ECO sensor change-out to delay STS-121

by Chris Bergin

Sources this morning are noting that there is a “99 percent” chance that the LH2 ECO (Engine Cut Off) sensors in ET-119 will need to be replaced, after a fault was discovered.

The change-out will involve a 17 day schedule hit on the STS-121 processing flow, which, with no contingency days to spare, eats into the whole of the May launch window of opportunity for Discovery.

While a launch date hasn’t been officially set for May, the next launch window is in July.

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ET-119 arrived at the Kennedy Space Center last week, but following inspections and tests on the tank – prior to being lifted out of its checkout cell in the VAB (Vehicle Assembly Building) a phase shift was noted on the LH2 ECO sensors.

This was alerted to the Michoud Assembly Facility in New Orleans, where Lockheed Martin engineers and technicians recommended the changeout of the sensors.

‘I believe they had a phase shift of two on one of the sensors,’ said a source to this site this morning. ‘There may be a shelf life issue also – and they are about to run out.’

The tank will be worked on in the VAB, with Lockheed Martin technicians assisting the change-out. The tank can’t be stacked and mated with the SRBs (Solid Rocket Boosters) until the change-out has been completed.

‘They want to do it (at KSC) because it is easier to do it in the vertical,’ added a source. ‘Lockheed Martin technicians will do the work and KSC will support them.’

The ECO sensors were troublesome in the build-up to STS-114 last July.

During the first tanking test with Discovery and ET-120, a fault with an ECO sensor was detected during the tanking, with a second ECO sensor failing during the draining of the Liquid Hydrogen from the ET.

Following a second tanking test, which also incurred a problem with the LH2 pressurization relief valve (hydrogen vent valve) over-cycling – breaching Launch Commit Criteria (LCC) – on both tanking tests involving ET-120, the decision was made to roll-back the Shuttle into the VAB, to switch ETs (from ET-120 to ET-121 – which had been set to fly with Atlantis immediately after Discovery’s mission).

However, the ECO issue returned with the new ET, when one ECO (LH2 number 2) started to show an intermittent fault, less than two hours prior to the scheduled launch attempt. The launch attempt was scrubbed, and engineers immediately went looking for the cause – through a troubleshooting process.

Searching for the problem brought up several possibilities. One that has gained ground at the time was an apparent problem with the LH2 ECO number 2 SIM circuit.

Engineers have noted – while not in public – that the problem appeared when the SIM circuit is turned on, yet disappears when the SIM circuit is turned off. Yet the problem was classed as ‘intermittent’ by Shuttle manager Wayne Hale during a post MMT press conference, and later blamed on electo-magnetic interference as a potential cause. Officially, the ECO sensor issue was classed as a UA (unexplained anamoly). 

Other possible causes for the error came in the form of the sensor registering ‘0’ or WET, a problem believed to be with the wiring, possibly a short somewhere between the Point Sensor Box and the ECO sensor. The latest problem with ET-119 now points to a problem inherent in the External Tank itself.

The sensors are a vitally important element of the Shuttle. They become armed seconds prior to the planned MECO (Main Engine Cut Off) – both triggered by real-time shuttle velocity calculations. The danger of a miscalculation, or fault in the ECO sensors, could lead to the cutting off the main engines too soon or too late. Too soon and a risky abort may be called, too late and the Orbiter could be destroyed by its own engines – sucking up ullage gas, rather than propellant.

There has been talk prior and post STS-114 to reduce the requirement of all four ECO sensors needing to be in perfect working order, allowing a LLC of three out of four being in working order to proceed with a launch.

The four out of four methodology was determined after a very careful study by system experts, with the ECO sensors classed as critical flight hardware. However, a reduction in requirement to 3 out of 4 has already been looked at before – and dismissed.

A group of NASA engineers spent a huge amount of money composing a study on the effects of reverting to a 3 out of 4 requirement with the ECOs. Without consulting External Tank construction engineers and designers, the study made the IRN (Interface Revision Notice) stage.

Also apparently in the dark was the PSIG (Propulsion System Integration Group) – a consortium of NASA, Lockheed Martin, Boeing and Rocketdyne propulsion engineers who met several times a week to discuss such issues.

Once discovered, the study was bitterly opposed. It was eventually thrown out by high-level NASA managers, after the groups that had been left in the dark took their grievances to the very top of the NASA management structure.

This background practically ensures any faults with the ECOs will lead to the requirement to have four out of the four sensors in full working order, which now isn’t the case with ET-119.

Further updates will follow.



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