STS-133: Evaluating the concern of stringer foam loss – 2011 launch more likely

by Chris Bergin

The main drive to return ET-137 to flight status is continuing this week, with focus being placed on assurances a stringer defect does not result in foam liberations during ascent, with new documentation showing even small foam losses from the flange area has the ability to cause critical damage to the orbiter. Meanwhile, managerial memos have spoken negatively about the potential of launching prior to February.

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Discovery is sat patiently at Pad 39A, awaiting one of several outcomes. Currently she remains in a holding pattern in her flow, as managers continue to work flight rationale after the observations of four cracks in two stringers on ET-137.

While those cracks have since been repaired, a go to proceed with the next launch window opportunity – which ranges from December 17 to 20 – won’t be forthcoming until at least the end of this week.

With several NASA centers involved in the ongoing evaluations, Discovery is powered down and secured from ordnance operations, while engineers await an update from this week’s Program Requirements Control Board (PRCB) meeting on the forward plan.

“OV-103/SRB BI-144/RSRM 112/ET-137 (Pad-A): There was no work scheduled over the long weekend. The PRCB met last week and concluded that more time was needed for analysis, test, and development of flight rationale on the ET stringers,” noted the NASA Test Director (NTD) update (L2).

“Direction was given to secure from S5009 ordnance operations and power OV-103 down for the holiday. Solid Rocket Booster (SRB) ordnance was disconnected last Wednesday and remains disconnected. The vehicle is in a safe configuration. SS0071, OMS/RCS/MPS pressurization will be rescheduled.

“The PRCB is scheduled to meet again on Dec 2. The Program will work toward a launch window NET (No Earlier Than) Dec 17; however no formal launch date has been selected. The option to perform a tanking test is still under discussion.”

STS-133 Specific – Includes all GUCP and ET Stringer Issue – Articles:

Other options include slipping the launch until early next year, and/or rolling the stack back to the Vehicle Assembly Building (VAB) for scans of the inaccessible areas of the intertank flanges, which could even involve a tank swap. So far this week, managers have not placed much hope in making the December window.

“Another PRCB this week to talk status. Then talk with Gerst (Bill Gerstenmaier (Associate Administrator for Space Operations for NASA)) on Friday,” noted managerial notes within JSC this week (L2).

“Folks are really interested on the forward plan so that folks can appropriately make Xmas holiday plans. (There is hope) that this decision won’t get strung along so folks are strung along. Hopefully Friday we get direction.

“(Management) said Dec 17 is hard to meet work completion to correlate the models. Would not be surprised if we see the schedules can’t support Dec 17, because of how things have gone the last month. Launch windows early next year are not good with all the vehicle traffic. January is almost all out.”

Additional information is coming to light as to why last week’s PRCB meeting concluded with a need for additional evaluations into the root cause and risks associated with the stringers, relating to the concern of foam liberating from the flange area between the LOX and Intertank.

Such a concern exists partly due to the lack of a root cause, which negates the knowledge base of the Probabilistic Risk Assessment (PRA) data. In other words, it is not known how much foam would liberate from the flange in such a scenario.

“Critical Debris area with transport to RCC (Reinforced Carbon Carbon) and majority of underside Tiles. PRA is not credible. Difficult to estimate debris size without full understanding of root cause. Existing PRA for Void DP (Delta Pressure) is not applicable due to Mass & Time of Release,” noted one of the presentations from last week’s PRCB (available on L2).

However, based on the mass of projected foam liberations, the location of the flange to the orbiter, the time of ascent when the area would be under its highest loading/stress (around MaxQ) and the path such debris would take, the PRCB was told that even small amounts of foam are estimated to cause “critical damage” to the orbiter’s heatshield.

“Deterministic DTA (Debris Transport Analysis) completed to define transport / impact areas (+Z). Assumed 0.005 lbm, 0.010 lbm, and 0.025 lbm debris sizes. Close Proximity to Orbiter results in high likelihood of impact. Transport does not change significantly with debris size. Even relatively small debris estimated to cause critical damage,” the presentation continued.

“Assessed Airloads for +Z LO2 Intertank Flange area. Worst case is 65 sec to 70 sec Mission Elapsed Time (MET) (8.44 psid / 7.15 psid respectively). Represents High Loading to any exposed crack surface.”

The presentation also noted some the rules for the observation of issues with the flange foam, such as those seen during STS-133’s scrub due to the Ground Umbilical Carrier Plate (GUCP) leak.

“DIG (Debris Integration Group) position is that crack offset observed before MEI (Main Engine Ignition) is NO-GO for launch. Indication of debonded foam; no way to determine otherwise in time. In-family IT crack w/No Crack Offset is not expected to be a debris threat.”

Managers had already created a plan for camera observations of the flange’s circumference during the countdown, allowing controllers to watch out for any defects in the foam, during and after inspection by the Final Inspection Team (FIT).

“Imagery Coverage: Operational Television (OTV) and fixed Infrared (IR) assets can view majority of LO2 and LH2 flanges (some areas of both are not in view; details on subsequent charts). Existing lighting is adequate during night ops (night views in backup). All stringers are visible and not in shadow,” noted a separate presentation (L2).

“Thrust panels expected to be partially in shadow due to SRBs (Solid Rocket Boosters). Listed OTV and IR (Infrared) cameras are Ice/Debris controlled. Periodic scans of LO2 and LH2 flanges performed during tanking/post tanking.

“FIT portable assets provide a 360 degree inspection coverage of the LO2 and LH2 flanges with the following limitations. LO2 flange is not visible under cable tray and pressline. LH2 flange is not visible under LO2 feedline, cable tray and presslines.”

With this set up at the pad, managers have noted that they can be “confident can detect similar anomalies if they occur prior to lift-off.”

Meetings relating to finding a root cause for the cracked stringers, which will be the path towards flight rationale, will continue through this week and likely beyond.

(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, Lead: Within the article: via L2 acquired PRCB presentations).

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