1、Lessons Learned Entry: 0487Lesson Info:a71 Lesson Number: 0487a71 Lesson Date: 1997-01-16a71 Submitting Organization: JPLa71 Submitted by: C. GuernseySubject: Long Term Materials Compatibility in Propulsion Systems (1993) Abstract: This lesson provides several examples of incompatible materials in p
2、ropulsion systems that posed a major risk to Cassini and Mars Global Surveyor, and may have led to the loss of Mars Observer. Propulsion system components should be tested for long term chemical compatibility with propellants. Heritage systems should be requalified for mission environments for which
3、 they were not originally intended.Description of Driving Event: Incompatible materials in propulsion systems have posed a major risk to recent missions:Tungsten Carbide Ball Material: During JPL qualification test of the original candidate Cassini pressure regulator, the tungsten carbide/nickel reg
4、ulator ball was tested for compatibility with nitrogen tetroxide (NTO) and monomethyl hydrazine (MMH). Exposure to the NTO oxidizer for 28 days caused substantial corrosion of the ball, and a surface roughness corresponding to a 42% increase in surface area. Another regulator design was therefore in
5、stalled on Cassini; use of the original design would have caused serious pressurant leakage and possible system contamination problems.Flow Restrictor Braze Material: The pressure sensing ports in the pressure regulators contain flow restrictors which allow the regulator to sense and respond to down
6、stream pressure changes. During the Cassini and MGS regulator heritage reviews, it was discovered that the MO restrictors contained a copper-bearing (AMS-4774) braze and a wax lubricant, neither of which are compatible with NTO propellant vapor. Neither of these materials were listed on the material
7、s list for the regulator. Long term exposure to this vapor could potentially produce enough corrosion products to plug the restrictor orifice, which would likely lead to an inability to sense the downstream pressure and failure Provided by IHSNot for ResaleNo reproduction or networking permitted wit
8、hout license from IHS-,-,-to regulate tank pressure. Such extensive exposure of the same regulator to propellant vapors would not normally occur on an Earth-orbital mission where pressurization is only required for a short period immediately following launch. These materials were eliminated in the M
9、GS and Cassini regulator design.One of the credible hypotheses which had earlier been posed for the loss of the MO spacecraft was the plugging of identical flow restrictors due to long term reactions of propellant vapors with an unspecified incompatible material.Teflon Swelling: When the Galileo spa
10、cecraft executed 3 large delta V maneuvers with the main 400 Newton engine, restricted helium flow through the check valves caused significant pressure drops in the propellant tanks. The pressurization system is designed to hold the tank pressures constant during propellant flow. Pressure regulator
11、performance was found to be nominal, but the flight propellant tank pressure data indicated significant degradation of both the fuel and oxidizer check valves. Ground tests have shown that swelling of the TFE Teflon valve bushing and PFA Teflon seal due to months of oxidizer exposure duplicates the
12、flight behavior of the oxidizer check valve.Additional Keyword(s): Heritage Hardware, Mars Observer (MO)Lesson(s) Learned: 1. Propulsion pressurization system components suitable for missions where the system is isolated after a few days may not be suitable for interplanetary missions.2. Swelling of
13、 non-metallic components upon exposure to propellant vapors can result in propulsion system malfunction even if no chemical incompatibility exists.Recommendation(s): 1. Propulsion system components should be tested for long term chemical compatibility with propellants. The test should be performed b
14、y JPL or a qualified contractor if the component manufacturer lacks experience with propellants.2. Heritage systems should be requalified to verify their functionality in mission environments for which they were not originally intended.Evidence of Recurrence Control Effectiveness: N/ADocuments Relat
15、ed to Lesson: N/AProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Mission Directorate(s): N/AAdditional Key Phrase(s): a71 Hardwarea71 Parts Materials & ProcessesAdditional Info: Approval Info: a71 Approval Date: 1997-01-24a71 Approval Name: Carol Dumaina71 Approval Organization: 125-204a71 Approval Phone Number: 818-354-8242Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
