Troubleshooting task is complex

by Chris Bergin

Confusion still surrounds the cause of the intermittent failure of the LH2 Engine Cut Out sensor (number 2) during yesterday’s countdown – leading to a scrub and subsequent delay to ‘no sooner than’ late next week for the Return to Flight of the Shuttle fleet.

However, it is highly unlikely Discovery will launch anytime soon.

Troubleshooting started in earnest following Discovery’s scrub, although the length of time before another launch attempt will be undertaken depends of targeting what is still classed as an Unexplained Anomaly (UA).

Discovery is no stranger to an ECO sensor issue, following her double fault during the first tanking test. However, on that occurrence, LH2 ECO sensors 3 and 4 were at fault.

There are four LH2 sensors and four LOX sensors. They are checked like this: LH2 and LOX sensors number 1,2,3,4. The sensors kick in during the final seconds of the Shuttle’s assent into orbit, checking the level of propellant remaining in the External Tank.

Should a failure occur where a sensors fail to correctly note how much LOX and LH2 is remaining, the Space Shuttle Main Engines (SSMEs) could potential run out of propellant, begin ingesting ullage gas, cavitation would the result in the turbines over-spin and rip the Orbiter apart.

Speaking on condition of anonymity, one of the launch operators gave the full outline to the issue arising, the troubleshooting that is on-going, and potential scenarios to solve the problem.

“The sensors are checked, the LH2 and LOX sensors are checked in pairs – #1 ECO LOX is checked along with #1 ECO LH2.

“(During ‘replenish’) United Space Alliance (in control of the Shuttle pre-launch) did this with the #1 ECOs and both temporarily went ‘DRY’, then they switched them back on to read data. The input to the sensors for this check is provided by something called a ‘SIM circuit’.

“They did the test for ECO #1’s and it passed since they both indicated DRY. Then they did this for the #2 ECOs, and only the LOX ECO indicated ‘DRY’ – the LH2 ECO (#2) indicated WET. They repeated this test several times, all with the same result. All other ECO sensors passed the test successfully.”

“Then they did a special form of this test where all eight ECOs (all four LH2 ECOs plus all four LOX ECOs) were tested simultaneously, and all eight indicated ‘DRY’.”

“Before this test all four LH2 ECOs were working perfectly.”

“Bear in mind that when they switched the test ‘OFF’, the LH2 ECO #2 sensor indicated ‘WET’, as it should. This seems to indicate that the sensor was working properly. We’re thinking it was something about that test that failed.”

“They decided to leave the #2 ECOs switched in the ‘DRY’ test position (which meant that LOX ECO was indicating ‘DRY’ and the LH2 ECO was errantly indicating ‘WET’, with both tanks completely full).

“When the tanks were drained, they were hoping that LH2 ECO #2 would suddenly indicate ‘DRY’ as it should – then they could pinpoint the problem with the SIM circuit.

“Well, we were hoping that this would happen – that LH2 ECO #2 would indicate ‘DRY’ when the other three did – but it stayed wet, eerily similar to the first tanking test.”

Speaking today from the Kennedy Space Center, NASA manager Wayne Hale noted Engineers are discussing how the thermal environment of the avionics box affects its operation. If the box is at fault a replacement would take at least 10 days – most likely longer – to build.
However, the issue has not yet been pinpointed.

“LH2 ECO ‘wet’ indication is an indication that there’s an ‘open circuit’,” added the source. “which could be anywhere from the sensor itself to the wiring in the Orbiter.”

Related Articles