STS-135: Atlantis rollout – Tanking Test and debris investigation on tap

by Chris Bergin

Atlantis kicked off what was a historic evening for the Space Shuttle Program (SSP), as she rolled out of the Vehicle Assembly Building (VAB) – with first motion at 8:42pm local time. Now at the pad, she will be put through a nominal pad flow, prior to a Tanking Test on ET-138, to be followed by borescope inspections relating to an ascent anomaly observed during Endeavour’s ride uphill.

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Atlantis remains on track for her July 8 launch date, as much as the target remains preliminary ahead of the Flight Readiness Reviews (FRRs) which will be the focus of her mission’s paperwork over the coming weeks.

After a Memorial Weekend sat on top of the Mobile Launch Platform (MLP), the final platforms surrounding the vehicle have been retracted, in order to allow the final departure of a Space Shuttle stack from the VAB on Monday evening. 

The historic rollout event was witnessed by a huge crowd of fans, media and members of the Kennedy Space Center (KSC) workforce, as the Crawler Transport departed from High Bay 1 (HB-1), just hours ahead of Endeavour’s landing at the nearby Shuttle Landing Facility (SLF).

“V1149 Orbiter/ET cavity purge was completed on Friday. A5214 Rollout to Pad A: VAB E and B platforms were retracted Friday. VAB D platform retraction is currently in work (since completed),” noted the NASA Test Director (NTD) report on Tuesday morning (L2).

“Rollout CTS (Call To Stations) is scheduled for tonight at 1600 EDT with MLP first motion at 2000 EDT.”

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Engineers also completed work on the only Interim Problem Report (IPR-38) of late, relating to Bus Monitoring Unit (BMU) – a data monitoring device attached to several of the orbiter’s systems (of the electrical bus).

“IPR 135V-0038 BMU update: Troubleshooting isolated the noise to the 189 housing. The 189 housing was R&R’d (Removed and Replaced). Retest of the 50 percent calibration was nominal,” added the NTD report. “All channels were stable. Engineering will verify data during S0009 and re-perform the S0008 BMU Data Verification.”

Preparations for Atlantis’ arrival at Pad 39A continued either side of the long weekend, with the final pad turnaround work – following Endeavour’s STS-134 launch – and waves of tanker trucks ferrying in the LH2 and LOX which will be loaded on to the STS-135 tank, ET-138.

It was thanks to both the good condition of Pad 39A – and the issue-free turnaround of the complex – which avoided STS-135 slipping into the middle of July, as some managers had expected.

“Pad A: Pad turnaround operations continue. Two waves of tankers for LH2 storage tank replenishment were completed Friday. SCAPE operation for hyper flowmeter R&R was completed early Friday morning,” updated the NTD. “LOX vaporizer weld repair was completed Friday. Four waves of tankers for LOX storage tank replenishment are scheduled for today (Tuesday).”

Normally, the LH2 and LOX would be loaded on to the tank on launch day. However, a Tanking Test in the middle of June will be conducted, based on ensuring ET-138 does not suffer from the cracked stringer problem observed on its related ET-137.

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Those cracks were observed on ET-137’s LO2 Flange TPS foam area, at the top of the intertank, during Infra Red camera observations on the tank, as it was being offloaded due to a scrub caused by a leak of its Ground Umbilical Carrier Plate (GUCP).

Once the tank was inert, and closer observations were made, a decision was taken to remove the foam, revealing a number of cracks on the actual stringers themselves. Following an instrumented Tanking Test, the STS-133 stack was rolled back to the VAB for the replacement of the cracked stringers, and the implementation of a radius block modification, designed to strengthen the stringers during both tanking and the ascent environments.

This modification worked as advertised, with no issues observed on ET-137 during tanking and ascent. This success was key, with one of the main threats of a cracked stringer during ascent relating to the potential of foam liberations during first stage flight, liberations which hold the threat of damage to the orbiter’s Thermal Protection System (TPS).

As a result, the modifications were made to ET-122 and ET-138, the latter of which is of greater focus, due to the close production dates and the potential ET-138 may have been fabricated with the same suspect stringer lot as ET-137, which was shown to suffer from a “mottled” material, which weakens the strength of the stringers.

With ET-138 fully examined in the VAB via Non Destructive Evaluation (NDE) X-rays, along with the installation of radius blocks around its circumference, it is highly unlikely Atlantis’ tank will suffer from any issues. However, the Tanking Test will be used to confirm the tank is ready for launch day.

Following the Tanking Test, engineers will use borescope cameras to check STS-135’s ET/SRB attach points are in a nominal configuration, after an unidentified piece of debris was caught on ascent footage during STS-134’s SRB separation.

Inspections have already taken place in the VAB, although access was limited, leading to the Engineering Review Board (ERB) opting against delaying the rollout, instead calling for inspections to take place once the vehicle has arrived at the pad.

“The ERB met Friday morning to discuss the results of the additional inspections that were performed as a result of the debris concerns regarding the object seen after SRB separation on STS-134,” noted the NTD report, which also focused on any potential leads into the actual STS-134 debris.

“Thus far inspections in the VAB and at Pad A have not revealed any missing hardware that could have contributed to the debris.

“Engineering is still evaluating high wind data that could have liberated hardware at the pad during the event. The going forward plan is to investigate the possibility of getting more focused BCI photos at the pad.

“As part of the final SRB inspections, the team plans to perform borescope inspections of the -Z side of crossovers from the forward skirt platforms. Both operations (BCI Photos and Borescope Inspections) will be performed after the Tanking Test and prior to S0007 (Launch Countdown).

Engineers will also carry out some initial borescope inspections as early as Thursday or Friday this week at Pad 39A, based on available access.

(Images: L2, Further updates on STS-135’s status will be provided as information arrives, driven by L2’s new and expanding STS-135 Special Section.)

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