AGU presentation

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9th International Conference on Space Operations,
Rome June 2006
SMART-1 EP OPERATIONAL EXPERIENCE
Presented by David Milligan, Spacecraft
Operations Engineer European Space Operations
Centre, Robert-Bosch Straße 5, 64293, Darmstadt,
Germany e-mail David.Milligan_at_esa.int
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PRESENTATION

• Spacecraft, Mission Ground Segment
• Smart-1 EPS description (Hardware Software)
• Post launch mission optimisation
• EP Re-boost Phase (Mission Extension)
• Operational Considerations of EP
• EP shutdowns Performance changes
• Power Throttling
• Interactions
• Conclusions and Remarks

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Spacecraft Platform Design
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Electrical Propulsion Hardware - Main Elements

• Xenon Tank
• PPS1350 Thruster
• PPU
• EPIRTU
• BPRU
• XFC
• FU
• HPSSPC
• PPIM

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Electric Propulsion Hardware - Thruster

• Stationary plasma thruster PPS-1350-G developed
  by SNECMA
• Input power range 649-1418 W
• At maximum nominal power set, Thrust 70.1mN,
  Isp 1609s

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Electric Propulsion Hardware - Thruster
Operating Principle
Ions
B
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Electric Propulsion Hardware - EPMEC

• Gimbals the EP thruster
• 2 axis /- 8deg
• Counteract disturbance torques

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EPS Subsystem Software
PREcard Software

• PREcard is responsible for Propellant Management
• Dedicated TCs can program variables
• Used in Venting and Automatic modes
• Controls Valve opening and closing times
  dedicated health checks
• Essentially regulates LPT pressure i.e. pressure
  in the plenum tank..

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EPS Subsystem Software
PPU - Variable Power and Inrush Limitation

• Executes in PPU Automatic mode
• Commands EPS power supplies (1 of 117 nominal
  power sets)
• Initiates controls ignition sequence
• Operates closed loop control of EPS
• Many monitoring checks..

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Ground Facilities

• Ground segment facilities are based on
  (extensions of) existing infrastructure

• ESA ground stations shared between ESA missions
  and external customers.
• Dedicated Control Room, incl. MCS workstations.
• Dedicated areas for FD, Network, etc.

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Operations Concept

• Mission operations team kept at a minimum
• Reduction of planned passes to reduce operations
  cost (originally planned to have 2x8hrs passes
  per week)
• Support during normal working hours only during
  the routine phase
• Sequences uplnked to cover 10 days
• No spacecraft controllers (low level of
  operations).
• Approach was adapted after launch (e.g. improved
  Lunar orbit doppler and ranging data for EP)
• Some lights-out operations

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Mission Description 7 Phases

• Launch and Early Orbit Phase. Launch on
  2003/09/27, initial orbit 7029 x 42263 km.
  Platform Commissioning - 4days (24hrs/day)
• Van Allen Belt Escape. Escape phase completed by
  Dec-2003 Jan-2004, orbit 20000 x 63427 km.
  Continuous thrust.
• Earth Escape Cruise. Mainly perigee thrusting to
  raise apogee.
• Moon resonance and Capture. Moon capture on
  2004/11/11, 310,000 km from the Earth and 90,000
  km from the Moon.
• Lunar descent. Target orbit of 2,200 x 4,600 km.

• Lunar Science (before and after re-boost phase).
  Until the end of lifetime around 2006/07, excl.
  one-month re-boost phase.
• Re-boost phase. 207 revolutions of about 5 hours
  each with 68 minutes thrust during both
  ascending, and descending arcs of the orbit.
  02/08/05 to 17/09/05

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In Flight Use SMART-1 Mission Evolved with
Experience

• Removal of 3 Swing-bys
• Earlier arrival Improved orbit (300x10000km ?
  300x3000km)
• Mission Extension (1yr)

• EPS excellent performance
• Power budget
• FD transfer optimisation / season

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MISSION EXTENSIONEP Re-boost Phase - Orbital
Perturbations

• With nominal mission approaching end S1 in good
  health mission extension foreseen
• Orbit unstable - main disturbance 3rd body
  Earth and Sun
• Retrograde argument of perilune drift
• Eccentricity also effected
• Decreases initially (up until line of apsides
  alignment with Lunar axis, at 270deg)
• Increases afterwards - perilune altitude affected
  (until perilune lt Lunar radius impact!)

