GUCP troubleshooting continues as MMT push for launch on June 17

by Chris Bergin

In a similar event to STS-119’s scrub, Endeavour’s launch has been delayed due to a significant leak registered in the region of the Ground Umbilical Carrier Plate (GUCP). Engineers will dismantle the troublesome hardware at the pad, with the aim to attempt a single launch opportunity on June 17.

STS-127 Scrub/GUCP Leak:

Tanking had been proceeding normally, with good voltages recorded on the Engine Cut Off (ECO) sensors during their regular checks as the tank was loaded with LH2 and LOX.

However, leak detectors tripped around the GUCP area once tanking had reached a similar stage as observed with STS-119 – with the hydrogen tank nearing the 98 percent filled – when a leak was also detected in the hardware associated with GUCP.

The leak was well above both the Launch Commit Criteria (LCC) of 2 percent concentration, and even much higher than the 4 percent level – which is the point a scrub is the only option available to managers.

At the time of the initial leak detection, engineers called for the GUP vent valve to be cycled four times, in order to see if the hydrogen leak could be reduced to acceptable levels, as they did with STS-119. Following this process, and with no resolution to the leak measurements, the call was made to scrub the launch and detank the ET.

c2The forward plan is to dismantle the GUCP hardware once the tank is inert – a process that can take nearly a day.

Once the QD (Quick Disconnect) has been demated, engineers will be able to take a close look at both the alignment of the hardware, and the seal – with the latter a likely cause of the leak.

The Mission Management Team (MMT) will meet on Sunday to discuss launch options, which will include discussions over the status of the range – due to be handed over to the Atlas V/Lunar Reconnaissance Orbiter (LRO) launch, currently targeting the same date as the new STS-127 NET.

SUNDAY UPDATE: Engineers are now working on the tank, after the remaining LH2 had boiled off, making the tank inert and safe to work on. The plate will be unmated from the tank once the associated QD bolts and covers have been removed.

“GUCP ET disconnect complete, GH2 ventline disconnect complete, GUCP R&R in work,” noted status on Sunday, as engineers closed in on checking the health of the seal on the GUCP – which is a leading candidate as being the cause of the leak.

The MMT began their meeting at 2pm local time at the Kennedy Space Center, where managers decided to push for a single launch attempt on June 17, before standing down for the LRO launch to take over the range. Confirmation of the schedule will be finalized on Monday.

ET-131 GUCP Installation Issue:

A problem with the GUCP seal and alignment between the Quick Disconnect and the External Tank would still be surprising, following extensive checks – especially on Endeavour’s ET-131 tank – as part of the STS-119 investigation into root cause.

Indeed it was the rollout to Pad 39B as part of Endeavour’s STS-400 Launch On Need (LON) requirement that allowed engineers the opportunity to take a very close look at the GUCP mating process, required due to the lack of a GUCP test fixture at the Kennedy Space Center.

Part of the mitigation process mainly related to measurements, and included tape being added to the hardware to ensure the alignment of the hardware and the ET remained the same over the period the stack remained at the pad.

c4However, an issue was already noted on the STS-127 Agency Flight Readiness Review (FRR) presentation by Lockheed Martin/ET, relating to the installation of the GUCP hardware on Endeavour’s tank.

“ET-131 Ground Umbilical Carrier Plate (GUCP) Installation, Issue: Interference between GUCA (ground umbilical carrier assembly) and ET-131 right hand hinge support observed during mate GUCP (Ground Umbilical Carrier Plate) installation in VAB (Vehicle Assembly Building),” noted the FRR presentation, available on L2.

“Installation sequence verifies contact with hinge supports and no interference between pivot assembly and hinge bracket. Interference occurred on RH (Right Hand) side of RH pivot assembly to RH inside of bracket.

“Concern: Interference not allowed at hinge location per engineering. Potential to induce un-intended loading on pyro bolt assembly and potentially affect separation mechanism

“Actions Taken / Status: GUCP was removed and different unit installed and interference remained. Both GUCP assemblies measured and within engineering tolerance. Inspection performed at KSC to assist in investigation of root cause.

