OPERATIONAL WORKAROUNDS FOR THE SPACE STATION BETA GIMBAL ANOMALY

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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/spacesta
  tion/

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Outline

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

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Introduction ISS PV Arrays
PV 2B
BGA
PV 4B
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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 ß
  angle, vehicle attitude, and BGA position

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Solar ß Angle
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XPOP Attitude
ISS Flight Attitudes
XVV Znadir Attitude
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Bearing Motor Roll Ring Module

• Transfers electrical power across a rotating
  joint
• Positions the solar arrays

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

• Ch. 4B BMRRM motor current (amps) over 5 hour
  period Performance after 635 cumulative rotations

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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)
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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

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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

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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
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EVA to Install BGA BlanketEVA Crew Moving
Towards the Worksite
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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)
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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)
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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)
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BGA Thermal Blanket
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Dual Angle Mode Operational Concept
Channel 2B BGA motor current (amps) performance
during dual angle mode testing (30 hours, June
21-23, 2002)
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)
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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 ß angles in the XVV Znadir attitude when
  the Space Shuttle is not docked to the
  International Space Station

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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 gt
  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
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Power Demand Per Channel And Power Generation Per
Channel
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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 RR, 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
  teams 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