Discovery rollover Sunday – STS-120 launch could avoid delay

by Chris Bergin

Discovery will rollover on Sunday morning, following the completion of repair efforts on her right main landing gear strut – well ahead of schedule.

A superb effort by engineers has seen the completion of the changeout of the seals on the strut, with a decision on how much re-testing is required to determine the exact rollover date. At present it appears NASA will be able continue to target October 23 as the launch date, although no contingency days will remain.

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STS-120/Discovery status

The issue was spotted during Center of Gravity checks on Discovery inside her OPF-3 (Orbiter Processing Facility) last Thursday. A decision was made – following a number of cycles to try and seal the leak – to replace the seals.

‘Leakage occurred on 13th of September when vehicle assumed weight on wheels (WOW) for weight and C.G. measurements. First time vehicle has been with WOW and pressurized strut since STS-116,’ noted an overview of the issue.

‘Strut pressurized to 453 psi for flight about 1 week ago. Installed seals are ~20 to 24 years old. Installed seals in hydraulic applications (wetted) are good for 27 years. Strut fluid leaking from the RH MLG (Right Hand Main Landing Gear) Strut observed. Leak rate was 285 drops/hr – allowable is 1 drop per hour.’

A repair was required, due to the hazard it may have caused during Discovery’s landing at the end of her STS-120 mission, with ‘excessive leakage causing decreased/no damping ability of the shock strut during landing (Crit 1/1 failure mode),’ noted as the problem.

The cause of the leak appears to have been debris on the surface of one of the seals, as opposed to the seals themselves. Regardless, the seals have been removed and sent off for closer inspections. New seals were placed into the strut this morning.

The process of removing the strut to get to the seals for replacement was no easy task. However, United Space Alliance engineers, related shops, and technicians – ably assisted by a team from Goodrich – managed to complete the process in one morning, as opposed to a number of days as predicted.

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Once the strut was removed from the body of the main gear, the only issue spotted on the seals was a very small particle on one of the four seals that have since been replaced. The strut is now being put back together on the MLG.

This will see the continued hydraulic and pneumatic servicing of the strut, followed by brake installation, wheels, brake hydraulics and instrumentation being connected back into place.

‘The replacement strut seals were delivered to the OPF. The vendor and the strut changeout team got an early start this morning 9/19. We got the RH MLG Strut depressurized and deserviced. Inspected the old seals which I was told from initial inspection were in good shape, however, they have been sent to the M&P folks for further inspection and closer scrutiny,’ noted processing information.

‘We replaced those with new seals (the new package was dated 1987) approved by the vendor after inspection. The strut has been re-installed post seal R&R and we are currently doing strut linkage re-assembly to be followed by Hydraulic level servicing.’

The importance of completing the process ahead of schedule is apparent in memos that have been flying around shuttle managers today, with optimism of managing to complete rollover and mating of Discovery in the Vehicle Assembly Building (VAB) in time to give NASA an opportunity to launch STS-120 on schedule.

‘Orbiter rollover from the OPF to the VAB for stacking could occur as early as September 24, provided the strut work (including retest) proceeds nominally,’ noted an earlier memo.

‘Based on OPF rollover on September 24, an October 23 launch date is still feasible, although there are no contingency days remaining in the schedule. Everyone needs to continue to work towards their current schedules for an October 23 launch.’

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The optimism based on the ever-improving repair schedule even noted that rollover could occur as early as Sunday, saving another day on the flow. 

‘Looks like they are well ahead of schedule and what remains now is leak checks, brakes, wheel, instrumentation re-assembly,’ added another memo around midday on Wednesday.

‘There will be one issue remaining is whether the program determines if we need to do a full Hydraulic retest, and that will be discussed later today. This will require vehicle power and access to the aft compartment and more time.

‘However, with the schedule of events going smoother than expected and if full hydraulic retesting is not needed, we could see the OPF to VAB rollover sooner. The vertical flow will now face a challenge (with no contingency days) if the 10/23 launch date can still be accomplished.’

Further news will follow after the decision by JSC Engineering on the re-test is taken, along with a possible solid rollover date. This decision may be taken by Thursday morning’s Shuttle Standup meeting.

UPDATE: Sunday rollover confirmed.

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