NASA OIG: Final Memorandum on the Review of the Space Shuttle Liquid Hydrogen Fuel Tank Sensors


National Aeronautics and Space Administration
Office of Inspector General
Washington, DC 20546-0001
January 5, 2009

TO: Project Manager, External Tank Project Office

FROM: Assistant Inspector General for Auditing

SUBJECT: Final Memorandum on the Review of the Space Shuttle Liquid Hydrogen Fuel Tank Sensors (Report No. IG-09-009; Assignment No. S-08-011-00)

Since March 2006, Marshall Space Flight Center's (MSFC) Engineering Department has been performing assurance testing on the cryogenic liquid level sensors used in the Space Shuttle Program's liquid hydrogen (LH2) fuel tank. The MSFC Engineering Department screens the sensors after the supplier delivers them to the prime contractor and after the contractor completes normal acceptance testing on the sensors. The MSFC Engineering Department found, as a result of their screening, that a number of LH2 sensors did not conform to the required design specifications, even though the sensors had passed the contractor's acceptance testing.

The overall objective of this review was to determine whether the Space Shuttle Program appropriately accounted for, used, or disposed of nonconforming LH2 sensors. We focused our review on actions taken by the External Tank Project Office (Project Office) after it identified nonconforming LH2 sensors. In addition, we reviewed the results of the LH2 sensor screening and ranking performed by the MSFC Engineering Department's Electrical, Electronic, and Electromechanical Parts Analysis Team, using the NanoFocus Lab,1 and the LH2 sensor inventory records maintained by the contractor. (See Enclosure 1 for details on the review's scope and methodology.)

Executive Summary

We found that the Project Office initiated appropriate actions to identify and resolve LH2 sensor nonconformance issues by reviewing manufacturing and testing processes. As a result, the Project Office recommended that modifications be made to the supplier's manufacturing process and to the contractor's acceptance testing processes. In addition, we found that because of the Project Office's review, the contractor implemented a detailed inventory control measure that segregated the sensors into two inventories--Flight Ready inventory and 74L4-2 Parts inventory.

The 74L4-2 Parts inventory contains sensors not yet screened by the MSFC Engineering Department, sensors determined to be low ranking, damaged, questionable, and certain other sensors. To mitigate the risks inherent in using nonconforming sensors, the contractor labeled all the pre-process improvement LH2 sensors as nonconforming parts to ensure that questionable sensors were not used inadvertently in a mission critical position within the LH2 fuel tank. In addition, all LH2 sensors the contractor receives from the supplier are not considered flight ready until the MSFC Engineering Department uses the NanoFocus Lab to screen them. After the sensors pass the screening, the contractor assigns a new part number, which indicates they are flight ready. However, the contractor has not screened all the LH2 sensors and has continued to maintain low ranking, damaged, and questionable sensors in the 74L4-2 Parts inventory.

Although the Project Office and the contractor took appropriate actions to resolve quality- and inventory-control issues with nonconforming LH2 sensors, the 74L4-2 Parts inventory still contains sensors that have not been screened using the NanoFocus Lab, thus their usability is unknown, and also contains sensors that the Project Office does not intend to ever use. Maintaining parts in inventory that are not suitable or intended for use introduces unnecessary risk. In our November 4, 2008, draft of this memorandum, we recommended that the Project Office determine the usability of all remaining LH2 sensors labeled as nonconforming parts and dispose of the sensors not intended for use. In commenting on the draft of this memorandum (see Enclosure 2), the Program Manager, Space Shuttle Program, generally concurred with our recommendations and will transfer the nonconforming, pre-process improvement LH2 sensors, which are not intended for use, to MSFC to be held in bonded storage. In addition, post-process improvement sensors held in the 74L4-2 Parts inventory will be evaluated to determine their final disposition. Management's comments on the draft of this memorandum are responsive; therefore, we consider the recommendations resolved and will close them upon completion and verification of management's corrective action.

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