The final major modification to the External Tanks heads into its Critical Design Review (CDR) this weekend, as NASA aim to mitigate potential “threat areas” of foam liberation during the Shuttle’s ride uphill.
The Ice Frost Ramps (IFRs) have completed wind tunnel testing, with the new design set to be incorporated next week on ET-124 (STS-117 – Atlantis) – and on all future tanks.
**Over 350mb of STS-116 related presentions and mission documentation available on L2 – Further IFR images and several documents on IFR re-design are available on L2.
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**UPDATE: NASA CDR defers decision past ET-124**
An on site review of the new design is taking place on Saturday, which is part of the three day CDR that will end with the approval and immediate implementation of the design on ET-124. Work will start the day after the CDR concludes.
That tank is currently undergoing final production work at the Michoud Assembly Facility (MAF) in New Orleans, as it heads for a mid December shipping date to the Kennedy Space Center in Florida.
Each External Tank has 34 ramps, which house two pressurization lines that run alongside the tank. However, only the top four ramps on the Liquid Hydrogen area of the tank are deemed at risk of causing damage to an orbiter’s TPS (Thermal Protection System) from foam liberating off the ramps during ascent.
Getting to this stage of heading into the CDR has been a delayed process, with some test results having to be refined due to small amounts of chipping observed in the wind tunnel – which itself was unavailable for a period of time, adding to the delays.
However, aligned with the new schedule for the Shuttle fleet, Modifications on ET-124 can start next week, with plenty of time in adhering to the required shipping date to get the tank to Florida in time for Atlantis’ LON-317 requirement next year.
The modifications to ET-124 will involve the top three IFRs being ‘cut down’ to a thin collar, which is the preferred design. At one stage it appeared that leaving the ramps ‘as is’ was favored by engineers, based on the excellent performance of tank on the last two flights.
However, given NASA wishes to continue to reduce the risks associated with foam loss on ascent, this extra move was seen as prudent, pre-empting any potential issue with the unmodified ramps on future flights.
STS-116, launching in a window that begins on December 7, will be using ET-123, which does not have the modification of the new IFR design, but the confidence gained from the last two flights mitigate any concerns in regards of the unmodified ramps.
Further changes to the ETs during these final few years of the Shuttle program aren’t ruled out, with Shuttle managers stressing tha the ET foam is not an exact science, with lessons learned on every ascent uphill.
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