STS-135: Atlantis in smooth flow as ET-138 undergoes mods – Hale on stringers

by Chris Bergin

Atlantis is continuing to enjoy a smooth processing flow inside her Orbiter Processing Facility (OPF-2) for the late June STS-135 mission, while her External Tank (ET-138) undergoes radius block modifications to avoid the potential of cracks forming in the intertank stringers. Former SSP manager Wayne Hale also noted the observation of such cracks is a classic example of a challenge for what is still an experimental vehicle.

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Although there are still some political clouds hanging over STS-135 – mainly due to the uncertainty caused by Continuing Resolutions (CR) and the need to complete STS-134 without a large-scale delay which would push Atlantis’ mission way past allocated funding cycles – the vital ISS supply run is likely to be staunchly defended by NASA managers.

The mission will be the last opportunity to stock up the International Space Station (ISS) via the vastly superior upmass capability of the orbiter, which will be carrying a full Multi-Purpose Logistics Module (MPLM) and a LMC (Lightweight Multi-Purpose Carrier) – also utilizing the downmass ability of the vehicle, which is already manifested to return the failed Pump Module (PM) for vital examinations on the ground.

The official launch date continues to dance around the end of June on various shuttle and ISS manifests, with the 28th of that month currently being used for planning purposes (Latest “All Vehicles to ISS” 2011-2012 manifest presentation (L2) screenshot, left).

It is currently understood that “preferences” for the mission to launch later – in order to provide some schedule relief for the fabrication of one or two additional Orbital Replacement Units (ORUs) to hitch a ride with Atlantis – is no longer a major discussion point, with ISS managers now planning for Atlantis to arrive at the orbital outpost in the early summer.

Atlantis’ processing inside OPF-2 continues to show good margin in achieving the end of June launch date, with only 30 Interim Problem Reports (IPRs) charged against the flow, which is now starting to ramp up after a period of Thermal Protection System (TPS) work.

“Tile processing and Orbiter closeouts continue. IMU (Inertial Measurement Unit) functional checkout and calibration was completed. SCAPE (Self-Contained Atmospheric Protection Ensemble) ops APU (Auxiliary Power Unit) catch bottle venting was completed,” listed an example of the short NASA Test Director (NTD) flow reports (L2) earlier this month.

An IPR was noted last week, relating to observations during the APU gearbox/GN2 pressurization task. likely caused by a problematic pressure transducer. The APU decay checks are currently in work, with the 24 hour check passing without incident, while the 48 hour check was being carried out over the weekend.

“IPR-0029 (APU 1 fuel tank outlet pressure anomaly) troubleshooting was performed. Results show that OI components in the measurements’ data stream are operating properly and the problem is likely a biased pressure transducer,” noted the NTD. “Data has been turned over to APU engineering for evaluation.”

The latest IPR is an example of how minor issues can be added to the list, sometimes through no fault of the orbiter. In this case, an incorrect connector was used on a heater loads test on the Orbiter Boom Sensor System (OBSS).

“IPR-0030: During OBSS Keep-alive test, readings for the ITVC (Camera) and PTU (Pan/Tilt Unit) heater load tests were taken at the Break-Out-Box attached to connector J602, should be J601,” the NTD noted. “Engineering evaluation and discussions with vendor concluded that no damage was caused due to the error. IPR will be closed as human error.”

No weekend work – other than leaving the APU to complete its 48 hour decay test – was carried out on Atlantis, as engineers prepare to enter the payload pre-mate testing stage of the STS-135 flow.

“OV-104 (OPF Bay 1): Sampling and top-off of the Freon coolant loop is in-work and complete. ROEU (Remotely Operated Electrical Umbilical) installation was successfully completed. Payload pre-mate testing will be performed next week. Weekend Work: None planned.”

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ET-138 Modifications:

Work has also ramped up inside the Vehicle Assembly Building (VAB), where preparations for building up the STS-135 stack are taking place. While MLP-3 (Mobile Launch Platform) is undergoing post launch inspections and repairs, the Solid Rocket Boosters (SRBs – SRB BI-146/RSRM 114) are being processed inside the Rotation Processing and Surge Facility (RPSF).

Atlantis tank, ET-138, is currently the center of attention in the VAB’s High Bay 2E (HB-2E), as Lockheed Martin engineers from the Michoud Assembly Facility (MAF) began work on installing radius blocks around the LO2 circumference of the intertank flange.

Debuting on ET-137 – following a multi-center effort to determine the best course of action on the cracks which were observed and found following the GUCP-scrubbed November launch countdown – the radius block modification was shown to successfully provide the the required mitigation during the cyro-cycle and pressurization of tanking, along with the ascent stresses of the vehicle riding uphill.

Although STS-134’s ET-122 is an “older” tank and probably does not suffer from the weakened “mottled” stringer material, managers decided to carry out the radius block modification for additional margin. Because ET-122 is unlikely to have any issues during the loading of propellants next month, no Tanking Test will be added as an extension to the S0007 Launch Countdown operations.

However, ET-138 will require a Tanking Test – per notes sent to the Kennedy Space Center (KSC) team last week – due to the commonality between ET-137 and ET-138 during their production phase and subsequent use of similar material stock. Again, the addition of radius blocks, in order to strengthen the stringers, is also expected to result in the same positive results seen with ET-137, post modification.

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Beginning on the +Y side of the tank, Lockheed Martin specialists opened the modification work last week by carrying out the removal of fasteners and match drilling on three panels, while a foam trim was in progress on another panel. By the end of the week, work had been completed on three panels.

“TPS ET-138-ST-003 ET-138 Stringer Mod: Radius block installation is complete on Panels 1, 2, and 3,” noted the latest NTD report. “Fastener removal on odd-numbered stringers is in work on Panel 6.”

Highly renowned for his work on the mitigation of foam liberation from the ETs during the post Return To Flight missions, along with the management of the highly eventful pre-launch STS-122 issues, former SSP manager Wayne Hale noted that the stringer issue came out of the blue, as much as it was a typical example of the challenges that come with what is still an experimental vehicle.

“This is classic and it’s very typical – this is a high-tech, high-performance vehicle, and it’s still in the experimental stage after 130 flights,” noted Mr Hale during an interview with’s Philip Sloss.

“The one thing you learn very quickly is that there are going to be new issues everyday, and some of them are trivial and some of them are major. Did I anticipate this particular problem? No. (It was) out of the blue. Did I anticipate that there would be significant problems? Absolutely, guaranteed.”

Mr Hale added that due to the nature of working with such a complex vehicle, the ET stringer issue is unlikely to be the last challenge faced by the Program. The hope is the remaining issues won’t result in delays in the region of several months.

“I predict that before the last Shuttle mission we’ll probably have at least two more. Hopefully they won’t delay the launches by four or five months, but there are going to be issues – there are issues everyday,” Mr Hale added.

“If you read those FRR (Flight Readiness Review) briefings, they’re just chock full of ‘we had this problem and here’s what we did about it and here’s why we think it’s OK to go fly.’ There’s something new everyday. It is a complex vehicle, it’s got very small margins, and flies in a really unforgiving environment.

“This is not your airplane down at the airport that you can just hop on and take off.”

(Numerous articles will follow. L2 members refer to STS-135 coverage sections for internal coverage, presentations, images and and updates from engineers and managers. Images used: Larry Sullivan MaxQ Entertainment/, ET/ISS documentation on L2, and for the APU).

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