Scientific Mission Applications

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Scientific Mission Applications

• P. K. Toivanen, P. Janhunen, and J.-P. Luntama

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Outline

• Example mission to Mars
• Optimal orbit to Mars
• Optimal operation of the sail
• Optimal operations and real solar wind
• Solar wind variations and sail performance
• Density variations
• Wind speed variations
• Average performance
• Tether voltage and navigation

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• Electric sail and science missions
• About mass budget of electric sail
• About economics of electric sail missions
• Interstellar Heliospheric Probe (IHP)
• Kuiper/centaur flyby mission
• Asteroid tour
• Space weather monitoring

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Optimal orbit to Mars

• Mengali, Quarta, and Janhunen
• Journal of Spacecraft and Rockets, 2008.
• Solar wind speed, 400 km/s
• Density, 7.3 cm-3
• Electron temperature,12 eV
• Radial scaling laws for the solar wind
  parameters
• Total mass 200 kg

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Optimal operation of the sail

• Optimal solution includes
• Initial acceleration of about 0.5 mm/s2 (Earth)
• Coasting phase (shading)
• Constant thrust angle of 20 deg
• Acceleration at Mars of about 0.3 mm/s2
• Travel time of 600 days

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Optimal operations andreal solar wind

• Varying density and speed
• Acceleration varies about 40 around the average
• Mars missed!
• But s/c kind of got there

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Solar wind variations andsail performance

• Some severe weather conditions
• Densities higher than 30 cm-3 may occur
• Solar wind speed may be higher than 1000 km/s
• Variations in acceleration far more mellow than
  those of the solar wind driving the sail

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Density variations

• Acceleration limited
• Electron current to the tethers increases
• Electron gun power limited by the given solar
  panel power
• Tether voltage drops

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Wind speed variations 1

• Acceleration is regulated
• Solar wind speed drive not linear

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Wind speed variations 2

• For small wind speed values
• Solar wind kinetic energy less than the tether
  electric potential
• Dynamic pressure term dominates
• For large wind speed values
• Solar wind kinetic energy larger than the tether
  electric potential
• Solar wind penetrates to the tether potential
  structure

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Average performance 1

• 3-month averaged thrust in cases of
• Limited tether voltage (40 kV, thick)
• No tether voltage limitation (thin)
• Variations relatively small around average at 70
  nN/m
• Missions can be desinged for the minimum thrust
  (dotted) without missing much of the maximum
  thrust (dashed)

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Average performance 2

• Thrust vs. solar panel power
• For small power values, difference between the
  maximum and minimum thrust not large
• For large power values, the minimum thrust
  saturates

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Average performance 3

• Thrust vs. averaging window
• Down to averaging over about ten days, difference
  between maximum and minimum thrust does not
  change dramatically
• Averages below ten days are not relevant in
  mission time scales

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Tether voltage and navigation

• Simple navigation procedure
• Onboard accelerometer
• Time-integrate measured acceleration for
  spacecraft speed, ?Vsc
• Compare hourly Vsc with speed at optimal orbit,
  V0
• If Vsc lt V0, increase tether potential by 5kV for
  the next hour
• If Vsc gt V0, decrease tether potential by 5kV for
  the next hour

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Electric sail and science missions

• High delta-v for small payloads
• Interplanetary Heliospheric Probe (IHP)
• Kuiper/Centaur flyby mission
• Asteroid tour
• Space weather monitoring
• Other missions
• Near-solar missions
• Planetary missions

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Electric sail propulsion system

• 100 X 20 km aluminium four-fold Hoytether
• Tethers 7.3 kg (20 µm)
• Reels 22.0 kg (3 X tethers)
• Electron gun radiator 1.5 kg (40 kV 1kW)
• High-voltage power source 2.0 kg
• Avionics tether direction sensor 7.0 kg
• Solar panels 6.0 kg (1.1 kW)
• Battery Li-ion 1.0 kg (8 Ah)
• S/c frame with thermal isolation 4.5 kg
• AOCS thrusters 1.0 kg
• Total 52.3 kg

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About economics of electric sail missions

• Payload more expensive than the launch
• Soyuz-fregat 1.3 ton payload to escape orbit
• Electric sailer with 1.3 ton payload
  accelerates slowly
• Smaller booster saves no that much
• 4-6 electric sailers per launch
• Piggybag

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Interstellar Heliospheric Probe

• Fast flight to interstellar medium
• Formation of the heliosphere
• Pioneer anomaly
• Present proposed mission time is tens of years
• Electric sailer is an enabling technology
• Reduced travel time
• Weight issue
• Use of several electric sailers

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Kuiper/centaur flyby mission

• Properties of primoidal objects
• Group of flyby probes, target per probe
• One launch with Siamise Twins spin-up for each
  pair
• Small payload (total mass 150-200 kg)
• Minimal instrument set only to study the target
• Fast travel time
• Fast flyby, data into memory and slow downloading

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Asteroid tour

• More for the same money
• Single electric sailer can visit several
  asteroids
• Water/hydrogen on asteroids
• Mineral composition
• Morphology
• Imager, radar, and spectroscope (infrared,
  neutron, and gamma)
• Shoot bullet with a railgun
• Laser heating
• Micrometeor flashes on dark side

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Space weather monitoring

• Off-Lagrange point monitoring
• Propellantless operation needed
• Longer than the 1-hour time delay to Earth (solar
  wind)
• Solar wind monitoring for other planet missions
  (as a piggybag)
• Tether voltage cycled
• off during monitoring
• on during orbit control