NASA Technical Reports Server (NTRS) 20030014592: Space Station Power Generation in Support of the Beta Gimbal Anomaly Resolution

16 Pages·1.2 MB·English

by  NASA Technical Reports Server (NTRS)

NASA/TM—2003-212012 AIAA–2003–0758
Space Station Power Generation in Support
of the Beta Gimbal Anomaly Resolution
Ann M. Delleur
Glenn Research Center, Cleveland, Ohio
Timothy W. Propp
The Boeing Company—International Space Station, Houston, Texas
January 2003
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NASA/TM—2003-212012 AIAA–2003–0758
Space Station Power Generation in Support
of the Beta Gimbal Anomaly Resolution
Ann M. Delleur
Glenn Research Center, Cleveland, Ohio
Timothy W. Propp
The Boeing Company—International Space Station, Houston, Texas
Prepared for the
41st Aerospace Sciences Meeting and Exhibit
sponsored by the American Institute of Aeronautics and Astronautics
Reno, Nevada, January 6–9, 2003
National Aeronautics and
Space Administration
Glenn Research Center
January 2003
Acknowledgments
Many people have made significant contributions to the BGA ART. Although there is no way
to acknowledge everyone, the authors wish to recognize the following core members of the
BGA ART whose diligent work we had the challenging job of summarizing in an 11 page
technical paper: George Gafka, Kevin Meehan, Mark Mulqueen, Chad Blodget, Scott West,
Dave McCann, Robert Davis, Chuck Haynes, Robert Puckett, Caryn Preston, Carlos Pagan,
Bob Levy, Curt Carlton, Ed Mohr, and Tony Delucia. The authors would also like to thank
Dave McKissock and Jeff Hojnicki for their invaluable assistance, and the GRC EPS
analysis team for their peer reviews during the BGA ART process.
Available from
NASA Center for Aerospace Information National Technical Information Service
7121 Standard Drive 5285 Port Royal Road
Hanover, MD 21076 Springfield, VA 22100
Available electronically at http://gltrs.grc.nasa.gov
SPACE STATION POWER GENERATION IN SUPPORT
OF THE BETA GIMBAL ANOMALY RESOLUTION
Ann M. Delleur
National Aeronautics and Space Administration
Glenn Research Center
Cleveland, Ohio 44135
Timothy W. Propp
The Boeing Company — International Space Station
Houston, Texas 77059

ABSTRACT INTRODUCTION
The International Space Station (ISS) is the largest and When fully assembled, the U.S. EPS will include four
most complex spacecraft ever assembled and operated PV modules1 that will be capable of producing a
in orbit. The first U.S. photovoltaic (PV) module, combined average power output of 75kW. The arrays
containing two solar arrays, was launched, installed, power not only electrical loads when the station is in
and activated in early December 2000. After the first the sun, but also charge batteries that supply the
week of continuously rotating the U.S. solar arrays, station’s power demands in eclipse. To maximize the
engineering personnel in the ISS Mission Evaluation power available for core system electrical loads, unique
Room (MER) observed higher than expected electrical ISS assembly activities, and scientific experiments, the
currents on the drive motor in one of the Beta Gimbal arrays need to be pointed towards the sun throughout
Assemblies (BGA), the mechanism used to maneuver a the sunlit period of the orbit (insolation). Continuous
U.S. solar array. The magnitude of the motor currents sun pointing is accomplished via target positions
continued to increase over time on both BGA’s, supplied to the BGA’s by the on-board Guidance,
creating concerns about the ability of the gimbals to Navigation, and Control (GN&C) system. The BGA
continue pointing the solar arrays towards the sun, a target positions are updated once every second by the
function critical for continued assembly of the ISS. A GN&C software.
number of engineering disciplines convened in May
2001 to address this on-orbit hardware anomaly. This In the early ISS assembly configuration, as depicted in
paper reviews the ISS electrical power system (EPS) Figure 1, the solar arrays on the first U.S. PV module,
analyses performed to develop viable operational designated P6, can only rotate in one axis. The solar
workarounds that would minimize BGA use while arrays, designated PV (or Channel) 2B and PV 4B, are
maintaining sufficient solar array power to continue electrically isolated. The percentage of the maximum
assembly of the ISS. Additionally, EPS analyses solar array power available for electrical loads can be
performed in support of on-orbit BGA troubleshooting approximated by the cosine of the sun incidence angle.
