingency time remains, but that takes into account a cease to pad operations during the launch of the Atlas V carrying the Wideband Global SATCOM satellite Wednesday night.
‘An Atlas V is currently scheduled for launch from Complex 41 on Wednesday October 10th with a targeted T-0 at 2022. The launch window extends from 2022 – 2133. The launch vehicle is flying in the 421 configuration with a 12 foot fairing, two solid rocket boosters and a single engine Centaur upper stage.
‘This configuration allows for a concern for Toxics in the LC 39 area. Pad A will be cleared at 1700 Wednesday and Pad B will remain open.’
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