STS-131: ET-135 arrives at KSC as engineers evaluate intertank foam losses

by Chris Bergin

External Tank ET-135 has arrived at the Kennedy Space Center (KSC), following its journey from New Orleans in the Pegasus Barge. The tank will remain inside the barge until the new year, ahead of being towed into the Vehicle Assembly Building (VAB) for processing. Meanwhile, engineers are continuing to evaluate the foam loss observed from STS-129’s ET-133 intertank during flight.

STS-131 Processing Latest:

Discovery is currently targeting launch on March 18, 2010, the mission will deliver supplies to the International Space Station (ISS) via the Multi-Purpose Logistics Module (MPLM) Raffaello and the Light-weight Multipurpose Experiment Support Structure Carrier (LMC).

She was powered down for the Christmas holidays during the week, following late work on checking her ET Doors and a retest of a Multiplexer Demultiplexer (MDM) – along with the installation of sensors that will gain additional data on Main Engine Ignition (MEI) Acoustic and SSME (Space Shuttle Main Engine) Ignition Overpressure (IOP) Environments.

“The vehicle was powered up to support ET Door Functional preps and remained up to support Flight Control RCS (Reaction Control System)/Sensor testing (MDM FA4 retest). Once all powered testing requirements were completed, the vehicle was powered down for the remainder of the year,” noted processing information on L2.

“The side hatch was closed in support of waterproofing operations. MEI (Main Engine Ignition) work on the LPOD stinger door in work.”

Stacking of STS-131’s Solid Rocket Boosters (SRBs) will also continue when engineers arrive back after the holidays, with the left booster currently being secured on the Mobile Launch Platform (MLP) in the VAB.

STS-131 Specific Articles:

“The Left Aft Booster was set down on the MLP as scheduled; work to install Hold Down Post Studs and perform tensioning continues.”

ET-135 arrived in the Turn Basin near the VAB on Christmas Day, ahead of being towed into dock. There it will remain for an extended period of time – last seen with ET-117, which spent 24 days on the barge at KSC – prior to being removed from the barge and towed into the VAB.

“ET-135, aboard Pegasus/Freedom Star, departed Gulfport Monday and are heading for Port Canaveral. Anticipated arrival at the VAB Turn Basin is 12/25,” noted mid-week information. “Offload is scheduled for 1/5/10 with the lift into the VAB Checkout Cell scheduled for 1/6/10; both support the STS-131 timeline.”

ET-133 Performance:

With ET-134 already stacked to the STS-130 vehicle, two tanks at KSC will benefit from the lessons learned from the previous flight to launch from the Cape – STS-129.

A2ET-133’s performance was reviewed at the end of mission IFA (In Flight Anomaly Review) at a mid December Program Requirements Control Board (PRCB) meeting, which found the tank’s overall performance was excellent.

“16-Nov-09, STS-129 / ET-133: Launch at 1928 GMT on 11/16/09 (1328 hrs Central time on 11/16/09): Pre-launch Performance Assessment: Electrical and Propulsion systems performed nominally. No leakage detected at the GUCP (Ground Umbilical Carrier Plate) during the transition to topping,” noted the Lockheed Martin ET-133 IFA presentation – available with the other 12 IFA presentations on L2.

“TPS (Thermal Protection System foam) performance nominal or acceptable. Three typical TPS cracks on -Y Vertical Strut. No I-T Acreage valley cracks observed. Crack on Feedline Support Strut foam at Xt 1871. Frost formations at the umbilical side of LH2 Recirc Line Closeout.

“Post Flight Performance Assessment in work: Electrical and Propulsion systems performed nominally. Structural System performance nominal. ET/Orbiter separation and ET Disposal nominal.”

A6Foam liberations during early ascent are obviously the main concern for the performance of the tank from a non propulsion standpoint during launch, with the presentation showing the Michoud Assembly Facility (MAF) engineers once again handed NASA a great tank, with only minor liberations observed via an area of onboard cameras – including the debuting flight deck camera.

“TPS assessment: Five observations from initial ET Camera review. Three loss locations identified in Umbilical imagery. No anomalous observations noted from the Orbiter cockpit camera. Crew Handheld assessment identified nine additional areas of interest. No anomalous observations noted from the 16mm film.”

All of the tank’s LH2 and LO2 IFR (Ice Frost Ramps) – the latter filmed by the new flight deck camera – were shown to be in excellent condition after ET sep, after the IFRs had suffered from a few minor liberations during recent flights.

A5Also an item of interest, the foam losses from the intertank region – observed on ET-131’s flight with STS-127 – resulted in “pull tests” and “plug pulls” on subsequent tanks, concentrating on the top side that holds the greatest threat to the orbiter in the event of liberations.

As expected, ET-133 did not lose any foam from the top side of the tank, but did lose several areas of foam from the backside of the intertank – known as the -Z side – which appear to match the locations of foam loss from ET-131 – at least on the backside of the tank – and the potential mechanism that caused the losses from STS-127’s tank.

“Eight losses identified on -Z Intertank: Preliminary assessment indicates these losses are consistent with ET-131 intertank foam loss per the special investigation,” added the presentation.

“Losses are not to the same extent or degree, but appear to have the same failure mode. There are indications of primer in most locations, but imagery is not conclusive. Coincident with similar locations as those observed on ET-131.”

A4Those losses from ET-131 are understood to have been caused by a contaminant – likely to be dust – making its way on to the structure of the tank ahead of the application of the TPS foam, thus reducing the foam’s adhesiveness. Another investigation was called for ET-133’s intertank events, with the hope that processing photographs may confirm a similar situation.

If so, it’s unlikely to be a problem with the remaining tanks on the manifest, as ET-133 was the last tank to undergo intertank production prior to the findings from the ET-131 investigation – resulting in new procedures for intertank production.

Regardless, pull tests are still likely to be carried out on future tanks after they arrive at KSC, under the order of SSP (Space Shuttle Program) managers – who have changed the “risk” status accordingly.

“IFA recommended for foam loss on -Z Intertank: Hazard Reports have been modified to address increase in risk associated with this failure mode from ‘remote’ to ‘infrequent’,” added the IFA presentation. “Will add this occurrence to existing documentation regarding IFA from ET-131 Intertank foam loss to document and track accordingly.

“Failure mode is believed to be the same as observed on ET-131, but to a lesser degree. Actions have already been taken to perform additional bond adhesion tests on ET-134 and beyond in response to this occurrence.”

However, the intertank events should not detract from the excellent performance of the tank’s TPS foam, especially after all the related challenges since Return To Flight. None of the foam losses exceeded the strict safety limits, which ultimately resulted in the extremely clean heat shield on Atlantis after she made it into orbit.

“Summary: Preliminary assessment of ET flight performance on STS-129 complete,” the presentation concluded. “TPS system performance assessment ongoing. Three losses of interest in critical debris zone: 8 Losses observed on -Z intertank acreage being evaluated. No loss exceeds safety risk assessment mass. All other systems performed nominally.”

L2 members: Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size

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