Keeping ECO at 4 for 4 – a must for some

by Chris Bergin

With the Engine Cut Out sensors continuing to gain more attention with regard to Shuttle Discovery’s long awaited launch, NASA managers have been mulling over the idea of reducing the requirement to 3 out of 4 – to enable a launch to proceed.

When one ECO (LH2 number 2) started to show an intermittent fault less than two hours prior to the scheduled launch last Wednesday, engineers immediately went looking for the cause – through a troubleshooting process.

Most passionate are the people who want the requirement to remain at 4 out of 4 working ECOs. At Wednesday evening’s press conference, NASA managers believe the issue is electo-magnetic based and will be prepared for ‘expected issues’ with the ECOs. Any unexpected issue will result in a launch scrub.

NASA has set a new tenative launch date for the shuttle Discovery – Tuesday, July 26.

“Right now we think we have eliminated all possible causes related to the glitch,” Shuttle manager Bill Parson said.

Even though there have been numerous improvements and enhancements over the years, the Space Shuttle is still a phenomenally complex vehicle. The electronics are staggeringly complicated, highlight not least by the current inability of Kennedy Space Center engineers continuing to find out why – after the issue from Tanking Test 1 – the issue has not been solved. During and immediately after Tanking Test 1, two ECO sensors (3 and 4) failed in similar fashion to ECO 1 during the launch attempt last Wednesday.

Searching for the problem has brought up several possibilities. One that has gained ground this week is an apparent problem with the LH2 ECO number 2 SIM circuit. Engineers have noted – while not in public – that the problem appears when the SIM circuit is turned on, yet disappears when the SIM circuit is turned off. This at least – if found to be top of the fault tree – gives hope that the problem may not be as “intermittent” as Shuttle manager Wayne Hale noted this week during a MMT press conference.

With the error with the ECO coming in the form of it registering “0” or WET, the issue may also be down to wiring, possibly a short somewhere between the Point Sensor Box and the ECO sensor.

Keeping the 4 out of 4 ECO sensor requirement could be perceived as ensuring the general public don’t feel there is a reduction in safety – especially with this being the first launch since the loss of Columbia on STS-107 two and a half years ago.

With the ECO sensors becoming armed seconds prior to the planned MECO (Main Engine Cut Out) – both triggered by real-time shuttle velocity calculations, the absolute requirement for the ECO system is so important due to the dangers of a miscalculation in 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.

The four out of four methodology was determined based on a very careful study by system experts, originally studied for the Apollo Program – which the Shuttle was born out of in later years – and the ECO sensors are classed as critical flight hardware.

The problem with this particular hardware is due to two ECO sensors being on the same electrical bus. If that bus was lost, the Shuttle could find itself with serious problems.

However, a reduction in requirement to 3 out of 4 has already been looked at before.

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.

Any decision that will come on the ECO sensor requirements is likely to dig up these previous issues. A solution to the current fault that is undergoing troubleshooting would be the best conclusion to those that wish to see the current ECO requirements remain.

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