Engineers believe they have closed in on the culprit for the ECO (Engine Cut Off) sensors issues during their two launch attempts with STS-122 last week.
An expansive presentation was reviewed by Thursday’s Program Requirements Control Board (PRCB), which listed the findings from the External Tank project, which classes the LH2 Feed Through Connector as the only probable ET-related cause for the problems. Meanwhile, the latest NET launch date has moved to January 10.
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Work started at the pad on Thursday, after the stack was taken out of its launch configuration earlier this week. Troubleshooting remains on track for the tanking test to be conducted next Tuesday.
Thursday’s work involved ‘tapping’ into LH2 ECO sensors 1 through 4, and the 5 percent sensor wires near orbiter monoball. Resistance checks on ET side of circuit were also conducted, with the use of the Time-Domain Reflectometer (TDR) on all 5 circuits – with full TDR testing due tomorrow. In readiness of more tests on Friday, engineers also spliced into wiring downstream of the orbiter to the MLP (Mobile Launch Platform).
Friday will involve the baseline TDR run and functional test, while Atlantis is powered up, before the aft is closed ahead of the tanking test next Tuesday – due to begin at 7am local time. The Rotating Service Structure (RSS) will be moved to its park position at 21:30 local time on Monday.
Thursday saw the fascinating presentation from the ET Project to the PRCB, which covered a vast amount of current thinking, data and background from ECO related events since Return To Flight and STS-122.
The presentation covered the known theories on the initial ECO sensor problems which first showed up during STS-122’s countdown. The lack of problems in recent pre-launch countdowns had raised hopes that the solution – involving the ‘hand picking’ of the best sensors – was the end to the problems on this system.
‘Investigation performed and identified most likely cause attributed to loose swages on ECO sensors,’ noted the pre STS-122 ECO events. ‘Loose swages provide opportunity to break contact between lead wire and sensor. Process enhancements implemented on swages to improve contact.’
Moving swiftly on to the current problem, the ET project outlined their latest findings on what may be causing the problematic ECO sensor readings during the two launch attempts of Atlantis/STS-122 – assuming the fault is with the External Tank, and not downstream with the orbiter herself (deemed unlikely).
‘Assessed potential ET hardware common cause failure scenarios that could result in STS-122 circuit behavior,’ added the presentation, which showed the LH2 Feed Through Connector is now the only ‘possible’ cause of the ECO issues, via tank hardware.
‘Level sensors failures – Remote to Unlikely. Wire Harnesses / Splices – Remote to Unlikely. Feed Through Connector / Plug â€“ Possible,’ the document added, before explaining in relation to the connector: ‘Requires either contamination of contacts and/or physical motion / damage of mating surfaces during tanking fill and drain.
‘Most likely ‘Common Cause’ for STS-122 ECO Sensor Circuit Anomalies Attributed to Feed Through / Connector (ET Project Assessment).’
Originally, all ET hardware elements were not believed to be a possible cause of the ECO sensor faults. While the connector is now in focus as a suspect in the fault tree, the ET Project emphasized that background information on the component showed no issues during testing.
‘Review of feed through connector design certification and as-built documentation in-work – No issues identified to date,’ the presentation added, whilst noting expansive background information on the connector, which included: ‘Acceptance testing complete with no issues. Visual inspections and pin retention tests performed prior to assembly.
‘Original design qualified to cryogenic environments with random/sinusoidal vibration loading at 37 psig (max flight pressure ~34 psig). Thermally shocked 5 times prior to test (-423 degrees F – +200 degrees F) at 37 psig. ET-120 feed through x-ray and visual inspection performed – No issues identified.’
The only ‘recent’ change to the connector was noted as a ‘glass material change,’ which was implemented in 1999 (ET-120 and subs). However, this change was qualified by similarity analysis (re-qualification not required), with no significant change in mechanical properties.
The presentation also gave a new in-depth look at the ECO sensor readings during both scrubbed launch attempts, characterizing it as ‘significant new data.’ This new data – according to the presentation – has led to the added focus on the LH2 Feed Through Connector as a potential key element in the fault tree.
‘Characteristics of the ECO System Anomalies: Timing of the failed wet indications appears related to feed through connector thermal transients during chill down and warm up. Failures are isolated to the sensor circuits in the LH2 bottom connector even though hardware, circuits, and temperatures are very similar to the top level sensors.’
‘Postulated Failure Mechanism Summary: Circuit break caused by contamination of contacts between external plug and feed through pins and internal movement of plug sockets. Contamination is the result of condensing and freezing of air and water vapor within connector on feed through pin surfaces. Solid air and ice are effectively electrical insulators. Cryopumping of ambient air is expected through harness.
‘Plug socket movement is the result of induced loading on plug sockets from forces exerted by vacuum pressure, CTE-motion, and deformation of socket restraint materials (grommet cracking). Circuit opens due to contacts being moved over contamination on pin surface. During drain back – temperature increases, contamination melts, socket contacts reconnect pins, anomaly clears.’
Additional data is noted, which concentrates on the connector during both of STS-122’s tankings. The findings provide an interesting insight into a three point timeline for the potential of the connector to become compromised by ice contamination, as the tank drops in temperature during cryogenic conditions.
Evaluations can be found in the document, which lists events and observations, via new data, for all four events relating to the launch attempts (tanking and draining). For the first launch attempt, the following observations were found, specific to the connector:
Event 1) (Tanking begins): Temperature of pin in feed through connector decreases to < 32 degrees F. Moisture/vapor freezes on pins resulting in ice contamination.
Event 2) (Just prior to ECO sensor readings failure): Pin temperature decreases to < -320 degrees F. Ice contamination increases by freezing air within cavity (now under pressure of 14.7 psid.)
Event 3) (Sensors fail): Cavity temperature < -320 degrees F. Grommet material cracks / deforms and degrades capability to maintain sockets in place. Sockets move onto pins entrapping contamination and causing ‘open’ in connection. Cryopumping continues into all available cavities.
‘Supporting Rationale for Postulate Failure Mechanism: Thermal analysis results show that predicted temperatures consistent with air / water vapor phase change correlate with sensor circuit anomalies. Cryopumping through wire harnesses and restrictive leak paths well established through tests,’ added the presentation.
‘Bench test results show open circuit can be generated by moving socket over pin with solid air on pin surface. Analysis of constituent material CTE deltas show socket motion cannot be precluded. Delta pressure and thermal gradient effects will provide forcing function for socket motion.’
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Interestingly, the ET Project gave potential flight rationale for the connector after the tank is ‘fuelled’ for flight. This may remove concerns about the ECO sensor system malfunctioning during the eight and a half minute ride uphill. This – pending further data via Tuesday’s tanking test – may be the key to launching early in January.
‘Physics-based understanding supports stable performance following thermal equilibrium. Thermal conditions and forcing functions remain constant through ascent. Cryopumping of ambient air will effectively pot plug with solid air. No further degradation to pin / socket contact – Steady-state condition already achieved,’ added the presentation.
‘Tanking test should provide additional data to support understanding of scenario. No further degradation of system (ECO number 3 only fails or all sensor functional). Additional failures should match same time / temperature relationship as previous failures. After Steady-State Tanking Conditions are Reached, no Degradation of System Expected.’
There’s still a long way to go before engineers are confident they may have found their root cause, never mind being able to push forward towards launching STS-122 without any concerns relating to the ECO system. However, they are making huge strides in just these opening days to achieving that goal.
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