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OPERATIONAL WORKAROUNDS FOR THE SPACE STATION BETA GIMBAL ANOMALY: 

OPERATIONAL WORKAROUNDS FOR THE SPACE STATION BETA GIMBAL ANOMALY Timothy W. Propp The Boeing Company – International Space Station Ann M. Delleur NASA Glenn Research Center http://space-power.grc.nasa.gov/ http://www.boeing.com/defense-space/space/spacestation/

Outline: 

Outline Introduction Beta Gimbal Assembly Performance Anomaly Power Generation Scenarios Dual Angle Mode Concept Flight Attitudes Concluding Remarks

Introduction: ISS PV Arrays: 

PV 4B PV 2B BGA Introduction: ISS PV Arrays

Introduction: PV Array Tracking: 

Introduction: PV Array Tracking PV arrays designed to continuously track the sun Beta Gimbals provide 360º single axis solar array rotation per orbit in XVV Znadir attitude PV arrays originally planned to track the sun continuously once pointing data became available Solar array off-pointing is a function of solar β angle, vehicle attitude, and BGA position

Solar β Angle: 

Solar β Angle

XPOP Attitude: 

XPOP Attitude XVV Znadir Attitude ISS Flight Attitudes

Bearing Motor Roll Ring Module: 

Bearing Motor Roll Ring Module Transfers electrical power across a rotating joint Positions the solar arrays

BGA Performance Anomaly: 

BGA Performance Anomaly Ch. 4B BMRRM motor current (amps) over 5 hour period Performance after 635 cumulative rotations Ch. 4B BMRRM motor current (amps) over 5.5 hour period Performance after 20 cumulative rotations

Stall Current Signature: 

Stall Current Signature Ch. 4B BMRRM motor current (amps) performance during a stall event (5.5 hours) Ch. 4B BMRRM motor current (amps) performance in the XPOP attitude (24 hours)

Anomaly Resolution Activity: 

Anomaly Resolution Activity Conducted tests to isolate the root cause; results were inconclusive ISS program decided against a BMRRM replacement due to risks Unpowered components predicted to exceed lower temperature limits Thermal gradients and fit tolerance of a new BMRRM or re-installation of old BMRRM Precise choreography of both IVA and EVA Crewmembers plus ground personnel

Reduce BGA Wear: 

Reduce BGA Wear The BGA ART was directed to reduce BGA rotations while minimizing impacts to ISS assembly and scientific experiments Hoped to manage U.S. electrical loads such that Channel 4B BGA could be parked for extended periods in Xvv Znadir attitude Analysis showed that if a subset of Channel 4B electrical loads were transferred to Channel 2B, only the 2B BGA must be able to reliably rotate to successfully complete the next three assembly missions

Power Generation for Parked Solar Arrays: 

Power Generation for Parked Solar Arrays Parked solar array power generation at various solar ß angles and BGA park angles Power generation and expected load demand for Ch 4B XPOP rate mode test with Ch 2B parked

EVA to Install BGA BlanketEVA Crew Moving Towards the Worksite: 

EVA to Install BGA Blanket EVA Crew Moving Towards the Worksite

Power Generation during EVA Case 1: Begin Reverse Rotation At Orbit Sunset: 

Power Generation during EVA Case 1: Begin Reverse Rotation At Orbit Sunset Case 1: BGA position for BGA blanket installation Case 1 solar view factors (Albedo view factors not shown) (Gray bars denote eclipse)

Power Generation during EVA Case 2: Begin Reverse Rotation at Orbit Sunrise: 

Power Generation during EVA Case 2: Begin Reverse Rotation at Orbit Sunrise Case 2: BGA position BGA blanket installation Case 2: Solar view factors (albedo view factors not shown) (Gray bars denote eclipse)

Case 1 Flown: 

Case 1 Flown Electrical Systems Flight Controllers selected Case 1 because of the more favorable Channel 2B battery SOC performance early in the EVA Post-flight analysis showed a significant reduction in the magnitude of the thermally induced cyclic motor current signature, but the thermal blankets did not resolve the overall anomaly Battery State of Charge on Channels 2B and 4B for Case 1 and Case 2 (gray bars denote eclipse)

BGA Thermal Blanket: 

BGA Thermal Blanket

Dual Angle Mode Operational Concept : 

Dual Angle Mode Operational Concept Dual angle mode (90º and 60º sweep) Channel 4B BGA motor current (amps) performance during dual angle mode testing (24 hours, May 7-8, 2002) Channel 2B BGA motor current (amps) performance during dual angle mode testing (30 hours, June 21-23, 2002)

Dual Angle Mode Status: 

Dual Angle Mode Status Dual angle mode was tested successfully in the summer of 2002 Dual angle mode is currently meeting ISS power needs on Channels 2B and 4B at low to medium solar β angles in the XVV Znadir attitude when the Space Shuttle is not docked to the International Space Station

Low Solar ß XPOP: 

Low Solar ß XPOP The most favorable BMRRM motor currents were observed with the ISS in the XPOP attitude XPOP attitude initially certified for |solar ß| > 37º ISS Subsystem teams screened hardware performance down to solar ß = +/-10º (“low solar ß”) On-orbit low solar ß XPOP test was executed successfully Nov. 2001 ISS Program approved low solar ß XPOP in Jan. 2002 Implementation of low solar ß XPOP projected to save well over 1000 rotations on each P6 BGA Expansion of XPOP Attitude to Low Solar ß Angles

Power Demand Per Channel And Power Generation Per Channel: 

Power Demand Per Channel And Power Generation Per Channel

Summary: 

Summary The on-orbit BGA motor current anomalies introduced a significant threat to the continued assembly of the ISS A multi-faceted team was formed to address root cause, BMRRM R&R, and operations issues Over the course of a year, the operations team developed successful methods of managing ISS energy balance while supporting the root cause team’s troubleshooting efforts and the overall team goal of minimizing cumulative BGA travel The Dual Angle technique and low solar ß XPOP flight attitude continue to be used successfully The Dual Angle and low solar ß XPOP techniques have been modified to reduce vehicle drag and save on-orbit propellant resources