Multiple External Tank modifications cancelled

by Chris Bergin

The ET project has cancelled several proposed modifications to the downstream External Tanks, based on post Columbia flight history, costs and manifest requirements.

The cancelled modifications are not related to the current ET delivery pressure on the near-term manifest. However, the changes will further ease the work required on tanks for missions in 2009 and 2010.

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History behind post-Columbia modifications:

The External Tank Project has been developing a phased approach to debris minimization for a number of years, in response to Columbia Accident Investigation Board (CAIB) recommendations. These recommendations were re-iterated during STS-114 IFA (In Flight Anomaly) investigation.

Phase I identified early candidates for continuous improvement of ET systems to mitigate foam and ice debris. Phase II (2007) evaluations assessed ‘Risk and Benefits’ to determine implementation practicality (Specific Study) – ahead of the March, 2008 overview which saw 10 modifications being cut as a result of the studies.

In total, the level of modifications to the tanks has been nothing short of dramatic. Return To Flight (RTF) modifications eliminated the culprit Bi Pod foam that ultimately caused the Columbia disaster, though STS-114 proved to be another key junction for the changes to the tanks.

With a large piece of the Protuberance Air Loads (PAL) ramp liberating during Discovery’s ascent, the fleet was again grounded while the ramps were removed from all future tanks.

The move was heavily dissented at the time, and to this date it has never been confirmed if liberation event during STS-114 was actually anything more than a one off, after images of the area in question during processing at the Michoud Assembly Facility (MAF) showed it was actually a repaired area – thus had risk of ‘popping’ off via cryopumping during the ride uphill.

Regardless, the PAL ramp removal eliminated any chance of the event occurring again, with the second Return To Flight mission (STS-121) used to prove that decision was the correct call.

Although it was a full year until Discovery again launched – this time with the additional modification – it could have been a lot longer, given just weeks after Discovery had returned from STS-114, Hurricane Katrina hit the New Orleans MAF plant.

Despite MAF mainly surviving intact, the main region in which the workforce resided was devastated. Regardless, the workforce returned to work to ensure no major impact was felt by the Shuttle Program, earning the praise of then Shuttle manager Wayne Hale. Even today, some of the workers have not been able to return to their houses.

Upcoming modifications:

Further modifications followed, as the program learned from each new flight, with STS-124’s mission to debut another set of modifications that has been a long time in development.

ET-128 – which will ride with Discovery – will sport the modified LH2 IFRs (Ice Frost Ramps), welcomed after 32 divots and three related LH2 acreage foam losses have been registered since RTF.

The change to the ramps will result in a ‘reduction in cracks and delaminations which could lead to cryopumping foam loss,’ according to a presentation (available on L2) outlining the changes to the tank that have been approved, and a ‘reduction in voids which could lead to void/delta P foam loss.’

ET-128 will also debut the LO2 Feedline Titanium Yoke and TPS (Thermal Protection System) Redesign, following a flight history of 23 occurrences of yoke losses.

The design, currently authorized under contract direction (Task Authorization), replaces Aluminum with Titanium on the LO2 Feedline Yoke, leading to a ‘reduced ice formation, and a secondary benefit of ‘reducing the amount of TPS and reduces possible mechanical interference during articulation due to ice between the yoke/strut and Feedline.’

By ET-131, the LO2 Feedline Base Closeout will be added as a modification to the tanks, following flight history of 13 occurrences of foam loss, and 12 LH2 Acreage foam losses adjacent to the LO2 Feedline Base Closeout, and one OB Base Closeout.

“Reduction in cracks and delaminations which could lead to a cryopumping foam loss,” noted the description of the change. “Reduce design induced defects associated with the LO2 feedline base support closeouts.

“Add layer of BX-265 in contact with aluminum substrate prior to PDL closeout. Implementation on station with high rate of incidents and high benefit (station) Xt1129.”

The final modification now set to take place on the tanks will debut with ET-135 – a foam removal in the area of the +Z Aerovent in the intertank region.

While this area has only suffered one element of foam loss (on STS-118), the directive of the CAIB comes into affect: “Basis for Recommendation: CAIB directed the elimination of all non-thermal TPS and reduces debris potential.”

“Removal of foam-over-foam application locally around the +Z Aerovent near the forward flange of the Intertank,” noted the description of the change. “BX spray currently used to build up the TPS locally within the stringer valleys will be omitted and the remaining acreage will be added to Intertank Cell “L” machining and venting operations.

“Aerovent, still necessary for proper Intertank venting, will remain open, with the surrounding foam, trimmed flush with the adjacent stringers, having a 40 degree slope forward and aft of the aerovent.”

While there is minimal risk associated with this area of the tank, the ease of the process to carry out the modification has allowed this change to remain in place. However, a large number of additional modifications have been removed.

No longer recommended modifications:

Ten modifications are no longer recommended for implementation to downstream tanks, mainly due to “minimal reduction in risk, significant testing and analysis required, negatively impact schedule.”

These include the LO2 IFR on the intertank region, which has a flight history of five occurrences of foam loss outside of the normal erosion.

This modification was due to be implemented by ET-138, but given the minimal reduction in risk, and the change only making the very end of the shuttle program, this is no longer recommended. This will also save the program around $750,000.

A good example of a modification no longer making the cut is a change to the Helium Inject Box TPS. This was set to debut on ET-136 onwards, though the initial recommendation was not based on any visible flight history (due to orientation).

“Reducing thickness of molded PDL foam on the injector box. PDL is manually shaved for reduction. Quantity of foam reduction would be dependent on results of ice testing,” noted the description of the change.

Given that requirement for manual shaving to this region of the tank and extensive testing – involving Icing tests, high fidelity mock-up to replicate environments, analysis, aero, loads, materials, stress and thermal, plus SE 66 TPS Testing – the benefits would be out weighed by increasing stress on the downstream manifest due to workload.

Also no longer recommended are changes to the Aft Lower ET/SRB Fitting (ET-136), LO2 Umbilical Cable Tray (ET-137), SRB PAL Ramp (ET-137), Diagonal Strut TPS (ET-136), GSE Shipping Strut & Diagonal Strut Closeout (ET-132), Aft GH2 Pressline Fairing and Vertical Strut TPS (Both impossible to carry out due to lack of workforce).

One surprise deletion from the recommended changes is the LO2 Feedline Bellows Heaters installation into ET-136 and beyond, which originally had a lot of support in the engineering community.

The addition of heaters to the aft bellows was based on 35 occurrences of missing ice/frost – which are debris hazards to the orbiter during ascent. The heaters would have completely eliminated such ice/frost build-up during tanking and pre-launch.

“Install 1 heater (2 places) on LO2 Feedline at Mid and Aft Bellows (stations) Xt1979 and 2026,” noted the description of work that would have been involved. “Heater similar to (station) Xt1106 materials, processes and power split.

“New TPS closeout required (wire bonding). Major lead wire tab is on the end shield side and the silicone gasket is wider to accommodate more deflection of the bellows. Eliminate LO2 Feedline Drip Lips all locations (3 places).”

Regardless, the risk reduction is again seen as minimal, with the negative production schedule impact (significant increase in production processing time), putting pay to this change to the tanks.

These modifications – or the addition of new modifications – will continue to be under review, based on the performance of the tanks on the upcoming missions.

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