Interesting FRR for STS-118 upcoming – main topic isn’t the O-rings

by Chris Bergin

NASA is conducting an agency-wide effort to check into issues relating to the LO2 level sensors in ET-120 – the External Tank required for LON (Launch On Need) support of STS-118.

While another issue – relating to the O-rings on the twin SRBs (Solid Rocket Boosters) – is not deemed as a constraint to launch, NASA managers are evaluating all outstanding issues ahead of the upcoming Flight Readiness Review (FRR).

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The sensor issue relates to observed ‘high resistance’ in the LO2 sensors inside the LOX tank of ET-120. These sensors check the levels of propellant inside the External Tanks during tanking, and as the tank empties during ascent.

Sensors – in both the LH2 and LOX tanks – use ‘resistance’ to record if they are ‘wet’ or ‘dry’ – sending information to launch controllers like a fuel gauge. High resistance in the sensors may inhibit their ability to give an accurate reading, primarily during pre-launch tanking.

The issue appears to be similar to what was observed with the LH2 ECO (Engine Cut Off) sensors during tanking on the Return To Flight (RTF) missions, which ultimately led to the changeout of what was classed as a ‘bad batch’ from all future tanks. It was a resistance issue with LH2 sensors in STS-121’s tank that lead to the mission being delayed – along with the replacement of the sensors.

Given there wasn’t any recorded problems with the sensors in the LOX tank during the sensor issues of the past, the LO2 sensors weren’t removed and replaced. However, it now appears that a similar problem has been spotted, late in the processing of ET-120, although evaluations are continuing.

‘Working two issues: impacts to external surface (repairs in place) and two liquid level sensors in LO2 tank showing high resistance (double checking this),’ noted processing information. ‘Team isolated sensor issue to specific section of hardware.’

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ET-120, which was due to ship at the end of last week – no confirmation of shipping has been forthcoming at this time – is required at the Kennedy Space Center (KSC) by the end of this week, in order to support STS-118.

ET-120 will fly with Discovery on her primary mission of STS-120 in October, but it is also on LON standby support of Endeavour – and is required to be ready within a tight processing schedule, in case Discovery needs to mount a rescue mission.

‘Entire Agency engaged in LO2 sensors,’ noted the latest integration report, noting the importance of working rationale on the tank. ‘Appreciate everyone’s help on this. Objective is to minimize impact to the tank.’

It’s not known if this effort includes working on potential commonality issues with Endeavour’s tank (ET-117), although no issues were noted during tank processing, and any future issues would show up during tanking. There would be no risk of Endeavour launching with faulty liquid level sensors.

In reference to ET-120, it appears NASA will carry out electrical tests on the tank once it arrives in the checkout cell inside the Vehicle Assembly Building (VAB). This may be used to increase rationale for using the tank ‘as-is’. However, with the FRR due on Wednesday and Thursday of this week, engineering discussion is likely raise the issue of ET-120 being able to support LON, before following any further evaluations.

‘Sensor circuits on LO2 tanks had resistance shifts after MAF (Michoud Assembly Facility) went through checks; folks across SSP (Space Shuttle Program) engaged in this,’ added processing information.

‘If these two circuits aren’t reusable, will need to determine fault tolerance for loading and operational workarounds. Determining whether circuits would still work if had resistance shifts. If don’t find problem, circuits may still work at pad.’

ET-120 is one of the tanks that was sent back to MAF for RTF modifications following the loss of Columbia. A set of two periods of extensive modifications saw the tank endure processing delays, past its original shipping date of May. However, engineers at the New Orleans plant managed to edge back the shipping date, buying back some of the processing schedule for LON support.

Meanwhile, the O-rings on the SRBs for STS-118 have been the subject of media attention over the past couple of days, although this is not deemed a major topic of engineering discussion at this time. The seals on the boosters hardly gained a mention in the latest SSP processing overview.

‘Testing for O-ring resiliency is ongoing and will be completed this weekend,’ was one of the few mentions in processing information, and is not deemed a big issue by engineers at KSC. Further reports will follow if that status changes.

Pre-FRR engineering evaluations mainly relate to other areas of the shuttle, such as the analyzing of a cold plate ding, a hydrogen recirculation pump issue, and ‘a gap between two nose landing gear door tiles was a bit less than should have been,’ which all appear to be minor issues.

A fuel cell pump epoxy crack issue is also being evaluated, among other items of interest, which will likely go past the FRR into the L-2 day Mission Management Team (MMT) meeting.

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