1、Lessons Learned Entry: 1598Lesson Info:a71 Lesson Number: 1598a71 Lesson Date: 2005-06-27a71 Submitting Organization: JPLa71 Submitted by: David Oberhettingera71 Authored by: Allan LeeSubject: Manage Reaction Wheels as a Limited Spacecraft Resource (2002) Abstract: After two and one-half years of op
2、erational use, a bearing cage instability trend developed in a bearing in one of three Cassini reaction wheels. JPL responded to the indication of life-limiting wear through steps to manage RWA use, including tracking RWA performance, limiting RWA usage, using a software tool to manage reaction whee
3、l biasing events, and providing a reaction wheel drag torque estimator to identify anomalous bearing drag conditions. Description of Driving Event: Attitude control for the Cassini spacecraft, launched in October 1997, is accomplished during the Tour phase of the mission with a set of three reaction
4、 wheels (RWAs) (with a fourth provided as a backup). During most of the inner and outer Cruise phases of the mission, spacecraft attitude was controlled by reaction control thrusters. Beginning in October 2002, after two and one-half years of operation, a bearing cage instability trend developed in
5、at least one of the two bearings in Cassini reaction wheel assembly No. 3 (RWA-3) (Reference (1). The Cassini reaction wheels were first used for spacecraft attitude control in March 2000; after the trend was identified, they were used only when needed for precise and stable science-pointing. A hori
6、zontal bar illustrates the timeline described in the first two paragraphs of the Description of Driving Event. It begins with the October 1997 Cassini launch, and depicts milestones for the March 2000 initiation of RWA operational use and the October 2002 beginning of RWA-3 instability, and the time
7、line ends with the July 2003 replacement of RWA-3 with a backup. Cassini Bearing Cage Instability Timeline Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-The problem was detected because Cassini reaction wheel performance has been continuously track
8、ed via in-flight drag torque characterizations. When the data showed a 9-month period of intermittent RWA-3 bearing cage instability, this reaction wheel was replaced with the backup (RWA-4) in July 2003. Although the performance of the remaining three operational reaction wheels has been nominal, t
9、heir identical bearing and lubricant design suggests that they too could be subject to the cage instability observed on RWA-3. Hence, the Cassini project has taken the following steps to control reaction wheel performance degradation: 1. Kept RWA use to an absolute minimum during the outer Solar cru
10、ise phase to ensure RWA availability for science operations during the primary mission. 2. Uses a JPL-developed Reaction Wheel Bias Optimization Tool (RBOT) to analyze optimal RWA bias rates so that the total RWA dwell time inside the problematic low-rpm region can be minimized. The deleterious effe
11、cts of low-rpm operation are discussed in References (2) and (3). 3. Continues to monitor and trend the performance of the reaction wheels (including RWA-3, which now serves as the backup). Bearing cage instability is typically heralded by audible noise, a significant spontaneous increase in the bea
12、ring drag torque, and excessive drag torque “roughness.” The failure mechanism promotes energetic vibration of the retaining cage that leads to localized heating and polymerization of bearing lubricant (i.e., “gelled” oil) and, ultimately, premature bearing failure (Reference (3). As bearing cage in
13、stability in spacecraft reaction wheels is not uncommon (Reference (4), it is likely that similar persistent problems may be experienced by future missions that use reaction wheels for attitude control. References 1. JPL Incident Surprise Anomaly No. Z78007 (Cassini), October 22, 2002. 2. “Cassini R
14、WA Flight Anomaly Due to Extended Use at Slow Speed,” Lesson Learned No. 1416, May 28, 2004. 3. Allan Lee, Gene Hanover, & Sam Sarani, “Cassini RWA Status at Launch +8 Years,” Presentation to JPL SCO AACS Team, July 2004. 4. Tungseth, A.E., “DSP Wheel Recent Experience,” Proceedings of the 17th Annu
15、al AAS Rocky Mountain Guidance and Control Conference, Keystone, Colorado, February 2-6, 1994, pp. 593-606. Additional Key Words: bearing failure, momentum wheels, excessive torque, torque instability, lubricant loss, tribology Lesson(s) Learned: Provided by IHSNot for ResaleNo reproduction or netwo
16、rking permitted without license from IHS-,-,-As complex mechanisms critical to spacecraft operations, reaction wheels should be treated as a limited resource, even when there is no indication of anomalous performance or life-limiting wear. Recommendation(s): Manage reaction wheel assemblies as a lim
17、ited resource: 1. Track RWA performance, beginning with acceptance test and throughout mission operations, to identify potential limitations on reaction wheel lifespan. 2. Where RWA performance data indicates that the RWA lifespan may be constrained, take measures to mitigate the mission impact (sav
18、e the weaker RWA for end-of-mission, avoid operating bearings at low-rpm regimes conducive to metal-to-metal contact, etc.) 3. To assist in minimizing bearing operating time within the low-speed region, consider using a ground software tool (e.g., RBOT) to carefully manage reaction wheel biasing eve
19、nts. 4. Implement a reaction wheel drag torque estimator in the flight software design to identify any anomalous bearing drag conditions such as cage instability. Mitigate the potential effects of such conditions on overall spacecraft attitude performance by compensating the attitude control torque
20、by the estimated drag torque. (Both the Galileo and Cassini attitude control software were designed to perform this compensation.) Evidence of Recurrence Control Effectiveness: JPL has opened Preventive Action Notice (PAN) No. Z86936 to initiate and document appropriate Laboratory-wide action on the
21、 above recommendations. Documents Related to Lesson: JPL Procedure: “Mission Planning - Operations, Rev. 1,” JPL Document 31912, March 05, 1999.Mission Directorate(s): a71 Exploration Systemsa71 Sciencea71 Aeronautics Researcha71 Space OperationsAdditional Key Phrase(s): a71 Flight Equipmenta71 Flig
22、ht Operationsa71 Ground OperationsProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-a71 Hardwarea71 Partsa71 Risk Management/Assessmenta71 Safety & Mission Assurancea71 Spacecrafta71 Test & Verificationa71 Materials & ProcessesAdditional Info: a71 Year of Occurrence: 2002Approval Info: a71 Approval Date: 2006-06-28a71 Approval Name: dkruhma71 Approval Organization: HQProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
