New issue found on Atlantis’ removed ET Feed-through connector

by Chris Bergin

A new issue with Atlantis’ External Tank LH2 Feed-through connector hardware will require further investigation, after cracks were found on the glass seal below the pins on ET-125’s removed external connector.

At present, engineers do not believe it is related to the Engine Cut Off (ECO) system anomalies, as the process begins at the pad to install a replacement Feed-through connector system into the tank, in time to allow STS-122 to launch in early February.

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The surface cracks were observed on the vitreous glass seal, via initial inspections over the weekend at the Marshall Space Flight Center (MSFC). Engineers used a magnification of x50 to gain imagery of the ‘crazed’ cracks.

The concern relates to the potential that unacceptable amounts of liquid hydrogen could leak through those cracks during ascent, which would breach safety rules. As a result, engineers are being tasked with understanding and eliminating the concern via testing.

‘Provide comprehensive plan to utilize all available connector hardware in the development of flight rationale for the existing feed through connector design. Surface cracks observed on vitreous glass seal using 50x visual inspection on ET-125 feed through connector,’ noted information acquired by L2.

‘Surface cracks could result in unacceptable LH2 leakage during ascent (Crit. 1). Rationale required prior to installation and flight.’

This is currently the main focus of the engineering evaluations on the cracking, after opening results showed that there is neither the flight history, nor the data, that supports any correlation between the observation of cracked glass and the ECO sensor system anomalies.

As a result, MSFC engineers are now tasked with a separate investigation, likely in cooperation with the vendor, to pinpoint the exact cause of the cracks, along with a full understanding of any potential safety implications during the ride uphill.

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Opening data shows that engineers have been aware such cracks can exist, and are the result of processing. Sources believe that certain levels of observed cracking leads to hardware being rejected – via a quality control process. However, such cracking has only when spotted via inspections that are to a certain level of magnification.

Due to the intense focus on ET-125’s connector, the cracking was observed at magnification levels of up to the power of x50. This may add confidence, due to the likelihood that cracking is typical – though not always spotted – yet has not exhibited a problem on any flight of the shuttle.

However, NASA aren’t taking any chances with the connector hardware, following the problems associated with STS-122. A full investigation will be utilized in parallel with the MSFC cryogenic testing of ET-125’s connector, as engineers hope to recreate the error seen during the two launch scrubs and subsequent tanking test, with an eye on confirming that the removed external connector is at fault.

‘Test-Based Verification Approach: All available hardware assets being used to provide timely data to support two critical milestones – Production Readiness Review (PRR) and Flight Readiness Review (FRR). Test plan structured to provide sufficient data to proceed with confidence at PRR,’ noted information.

‘Data includes multiple leak tests with 50x visual inspections of ET-125 connector. ET-125 connector experienced multiple cryogenic cycles with pressure loading. Provides confidence that cracks are benign.’

Such is the level of testing that is being carried out on this particular issue, hardware from the X-33 – the technology demonstrator for the VentureStar program back in the 1990s – will be called upon, due to the commonality of the connector hardware.

‘(Testing) planned for X-33 connector. X-33 connector qualtest-demonstrated (>6 cryocycles, delta pressure, and vibration),’ added information. ‘Provides evidence that glass structural integrity not compromised by cracking under combined environment loading.

‘Test plan builds on existing database to further increase confidence prior to FRR. Includes additional vibration testing of ET-125 connector with cracks. Ultimate pressure testing at cryoplanned for ET-120, ET-125, Qual, and 1st article units. DPA planned following ultimate pressure testing. 50x visual inspections planned at appropriate points during testing.’

Confidence is high that this new issue won’t cause a problem with the current efforts of installing the replacement Feed-through connector – which is an opening process of the forward plan to bring STS-122 back into a launch posture, with rationale already created to ‘proceed’ with the current plan.

‘Rationale for Proceeding with ET-125 Installation: 50x visual inspections show no change in crack characteristic following cryocycling with leak tests (ET-125). Leak testing performed at pressures – flight limit pressure (100 vs. 34 psig) (ET-125). Leakage within acceptable limits.

‘X-33 qualification article (cryo, pressure, vibe) contained cracks consistent with ET-125. Additional cryogenic cycles and pressure testing performed. No crack growth observed following 100 psi leak test at KSC. X-33 connector dissection data to show nature of cracks as superficial / benign.’

The plan is to ship the X-33 connector hardware to the Michoud Assembly Facility (MAF) early this week, for dissection and testing by Thursday. At the same time, further inspections will be carried out on the ET-125 connector at MSFC on Tuesday.

‘Coordination with vendor suggests cracks most likely caused by processing and exacerbated by CTE (Coefficient of Thermal Expansion)-effects at cryogenic temperatures. ET-125 and X-33 connectors required to provide data for PRR,’ added information.

‘ET-125: Leakage and crack characteristics (MSFC) X-33: (Testing) to assess extent of cracking (MAF). Additional testing planned for ET-125, ET-120, and verification test articles. ET-125: Cryogenic vibration followed by ultimate pressure test (MSFC) and testing (MAF) – (Same for ET-120).

‘Detailed schedule in work to show timeline for additional scope required for flight rationale.’

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