kepler dust cover ejection event design and optimization chris zeller and david acton ball aerospace...
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Kepler Dust Cover Ejection EventDesign and Optimization
Chris Zeller and David ActonBall Aerospace & Technologies [email protected]@ball.com
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Outline
Kepler mission overview Summary of problem How and why project used AGI software Optimizing dust cover release attitude Key risk reductions from using AGI software
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Kepler mission overview
NASA mission launched March 2009
Search for Earth-size planets
– In/near habitable zone of solar-like stars
Highly sensitive photometer
Continuously and simultaneously measures brightness of >100k stars
Flight segment design and fabrication at Ball Aerospace & Technologies Corp.
Scientific Operations Center at NASA Ames Research Center
Mission Operations Center at LASP – University of Colorado
Variation in star brightnessindicates planet transit
Planet transit
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Summary of problem
Ensure ejected photometer dust cover (DC) does not return to strike flight segment (FS)
Determine release attitude to maximize FS-to-DC distance over mission duration– Must meet power, telecom, and sun-avoidance constraints
Ensure validity of solution considering uncertainties– DC ejection direction and velocity– DC surface properties– DC release date
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Kepler trajectory description
Helio-centric Earth-trailing orbit avoids obscurations– ~0.5 AU range from Earth
after 4 years
No traditional V maneuvers required
Periodic reaction wheel desaturations– Via RCS thruster pulses
– Small but measurable effects on trajectory
– STK excellent modeling fit
Autumnal Equinox
Vernal Equinox
Summer Solstice
Winter Solstice
Fallroll period
Summerroll period
Springroll period
Winterroll period
Orbital direction
Kepler’s orbit
Projection of photometer axis onto
the ecliptic
Earth’s orbit
Sun
Earth on March 6th
Kepler 1 year later
Kepler 4 years later
Launch
View from the ecliptic North Pole
Earth’s orbit
Kepler’s orbit
Kepler’s position on March 6th of each year
Roll period
Earth at launch
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Dust cover design and release
Protects photometer– Contamination prior to, and during launch– Stray/direct sunlight during launch and early commissioning
Deployment mechanism– Single latch, single fly-away hinge, and pre-loaded screws
Nominal release– Along vector ~ 8º from sunshade
normal, towards hinge side– Relative velocity ~0.5 m/sec– Variations must be considered
Constraints on release attitude– Power – Telecom– Photometer Sun-Avoidance
Photometer field-of-view (~100 deg2)
SunlineCover position att = 2.4 sec after release
t = 2.9 sec
t = 3.3 sec
t = 3.9 sec
t = 4.2 sec
Sunshade normal
Dust cover
Photometer
Spacecraft
Dust cover
Latch
Hinge
Sunshade assembly
Pre-load fittings (x4)
> 14 kg1.7 x 1.3 m
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STK allowed efficient and accurate analysis for important Kepler issues
STK as standard trajectory modeling and analysis tool– Chosen early in the project– Ease of use, flexibility, visualization, accuracy, and familiarity to analysts
Used for a variety of analyses– Power estimates, telecom range and angles for duration of mission– Initial Acquisition timing and angles– Deep Space Network station view periods– Optimization of quarterly roll windows– Verification of commissioning attitudes– Dust Cover Ejection event (this presentation)
Allowed validation of similar customer analyses
This analysis – STK Professional, Astrogator, Chains, and Analyzer– Astrogator provided unique features to tailor deep space analysis
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Baseline trajectory model
Trajectory modeled using Astrogator– Initial conditions at launch vehicle separation– Near-Earth perturbations with Earth-moon gravity model– Dust cover separation reaction modeled as a maneuver– Desaturation burns (every 3 days) using sequence loops– Deep Space propagation (6 years)
Kepler-Earth body-body rotating reference frame
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Validation of the STK Kepler model
Validated model with JPL Navigation Team’s MONTE Tool– Tailored Astrogator propagator to determine which physics to model– Updated STK to latest planetary ephemeris to match JPL inputs
Final result – highly accurate STK trajectory model
0
5000
10000
15000
20000
25000
30000
