REG NASA-LLIS-6036-2011 Lessons Learned - Ground Umbilical Carrier Assembly Gaseous Hydrogen Leak Investigation.pdf

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1、Public Lessons Learned Entry: 6036 Lesson Info: Lesson Number: 6036 Lesson Date: 2011-08-3 Submitting Organization: KSC Submitted by: Christopher Broadaway Subject: Ground Umbilical Carrier Assembly Gaseous Hydrogen Leak Investigation Abstract: A gaseous hydrogen (GH2) leak occurred on 03/11/09, dur

2、ing the STS-119/ET-127 cryogenic loading for flight, and was repaired with a fix to fly approach by replacing the flight seal and the Ground Umbilical Carrier Assembly (GUCA) quick disconnect (QD. There was no attempt to determine the cause of the leak before the repair. An anomaly resolution team w

3、as assembled after the launch to determine the long-term approach. In successfully mating the GUCA QD to the External Tank Carrier Assembly (ETCA), the following factors play key roles: Ground Umbilical Carrier Plate (GUPC) concentricity, QD probe/flange concentricity, two-piece seal, the GUCA QDs I

4、nconel bellows, vent line loads, alignment between the ETCA and the flight carrier plate, alignment of the hinge support brackets, and the pyrotechnic bolt. Of these eight factors, GUCP concentricity and QD probe/flange concentricity can be adjusted within limitations. Attached to this lesson is the

5、 GUCA GH2 Leak Investigation White Paper, which provides details of the investigation and process changes. Click to view white paper Description of Driving Event: On 03/11/09, GH2 leaked at the GUCA (greater than 40,000 ppm, calculated to 61,000 ppm) during the topping portion of loading resulted in

6、 a violation of Launch Commit Criteria (LCC) and ultimately, a scrub. IPR 119V-0070 (PR SS20-1-0106) was initiated. On 06/12/09, STS-127s first launch attempt was scrubbed because the GUCA was leaking approximately 61,000 ppm. The vent line was demated and the GUCA QD (s/n 9) was removed and sent ou

7、t for inspection and teardown. The inspection and the teardown of the QD did not reveal any anomalies. The GUCP (s/n 8) was realigned and the pivot assemblies were modified to allow for better alignment as follows: 0.100“ was shaved from the inboard side the left-hand pivot assembly foot, and a 0.02

8、5“ shim was installed on the outboard side of the right-hand pivot assembly (previously modified). The replacement GUCA QD (s/n 5) was installed (the QD was rebuilt according to the post-STS-119 inspections) and the vent line was connected. On 06/15/09, STS-127s second launch attempt was scrubbed be

9、cause the GUCA was leaking approximately 41,700 ppm. On 11/05/10, during reduced fast fill to topping of the LH2 tank, a leak exceeding 40,000 ppm was detected on leak detectors 23 and 25, and IPR 133V-0068 was initiated. The following was performed for the GH2 leaks for STS-119 and STS-127. A detai

10、led disassembly plan was developed for use in determining the root cause. Dimensions were recorded step by step, and measurements were taken with strain gages and LVDTs. Borescope inspections were performed and optical measurements were taken with a Faro arm to record configurations and changes duri

11、ng disassembly. As a result of the disassembly and knowledge gained from the STS-119 anomaly resolution teams investigation, the following enhancements were implemented: Used a concentricity tool to verify proper alignment of the GUCP. Used higher-tolerance alignment pins (0.485“ to 0.518“ diameter)

12、 to verify that concentricity is maintained. Manufactured customized pivot assembly feet to the ET-131 geometry. Installed washers on the pivot assembly pins to limit movement. Replaced the current one-piece flight seal with a two-piece seal (used previously on STS-116 and STS-117), which improves s

13、ystem compliance. The GUCA QD (s/n 5) was removed and inspected, with nominal results. The QDs probe height and concentricity were verified and found to be within the specifications and experience base. The one-piece flight seal was removed and sent to MAF for analysis. The results were similar to t

14、hose from previous analyses of leaking flight seals. It was decided to Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-install the two-piece flight seal that had been used on STS-116 and STS-117. The purpose was to improve the compliance in the GUCA

15、and vent line systems to the normal excursions experienced during processing and loading. The team developed a concentricity tool that attached to the bolt holes of the flight seal retainer before the flight seal and retainer are installed. The tool measures the GUCPs concentricity to the ETCA QD. I