exercises is reviewed. EPS capability analyses were This angle is measured between the normal to the active
performed using SPACE, a computer code developed surface of the array and the sun vector. The magnitude
by NASA Glenn Research Center (GRC) for the ISS of the sun incidence angle is influenced by three
program office. factors: the vehicle orientation (or attitude), the array
position, and the solar ß angle, which is the angle
measured between the sun vector and the orbit plane
(see Figure 2).

NASA/TM—2003-212012 1
BGA
PV 2B
PV 4B
VELOCITYVECTOR
Z-NADIRVECTOR
X
Y
FIGURE 3 – ILLUSTRATION OF THE ISS ORBITING IN THE
XVV ZNADIR FLIGHT ATTITUDE
Z
FIGURE 1 - LOCATION OF THE GIMBALS THAT ROTATE In the XPOP attitude (see Figure 4), the ISS X-axis
THE U.S. PHOTOVOLTAIC ARRAYS remainspointedinadirectionperpendiculartotheorbit
plane. TheXPOPattitudeismoreconduciveforpower
generation because the sun incidence angle remains
nearlyperpendiculartotheactivesurface(frontside)of
theP6solararraysthroughouttheorbit. Inthisattitude,
the BGA’s travel approximately 4.5º per day, which is
roughly equivalent to the daily rate of change of the
Orbit solarßangle.
Noon
X
XPOP Y
XPOP
β angle
Sun
Z
XPOP
FIGURE 2 – SOLAR ß ANGLE
Y
X XPOP
XPOP
The ISS generally orbits in one of two attitudes: Xvv
Z
ZnadirorXPOP. IntheXvvZnadirattitude(seeFigure XPOP
3), the ISS +X-axis points in the direction of the
velocity vector, while the ISS +Z-axis points nadir. In FIGURE 4 – XPOP FLIGHT ATTITUDE
this attitude, the BGA’s rotate each solar array through
afull360°sweepaboutitslongitudinalaxisonceevery
BETA GIMBAL ASSEMBLY ANOMALY
orbit,16orbitsperday,totrackthesun. WhentheISS
is flying in this Xvv Znadir attitude, the power
generated from the P6 solar arrays decreases as the The BGA provides two critical capabilities to the ISS:
magnitude of the solar ß angle increases. This occurs 1) transfer of electrical power across a rotating joint;
because the sunincidence angle onthe arraysincreases and 2) positioning of the solar arrays. The primary
withincreasingsolarßangle. component of the BGA, illustrated in Figure 5, is the
Bearing Motor Roll Ring Module (BMRRM). The
BMRRMcontainsauniquebearingassemblythat,prior
to the ISS, has never beenoperated inspace. Because
NASA/TM—2003-212012 2
of this, it is difficult to determine what constitutes an the ISS MER observed higher than expected electrical
anomalousmotorcurrentsignature. currents on the drive motor in the Channel 4B
BMRRM. The 4B BMRRM performance steadily
decreased over the next four months, as the frequency
ofexcursionsover0.5amperesincreasedwithtime(see
BGA Figure7).
Housing

FIGURE 7 – CH. 4B BMRRM MOTOR CURRENT (AMPS)
OVER 5 HOUR PERIOD - PERFORMANCE AFTER 635
Platform CUMULATIVE ROTATIONS
BMRRM
In early May 2001, the 4B BMRRM experienced the
FIGURE 5 – BETA GIMBAL ASSEMBLY
firstmotorcurrentinexcessof1.0amperes. Thiswasa
significant milestone because it indicated the BGA
Industry standards state that the system should operate
motor needed two-thirds of its maximum capability to
below 25% of full capability for average current, and
maintain rotation. At the same time, the Channel 2B
below 50% of full capability for peaks. The BMRRM
BMRRM, which had been operating nominally, began
motor can supply a maximum output current of 1.5
experiencingmotorcurrentsinexcessof0.5amperes.
amperes. Applying the industry standards, a BMRRM
motor current < 0.38 amperes is considered an average
After 1200 cumulative rotations, the 4B BMRRM
or nominal value, while a BMRRM motor current
beganto periodicallystall. A stallevent(see Figure 8)
> 0.75 amperes is considered anomalous and is often
occurswhenevertheBMRRMvelocitygoestozeroand
referred to in the BGA community as a motor current
thepositionerror(error=actualposition–commanded
“spike”.
position) exceeds 10 degrees for more than three
minutes. Themaximumcurrentthemotorisallowedto
drawduringastallconditionis1.1amperes.