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EP Re-boost Phase Maximise use of xenon

• Residual xenon was 1.8kg (cant be used) of
  original 82kg
• But after analysis - innovative new procedures
  developed to push system beyond its original
  design (down to last 280g)
• Ultimate limit at very low pressure
• Propellant flow system

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EP Re-boost PhaseMission Extension Implementation

• Studies and sims gave good confidence ? the most
  ambitious re-boost strategy was selected (of
  possible 3)
• Re-boost August 2nd -September 17th, 2005.
• Included 3 pauses (total 10 days) - double
  star-tracker blinding
• The thrust arcs outside the perilune and apolune
  rotate as well as raise the orbit
• Some FDIR disabled
• Other operational tricks
• Insertion of filling procedures at perilune and
  apolune

Operational Orbit selected for Mission extension
Distance from the Moon Centre
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EP Re-boost Phase Operations and Results

• Ultimate limit when the Anode current falls below
  1.35A, the PPU software turns off the thruster
  power supplies and flags a FLAME OUT in
  telemetry.
• BB valves push well beyond qualification for
  mission extension performed very well

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Orbit at end of EP re-boost 1

• Orbit largely reset to initial conditions ?
  most favourable trade-off for lunar science
• First Science phase, argument pericentre 286.2
  ? 243.8
• Re-boost, 243.8 ? 292.9
• Residual xenon mass reduced from 1.8kg to only
  280g
• Second (much smaller re-boost phase foreseen).

From October 1st 2005 the Science Activities were
resumed and they will finish just before the Moon
impact scheduled on August 17th 2006 ? now
September 3rd 2006
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General EP Ops experience - EPS Unplanned
Shutdowns

• Initial orbit raising
• Unplanned shutdowns out of coverage
• Mainly radiation induced
• SEUs in software etc
• EP not primary source
• Less than half were OSETs
• All solved after launch
• e.g by patching various software on board

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General Ops Experience - Unplanned EP shutdown
effect

• Antenna pointing errors
• Infrequent passes
• Missing impulse ? early AOS

GS pointing error
S1 Thrust arcs
Expected position
Actual position
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Orbital Uncertainties
Variable Thruster performance and switch OFF out
of coverage

• Infrequent passes on orbit raising missions is
  challenging
• Unplanned shutdowns can create very large TOVs.
  Need to be able to deal with them (impact search
  routines - scheduling)
• Small thrust variations create small TOVs
  increases required frequency of orbit
  determination
• SMART-1 GS developed new search routines and
  demonstrated tolerance to performance uncertainty

S1 thrust performance
PPS1350 Anode rms Current
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EP Thrust Performance

• Thrust levels from ranging Doppler cf Nominal
  Power Set

Drawing provided by ESOC Flight Dynamics
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PPS1350 Anode rms Current
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Ops Experience - OSETs

• Unexpected EP shutdowns occurred, with the PPU
  software bit Flame Out triggered in TM
• Strongly correlated with the Halloween solar
  flare 10/2003
• Sensitivity identified in an optocoupler in the
  PPU (single events)
• Recovery action is after status check of power
  supplies to restart the thruster OBSW patched
  developed to automate process

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Ops Experience - OSETs

• OBSW patch developed that detected and
  automatically restarts EP

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General EP OPS experience - Power Dependency 2

• Generated Solar Array Power with the seasonal
  effect removed
• Distance affects available W/m2
• Drop caused by Van Allen Belt Crossing ? 8
  (7.5)..

Radiation exposure
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EP Power Throttling

• Goal safely maximize power into EP
• SA output and platform consumption predicted
• EP power levels (?thrust/Isp) given to FD for
  Orbit Propagation..

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EPS Performance Summary
From October 1st 2005 the Science Activities were
resumed and they will finish just before the Moon
impact scheduled on August 17th 2006 ? now
September 3rd 2006 see S1 impact presentation
on friday
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CONCLUSIONS AND REMARKS

• SMART-1 Mission performance - a great success..
• First EP use to escape Earth centred orbit (weak
  stability crossing)
• The orbital strategy, the performance operation
  of thruster and power subsystems, allowed
• Shorter transfer
• Improved Science orbit
• 1 Year mission extension
• Ground segment for EP orbit raising demonstrated
  (tolerant to switch offs performance changes)
• Low pressure ops demonstrated Tank residual
  1.8kg ? 280g
• SA degradation due to VA belts 8 (with X-class
  solar flare)
• Plasma of EP caused no operational impact

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QUESTIONS

• ?
• Further questions david.milligan_at_esa.int
• Smart-1 Science and Technology Website
• http//sci.esa.int/science-e/www/area/index.cfm?fa
  reaid10