1、Lessons Learned Entry: 1040Lesson Info:a71 Lesson Number: 1040a71 Lesson Date: 2001-08-22a71 Submitting Organization: WSTFa71 Submitted by: Jess WallerSubject: Dimensional stability of dissimilar materials in operational and non-operational environments where excessive or unintentional heating may o
2、ccur Description of Driving Event: In order to mitigate pilot seal extrusion-related failures of the Primary Reaction Control System (PRCS) pilot-operated valve (POV), effort were made to develop a redesigned POV (RPOV) pilot seat assembly. The POV controls the flow of hypergolic liquid propellants
3、to the Space Shuttle Orbiter attitude control thrusters. During the redesign, specific factors were found to be essential in fabricating a dimensionally stable pilot seal.Seal Extrusion. Seal extrusion (Figure 1) is caused by thermal expansion mismatch between adjacent PTFE and metal parts in the PO
4、V or RPOV, and is potentially aggravated by:a71 excessive internal stress or internal stress gradients in as-fabricated pilot sealsa71 low seal exit area relative to entrapped seal volumea71 thermal cycling during seat assembly (e.g., retainer welding)a71 vacuum bake-outs during periodic thruster ma
5、intenancea71 heat soak-back after thruster firing, especially multiple sequential burns or long burnsa71 any presence of oxidizer vapor close to a fuel pilot sealData on semi-trapped PTFE specimens simulating a POV seal configuration show that extrusion is:a71 incremental and irreversiblea71 increas
6、es with the size of the thermal excursiona71 decreases with successive thermal cycling (stress relief)a71 is accompanied by gap formation in the seal cavityThe above factors must be considered when developing a viable pilot seal.Provided by IHSNot for ResaleNo reproduction or networking permitted wi
7、thout license from IHS-,-,-refer to D descriptionDFigure 1. Impression replica profiles of an extruded Shuttle Orbiter PRCS POV pilot seal (left) and an unextruded, unused POV pilot seal (right) (63 ? magnification)Seal Recession. Seal recession (Figure 2) is characterized by a loss of seal proud he
8、ight around the circumference of the seal. Recession deprives the sealing interface of material thus leading to possible leakage. Recession arises from negative internal stress during differential thermal contraction of seal and surrounding metal. Although recession was observed after valve level ac
9、ceptance testing of RPOVs, it is conceivable that recession could also be caused by overheating during fabrication, or use of too small of a PTFE preform.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionDFigure 2. A sectioned RPO
10、V pilot seat assembly showing seal recession (within boxed region) after initial valve level acceptance testing. The seal has receded into the cavity by approximately 0.050 mm (0.002 in.).PTFE Susceptibility to Oxidizer. When exposed to oxidizer (dinitrogen tetroxide), PTFE undergoes significant yet
11、 reversible changes in mass, hardness, density, and crystallinity 1. The effect of fuel (monomethylhydrazine) on PTFE properties is negligible by comparison (Figure 3).Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionDFigure 3. M
12、ass uptake and desorption data for PFTE Teflon 7A exposed to liquid oxidizer (solid symbols) and fuel (open symbols).Reference:1. Pasternak, R. A., Christensen, M. V., Heller, J., Macromolecules, 3, 366 (1970).Lesson(s) Learned: The effect of operational and non-operational environments on seal dime
13、nsions must be anticipated. This is especially true for dissimilar materials subjected to excessive and unintentional sources of heat, or materials whose physical or mechanical properties may be altered, even reversibly, during service.Recommendation(s): Since the coefficients of thermal expansion o
14、f plastics and metals differ by an order of magnitude, preventing unwanted seal extrusion entails implementation of appropriate engineering rather than materials approaches. For example, more effective seal entrapment design strategies could be implemented (for example, use grooved or barbed retaine
15、r surfaces, or different seal cavity entrapment geometries). Alternatively, the time at temperature during the seals service lifetime could Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-be minimized. Unwanted seal recession could also be prevented
16、by optimal seal preform geometry and use of lower process temperatures.Evidence of Recurrence Control Effectiveness: N/ADocuments Related to Lesson: N/AMission Directorate(s): a71 Exploration Systemsa71 Aeronautics ResearchAdditional Key Phrase(s): a71 Hardwarea71 Parts Materials & Processesa71 Rese
17、arch & Developmenta71 Test & VerificationAdditional Info: Approval Info: a71 Approval Date: 2001-12-12a71 Approval Name: Jan Jungewaeltera71 Approval Organization: WSTFa71 Approval Phone Number: 505-524-5544Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