16、n addition, higher-tolerance GUCP alignment pins (ranging in diameter from 0.485“ to 0.518“) were manufactured to replace the single-dimensioned pins (0.485“ diameter). The higher-tolerance pins helped verify that the GUCP-to-ETCA QD concentricity was maintained throughout the processing of the GUCA

17、. Use of the higher-tolerance alignment pins and the concentricity tool allowed the GUCP (s/n 8) to be realigned. Once the GUCP was aligned, neither the standard pivot assemblies nor the modified pivot assemblies could be used because of the positioning of the ETCA hinge brackets on ET-131. Therefor

18、e, custom pivot assembly feet were manufactured and washers were installed on the ETCA hinge brackets to limit the movement of the pivot assembly feet. The measurements of the GUCP verified concentric alignment, and GUCA QD (s/n 5) was reinstalled. Post-QD mate measurements of the GUCP verified that

19、 concentric alignment was maintained, and the vent line was connected. Investigating the GH2 leaks for the STS-133 mission, the team observed no anomalies in the vent line assembly but did observe that the GUCP had shifted since the final measurements taken before S0007. The plate was originally off

20、set (0.049“) to the 8 oclock position and had shifted toward the 7 to 8 oclock position. The first concentricity value after disassembly was 0.061“ to the 8 oclock position. After the GUCA QD was removed, measurement with a Faro arm revealed that the QDs probe had an offset of 0.024“ to the 6 oclock

21、 position as compared to the QD flange center. On the probe, a witness mark of the flight seal was observed. Two minor contamination marks were also observed outside of the sealing surface. Samples were taken for analysis, but the results were inconclusive. The GUCA QD (s/n 4) was then disassembled

22、and inspected but no anomalies were found. The flight seal underwent tactile and visual inspections by three independent, experienced individuals. The inspections found some circumferential deformation at the 7 to 9 oclock position. In addition, the gap of the seal edge to the retainer ring appeared

23、 smaller at the 7 to 9 oclock position. These observations reinforced the initial finding that the GUCP had shifted toward the 7 to 8 oclock position. GUCP (s/n 2) was removed and 3-D scanned. The results indicated no anomalies, and the plate met all drawing requirements. The 3-D data was used in so

24、me later modeling effort. The flight seal was removed and sent to a KSC lab for analysis (Ref. KSC-MSL-20 10-0349). Chemical and nondestructive examination revealed no anomalies that would have contributed to the leak. Magnified visual inspection of both pieces of the flight seal (PTFE and spring) r

25、evealed no anomalies. Chemical analysis of the wipes taken during tactile examination revealed unidentified organic materials. Debris analysis from the seal removal revealed a variety of small metallic and organic particles. Analysis of the swabs taken from the QD thin film found no organic constitu

26、ents. After a review of these results with the engineering team, none were determined to contribute to the leak. Dimensional analysis of the PTFE jacket found one minor dimension to be slightly undersized. Analysis of the metallic spring found its outer thickness to be slightly undersized. Additiona

27、l analysis determined this observation to be consistent around the spring, indicating that the spring was not out of round. The engineering team reviewed these results and determined all observations to be insignificant. The flight seal was then sent to MAF for further evaluation and possible destru

28、ctive evaluation but was later determined to be of little value since nothing was discovered at the KSC lab and the team had developed a leak mitigation strategy. The flight sealing surfaces on the ETCA were inspected and found to have no anomalies. A new two-piece flight seal was installed. The fin

29、dings from the investigations revealed that the GUCA QD probe offset needed to be considered when determining the optimum system concentricity. The initial STS-133 configuration that leaked during cryogenic loading had the GUCP offset to the 7 to 8 oclock direction and the GUCA QD offset to the 6 oc

30、lock position. This was compounded by the loads from the vent lines in the 7 to 8 oclock direction. These offsets by themselves in the GUCP and GUCA QD would not have been detrimental if the offsets had been smaller. With the initial GUCP offset of 0.049“ in the 8 oclock position and the GUCA QD of

31、0.024“ in the 6 oclock position, the resultant GUCA offset was 0.065“ in the 7:20 oclock direction. This resultant offset well exceeded the GUCP installation concentricity requirement of 0.050“. As stated, the load imposed by the vent line increased the misalignment. The decision was made not to use

32、 GUCP (s/n 2) because of its significant offset (0.049“) when mated to ET-137. GUCP (s/n 3) was selected because it had been fit-checked on ET-137 at MAF. The concentricity was 0.011“ to the 9:00 oclock position at MAF. The final installation concentricity for GUCP (s/n 3) at the Pad was 0.014“ to t

