Leak on Endeavour’s Main Landing Gear requires repair

by Chris Bergin

Reminiscent of a leak on Discovery’s landing gear ahead of STS-120, a diaper has been placed around the Left Hand Main Landing Gear (LH MLG) of Endeavour, after it was observed to be leaking fluid.

Endeavour is tasked with the roles of the Launch On Need (LON) rescue orbiter for STS-122, while she prepares for her primary mission of STS-123 – currently scheduled to launch in February.

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The gear on Endeavour had been retracted over the weekend, as her processing flow inside OPF-2 (Orbiter Processing Facility) continues to push towards rollover next month (January 9). The bulk of work has been completed on the orbiter, with forward, midbody and aft closeouts on schedule ahead of the holidays.

As part of the closeout procedures, Endeavour’s gear was lowered, at which point engineers noticed that the LH MLG was leaking fluid around the gland nut area. Such was the rate of leaking, a puddle had formed on the gear, in the scissors of the strut, and down the side of the MLG door.

Ironically – and fortunately – a couple of specialist engineers were already on site, completing work on Discovery’s landing gear. ‘R&R’d main landing gear strut seals on weekend. The LH seal was upside down. Doing a hydraulic service check and leak check on those struts to be in position to complete testing,’ confirmed processing information.

The engineers – from Goodrich – immediately wrapped a diaper around the leaking strut, and are currently evaluating the leak rates in both the vertical and horizontal positions.

Managers met to discuss the forward plan, with the consensus pointing towards a full R&R of the seals in the strut, rather than leaving as-is with flight rationale. As of Wednesday, both LH and RH struts will be R&R’ed. All the wheels have been removed ahead of the effort.

The Orbiter landing gear systems use an oleo-pneumatic shock absorber strut design to provide landing load attenuation. Strut design consists of a piston/axle assembly and outer housing filled with hydraulic oil on top of nitrogen gas, separated by a floating internal piston.

The system has performed without any major issue since STS-1, though the shock strut static and dynamic seals – which prevent external leakage of hydraulic oil – are now starting to come into focus.

The incident with Endeavour mirrors a similar leak on Discovery’s Right Hand MLG strut, which slightly delayed her rollover ahead of STS-120. A United Space Alliance (USA) investigation into Discovery’s leak discovered the RH MLG strut dynamic seal assembly was not installed ‘per print’ configuration, and hadn’t been for 27 missions.

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‘Potential for strut hydraulic fluid leak past strut seal. Excessive leakage could cause decrease/loss of damping ability of the shock strut during landing (Crit 1/1 failure mode),’ noted the presentation associated with the investigation, explaining the recent R&R work inside the OFP.

‘Left hand (LH) MLG strut seal on OV-103 (Discovery) also suspect as it was installed at the same time period as the RH (Right Hand) side.’

It was the fine work by the Goodrich and USA engineers that enabled the changeout of the suspect seals on Discovery to take place without a major delay to STS-120’s rollover. That experience is likely to prevent any delay to Endeavour – though the actual rollover will be dependant on the ongoing STS-122 troubleshooting.

It was also Goodrich engineers that spotted the incorrect installation of the seals, after the R&R effort on Discovery’s leaky strut. The fact that it was only spotted recently has a lot to do with the complexity of the system, which uses both static and dynamic seals to prevent the external leakage of hydraulic fluid.

The Dynamic seal assembly consists of to Teflon rings and a Buna-N O-ring, housed in the lower bearing assy ID, which seals against the sliding main piston/axle assembly.

The wedge shaped outer ring provides added forces on Teflon foot seal during gear touch down pressure spike, while the Buna-N O-ring acts as a static seal and spring, to maintain the Teflon foot seal against the piston.

The Static seal is housed in the lower bearing assy OD, sealing against the fixed outer cylinder ID. Middle & bottom – spare seals provide field R&R capability.

Regardless of the incorrect installation of the seal, the orbiters weren’t under any threat of a problem at landing, following the findings of the investigation.

‘MLG technical community assessment (NASA, contractor and suppliers) concluded that the dynamic seal assembly will function as intended even without Teflon wedge ring in the proper location,’ noted the presentation.

‘Seal is contained in groove – dynamic hydraulic pressure spike at MLG touch-down forces Buna-N O-ring and primary Teflon seal against shock strut. Wedge provides additional sealing capability for high pressure spike. No expectation of gross seal failure based on failure analysis findings.’

What will be of interest to engineers is the potential of any commonality with the leak currently observed on Endeavour, given the incorrect installation findings only referred to Discovery. This information may be forthcoming via the R&R effort on Endeavour this week.

Another note relating to Endeavour’s landing gear was mentioned in processing information this week, this time with her nose gear. This will also be repaired this week, as processing continues along the ‘green’ timeline for STS-123 and LON support of Atlantis.

‘Were trying to do landing gear functional on weekend, but had a cracked tile at nose landing gear which must be repaired first. Want to do integrated hydraulic operations Friday and then start landing gear functional,’ added processing information, before rounding up other key elements of the march towards launch.

‘Have 20 tile cavities left, which is very manageable between now and rollout. SRB stacking was completed, are just doing closeouts. On ET, all four feedline brackets have been sprayed. All work on this ET is on schedule (pending associated troubleshooting relating to STS-122’s ET). Will standby to see what work is needed on ET.’

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