0 500 1000 1500 2000 2500 3000
Relativity On
Standard HPOP, Helio
HPOP No Moon
Standard STK HPOP
HPOP CIS Lunar Helio No Rel
CIS Lunar Helio No HPOP
Cis Lunar Helio Rel
J2 Helio
HPOP Lite Helio
J2 Moon Sun Helio
JDM3 HPOP Helio
J2 8x8 JDM3
J2 2x0 JDM3
Selected Propagator:• Earth J2 with Moon + Sun 3rd bodies • Heliocentric + all 9 planets after 9.25E+5 km from Earth
Alternate Selection:• Earth HPOP + Sun/moon 3rd bodies• Heliocentric + all 9 planets after 9.25E+5 km from Earth
Validation was essential to provide customer confidence in solution
Range Difference Between JPL and STK SolutionsR
ange
(km
)
Days After Release
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Features of the dust cover ejection model
Coordinate system selected for fixed attitude with respect to Sun
– Provided fixed constraints for photometer sun-avoidance & power
– STK Vector Geometry Tool validated antenna, star tracker, photometer FOV constraints
Target pointing attitude selection used to determine release attitude
Baseline DC trajectory returned towards FS several times
– Oscillatory behavior– Suggested we perform optimization
and sensitivity analyses
VNC(Sun) = Velocity, Normal, Co-Normal, centered on Sun
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Analyzer Carpet Plot was generated tooptimize release directions
Appropriate Figure-of-Merit was crucial– Oscillatory behavior of DC motion required careful FoM choice– FoM chosen as minimum range after initial “drift-away” period
Note: Not all options were good ones
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Optimum release direction
Optimal release direction maximized minimum range
– But did not meet Earth and Sun constraints
– Selected next best option – Nominal minimum range after
drift away is 40,820 km
Desaturation impulses help– Tend to push FS away from
DC over time
Attitude computation– Target Pointing attitude and
custom reports used to compute VNC-Body quaternion
Kepler Dust Cover - FS Relative Range
0
100,000
200,000
300,000
400,000
500,000
600,000
Mar-2009 Mar-2010 Mar-2011 Mar-2012 Mar-2013 Mar-2014 Mar-2015
Date
Ran
ge
(km
)
Actual 0 Az,35 El
Optimum 0 Az, 0 El
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Sensitivity analysis using Analyzer
Monte Carlo tool to investigate variations in parameters – Release angle, release
velocity, and DC reflectivity Verified large minimum range
met under even 3 conditions Reduced risk that inaccuracy
in any one parameter could throw us “off the cliff”
0
5
10
15
20
25
30
34,090 35,511 36,932 38,354 39,775 41,196 42,618 44,039 45,461 46,882
Min Range (km)
Fre
qu
ency
Mean 40,814 Km- 3 34,327 Km+3 47,302 Km
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Sensitivity analysis for DC release date
Reduce impact of commissioning schedule changes Necessary to run manually
– Analyzer could not handle variations in epoch dates Determined release date variations acceptable
– Within range of dates considered
DC-FS Range With Varying Release Date Vs. March 28th 2009
-20,000
-15,000
-10,000
-5,000
0
5,000
10,000
0 500 1000 1500 2000
Days After Release
Ran
ge
Dif
fere
nce
(km
)
29-Mar-09
30-Mar-09
31-Mar-09
1-Apr-09
2-Apr-09
3-Apr-09
4-Apr-09
5-Apr-09
6-Apr-09
7-Apr-09
8-Apr-09
9-Apr-09
10-Apr-09
Worst Case DC-FS range > 40,000 km
Dust cover successfully released onApril 8, 2009
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STK provided key risk reduction for dust cover ejection
STK allowed efficient analyses of complex problem– Reduced cost and time to address important Mission Design concern
Ability to fine-tune trajectory estimates during independent validation with customer solutions lowered risk of errors
Cost-benefit of Analyzer was important– Significantly reduced time for optimization and Monte Carlo analyses
3D visualization provided simple visual verification of all results– Lowered risk of violating flight rules– Easy to communicate results across program and to stakeholders
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Acknowledgements
AGI Tech Support – For their helpful dedication and long hours helping sort
out the best way to approach the problem Jeff Baxter Dana Oberg Luis Montano
Ball Aerospace colleagues– For their insightful consultation
Scott Mitchell Adam Harvey
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Contact information
Chris Zeller– Senior Systems Engineer– Ball Aerospace & Technologies Corp.– Boulder, Colorado– [email protected]– 303-939-4636
David Acton– Senior Systems Engineer– Ball Aerospace & Technologies Corp.– Boulder, Colorado– [email protected]– 303-939-4775