FIGURE 6 – CH. 4B BMRRM MOTOR CURRENT (AMPS)
OVER 5.5 HOUR PERIOD - PERFORMANCE AFTER 20
CUMULATIVE ROTATIONS
FIGURE 8 – CH. 4B BMRRM MOTOR CURRENT (AMPS)
PERFORMANCE DURING A STALL EVENT (5.5 HOURS)
Duringthemajorityofthefirstmonthofoperations,the
BGA’s remained parked in a stationary position that
was selected to optimize power generation. The arrays
were not required to rotate because the U.S. power Torecover froma stallevent, ElectricalSystemsFlight
demands were low. The primary incentive for parking Controllers in the Mission Control Center (MCC)
the solar arrays was to reduce the drag on the vehicle. command the BMRRM motor back on, rotate the array
As a result, the rate of degradation of the ISS altitude backwards, then continue forward motion. During the
was decreased, and on-orbit propellant supplies were five-month period of January through May 2001, the
preserved. The electrical loads increased significantly motor current performance of both BGA’s remained
with the installation of the U.S. Laboratory module pristinewhentheISSwasorbitingintheXPOPattitude
Destiny in January 2001. The power system began to (seeFigure9).
relymore heavilyonthe BGA’s to rotate whenthe ISS
wasorbitingintheXvvZnadirattitude,allowingtheP6 NeitherBGAhaseverbecomestuck,unabletomovein
solar arraysto provide the power necessaryto meet the either direction with a motor current of 1.1 amperes
increased electrical load demands. After one week of applied. The ISS MER mechanisms experts believe a
continuous rotation, engineering support personnel in stuckBGAisanunlikelyeventbecauseoftheavailable
NASA/TM—2003-212012 3
0.4 ampere motor current margin and the history of POWER GENERATION ANALYSES
successfulstallrecoveries.
The BGA ART operations team coordinated the efforts
within the ISS program to identify feasible options for
reducingBGArotationswhilecontinuingtosupporton-
orbit troubleshooting. Toward this end, a group of ISS
power resource experts convened to begin determining
the projected U.S. segment power demands and power
FIGURE 9 – CH. 4B BMRRM MOTOR CURRENT (AMPS) generation capabilities for a defined set of operational
PERFORMANCE IN THE XPOP ATTITUDE (24 HOURS) scenarios. The ISS EPS analysis team at NASA’s
Glenn Research Center (GRC) used System Power
Early projections of the motor current trends indicated Analysis for Capability Evaluation (SPACE)2,3,4 to
frequent stalls could be realized within 6 months. The predict the EPS power generation capabilities for each
ISS program office became concerned about the of the proposed operational scenarios, covering a range
feasibility of continuing the assembly of the ISS using of ISSstages,flightattitudes,arraycontroltechniques,
degraded BGA’s, primarily because the majority of and solar β angles. Four of the operational scenarios
assembly missions occur during solar ß periods that andoneofthetestconditionsthatwereanalyzedwillbe
require the ISS to fly in the Xvv Znadir attitude. A reviewedinthispaper.
BGA Anomaly Resolution Team(ART) was formed in
early May 2001, and consisted of three sub-teams:
Scenario 1: PV 4B BGA Parked in Xvv Znadir
1)rootcauseteam,2)operationsteam,and3)BMRRM
remove&replace(R&R)team.
