STS-135: Atlantis and ET-138 preparing for Wednesday’s Tanking Test

by Chris Bergin

Interrupting a nominal pad flow, engineers are set to begin S0037 operations – otherwise known as a tanking test – on Monday, ahead of tanking of External Tank ET-138 on Wednesday. The test will check the health of the tank’s intertank stringers, following the issue of cracks being found in the related ET-137, after STS-133’s scrubbed countdown.

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Several processing tasks have been completed out at Pad 39A, along with one required element of troubleshooting – related to the removal and replacement of a Window Cavity Conditioning System (WCCS) dessicant, which was completed over the weekend.

“On OV-104 (Atlantis), on the pilot-side window cavity conditioning system, the desiccant beads showed that moisture had gotten into that system,” noted the Orbiter Project Office (OPO) on the Shuttle Standup/Integration report (L2). “The team will change out the beads and make sure the system is dry and leak-tight.”

This issue, as with all the other minor Interim Problem Reports (IPRs) charged against Atlantis during her pad flow, holds no impact to the current launch date target at this time.

“OV-104 (STS-135) Processing continues to go well going towards the launch on July 8, 2011. In work with the left-hand forward WCCS desiccant change out,” added Ground Operations via the Standup report going into the weekend.

“Removing the ET blank off plates. Will pick up with preps for tanking test and aft closeouts. Call to stations for the tanking test is next Monday at 20:30, with the actual tanking starting at 07:00 Wednesday morning. Payload to the Pad will follow tanking next Thursday.”

The Tanking Test is not expecting to reveal any problems with the stringers – specifically at the top end of the intertank around the LO2 flange – after full NDE (Non Destructive Evaluation) testing – inside the Vehicle Assembly Building (VAB) and the implementation of the radius blocks which strengthen the stringers.

The modification has shown its value, working without issue on STS-134’s ET-122, but also on ET-137 (STS-133), mitigating any further cracks in the stringers after the radius blocks were installed around the circumference of the LO2 flange following rollback to the VAB over the Christmas holidays.

With ET-138 heavily related to ET-137 – by way of production dates at the Michoud Assembly Facility (MAF) in New Orleans – it is possible ET-138 has some of the weaker stringers, found to be made from a “mottled” material stock.

However, with the radius blocks already installed, all the stringers are now likely to be strong enough to avoid cracking during the cryocycle of tanking and the pressurization of the tanks, even if some of the support beams suffer from the mottling effect.

The Tanking Test is a validation test, and a failsafe from a surprise during the actual launch day countdown, with Wednesday’s test a dress rehearsal for ET-138. As such, the tank’s portion of pad flow will almost mirror the tasks it will be put through during STS-135’s S0007 launch countdown next month.

“OV-104/SRB BI-146/RSRM 114/ET-138/MLP-3 (Pad A): S0037 tanking test preps began Thursday. The tanking test operations are scheduled to begin on 1st shift Monday with tanking on Wednesday,” added the NASA Test Director (NTD) report (L2).

“S1287, Orbiter aft closeouts (in work, continued through weekend) and the pre closeout snapshot was completed. The final confidence checks are scheduled for Monday 1st shift. ET purges and LO2 and LH2 system dewpoints completed. The GH2 and GO2 blank off plate removals were completed. The PV-13 (fill and drain) leak check was successfully completed.”

With the S0037 Tanking Test ops beginning with Call To Stations (CTS) at 20:30 local time on Monday, the opening tasks will include Power Reactant Storage and Distributation (PRSD) loading and Orbiter Mid Body Umbilical Unit (OMMUU) securing.

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As would be the case for tanking during the actual launch countdown, the Rotating Service Structure (RSS) will be retracted at around 21:30 local time on Tuesday. Tanking will begin – pending acceptable weather – at 7am local on Wednesday, with the go to drain the tank expected at 12:30.

Once the tank has been drained and allowed to boil off to an inert state, technicians will perform NDE scans of the tank’s stringers to confirm no cracks have formed during the tanking test or subsequent drainback. This operation will be complete by 22:00 on Saturday, allowing for initial results to be presented to managers on Monday.

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As previously reported, plans have been put together to modify the External Tank camera which will ride with Atlantis’ ET-138, allowing it to beam back video of the tank’s disintegration after parting ways with the orbiter at MECO (Main Engine Cut Off).

The modification, which is on the schedule to be carried out after the NDE scans of the LO2 flange following the tanking test, relates to the camera’s timer, which normally shuts down the camera not long after the tank and orbiter part ways.

The resulting video has the potential to capture the tank as it vents and re-enters on the other side of the planet, depending on how long the camera survives during the subsequent break up as it hits the atmosphere.

With the departmental Flight Readiness Reviews (FRRs) taking place ahead of the SSP level FRR later this month, more information is starting to filter through on the go-forward plan to allow the camera to continue providing imagery, with confirmation that the plan will involve the placing of assets downrange to provide additional communications ability after the usual Loss Of Signal (LOS) with ground stations at around T+15 minutes.

“On the ET feedline video camera, there has been discussion of leaving it on after ET separation and trying to get some assets in place down range to get the video during reentry,” noted one departments FRR notes (L2).

“The team is also looking at using aircraft to not only receive the video but also get some radar data as well as infra-red data for the breakup.”

The plan, which was taken to the Chief Engineer’s (CE’s) Review Board, may utilize a P3 Orion aircraft, and/or a Halo II aircraft – a modified Gulfstream IIB – which provided a stunning “de-convolved Visible Track” Video of STS-121’s re-entry (L2), as Discovery raced back to Earth between the Mach 10-8 period.

Any footage of the tank re-entering is extremely rare, with all recent footage mainly coming from the orbiter’s crew taking handheld video and photography, for the purpose of checking the condition of the tank’s Thermal Protection System (TPS) foam, allowing any potential liberations on the tank to be matched with potential impacts on the orbiter via the Damage Assessment Team (DAT) evaluations.

As with most mission related events, final approval is likely to be required at the SSP and Agency level FRRs.

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

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