Long Term Preservation of Mars Express

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Long Term Preservation of Mars Express

• Zeina Mounzer
• June 2006

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Can the mission be extended another 10 years?

• Do we have enough fuel on board?
• Will the power subsystem survive another 10
  years?
• Do we have to change the orbit?
• Can the critical units be preserved?
• Communications Subsystem
• Attitude and Orbit Control Subsystem
• Data Management Subsystem
• Thermal Subsystem
• Will the mission scenario have to be modified?
• How to preserve the Mission Teams?
• How to maintain the overall system?

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The Available Propellant

• Bi-propellant Reaction Control System
• 2x4 Thrusters Main Engine
• 475.5 kg of propellant at launch
• 35.2 kg (24.3 kg oxidant, 10.9 kg fuel) today
• 1.5/1 usage ratio ? 28.7 kg available
• Uncertainties due to
• Bookkeeping (1)
• Fuel trapped above membrane (4.6 kg)
• Usage per year 2.7 kg
• Estimates remaining fuel good for 4 to 10 years!

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Preserving onboard Propellant

• Reduce uncertainties by applying Thermal
  Propellant Gauging Method
• Define momentum neutral guidance strategy
• No wheel off-loadings needed
• Impact on science
• Avoid large gravity gradient torques
• Control solar radiation pressure torques
• Optimise wheel off-loading attitudes
• 50 propellant saving for WOL
• No pointing constraints on instruments
• Loss of accurate long term control of the ground
  track

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Battery Status and Energy Management

• 3 Li-Ion batteries First use for complex
  mission profile
• ABSL performing real time ground test
• Degradation due to
• Cycle Life ? DoD and cycling rate
• Calendar Life ? SoC and temperature
• A number of models used
• BEAST Predicts electrical and thermal
  performance
• LIFE Predicts ageing
• Electric / Thermal model Predicts Voltages

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Battery Models

• Models provide good fit
• Current degradation (18) can be deduced
• But
• No predictive ageing effects
• Therefore
• Engineering studies

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Engineering Studies for Battery Degradation

• Engineering studies based on extrapolations from
  qualification tests
• Li-Ion batteries do not fail catastrophically
• Engineering studies are reasonable

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Maximum Survivable Eclipse Duration

• Eclipse duration acceptable if
• Batteries fully recharged before start of next
  eclipse
• Enough power to cope with Safe Mode at end of
  eclipse
• Eclipses will become shorter 1.5 hrs in 2005 to
  0.5 hrs in 2015
• Maximum DoD driven by Solar energy

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Energy Management

• Battery preservation
• Maintain SoC
• Maintain batteries at low temperatures
• Only outside power limited seasons
• Eclipse Reduction
• Can be achieved by modifying the orbital period
• Requires fuel
• Could have an impact on quality of orbit for
  Science
• Trade-off required

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Earth Mars Communications Subsystem

• 2 x Dual band transponder, 2x LGA S-band, HGA
  X-band
• 2 x Travelling Wave Tube Amplifier
• 2x Waveguide Interface Unit
• Status Nominal
• Total ON/OFF cycles 2900 ? 1.24 per day
• TWTA qualified for 20000 cycles ? not before 2020
• DRIVE-OFF strategy tested and will be implemented
  during power permitting seasons
• WIUs and RFDUs identical to Rosetta ? Life Time
  12 years
• Tests ongoing with additional ground stations to
  ensure downlink bandwidth
• Agreements for provision of S-band bandwidth till
  2008. X-band Safe Mode might have to be defined

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MEX Lander Communications Subsystem

• Designed for communications with Beagle-2, and
  cross-support with NASA landers
• Ultra High Frequency Transponder, Proximity-1
  protocol
• MELACOM re-programmable to allow for future
  protocols
• Communications tests with MERs successful
• DC-DC converter life limiting unit, designed
  for 50kRAD total radiation dose
• Further analysis required, yet initial
  engineering judgement by manufacturer is positive

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Attitude and Orbit Control

• Reaction Control Subsystem
• 414 N Main Engine - isolated
• 2x4 10 N Thrusters all manoeuvres can be done
  by 4 thrusters
• 2 Star Trackers, 30 deg angle between their
  optical axes Identical to Rosettas
• Reaction Wheel Assembly 4 wheels in skewed
  configuration
• 2 Inertial Measurement Units each including a
  set of 3 gyroscopes
• 2 Solar Array Drive Mechanisms Mechanism and
  Electronics identical to Rosettas
• 2 Sun Acquisition Sensors Identical to Rosettas

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Reaction Wheels and IMU

• 3 of the 4 Reaction wheels are sufficient for
  attitude control
• A thruster control mode is available (Safe Hold
  Mode)
• Reliability figures for the Reaction Wheels given
  w.r.t operations temperatures
• MEX RWs operate below these temperatures
• IMUs and Gyroscopes can be used in all
  combinations
• IMU 2 being used to maintain balance in usage
  periodic swaps foreseen

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IMU and Gyroscope Lifetime
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Data Management Subsystem

• Central Management Unit Identical to Rosettas
• Remote Terminal Unit Identical to Rosettas
• AOCSM Interface Unit Identical to Rosettas
• Solid State Mass Memory Extrapolation of
  reliability figures shows 0.95 reliability at 12
  years from launch, yet still providing at least
  half the memory capacity.

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Thermal Subsystem

• 5 heaters controlled by instruments
• 9 heaters payload interface thermal control
• 4 heaters no longer used since arrival at Mars
• 6 heaters used for AOCS equipment
• 8 heaters used for CPS lines and platform
  equipment
• Platform heater groups controlled by thermostats
• Thermostats mechanical lifetime of 1 million
  cycle at 2-3A

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Thermostat Fast Cycling

• Oscillations at stable thermal conditions
• Work-around implemented switch OFF affected
  circuit for 30 mins every 1.5 hour (leaving
  redundant circuit ON)

• Nr of cycles a day from 1200-1500 to 300-500

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Extension Feasibility

• Technical feasibility for long operational
  scenarios demonstrated
• Resulting mission scenarios will require
  adaptation of the science mission
• Trade-off required
• Structure required for preservation of the
  mission team
• Maintenance and preservation of the overall
  system will be needed

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Conclusions and Recommendations

• Obtain a more accurate estimation of the amount
  of propellant left onboard
• Investigate further the possibilities to optimise
  Reaction Wheel Off-loadings for minimum
  propellant consumption
• Optimise the fuel/oxidant mixture ratio if
  manoeuvres are foreseen
• Store the batteries at lowest possible
  temperature and State of Charge
• Trade off pros and cons of modifying the orbit to
  decrease eclipse duration
• Investigate ways to optimize heater power
  consumption
• Obtain lifetime test and/or analysis data from
  manufacturers of the various critical units
• Establish preliminary operational concepts and
  corresponding ground resource profiles that would
  allow actual implementation of the selected
  strategies

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Conclusions and Recommendations 2

• No show stoppers identified
• Active strategies required to preserve resources
  and units
• Impact on Science Mission
• Sooner rather than later