The final Flight Readiness Review (FRR) has approved October 23 as the launch date for STS-120, with the only one major outstanding issue receiving flight rationale.
Concerns over the coating on three of Discovery’s RCC (reinforced carbon-carbon) panels – noted by two engineers at NASA Engineering and Safety Center (NESC) who dissented through the Chief Engineer – were downplayed, after it was revealed NESC used data from Endeavour for their thermography data. Flight rationale has also been presented.
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Due diligence on the issue is paramount for the program, with cross agency and contractor discussions taking place over the past week and through the weekend, ranging from NASA HQ to the United Space Alliance (USA).
NESC’s concern – as exclusively revealed by this site early last week – relates to three panels on Discovery, which they recommend should be replaced, after they were found to have ‘weakened SiC to carbon substrate adherence,’ or ‘debonding’ of the top layer. The issue appears to be limited to areas around the edges of the panels which are protected by the T-seals.
The RCC panels in question are on both leading edges of Discovery’s wings, namely panels 9R, 12L, and 13R. Replacing these panels cannot be conducted at the pad, and would lead to the rollback of Discovery and a major launch delay.
It is understood that NESC took pre-flight data of RCC panels and compared them with the current health of Discovery’s panels. However, it appears NESC compared Discovery’s RCC with thermography data from Endeavour’s RCC – which in turn rules out commonality and flight history with Discovery.
More importantly, it is assumed that this also removes the fear that the heat of re-entry is causing the problem, which downplays the concern that burn-through – as raised by NESC – is the driving force behind the call to replace the panels – panels which may have always been as the data shows, and not ‘worsened’ by her recent re-entries, in turn removing NESC’s concern.
‘In relation to pre-STS-114 RCC panels and three panels in question on STS-120 (OV-103/Discovery), found out that NESC pulled OV-105 (Endeavour) thermography data as the pre-flight thermography data,’ noted OPO information acquired by this site.
‘Accounting for this, there is not such a significant difference between what was seen pre-STS-114 and on the three panels on STS-120,’ which adds confidence that Discovery’s RCC panels have not worsened during a number of her previous flights.
NASA HQ asked to be fully briefed on the RCC information, which was handed over by OPO (Orbiter Project Office) over the weekend.
‘Asked how many OV-103 panels have 0.15 or greater reading. Three panels >0.21 (9R, 13R, 12L), one at 0.19 (9L), and one at 0.15 (11L). Remaining 27 panels are 0.13 or less. Of the 27, 17 are <0.1,’ which aligns with NESC’s note on the three panels in question.
It is also understood that a NESC panel member spoke to the STS-120 crew on their findings before further clarification was forthcoming. This may be related to a note that spoke of a crew request.
‘Asked if we can make quantitative inspection criteria as crew requested,’ noted information. ‘Can be done, but not until we get to root cause.’
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That root cause still remains a mystery. Engineers are looking at the manufacturing process to see if that may be the reason for the results of the thermography data. The initial thought process on the root cause was understood to be related to oxidization within the panels that causes the decrease in the adhesive bond that holds the SiC layer in place.
‘Thermography on spare RCC panels: Have no indications yet on the 3 or 4 panels inspected (as expected for these new panels),’ noted information on the latest Stand-up/Integration report this week.
‘(Deputy Shuttle Manager) Mr. (John) Shannon said when crack showing in the microscopy was buffed down further, it was not a connected crack across all the crazed cracks.
‘Steve Poulos (OPO) spoke about this on Friday, but this does not change situation, because the crazed crack does bi-sect the interface between the substrate and the coating for about one inch, and could liberate.’
Poulos also presented flight rationale for STS-120 at the FRR, as the mission moves towards final approval for October 23.
‘Low likelihood of coating spallation occurrence at a flight critical time period. Abort cases combined with probability of occurrence is ~ 1/8000. On-orbit inspections mitigate risk of re-entry with undetected spallation. LDRI – FD2/Late Mission Inspection.
‘Microscopy data since FRR indicates the separation on Panel 8R apex region is not continuous. Initial concern was that the separation was continuous along the apex length. Flight history indicates minimal heat damage at two previous spallation sites indicating late timeframe of event. Also consistent with CTE mismatch stresses which occur during cool down following peak heating.
‘No spallation events have occurred during flight since IR. Thermography introduced as a process control pre-STS-114. 6 flights total – 1 panel removed (8R OV-103) due to IR indication. Panels with IR readings of ~ 0.2 have been stable the past several missions.’
Further articles will follow this week.
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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