Atlantis’ External Tank LH2 ECO (Engine Cut Off) sensors have again suffered problems during the tanking test, allowing engineers to take data on the potential root cause. Initial TDR testing has pointed to a fault with the LH2 Feedthrough connector.
STS-122 already has flight rationale in relation to the sensors. However, should this specific element of hardware require replacing – at the pad – a R&R timeline also shows January 10 is still possible.
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Key Events (local KSC time):
S0037 Monday- RSS OWP panels have been retracted ahead of RSS retract.
S0037 Monday- RSS MOVE TO PARK 2130 T-11 BIH AT 1900 (DURATION 6 HOURS) – complete.
S0037 Tuesday- T-11 COUNTING AT 0100 /0230 PAD CLEAR /TANKING TO BEGIN 0700 – Tanking commenced at 7:20am, with chilldown, following a slight delay due to minor troubleshooting.
Following 5 percent level, LH2 ECO sensors number 2 – for the first time during STS-122 – and number 3 showed out of family voltage readings, but have not yet hard failed. Shortly after, LH2 ECO Sensor 1 hard failed, open circuit.
Troubleshooting (TDR) to begin shortly – though delayed due to LOX stable replenish primary pump (blown fuse). Backup pump has now taken over stable replenish.
TDR tests on LH2 ECO (Low Level/5 percent) sensors now completed by a team of engineers inside the MLP. TDR will remain active and monitored by consoles in Firing Room 2 during detanking.
Multiple L2 sources note that the opening TDR test results point to the problem being related to the LH2 Feedthrough connector – as previously noted as the main culprit prior to the tanking test.
‘Based on Time Domain Reflectometry (TDR) testing at the MLP, readings indicate an open circuit for ECO sensor #1 at the feed through connector,’ noted part of a memo on L2. ‘All readings matched all characteristic profiles based on previous lab testing.’
At around 12:45pm, LH2 ECO sensor #3 hard failed. Detanking has commenced following the gathering of additional TDR testing on the aforementioned sensor.
Discussions taking place on R&Ring the connector at the pad (timelines on L2 show five to six days work).
All above confirmed by Shuttle manager Wayne Hale at the post tanking test press briefing. Further articles will follow on the forward plan.
Tuesday’s next step in the troubleshooting process will see engineers fill shuttle Atlantis’ External Tank with super cold propellants, after which Time-Domain Reflectometry (TDR) equipment will be used to test for the presence of open circuited components in the LH2 ECO sensor system.
Engineers believe the ET feed through and connector assembly is the most likely source of the problems. The Dec. 6 and Dec. 9 launch attempts produced previously unavailable time trending data that show sensor faults occurring shortly before and after the feedthrough and connector are immersed in the super-cold propellants.
In response, ground tests have confirmed that an open circuit can occur in the connector when the assembly is subject to the thermal changes of the tanking process.
Detailed plans, available on L2, have been drawn up by NASA engineers ahead of this week’s TDR testing on the filled external tank. These plans are summarized below.
The TDR system works by sending pulses of electrical energy along the suspect circuit. Changes in the circuit (crimps, cable joints, and most certainly open circuit connections) will reflect some energy back at the sender, indicating the presence of what could be a problem.
Engineers have connected this equipment into ECO sensor circuits upstream of the sensors themselves, to detect any open circuits between the orbiter computers and the sensors.
Before the tanked tests can be conducted, a set of results for a healthy ECO sensor system were collated to show engineers what a healthy ECO circuit looks like when viewed with TDR. Any changed in sensor circuit characteristics caused by the tanking process can then be identified.
These baseline results were gained via TDR testing on an upcoming external tank at the Michoud Assembly Facility (MAF). Over the weekend, the instrumentation group ran a series of simulation tests using the TDR unit and MSFC (Marshall Space Flight Center) supplied ECO sensors in laboratory conditions.
The tests were conducted under cryo conditions using LN2 and gained the baseline data to prepare for Tuesday’s testing, allowing engineers to more easily confirm the suspect case where the feed through connection is the source of the open circuiting.
Engineers also ran tests of the installation at the pad and are finalizing insulating the TDR wiring that has been spliced into the vehicle wiring. Testing the mated stack allows engineers to appreciate differences caused by the mated configuration or by manufacturing tolerances.
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Sensor cabling onboard Atlantis has seen TDR equipment inserted to test wiring and connectors downstream of the splice point. Documentation showed that three connection sites were proposed in planning for the TDR tests – two on the ET and one in the orbiter.
ET connection sites were not favored because of the requirement to remove and reinstate ET foam to access sensor cables and environmental measures that would need to be put in place at the pad. An ET connection site would also make routing the TDR cabling from the connection site relatively difficult.
Avionics bay 5 in the orbiter’s aft compartment was chosen as the preferred connection site, since it can be accessed relatively easily without requiring any foam removal. The TDR wiring is routed out of the 50-2 door at the aft of the orbiter and down the tail service mast to the TDR console on the MLP.
The tanking test will see the stack powered up, ordnance and pyros safed, boosters powered up and MPS helium tanks pressurized to 2000 psi. The vehicle will be brought to the T-3 hour configuration through the usual propellant loading process, during which ECO simulation commands will be sent and responses observed.
Engineers will be monitoring LH2 ECO sensor outputs (which will still be available with TDR apparatus installed) for any sensor failures during the lead up to stable replenish.
If sensor failures occur before stable replenish is reached, tanking will continue while and the system state (including any simulation commands to the sensors) will be maintained to preserve the failed sensor.
When the tank has reached stable replenish TDR testing of all LH2 ECO sensor circuits will be conducted even if sensor failures have not occurred at that time. Once all TDR tests have been concluded the tank will be drained of all propellants in the usual manner.
NASA documentation on L2 describe how the results of these tests will affect the flight rationale for launch in January:
Flight rationale, should all ECO sensor circuits operate normally during Tuesday’s tanking test, will be to ‘accept the theory that the sensor system either heals itself or remains the same on subsequent cryo loads’ and will ‘require 4/4 operational for flight.’
Should the ECO LH2 number 3 circuit failure reoccur with all other systems functional, then 3 of 4 ECO sensors will be required for launch, providing the only failure is that of ECO-3.
If ECO sensor systems, other than the ECO-3 circuit, fail during the tanking test then the flight rationale will be to ‘assume ECO sensor system will not properly operate during flight’ and to launch with a reduced payload weight ‘to further mitigate the probability of requiring the ECO system.’
In any case, should the TDR results from the tanked test reveal a faulty component or components, then ‘short term solutions’ will be investigated.
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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