Multiple sources have noted that attention has centred on an area of repair on External Tank 121 – the ET that flew with Discovery during Tuesday’s launch – a repair area that matches the section of foam on the Protuberance Air Loads (PAL) Ramp that was liberated shortly after Solid Rocket Booster separation.
“I was very surprised that the foam came off, that was my first reaction,” said Commander Eileen Collins during a radio interview from Discovery today. “But this is going to have to be fixed, we can’t fly again till this is fixed.”
However, the reasons to why that particular section – the largest liberation of foam noted during assent – came off the ET appears to be unique to ET-121. If confirmed, engineers may be in a position to put this event down to a repair that was conducted by ET engineers at the Michoud Assembly Facility (MAF) before it was transported to the Kennedy Space Center.
Images obtained by NSF yesterday clearly document where the repair was located on the ET, which match with images of the ET that show the foam loss following separation during STS-114’s assent.
Sources noted that MAF are almost certain this is the reason for the liberation, given Thermal Protection System (on the ET) debonds occur more easily in repaired sections. (C) Nasaspaceflight.com
Ironically, the reason this section was repaired, wasn’t due to a repair being required. That area of foam was purposely removed so that engineers could work in the ET’s “crotch” – the V-shaped groove in-between the LH2 tank and Intertank wall – and, by trying to make it safer, they in fact appear to have created a problem.
There are five “Tiger Teams” assembled at MAF to investigate this and other areas of foam loss – including acreage, ice/frost, and the PAL ramp. The evaluation process, once NASA gives us the thumbs up – expected next week – will lead to all PAL ramps will be removed from all ETs, and will not be placed on any future ETs.
Also under evaluation is the way that ETs are repaired, or have foam replaced, following any modifications to ET. It was a repaired section on the ET’s Bi-pod section that came loose and struck the leading edge of Columbia’s wing on STS-107 – dooming the flagship on re-entry.
It has also been revealed that ET-121 nearly scrubbed the launch of Discovery due to the number of LH2 Tank Prepress cycles. The LCC for ET-121 was set to 11 cycles as a precaution against the requirement for a third tanking test – it has been noted. During pre-launch of Discovery, 11 prepress bursts were counted, with only half a second to spare before a scrub on a 12th cycle.
While the cause for over-cycling on Tanking Test 1 was believed to be a new design of Diffuser (a double weave version – compared to the traditional single weave design), Engineers heard yesterday that evaluations have show that one of the values in the ground system on the MLPs (Mobile Launch Platform) had been replaced. The value was within specifications but had a different response time compared to its predecessor.
While more clarification is required to confirm this reasoning for the near over-cycling, further investigation will be sought on this issue.
Larger images and follow-ups can be seen here: