Discovery launches at first attempt – ET-135 debuting intertank mitigation

by Chris Bergin

Shuttle Discovery has launched at the first attempt, as STS-131 moves into Flight Day 1. Discovery enjoyed a smooth countdown – bar a slight issue with Fuel Cell 2 – prior to launching at 6:21am local time from the Kennedy Space Center (KSC). Meanwhile, ET-135 is the first tank to fly with new proceedures in place, following the intertank foam liberation events that have been observed since STS-127.

STS-131 Status:

For this mission, Discovery used ET-135, SRB (Solid Rocket Booster) BI-142, RSMR (Reusable Solid Rocket Motor) set 110, and SSMEs (Space Shuttle Main Engines) 2045, 2060, and 2054 in positions one, two, and three, respectively.

The Orbiter will once again use OI (Operation Increment) software 34 and carry no ballast material in her ballast boxes.

In all, Discovery launched with 22,760 lbs of OMS (Orbital Maneuvering System) propellant, 4,970 lbs of Aft RCS (Reaction Control System) propellant, and 1,912 lbs of Forward RCS propellant.

Moreover, in addition to the MPLM Leonardo and LMC (Lightweight MPESS Carrier) payloads, Discovery will also carry a new GLACIER unit, MERLIN, STL, AEM 1&2, and four “payloads of opportunity” with her on STS-131.

Furthermore, Discovery could perform a maximum of four (4) Detailed/Dedicated Test Objectives (DTOs) during STS-131.

These DTOs include the re-flight of the TriDAR rendezvous test (DTO 701A), a landing crosswinds performance demonstration (DTO 805) which would only be performed if wind conditions are acceptable at Discovery’s landing site, the second 0.35-inch protuberance flight of the Boundary Layer DTO (DTO 854), and a Reusable SRB DTO (DTO 900).

STS-131 Specific Articles:

No serious issues were reported during the count, aided by acceptable weather conditions for the majority of the countdown. Refer to live update pages for up-to-the-second news, with summaries of the pad flow added below.

Countdown: Fuel Cell 2 spike recorded and was evaluated at the pad. Appeared to be similar to event noted during STS-129 countdown – related to current in cockpit lighting. Teams met again at 11pm local to discuss the issue and decided to monitor it during the count. No further issues were reported with the Fuel Cell.

Tanking began once the pad cleared, at the later-than-scheduled time of 9:28pm local. ECO sensor checks via SIM commands all showing nominal voltages. The tank went into topping with no leak detectors tripped on the GUCP (Ground Umbilical Carrier Plate), marking an issue free loading of the ET.

New article will follow later on Monday reviewing early ascent data and orbiter status – which currently includes a failure of the Ku band antenna – at the end of Flight Day 1.

Late Items of Interest:

Should Discovery suffer from a 24 hour scrub on Monday, Tuesday’s launch will be without the TAL (Transoceanic Abort Landing) site at Zaragoza in Spain, due to a visit by King Juan Carlos – resulting in the Air Base having its air space closed down for security reasons.

TAL sites are referred to as augmented sites because they are equipped with shuttle-unique landing aids and are staffed with NASA, contractor and Department of Defense personnel during every launch.

Shuttle assets – such as two C-130 aircraft, used for search and rescue in the event of the crew bailing out on approach to the TAL site – have been moved to the near-by commercial facility.

“Zaragoza is RED as a TAL site on Tuesday 4/6. The King of Spain is going to be on base and the air space is closed. We met with Base Command and it is non-negotiable. We were able to salvage the mode VIII search and rescue capability by moving the two C-130’s to the commercial runway if we were to slip into Tuesday,” noted a Landing Support memo (L2).

“The ceremony is taking place within yards of our hangar and if we launch on Monday we are adjusting our pack and stow schedule to accommodate the restricted movement requirements.”

While all three sites are available on Monday, Moron Air Base, also in Spain, and Istres in France remain available for Discovery on Tuesday in the highly unlikely event they would be required. The availability of a TAL site is a strict rule in the Launch Commit Criteria for a shuttle launch.

The key L-2 Mission Management Team (MMT) meeting was mainly uneventful, thanks in part to a smooth Agency-level Flight Readiness Review (FRR) – aided by a complete review of the main items of interest at a preceding Special Program Requirements Control Board (PRCB) meeting.

With the regulators performing well during the late part of the countdown – aiding confidence the system is suffering from no ill-effects relating to the failed “open” Helium Isolation Valve on the Right Reaction Control System (RRCS) – only one FRR topic has required further work at the pad during S0007 launch countdown operations.

That one “non-standard” item of work required engineers to double check the torque settings on the ceramic inserts that surround the windows on Discovery, an item of interest since one of the inserts was observed as protruding on Endeavour during STS-130.

