STS-133: PRCB recommends 103 ET-137 stringer mods – Launch date slipping

by Chris Bergin

Thursday’s Program Requirements Control Board (PRCB) meeting overviewed the status of Discovery’s External Tank (ET-137), with recommendations made to install radius blocks on over 100 stringers. A new launch date will be entered as a Change Request (CR) next week, based on negotiations with the ISS on the ability to achieve either a February 24 or 27 NET (No Earlier Than) target.

STS-133 Latest:

Discovery final flight continues to be at the mercy of evaluations into the cracked stringers on her tank, although she was awoken from her slumber on Thursday to aid confidence checks on the replaced thermostat on her Auxiliary Power Unit (APU 3).

Engineers are also replacing a battery on her GPS system while the vehicle is powered up, while a proficiency run of S0044 Shuttle Final Countdown Phase Simulation is scheduled for Friday.

Over on ET-137, technicians and engineers are continuing to push forward with repair work on the latest cracked stringers, in parallel with doubler and radius block installation tasks. Currently, radius blocks are being added to just under a third of the tank’s stringers on the +Z and -Z sides of the LO2 intertank flange.

Work at present has been centered around three stringers, which were found to be cracked via X Ray scans of the entire circumference during the Christmas holidays. The original cracked stringers – observed during the tanking on the scrubbed tanking back in November – were repaired out at the pad.

“OV-103 / SRB BI-144 / RSRM 112 / ET-137 (VAB HB-1): ET-137 Stringer Investigation/Repair: Panel 6 Stringers 6, 7, and 11 removals are complete. S6-6 doubler installation is complete and radius block installation will follow later,” noted the NASA Test Director (NTD) update (L2).

“S7-6 and S11-6 doubler/radius block installation is on hold for two fasteners at the top of each stringer that will require a special tool to support the fastener removal. The removed stringers have been shipped to MAF (Michoud Assembly Facility) for further testing and analysis.”

Backscatter scans of the lower LH2 flange on the intertank are also continuing, and remain on schedule to be completed by the end of the week, despite an interference issue with a second backscatter machine, which arrived from the Marshall Space Flight Center (MSFC).

With the most recent plan continuing to focus on radius block modifications on 34 of ET-137’s stringers, foam application tasks have been allowed on two areas which hosted an array of instrumentation for the December Tanking Test.

“Foam application to the areas on panels 2 and 3 that were instrumented for the tanking test will be performed Thursday,” added the NTD report. “The areas are outside of the current authorized radius block reinforcement mod and are a “get ahead” task that will simplify the foam closeout process once the mod activities are complete.

STS-133 Specific Articles (click for numerous background content on the ET and relating to this article):

However, that foam is now expected to be stripped back off the tank, as Space Shuttle Program (SSP) managers recommend to press forward with the installation of radius block modifications to all of ET-137’s stringers – minus the five cracked stringers which will already sport doublers.

PRCB Status Update:

The amount of work being carried out on the investigation into the cracked stringers is highly impressive, with multi-center involvement utilizing everything from flight history to hands on evaluations into the actual ‘failed’ material.

One of the key objectives of the investigation is to understand the root cause of the cracks, which in turn provides – or at least aids – the mitigation path. Thursday’s PRCB meeting was presented with updated documentation which still classes the stringer incidents as an “Unexplained Anomaly”, although a potential root cause has been found when the stringers were closely examined at Michoud.

Found to have a “mottled” surface, analysis showed that this condition results in a 36 percent reduction in the overall capability of the stringer. Observations on standard stringer material found no signs of this “mottled” condition – leading to MSFC to request Al 2090 coil stock from MAF to work processing assessments to help better understand what in the stringer manufacturing process may potentially be responsible for the failure behavior on the stringers.

“Additional testing at MAF on single stringer samples were loaded to simulate the cryo induced radial shrinkage experienced during tanking at the launch pad.  Analysis of the test configurations shows reasonable approximation to the pre-launch steady state and the strain gage data collected during the tests has been consistent with tanking test data,” noted investigation notes (L2).

“The non-mottled stringers tested to date have demonstrated a factor of safety greater than 4 on deflection with and without induced defects however the mottled stringers showed a 36 percent reduction in overall capability.

On the positive side of the investigation, tests of the stringers – both mottled and in a nominal condition – once installed with the radius blocks, showed a healthy signs of mitigating the potential for cracks. This is important, as some source notes claim one of ET-137’s cracked stringers does not suffer from the mottled condition – a likely reason the root cause remains as an Unexplained Anomaly.