AnearlygoalfortheBGAARToperationsteamwasto
The root cause team coordinated several on-orbit and minimize rotations on the Channel 4B BGA. With the
ground tests in an attempt to isolate the root cause but ISS still in the early stages of assembly, it was hoped
results were inconclusive. Meanwhile the BMRRM that the U.S. electrical loads could be managed such
R&R team identified several risks associated with an thattheChannel4BBGAcouldbeparkedforextended
on-orbit replacement of a BMRRM. The primary risks periods of time when the ISS was in the Xvv Znadir
were: 1) hardware survival would be threatened with attitude. GRCpersonnelused SPACEto generatea set
nearly all of one U.S. power channel shut down during ofpowergenerationcapabilitiesasafunctionofparked
the R&R, as several unpowered components were solar array position across the nominal Xvv Znadir
predicted to exceed lower temperature limits within 6- solarßrange(seeFigure10).
10 hours; 2) thermal gradients and fit tolerance issues
would complicate successful installation of a new Using this data, the ISS power resource experts on the
BMRRM or re-installation of the old BMRRM; and BGA ART operations team determined that by
3) precise choreography of both intra- and extra- transferring a subset of Channel 4B electrical loads to
vehicular activity (IVA and EVA) as well as ground Channel2B, fromone arrayto the other (see Figure 1),
personnel activities would be required for a successful only one BGA must be able to reliably rotate to
R&R. In October 2001, the ISS program decided successfullycompletethenextthreeassemblymissions.
against a BMRRM R&R, primarily because the Beyond that, two reliable BGA’s would be required to
consequences of a failed BMRRM R&R could be continue assembly of the ISS. This was a significant
extremely high, possibly leaving the ISS to operate on finding for two reasons: 1) the Channel 4B BGA could
onepowerchannelandatreducedfunctionalitybecause remain parked, thus reducing the risk of further
of potential hardware failures. The BGA ART was degradationintheBMRRM;and2)itprovidedtheroot
strongly encouraged to identify and implement viable causeandBMRRMR&Rteamsadditionaltimetowork
operational techniques to reduce rotations on the their issues before ISS program management was
BGA’stothegreatestextentpossible while minimizing scheduled to reconvene to make a decision regarding
impacts to continued ISS assembly and scientific theR&RoftheChannel4BBMRRM.
experiments. Thescopeoftheefforttoreducerotations
extends for roughly two years, at which time both P6
solar arrays will be stowed until the PV module is
relocated to an outboard position on a future ISS
assemblymission.
NASA/TM—2003-212012 4
The expected electrical load demand on Channel 4B
during the XPOP rate mode test was ~4.5 kW, and on
Channel 2B ~6.4 kW. Figure 11 shows the results of
the power balance analysis. Positive power margins
wererealizedacrossthesolarßrangeofinterestforthe
proposed test. The data was presented to the BGA
ART on May 23rd, 2001, and the XPOP 4B BGA rate
modetestwassuccessfullyexecutedtwodayslater.
FIGURE 10 – PARKED SOLAR ARRAY POWER
GENERATION AT VARIOUS SOLAR ß ANGLES AND BGA
PARK ANGLES
Scenario 2: PV 4B BGA Constant Rotation in
XPOP - Test Condition
With the next ISS assembly mission just over one
monthawayandnorootcauseidentified,therootcause
teamcontinuedtoplanandexecuteon-orbittestsonthe FIGURE 11 – POWER GENERATION AND EXPECTED
Channel 4B BGA. Unfortunately, the solar β regime at LOAD DEMAND FOR CH 4B XPOP RATE MODE TEST WITH
thattimerequiredtheISStoflyintheXPOPattitudeto CH 2B PARKED
maintain positive energy balance. The BGA is not
required to travel more than a few degrees in either
Scenario 3: BGA Back-Drive Prior to Thermal
direction when the vehicle is orbiting in the XPOP
Blanket Installation
orientation. To assist the root cause team’s
troubleshooting efforts, the ISS power resource experts
wereaskedtoformulateaplanthatwouldrotatethe4B In July 2001, the root cause team called on the ISS
BGA at a fixed orbital rate of ~4º/minute in XPOP passive thermal analysis team to assess the possibility
while maintaining positive energy balance on the ISS. of thermal gradients contributing to the apparent
A rate of ~4º/minute was selected because the BGA’s friction in the BMRRM bearing mechanisms. The
normally move at that speed when tracking the sun in results of the thermal analyses led the BGA ART to
the Xvv Znadir attitude. To implement this request, propose building and installing a thermal blanket
one of the BGA software control modes, namely Rate around the BGA’s to provide a more thermally stable
Mode, was utilized to control the movement of the operating environment for the gimbals and attempt to
BGA at a commanded rate. SPACE was used to improve their overall motor current performance. ISS
determine how to optimize power generation program management approved the thermal blanket
capabilities for Rate Mode operations in XPOP. The proposal in late October 2001, and the blanket
analyses indicated that power generation could be installation was scheduled for an EVA on the next ISS
maximizedifthesolararrayrotationweresynchronized assemblymissioninlateNovember2001.
