ET over-cycling – Diffuser U-turn

by Chris Bergin

The Marshall Space Flight Center (MSFC) designed duplex screen in the External Tank (ET) LH2 diffuser will no longer be used in any of the remaining ETs on future launches.

The new diffuser screen causing nearly double the normal amount of cycles through the LH2 pressurization relief valve (hydrogen vent valve), breaching Launch Commit Criteria (LCC) – on both tanking tests, involving ET-120.
The diffuser is used to inject helium into the tank, a process that aids the correct pressure and temperature of the ET prior to launch.

The new dual-screen version was the cause of unusual behaviour – to a point that incorrect readings or actions resulted. This nearly led to too much over-diffusing of helium entering the ET. It has not been revealed just how dangerous this was to the ET and/or the Shuttle stack.

‘The diffusers will only use the single-shute-wire weave screen – the duplex screen will never be used again,’ said a Michoud Assembly Facility (MAF) source. ‘The problem with (the duplex screen) is that it almost over-diffused the GHe (Helium) entering the tank.’

While problems with the ECO (Engine Cut Off) sensors ultimately led to the rollback of STS-114 – with ET-120 being replaced with what was to be Atlantis’ ET (ET-121), set to fly the then-scheduled STS-121, MSFC requested a third tanking test to try and confirm ET-120’s diffuser was at fault.

With ET-121 using the old style diffuser, NASA managers decided to proceed to launch without another tanking test, as other readings from the ET stayed within LCC parameters.

However, after one launch scrub – due to another problem with the ECO sensors – Discovery proceeded along the countdown on July 26 – with the ECO sensors remaining nominal.

What wasn’t revealed, until later to this site – – was that ET-121 happened to be one cycle away from a launch scrub, cycling a total of 11 times, with the LCC set at 12 cycles.

‘This is why the cycle count is so crucial, because originally the LCC was developed to protect the tank against leakage,’ added the source. ‘With the over-diffusion of helium, we weren’t getting the pressure increase from each 0.5-sec burst – so, the incorrect screen type messed that up.’

What now appears to be the case was a conflict between the two screen types and the re-written requirement. Thus an incorrect procedure was to blame for the near over-cycle on ET-121, while the duplex screen was at fault on the over-cycling with ET-120.

‘The screen was actually correct in the spec,’ the source noted. ‘By the spec, the screen requirement was by pore count (i.e. number of ‘holes’ per inch), and you get an acceptable pore count with both simplex and duplex screens.

‘The requirement was re-written to clarify the screen type and wire count as well.

‘No chance that the incorrect screen will be used again.’

Ironically, the switch from ET-120 to ET-121 has been seen by some as more than unfortunate, given the resulting PAL ramp foam liberation from a previously re-worked area of the tank. However, it could have been worse, with ET-120 being used as a test specimen and showing hairline fractures along its own PAL ramp – believed to be the result the loading and unloading (twice) of super cold propellants during its two tanking tests.

All ETs will now fly without a PAL ramp, starting with STS-121, currently estimated to launch in May.

To follow up-to-the-minute news on Discovery’s preparations for STS-121, click here:

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