1、Lessons Learned Entry: 1416Lesson Info:a71 Lesson Number: 1416a71 Lesson Date: 2004-05-10a71 Submitting Organization: JPLa71 Submitted by: David OberhettingerSubject: Cassini RWA Flight Anomaly Due to Extended Use at Slow Speed (2001) Abstract: Three years after launch, higher-than-normal drag was d
2、etected on one of the Cassini reaction wheels. The likely cause was lubricant dispersal after prolonged operation of the RWA at a very slow speed. Treat reaction wheels as a consumable resource, and minimize the time they are operated at very slow speeds.Description of Driving Event: Three years aft
3、er the launch of Cassini, a higher-than-normal drag was detected on one of the reaction wheels employed in rotating the spacecraft. The excessive friction at one reaction wheel likely resulted from a reduction in lubrication after prolonged operation at a slow speed approaching zero rpm (Reference (
4、1). Excessive drag torque resulting from the frequent speed oscillations around zero rpm may also have contributed to the anomaly (Reference (2). The problem caused a 9-day suspension of scientific observations and an extra workload to prevent recurrence of the conditions. Operating the wheel at hig
5、her speeds apparently restored the proper dispersal of the lubricant and eliminated the drag. The Cassini flight team also established procedures for avoiding prolonged operation of the reaction wheels at relatively slow speeds. Bearing and lubrication systems are the primary determiners of reaction
6、 wheel assembly (RWA) life. It is difficult to analytically predict the failure of bearings in space mechanisms from lubricant starvation, and life testing provides the only accurate predictions. However, accelerated life testing does not accurately model the effects of bearing speed and temperature
7、 on bearing life. Hence, reliability predictions are commonly based on past flight experience with similar RWAs.References: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-1. JPL Incident Surprise Anomaly (ISA) No. Z70706, January 4, 2001.2. JPL Inci
8、dent Surprise Anomaly (ISA) No. Z70716, January 8, 2001.3. A. Lee, B. Shogrin, J. South, “Cassini RWA Operations Analysis and Recommendations,” JPL Presentation, March 1, 2001.Additional Key Words: bearing failure, lubricant failure, momentum wheels, SBA, excessive torque, torque instability, lubric
9、ant loss, tribology, low-RPM dwell, prolonged drag torque, COBE RWA Lesson(s) Learned: Maintaining RWAs at absolute zero speed (no motion) during cruise, with periodic exercises every 3 months, is benign. But dwell time inside the low-rpm region may contribute to failure mechanisms such as bearing “
10、blocking,” cage windup, lubricant dispersal, or lubricant polymerization. Operating RWAs in a mode that was not tested, may be viewed as a violation of test-as-you-fly and fly-as you-test.Recommendation(s): 1. Over the course of a mission, treat reaction wheels as a consumable resource:a. Develop an
11、 operations plan that reserves the RWA consumables (i.e., cumulative revolutions and cumulative low-rpm time) for the primary mission. Prior to the start of Approach science, consider the use of thrusters to control the spacecraft attitude, with RWAs used very selectively.b. Support implementation o
12、f the operations plan by performing periodic RWA drag torque characterization, and use a ground-based software tool to automatically detect occurrences of prolonged RWA drag torque settling time events.2. Bias RWA wheel speeds to eliminate frequent speed oscillations around zero rpm about zero rpm,
13、and to minimize time spent in the forbidden, low-speed range.Evidence of Recurrence Control Effectiveness: Corrective Action Notice No. Z84127 was opened by JPL on June 16, 2004 to initiate and document appropriate Laboratory-wide corrective action on the above recommendations.Documents Related to L
14、esson: JPL Procedure: JPL Document 31912, March 05, 1999. NPR 7120.5B, NASA Program and Project Management Processes and Requirements, November 21, 2002.Mission Directorate(s): Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-a71 Exploration Systemsa7
15、1 Sciencea71 Aeronautics ResearchAdditional Key Phrase(s): a71 Flight Equipmenta71 Flight Operationsa71 Ground Equipmenta71 Hardwarea71 Payloadsa71 Policy & Planninga71 Risk Management/Assessmenta71 Safety & Mission Assurancea71 Softwarea71 SpacecraftAdditional Info: Approval Info: a71 Approval Date: 2004-05-28a71 Approval Name: Carol Dumaina71 Approval Organization: JPLa71 Approval Phone Number: 818-354-8242Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