suchthattheactiveface(frontside)ofthearraypointed
directly towards the sun when the vehicle arrived at The lead Power Resource Officer (PRO) on the
orbit noon. Using this technique, the active face of the Electrical Systems Flight Control Team requested
array will generate most of the available power during assistance from the BGA ART operations team in
insolation. Power is also generated from direct solar analyzing power generation capabilities for the two
illumination of the backside5, and from albedo leadingoperationalscenariosproposedforinstallingthe
illumination on both the front and backsides6 of the BGA thermal blankets. The basic plan was to rotate
array. As a general rule of thumb, the backside of the both solar arrays backwards 360° at 10°/minute, then
U.S. PV arrays produces approximately 1/3 the power continue to rotate in the reverse direction until each
ofthefrontsideforequivalentsunincidenceangles. BGA had reached a pre-determined park position
NASA/TM—2003-212012 5
required to provide adequate clearances at the EVA Figure 14 illustrates the time-phased BGA position
worksites. At the conclusion of the 4 hour EVA to profile for Case 2, in which the backwards rotation is
install the blankets, the arrays would be further rotated initiated at orbital sunrise. Case 2 results in an
backwards to a new “thermal hold” park position that additional back-drive of ~50º-100º, further “resetting”
optimized power generation. The thermal hold would theBGAperformancepriortotheblanketinstallation.
consist of 1 insolation pass on Channel 2B and 3
insolation passes on Channel 4B. Figure 12 illustrates
the time-phased BGA position profile for Case 1, in
Backrotation
which the backwards rotation is initiated at orbital
Parked for
sunset.
Thermal Hold
Parked for EVA
Backrotation
Parked for EVA Parked for FIGURE 14 – BGA POSITION BGA BLANKET
Thermal Hold INSTALLATION EVA CASE 2: BEGIN REVERSE ROTATION
AT BEGINNING OF INSOLATION. (GRAY BARS DENOTE
ECLIPSE
ThedirectsolarilluminationviewfactorsforCase2are
illustrated in Figure 15. The view factorsindicate that,
FIGURE 12 – BGA POSITION FOR BGA BLANKET when compared to Case 1, starting the 360° backward
INSTALLATION EVA CASE 1: BEGIN REVERSE ROTATION rotation at the beginning of insolation leads to
AT ORBIT SUNSET. (GRAY BARS DENOTE ECLIPSE) significantly more cosine losses on both the front and
backsidesofthearray.
NASA GRC personnel used SPACE to generate view
factors for each case. The view factors for direct solar
illuminationareillustratedinFigure13. Theadditional
backwards rotation on the Channel 2B BGA, following
the initial 360° reverse rotation, resulted in no
illumination on the front side of the PV 2B array for
halfofthefirstinsolationperiod.
FIGURE 15 – CASE 2 SOLAR VIEW FACTORS (ALBEDO
VIEW FACTORS NOT SHOWN - GRAY BARS DENOTE
ECLIPSE)
Each P6 electrical channel is outfitted with 3 76-cell
Nickel Hydrogen batteries. The nameplate capacity of
an ISS battery is 81 Amp-Hours. The computer that
controls the P6 electrical systems computes a battery
FIGURE 13 – CASE 1 SOLAR VIEW FACTORS (ALBEDO stateofcharge(SOC)onceeverysecondforeachofthe
VIEW FACTORS NOT SHOWN - GRAY BARS DENOTE three batteries. The magnitude of the SOC is an
ECLIPSE)
NASA/TM—2003-212012 6