“Visual inspection indicates slight misalignment of carrier plate with panel cut-out. Laser measurements at KSC confirmed misalignment.”

This issue still shouldn’t be fully related to Friday’s scrub, after a good alignment was confirmed prior to the stack heading out to the pad. However, this tank – as a result of the aforementioned issue – was debuting a modification on the associated GUCP hardware.

c1“Pivot assembly modified (and successfully installed) by locally machining outboard surface (0.1” removed) to create required gap (0.035 gap provided). Stress analysis shows adequate FS for modified configuration (assuming single pivot assembly carries total reaction),” added the FRR presentation.

“Repair configuration does not impact GUCP release mechanism or sealing interface between flight and ground side. Installation sequence verifies proper alignment and rotational capability. Root cause investigation in-work – Most likely attributed to ETCA misalignment within panel cutout.

“Similar condition not observed at MAF (Michoud Assembly Facility) to date (ET-139 and ET-132 visual). Laser measurements being performed on all in-process tanks at MAF. Spare GUCP being used to support fit checks at MAF where possible (i.e. vertical orientation only).”

Interestingly, what appears to be a leak from the right hand pivot area of the assembly was captured on cameras monitoring the associated hardware prior to the scrub being called, although it is too early to know if this was the actual leaking event.

STS-119/ET-127 GUCP Overview:

The scrub related to STS-119’s ET-127 was only the second tanking that suffered a leak over the previous 31 loadings. The STS-127 leak is believed to have been larger than that seen during Discovery’s countdown.

c5“STS-119 / ET-127 Performance Summary: Pre-launch: 1st loading resulted in scrub/LCC (Launch Commit Criteria) violation due to GH2 leakage at GUCA (>40,000 ppm. Flight seal/QD (Quick Disconnect) replaced,” noted a post mission IFA (In Flight Anomaly) review presentation on L2, written by the MAF (Michoud Assembly Facility).

“Leakage occurred during transition from fast fill to topping. Vent valve opened when 98 percent level sensor indicated wet. Detected by leak detectors (LD 23 & 25) located in ground umbilical shroud. Isolates leak to either ground side quick disconnect (QD) or interface with flight seal.

“Contingency plans (vent valve cycling) unsuccessful in controlling leakage within acceptable limits. Launch scrubbed, flight seal/disconnect replaced. No GH2 leakage observed during subsequent loading. No other anomalous performance observed during loading.”

No root cause was confirmed, although the leading candidate has been noted on the recent documentation relating to the incident.

“No first-time hardware changes implemented for STS-119/ET-127 GH2 vent system. 31 previous loadings with only 1 leak observed (13,500 ppm). Previous leaks also observed during fast fill to topping transition,” added the MAF presentation.

c3“Most probable cause identified as momentary breach in flexible flight-seal to bellows probe due to ‘thermal shock’ of GH2/LH2 with vent valve in open position.

“Significant Disassembly Observations: Lower left pad was hard against skin. Other locations were not touching (0.014 – 0.030 gap / 0.001 requirement) indicating a pull downward and to the left. Peripheral seal compressed more on left side and toward bottom of GUCP.”

Interestingly, the pivot assembly – this time on the left hand side, as opposed to the right hand side with the aforementioned installation issues noted on the STS-127 FRR presentation – gains a mention in the STS-119 IFA review.

“Left side pivot assembly in hard contact with pivot pin (pin would not rotate). Stain observed on external surface of bellows guard and peripheral seal at 6 o’clock position. Flight-side seal asymmetrically compressed at 3, 7 and 8 o’clock positions.”

Although a root cause was not found, the leading candidates from the STS-119 event will provide helpful data to managers assessing the forward plan for STS-127’s next launch attempt – even if nothing obvious is found once the GUCP is demated.

Further updates will follow.

L2 members: Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.

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