33、he 9:00 oclock position. Two spare GUCA QDs (s/n 7 and 8) were measured with the Faro arm, and s/n 8 was selected because of its probe offset of 0.025“ to the 9:00 oclock position. The GUCA QDs can be clocked during installation in 90-degree increments. Therefore, to achieve the optimum resultant co

34、ncentricity, GUCA QD (s/n 8) was clocked 180 degrees so that the QD offset was at the 3:00 oclock position. This yielded a final concentricity of 0.011“ to the 3:00 oclock position. An ETCA/GUCP mockup was tested offline in the VAB, using similar equipment and procedures, which validated the premise

35、 of clocking the GUCA QD to optimize its probe offset (Ref Space Shuttle External Tank Hydrogen Vent System: Ground Umbilical Carrier Plate to Hydrogen Quick Disconnect Alignment Test Plan, Rev A.; and KSC-TA-11539, ET Hydrogen Vent QD/GUCP Alignment Investigation Team Final Report). Provided by IHS

36、Not for ResaleNo reproduction or networking permitted without license from IHS-,-,-Lesson(s) Learned: The most important lesson learned from these GH2 leaks is the need to understand and control process creep. Near the end of the Shuttle Program and after Hurricane Katrina hit Michoud Assembly Facil

37、ity, KSC received several ETCAs whose configurations were out of tolerance. Some hinge brackets showed evidence of offset and slight clocking, and in some cases, the plate itself showed clocking. These minor deviations in assembly created enough of a stack-up problem to prevent the GUCP from trackin

38、g with the ETCA. Even though master tooling was used to manufacture the ETCA components, slight variations (which could have been avoided) were allowed to be introduced because the components were installed without the use of any indexing tooling. Another important lesson is the possibility of unint

39、ended consequences when design changes are implemented. The CRES 321 bellows was replaced with the Inconel 718 bellows to eliminate a fatigue problem and to reduce the number of parts and the refurbishment time. But the Inconel bellows was less able to track with the out-of-tolerance ETCA component

40、installations. This change also added an unknown variable that was discovered during the investigation. Each bellows seemed to take a set during the QD assembly operation (minor offset from the flange to the probe). When the QD was stacked up with the out-of-tolerance ETCA and GUCP offset (misalignm

41、ent), the cumulative concentricity differences among the components prevented the assembly from sealing at cryogenic temperatures. Another lesson is the importance of concentricity between the sealing surface and the bellows probe, as well as the rest of the assembly that could have an effect on it.

42、 The closer the concentricity offset between the sealing surface and the bellows probe is to 0, the more able the system will be to comply with (accommodate) external forces (e.g., vent line loads, wind, weather, and cryogenic loading). This understanding was instrumental in resolving the problem of

43、 the out-of-tolerance ETCA installations since the flight side could not be modified. For a critical fluid interface, the design should be such that the system excursions are experienced away from the sealing surface. The GUCA was designed to pivot about the pyrotechnic bolt, which caused the sealin

44、g surface to be dynamic. The sealing interface should have been maintained by solidly attaching the GUCP to the ETCA and allowing the flexhose to handle all the system excursions. In addition, the sealing interface was a single seal with no secondary sealing capability. A redundant seal configuratio

45、n would have increased the systems compliance to excursions. Also by having a primary and a secondary seal, along with a leak check port, a better ambient leak check could have been performed. The leak check port would have greatly increased the fidelity (smaller leak check volume) and would have al

46、lowed for the use of a helium mass spectrometer, which can detect small leaks. On all GH2 leak anomalies, an ambient leak check of the system was performed, and no leakage was observed until cryogenic loading on launch day. The ambient leak check could verify only that the system didnt exhibit extre

47、me misalignmentit wasnt a good metric to determine if the installation was acceptable for cryogenic loading. Recommendation(s): 1. Thoroughly examine all stages in manufacturing and installation to identify where fidelity may be lost and make corrections. 2. Thoroughly evaluate design changes not on

48、ly for their intended purposes but also to identify unintended effects. 3. Design assemblies to achieve maximum concentricity and maintain seals through the use of solid attachments, redundant seals, and leak check ports. Evidence of Recurrence Control Effectiveness: N/A Documents Related to Lesson:

49、 GUCA GH2 Leak Investigation White Paper Click to view white paper Mission Directorate(s): Space Operations Additional Key Phrase(s): Manufacturing and Assembly Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Engineering Design (Phase C/D).Launch Systems Systems Engineering and Analysis.Planning of requirements verification processes Systems Engineering and Analysis.Engineering design and project processes and standards Pr