External Tank schedule spotlight

by Chris Bergin

Production and modification of the next two External Tanks (ET) at the Michoud Assembly Facility (MAF) in New Orleans is currently on schedule to support two Shuttle missions in 2006.

ET-118 will ship on May 30, while ET-123 is scheduled to leave MAF on August 7. ET-119 is set to mate with the twin Solid Rocket Boosters today (LIVE coverage – see link)

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The current flow of tanks, noted to be ET’s 119, 118, 123, 120 and 94 are ‘retrofit’ tanks, having been past a stage of construction that requires modification – most notably the removal of their PAL ramps, changes to their ice/frost ramps and LH2 Diffuser re-installation.

All tanks past ET-119 will include Developmental Flight Instrumentation (DFI) cable tray accelerometers, to aid the flow of information that will continue to be collated on the aerodynamic loads endured by a tank during ascent. These instruments will fly for the first time on ET-118, given ET-119 – which will fly with Discovery on STS-121 this summer – was shipped to KSC before the additions could be made.

ET-119 has completed the close-out procedures on the TPS (Thermal Protection System) foam in the aft dome region, following ECO (Engine Cut Off) sensor change-out, with a tanking test on June 1 to evaluate their performance and check for phase shifts – the initial reason for their change-out. However, that tanking test is still being protested by many within the tank community, due to concerns of stressing the tank via an extra cryo cycle.

Discovery is set to rollover from her OFP (Orbiter Processing Facility) to the VAB (Vehicle Assembly Building) on May 12, ready to be mated with ET-119 and the twin SRBs to make up the STS-121 stack.

ET-118 – with its primary role of flying with Atlantis on STS-115 – is still required to arrive in time to support Discovery’s mission by way of STS-300 requirements. The May 30 shipping date supports Discovery’s current mission window, which opens on July 1. Ultimately, the STS-300 schedule, needs to support the CSCS/LON (Launch On Need) date of August 4 for Atlantis – in case Discovery has a major problem during her mission.

Tests on ET-118 will continue at KSC in the VAB through to May 30 – including ‘Electrical Acceptance Testing’ in the checkout cell, a process that noted the issues with ET-119’s ECO sensors.



One note of concern on the current ‘To Go’ schedule of ET-123 is the large amount of work that is being concentrated on ET-118. A note on ET-123’s schedule claims ‘ET-118 recovery efforts could potentially jeopardize ET-123 production.’

While modifications on the tank’s ramps are on schedule, one red line appears to be – once again – with its ECO sensors, with the outstanding work on ‘perform electrical inspection and (refit) TPS covers’ stretching all the way to Aug 1 – the last work to be carried out before the Aug 7 shipping date. Modification work, post Wind Tunnel testing, could push ET-123 schedule further down the timeline.

That concern would only arise if NASA was confident on three missions in 2006, which is already highly unlikely. Such a concentration on ET-118 is fully justified by the STS-300 requirement, allowing at least one flight of the Shuttle in the middle of 2006. ET-123 would then eventually support STS-115 as part of the STS-301 stack near the end of the year.

The removal of the STS-300 requirement post STS-115 would then ease the tank requirements for what is currently a busy 2007 Shuttle schedule. However, everything will depend on a ‘clean’ STS-121 this year.

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