1、Lessons Learned Entry: 1487Lesson Info:a71 Lesson Number: 1487a71 Lesson Date: 2004-07-20a71 Submitting Organization: LARCa71 Submitted by: Charles E. HarrisSubject: Need for Ground Test Facilities Which Simulate the Flight Environment Abstract: One of the most difficult obstacles to overcome in res
2、olving the problem of the cracks found in the flowliners at the gimbal joint of the LH2 feedlines of the Orbiter fleet has been the disparity of the propellant feed systems between the ground-based test facility and the flight vehicle. Ground testing should be conducted in as close to flight configu
3、ration as possible. Where differences between ground and flight configurations or environments are necessary, every effort to correlate the ground test data to actual flight situations must be made. Description of Driving Event: In May of 2002, three cracks were found in the downstream flowliner at
4、the gimbal joint in the LH2 feedline at the interface with the Low Pressure Fuel Turbopump (LPFP) of Space Shuttle Main Engine (SSME) #1 of orbiter OV-104. Subsequent inspections of the feedline flowliners in the other orbiters revealed the existence of 8 additional cracks. No cracks were found in t
5、he LO2 feedline flowliners. A solution to the cracking problem was developed and implemented on all orbiters. The solution included weld repair of all detectable cracks and the polishing of all slot edges to remove manufacturing discrepancies that could initiate new cracks. Using the results of a fr
6、acture mechanics analysis with a scatter factor of 4 on the predicted fatigue life, the orbiters were cleared for return to flight with a one-flight rationale requiring inspections after each flight. OV-104 flew mission STS-112 and OV-105 flew mission STS-113. The post-flight inspections did not fin
7、d any cracks in the repaired flowliners. Even though the flowliner repair solution appeared to be successful, the NASA and contractor engineering team continued to investigate the problem. This continuing investigation was motivated by the fact that the actual cause of the original cracks had not be
8、en conclusively established. As part of the continuing investigation, two engine hot fire test series were conducted at Stennis Space Center to characterize the LH2 feedline flow physics and to measure the flowliner vibratory Provided by IHSNot for ResaleNo reproduction or networking permitted witho
9、ut license from IHS-,-,-response. The test results were somewhat alarming because the vibratory strains recorded by strain gages mounted directly to the upstream and downstream flowliners were considerably higher than anticipated. The strain gage data were used conservatively to develop a loading sp
10、ectrum to represent the ?worst-case? nominal flight. An updated damage tolerance analysis of the flowliner was then conducted by the Boeing Company based on the inspection crack detection limit of 0.075 inches adopted in 2002. The predicted values of residual fatigue life were only 0.8 missions for
11、a circumferential crack in the upstream flowliner and 1.4 flights for the downstream flowliner. These new results cast doubt on the validity of the flight rationale developed in 2002. Subsequently, the Orbiter Program Office sponsored an extensive effort to resolve this problem. Lesson(s) Learned: O
12、ne of the most difficult obstacles to overcome in the flowliner investigation has been the disparity of the propellant feed systems between the ground-based test facility and the flight vehicle. NASA does not have a Flight-Configured Main Propulsion System (MPS) Ground Test Facility that accurately
13、simulates the flight environment. Differences between the ground-based test facility and the flight vehicle can singularly and collectively degrade the fidelity of ground-based hot fire tests in characterizing the flight environment. Examples encountered during the course of this investigation inclu
14、de the following: a71 Differences in the feedline weld bead in feedline between flowliners and SSME interface;a71 Differences in thermal insulation (i.e., vacuum jacket vs. BX-250 SOFI) on the feedlines;a71 Differences in feedline layout (i.e., line lengths, bends, flex joints, valves, flow meters);
15、 anda71 Differences in sensors and sensor locations.The inability to accurately replicate the flight conditions of the MPS in a ground test has been a persistent difficulty in determining the fundamental root cause of the flowliner issue. Significant resources and schedule time have been expended to
16、 overcome the need for a test facility that replicates the collective environment (e.g., flow, structural, dynamic, thermal, etc.) of the actual flight vehicle. A facility dedicated to subjecting the engine and supporting feed system to a high-fidelity flight environment could be used for acceptance
17、 tests, green run tests, and fleet leader extension tests.The MPTA was useful in certifying the MPS for the shuttle, but was retired early in the program. It would not be necessary to configure the system to accommodate all the engines used in the MPS, but to replicate the fluid environment that one
18、 engine would be exposed to, with the flexibility of expanding the resolution of the system environment to include as many engines as needed. With regard to the MPS environment of subsequent vehicle programs, the instrumentation suite incorporated into the ground facility could be integrated into th
19、e propellant system without significant affect to the characterization of the system environment. Recommendation(s): Since NASA develops and operates unique, few-of-a-kind space access vehicles, the adequacy of test Provided by IHSNot for ResaleNo reproduction or networking permitted without license
20、 from IHS-,-,-facilities to support the entire vehicle life cycle including development, qualification testing, and operations must be addressed at the beginning of the vehicle development project planning. Evidence of Recurrence Control Effectiveness: N/ADocuments Related to Lesson: 1. NASA Project
21、 and Program Plans, NPG 7120.5BMission Directorate(s): a71 Exploration Systemsa71 Aeronautics Researcha71 Aeronautics Researcha71 Aeronautics ResearchAdditional Key Phrase(s): a71 Program and Project ManagementAdditional Info: Approval Info: a71 Approval Date: 2004-09-22a71 Approval Name: Leslie Johnsona71 Approval Organization: LARCa71 Approval Phone Number: 757-864-9409Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
