Tanking Test under consideration

by Chris Bergin

NASA is considering the inclusion of a Tanking Test into STS-121’s processing flow, in order to test ET-119’s new ECO (Engine Cut Off) sensors, which are currently being installed as replacements during change-out.

The inclusion of the Tanking Test requires approval from the KSC launch director – and discussions have been met with protests over potential foam defects which can result from “cryo cycling” the tank.

ET-120 went through two tanking tests prior to STS-114 last year – before being replaced by ET-121 for the mission. Following evaluations on ET-120’s PAL ramp, hairline cracks were found, which it is understood was a direct result of the two ‘cryo cycles’.

While a tanking test won’t affect the current NET (No Earlier Than) launch date of July 1, the rationale for adding a Tanking Test – soon after Discovery is rolled out to the pad – simply for the evaluation of the new ECO sensors, appears unbalanced on the pros and cons.

Should there be another problem with the ECOs, it will show up during the launch countdown – as they did during STS-114’s first launch attempt last July, thus deeming the Tanking Test as non-essential.

However, upon inquires on whether any other issues may have been adding pressure for a Tanking Test, sources note a wish to monitor the amount of cycles on the LH2 pressurization relief valve (hydrogen vent valve), which breached Launch Commit Criteria (LCC) – on both tanking tests, involving ET-120.

A new dual-screen diffuser (duplex) version, installed into ET-120, was deemed responsible for the over-cycling, which was to such an extent that it nearly over-diffused the amount of GHe (Helium) entering the tank.

ET-121 – which eventually flew with Discovery on STS-114 – used the traditional single-shute-wire weave screen on its diffuser, yet it appeared the problem remained, as the ET was only one cycle away from breaching the LLC during Discovery’s countdown.

Previous reasoning relating to the cycles on STS-114/ET-121 can be viewed here: **Related Diffuser Article**

Evaluations are still taking place on the request for a Tanking Test, with another MMT Sim set to take place next week (S0044). These Sims will evaluate the rationale on the requirement for further data.

Work is currently proceeding on changing out the LH2 ECO sensors in the aft dome area of ET-119, while at the other end of the tank, work on changing out the vent and relief valve has been halted due to foam damage, sustained when a portable light fixture fell on to the area near the top of the tank.

Images of the damage can be viewed on L2

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