While that insert remained in place during re-entry and landing, fleet-wide checks were required, due to the potential threat a liberating insert could hold enough energy to damage an OMS Pod on impact during launch and landing.

Incidentally, this became a flight rationale topic at the Agency FRR because of the similar inserts that are located along the Payload Bay hinge line, which could not be inspected on Discovery due to a lack of access at the Pad.

Thanks to no issues being found on Discovery’s two sisters – Endeavour and Atlantis – confidence in the inserts allowed for managers to approve flight rationale for STS-131.

ET Intertank Foam:

STS-131 may be the first mission for a while to avoid similar foam losses observed on the intertank region of the tank, thanks to a change of procedures at the Michoud Assembly Facility (MAF). While none of the liberations have been a threat to the orbiter’s Thermal Protection System (TPS), all foam loss events are undesirable – if not wholly unavoidable – early during flight.

“Background: STS-127 experienced significant foam loss across the Intertank. Investigation initiated a review of tank processing history and possible causes,” noted Flight Readiness Review documentation on L2 – 60 FRR presentations available for STS-131.

“Review identified contamination prior to foam application as the probable cause. Plug pulls were implemented as a process check to ensure no gross contamination issues prior to STS-128.”

Those plug pulls concentrated on the top side of the intertank, facing directly below the orbiter’s TPS, known otherwise as the +Z side. Imagery from STS-128 showed no foam losses of note from that side of the intertank, although no imagery was available for the backside – known as the -Z side – of the intertank.

“STS-128 had no foam losses from +Z side of Intertank. Crew Hand Held imagery was not available for -Z side of Intertank SRB video at separation provided limited views but no observed -Z losses. Plug pulls were continued as a process check for STS-129,” added the presentation.

And it was the -Z side that was the area of foam loss for STS-129, resulting in plug pull tests becoming desirable for all around the tank, although access becomes difficult once the shuttle is out at the pad.

“STS-129 had eight foam losses from -Z area with unknown times of release. Good Umbilical imagery showed no +Z Intertank foam losses. Additional plug pulls were added prior to STS-130 to sample platform access areas in addition to the high risk areas.”

STS-130 may be the final tank to have suffered from this run of intertank liberations – although in less frequency than previous flights – as new procedures at MAF had started on the following tank.

“STS-130 experienced two +Z Intertank foam losses. Currently tracking 21 observations of missing TPS on STS-130/ET-134 Intertank 2 Areas on +Z were determined to be adhesive failures similar to STS-127/ET-131 losses One occurred during ASTT (+Z losses observed at 119 sec and 485 sec MET),” noted STS-130’s findings.

Those new procedures at MAF included changing platform access for engineers tasked with cleaning the structure prior to the application of adhesive and foam to the intertank. Previously, some areas of the intertank were obstructed by the platforms, meaning they weren’t fully cleaned of dust contamination, which had built up on the already manufactured structure during the Return To Flight stand down.

With the new procedures in effect for ET-135 onwards, the Space Shuttle Program (SSP) will continue to request plug pulls, whilst keeping an eye on the size of any subsequent releases to ensure they fall well within their risk parameters.

“Intertank debris has improved significantly since STS-127, but flight history suggests random debris losses from this area will remain a SSP risk. ET process controls such as certification of cleaning processes, personnel, and spray processes provide confidence in overall TPS foam application and integrity,” noted the Systems Engineering and Integration Office (SE&I) STS-131 FRR presentation.

“Plug pulls provide confidence that a wide-spread process/contamination issue does not exist and provide additional assurance that weak bond issues do not exist in the high risk areas (critical debris zones). This rationale recognizes that plug pull tests are not controls and do not prevent foam loss but it does increase confidence in tested areas.

“Additional Plug Pulls done for STS-131/ET-135 in work platform areas where most losses have occurred. ET-135 is the last tank using platform cleaning. Imagery demonstrates that debris can liberate as a single piece, however flight evidence and testing show debris is likely to breakup into smaller pieces.

“Smaller foam pieces decrease the likelihood for critical damage to RCC (Reinforced Carbon Carbon). Breakup increases tile impact risk but smaller pieces decrease the potential for critical damage.”

The lessons learned since the largest intertank foam loss events of STS-127 have provided managers and engineers with a large amount of understanding of the characteristics involved with the liberations. No similar foam losses after ET-135’s ride into orbit will likely prove the mitigation process from the MAF change of procedures will have put the issue to bed.

Regardless, flight rationale is not a problem for SSP, providing plug pull tests continue to show good integrity between the foam and the structure, and no increase in liberations – or more so the mass of the liberations – are observed.

“STS-131 Risk is acceptable based on Qualitative Assessment and confidence gained by bond adhesion test performed in critical zone,” noted the FRR flight rationale for ET-135.

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