“At MAF, multiple mottled and non-mottled stringer samples have been tested at ambient and cryo temperatures. The testing determined that the non-mottled material exceeds minimum predicted load capacity and still has load capability with induced cracks whereas the mottled material with induced cracks has very low load capability,” the investigation notes continued.

“The tests were re-performed with radius blocks installed on both mottled and non-mottled samples and the testing indicated that the radius blocks provide a substantial increase in the load capability of both the mottled and nominal material for all various conditions tested.”

As previously noted, the part built ET-139, located at MAF, is providing test article data via its own stringers, again pointing towards the mottled condition – also seen on the S7 stringer on ET-139 – as the likely root cause, after it matched ET-137’s S7 failure during testing.

The materials issued on these tanks are being classed as “Stringers from Suspect Supplier Processing Time Period,” with the forward plan including “Additional tests planned to confirm data further investigate suspect material lots.”

With the installation of radius blocks a process that has regularly been carried out during tank production, and proof that the blocks do increase the overall capability of the stringers, this mitigation plan has a large amount of support throughout the program, bar one group which is still undecided over the modification’s certification plan as of Wednesday.

“Radius blocks mitigate high contributors / scenarios identified by (the investigation) as well as any potential undetected, collateral damage. Radius blocks reduce stress levels in stringer feet by increasing bending stiffness locally,” noted the expansive 82 page PRCB presentation on Thursday (available L2).

“Additional testing planned at MSFC (bend) and MAF (tension clip) to add confidence.”

Although the PRCB presentation outlined the recommendation for the installation on all of the remaining stringers on the tank, further data will continue to be evaluated through next week.

“Additional Recommendations Possible Based on Forward Work (Data and Analysis): Implement radius block design enhancement at remaining locations if remaining residual risk is unacceptable,” the presentation noted.

“Actions in-work to assess risk include: Mechanical property tests to characterize suspect material (i.e. modulus, strength, fracture toughness). Structural analysis if properties are off-nominal (stability and Mission Life). NDE (Non Destructive Evaluation) inspections of LH2 flange.

“Analysis to show prelaunch is effective screen for ascent loading (LO2 and LH2 flanges). Fail safe structural analysis (LO2 and LH2 flange locations). Debris assessments (LO2 and LH2 flange locations). Actions being reviewed through ET CERB (Chief Engineers Review Board) meetings this week and TIM (Technical Interchange Meeting) next week.”

Work will continue on ET-137, via the installation of the radius block design enhancement at on the LO2 flange at the +Z and -Z doubler panel locations, while preparations are made for adding the radius blocks to the remaining stringers.

The decision was key for STS-133’s launch date, as the plan moved from being able to accommodate the opportunity to aim for the launch window which opens on February 3, to one which moved the NET to later in the month.

The schedule outlined in the PRCB presentation shows a modification completion date of January 23 – which slips STS-133 into the next launch window which opens at the end of February.

“Preliminary Schedule for Radius Blocks on 103 Stringer Locations: TPS removal Engineering available to start activity on 1/9. TPS removal must/can start on 1/9,” added the presentation.

“USA (United Space Alliance) will provide resources/support effort for rough TPS removal. Radius Block installation Engineering available to start activity on 1/11. Additional LM (Lockheed Martin) Mechanical technicians available at KSC on 1/13. Access will allow minimum of two technicians per stringer panel on/at all panel locations. LM planned days off will be the 8th day of consecutive 12 hr work days (10th, 18th, 26th).

“Preliminary Schedule Summary Assessment. ATP workers must support start of additional tank activity on 1/9. Additional TPS removal effort starts 1/9 and finishes not later than 1/12. Mechanical repairs and radius block installations complete by 1/15. TPS final applications and trims completes on 1/21. Final shakedown and paper review/closure completes on 1/22 -1/23. Repair and reinforcement effort complete 1/23/11.”

However, no manifest announcement is expected until next week, as managers debate if the launch date will slip to NET February 27, or if the ISS program can accommodate changes involving visiting vehicles to allow Discovery to aim for a February 24 launch target.

(Further updates and articles will follow. Refer to live coverage threads linked above. L2 members refer to STS-133 live coverage sections for internal coverage, presentations, images and and updates from engineers and managers. Images used, via L2 acquired PRCB presentations).

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