ECO (Engine Cut Off) sensor anomalies are caused by “substantial amount of Krytox grease and silicon contamination” on the External LH2 Feed-through connector plug and pins, testing has found, as STS-122 marches – with confidence – towards launch next month.
A major discussion on the LCC (Launch Commit Criteria) has also been conducted, ahead of approval by the Flight Readiness Review (FRR), that will allow for slight sensor issues during the countdown.
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STS-122 ECO related news content: *Scrub 1* – *MMT Debate* – *Scrub 2* – *Hale Memo* – *Forward Plan* – *Culprit Found* – *Tanking Test* – *Repair Options* – *MAF Plan* – *PRCB Debate* – *Plan Approved* – *Repair Schedule* – *Launch Date* – *New Issue* – *Hale Rallying Call* – *PRCB Launch Dates* – *New Connector Installed* – *Positive Test Results* – *Manifest Impacts* – *Optimism with forward plan* – *Flight Rationale* – *Repairs Finalizing*
STS-122 Troubleshooting Latest: (Pre-FRR) – FRR Presentations (Jan 25) available to download in L2).
Thursday’s Program Requirements Control Board (PRCB) meeting was presented with a complete run down of the test results, showing the issue of contamination was deemed responsible for the issues during the two launch attempts and subsequent tanking test.
‘Substantial amount of Krytox grease and silicon contamination present in ET-125 external feed through,’ noted a PRCB overview presentation. ‘Krytox grease and has been proved to be an electrical insulator at cryogenic (-90 F) temperatures.
‘Electrical isolation upon solidifying coupled with pin/socket position variation are required for open failure. Verified in TS-300 cryo stat testing. Verified in LGN2 testing at KSC with DuPont representative.’
‘TDR (Time-Domain Reflectometry) testing resulted in determining that the ET-125 failure existed at the feed through and most probably at the external connector to feed through interface.
‘Failure signature (make/break) and reasonable cause hypothesis support external connector being suspect: Two events required for make/break – non conductive contamination at cryo temperatures and movement. Thermal effects, cryo pumping opportunity, Krytox, silicone contamination possibility point to external connector.’
Testing of the removed ET-125 External Feed-through connector at the TS-300 test stand located at the Marshall Space Flight Center (MSFC) also cleared the internal connector – which would have been a bigger problem, due to the requirement of rollback to remove the hardware.
‘Root cause positively identified as cryo induced contamination coupled with pin/socket movement which contamination findings support,’ added the test results.
‘Root cause is not transferable to internal connector because thermal profile doesnâ€™t support pin socket thermal gradients resulting in internal connector opens. Solid air, ice not possible inside tank. ET-125 Internal connector sockets are contamination free.’
The first of two Flight Readiness Reviews (FRRs) will be conducted on Friday, which will finalize the paperwork ahead of launch, following Atlantis’ stay at Pad 39A since November. No issues are currently being worked on the vehicle, as repair work is completed on ET-125.
‘Doing well on Orbiter-side at the pad. Work is planned out and they are flexible with this work at the pad,’ noted Deputy Shuttle Manager John Shannon at the latest Stand-up/Integration meeting.
‘ET work and trimming are complete. Enclosures were removed. Cures are up next week. In good position to get vehicle in launch configuration.
‘ECO efforts are complete. Working through one additional issue associated with fleet clearance for a TPS observation seen in production.
‘Expect clearance of this before FRR. Have a series of qual tests in work to support ET-125 ECO mods, all of which are on schedule.’
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Launch Commit Criteria:
No ultimate decision has been made on the LCC requirement for the ECO sensors during the STS-122 countdown, with the final debate heading for the Flight Readiness Reviews.
However, an interesting set of opinions, from related STS departments, showed an across-the-board change to the opinion that the LCC should remain as the standard 3/4 LCC for the sensors, rather than the 4/4 – as brought in for the second launch attempt in December.
Notably, departments, such as the conservative JSC Engineering – which had previously called for 4/4 – came into line, following the confirmation that a root cause had been found.
‘JSC Engineering Directorate Position on LCC for STS-122 ECO System is 3 of 4,’ noted their presentation. ‘Root cause identified and isolated to contamination on external pin socket connection. Very small residual risk exists that similar failure mode exists at internal pin/socket connection.
‘Design modification which includes solder the pin to socket, Teflon insert, and strain relief eliminate this common cause failure mode.
‘Comprehensive qualification test program implemented to demonstrate performance and reliability of the modified feedthrough connector. Great care and expertise used during flight unit soldering and assembly.
‘Flight unit post soldering inspections and high strength margins demonstrated by successful qualification testing ensure flight worthiness of flight units. Helps mitigate the fact that acceptance test on ET-125 feedthrough modification did not include vibration or cyro exposure.
‘Appropriate LCC is 3 of 4 to allow use of redundancy in the system to deal with other potential ECO system failure.
The Orbiter Project Office (OPO) at the Johnson Space Center (JSC) in Houston concurred, adding that the use of a fix, utilized by the Atlas Program – one which involved the soldering of the pins on ET-125 and ET-126’s connectors – added to the overall confidence.
‘This cause identification and corrective action is consistent with findings of Atlas Program tank feedthru connector failure resolution. There have been no recurrences of open circuits at this interface since this fix was put in place on the Atlas Program,’ added OPO’s overview.
‘The physics behind the failure mechanism is still being investigated. However, by soldering the external pin interface, the failure mode and its affects at this interface has been eliminated.
‘STS-122 tanking test data and the subsequent investigations have resolved previously theorized common cause concerns with other ET and Orbiter hardware (Point Sensor Electronics Box, Electrical monoball, production breaks, etc.,) that provide a high degree of confidence in the ECO system with the implementation of the feedthru connector fix
‘The ECO circuits always fail ‘WET’ per design during each tanking with anomalies. For tankings with anomalous sensor circuits, the remaining healthy circuits have always demonstrated functionality (transitioned to dry at the correct time during tank drain)
‘OPO recommends the use of the flight bypass for an ECO circuit known to be failed WET to preclude a DRY flash if the sensor circuit recovers following arming.’
Another article will follow Friday’s FRR. 500 pages worth in L2 – next article Saturday.
L2 members: All documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, updated live.
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