NASA are still in the midst of evaluating the viability of carrying out a repair on the damaged tile area on Endeavour – with overnight testing of mock-ups of the damage and continued risk trade-offs of carrying out the repair EVA. A decision will be made later today.
Meanwhile, Endeavour’s crew have taken images of a MMOD (micrometeoroid/orbiting debris) strike to window 2’s thermal pane. Evaluations class it as minor, after initially placing the use of the OBSS (Orbiter Boom Sensor System) on standby.
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MMOD strikes are commonplace for orbiting spacecraft, although Endeavour is docked in a position that normally provides her with a large amount of protection, with the debris shields of the ISS facing in the direction of travel.
Debris in space is one of the biggest threat to an orbiter, second only to launch and re-entry. The largest recent impact was in Atlantis’ cargo bay during STS-115, when a small piece of another expended vehicle punched a hole through a radiator panel – although it was only spotted after landing safely.
This impact, spotted during the opening hours of Flight Day 9, was evaluated by controllers on the ground. Memos at the time of the impact being spotted pointed towards the possible use of the LCS (Laser Camera System) on the OBSS for a closer look. This appears to have been cancelled after the ‘nick’ was classed as minor.
‘Requesting if LCS can image the MMOD window strike, which was described by the crew as being approximately 1mm deep,’ noted one memo. ‘Forwarded request for info to the Neptec (of the OBSS) flight support team for input.’
The LCS on the OBSS is the same system that took a close up 3D image of the tile damage on Endeavour’s belly, damage that is still awaiting a decision on if to repair or not, likely to be made at Thursday evening’s Mission Management Team (MMT). Managers are waiting on verification of thermal analysis and the final Arc Jet testing models to ‘cure’. No repair is currently favored.
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The whole basis of whether to repair or not rotates around just how much processing Endeavour will have to undergo once she’s back in her OPF (Orbiter Processing Facility).
A repair would be favored, for experience and a reduction of repair work in the STS-123 processing flow, but the risks associated with a first-of-its-kind EVA – and the potential hazard of making matters worse – continue to reduce the chances that an on orbit repair will be carried out. NASA insist the orbiter and the crew are in no danger, if they decide against a repair.
According to overnight evaluations, many risks are being traded with a repair EVA, including tool collision with Endeavour’s TPS (Thermal Protection System), the repair changing the thermal dynamics of the repaired area, the lack of vacuum-certified use of the T-RAD (Tile Repair Ablator Dispenser) repair technique, along with the risks of having a spacewalker on the end of the OBSS (Orbiter Boom Sensor System) in the repair area of Endeavour’s belly.
This is undergoing a huge level of risk analysis, with one example of the memos flying around the related NASA departments including questions such as: ‘EVA Safety and Team 4 asking if collateral damage to orbiter TPS from inadvertent release of EVA tools is being tracked.’
Two more tests have been conducted at Arc Jet on gaining more data on how the damaged area will perform during re-entry. Wednesday evening saw astronaut Scott Parazynski simulating a repair on the tiles, along with a test on another mock-up – called the ‘Case 2 thermal math model’ – used to verify thermal analysis.
The results may talk the better part of Thursday, due to a 24 hour cure time for the test articles.
So far, all the test articles and thermal analysis has shown that the damaged area did not breach the baseline safety requirements for Endeavour’s structure – which has been consistent with NASA’s evaluations over the past few days.
To ensure the test articles proved Endeavour’s ability to re-enter safely, the Arc Jet test articles were put through a ‘more severe’ re-entry simulation, partly because of the make-up of the facilities test equipment and partly to aid confidence in the results via added margins.
‘Differences between test article and flight: Test article is 1.0” thick vs. 1.12” thick. Test environment has thinner boundary layer and hence higher cavity heating factors. Flow (streamline) is ~10 deg off of flight. Structure thickness of test article is thinner than flight tbar (â€œsmearedâ€ thickness),’ noted an expansive Thermal Analysis presentation.
‘Fully turbulent inflow at the turbulent condition is more severe than flight. The 2â€ channel nozzle wall allows for possible shock reflections. Pressure in test is lower than flight. Heat rates for the damaged configuration were slightly lower than the baseline test. The heat load was lowered to account for the thinner tile on the test article by adjusting the duration of the heat pulse.
‘Aluminum temperature responses were benign. Baseline: average peak ~300 F. Damage: maximum peak ~340 F. Bondline temperatures. Baseline: maximum peak: 670 F. Damage: maximum peak: 857 F. Damage to tile down stream of damage region – likely initiated by high, very local heating on aft cavity lip.’
The MMT meeting has been put back to 3pm Central, to allow extra time for the continued evaluations.
Refer to L2 for expansive presentations on the tile damage (new additions today) and for hi res MMOD impact images.
L2 members: All documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, updated live.
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