Electric Sail Technology Status Review

1
Electric SailTechnology Status Review

• Pekka Janhunen
• Finnish Meteorological Institute,
• (Kumpula Space Centre)
• ESA/ESTEC
• May 19, 2008

2
Contents

• Tether manufacture
• Edward Haeggström et al., Univ. Helsinki,
  Electronics Res. Lab
• Tether reels
• Lutz Richter, DLR-Bremen
• Electron gun
• Mikhail Zavyalov et al., IKI-Moscow
• Tether Direction Sensors
• Greger Thornell et al., ÅSTC-Uppsala
• Dynamic Tether Simulations
• Numerola Oy company PJ
• Orbital Calculations
• Giovanni Mengali et al., Univ. Pisa
• Integration of components

3
Tether material tech selection

• Initial material technology study was made by
  Prof. S.-P. Hannula et al. at Helsinki Univ.
  Tech.
• Technology options covered
• Laser-cut tether from metal sheet (efficiency?
  quality?)
• Metal-clad fibres (CTE? radiation?)
• Wire-wire bonding
• Laser welding
• Ultrasonic welding
• Soldering (temperature range? CTE?)
• Glueing (reliability? CTE?)
• Wrap wire (not done at 20 um scale?)
• Ultrasonic welding selected, others are fallbacks

4
Wire metal selection

• Requirements Good yield strength, preferably at
  least steel-class conductivity
• No brittle-ductile transition at cold temperature
• Generally Alloying can improve yield strength,
  but usually destroys conductivity
• Good-conductivity alloys
• 90 Cu, 10 Ag Tensile strength 1000-1600 MPa,
  Density 9 g/cm3
• 99 Al, 1 Si Tensile strength 300 MPa, Density
  2.7 g/cm3
• Dense metal has better micrometeoroid tolerance?

5
Tether manufacture

• Prof. Edward Haeggström, Univ. Helsinki,
  Electronics Research Lab
• Presented by Henri Seppänen

6
Tether reels

• Preparatory work by Lutz Richter, DLR-Bremen
• Baseline plan
• Spinning reel, maybe with capstains
• Outreeling only, or reeling both in and out
• Ordinary or magnetic bearing
• Other ideas also considered
• Plan for proceeding
• TRL 4 level work can commence when at least few
  metre piece of tether is available (either
  final-type or mockup, this is TBD)

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Electron gun

• Prof. Mikhail Zavyalov, Pavel Tujrujkanov, E.N.
  Evlanov, Space Research Institute IKI, Moscow
• Three new designs produced, based on IKI heritage
  hardware
• 300 V low-voltage gun for ionospheric testing
• 20 kV/2kW baseline model for solar wind
• 40 kV/2kW enhanced voltage model for solar wind

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Main properties of designed guns
9
40 kV gun design
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Electron gun summary

• 40 kV, 2 kW, 50 mA gun Mass 3.9 kg including
  power supply (2 kg) and radiator (0.9 kg)
• LaB6 cathode lifetime theoretically should be at
  least 10 years in high vacuum
• Overall, electron gun situation looks good gun
  which actually exceeds our power requirement
  (400 W) several times has lt4 kg mass. Could have
  more than one gun for redundancy.

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Tether Direction Sensors

• Greger Thornell, Henrik Kratz, Ångström Space
  Technology Center, Uppsala
• Status Preliminary TRL 3 -level analysis done in
  collaboration with ÅSTC and PJ
• Initially, also Univ. Liege (P. Rochus et al.)
  looked at the topic
• Main idea Detect tethers optically with stereo
  camera, Reconstruct 3-D directions from images
  onboard
• Purpose Tether lengths must be actively
  fine-tuned to avoid their collisions. One must
  first detect them.

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Tether Direction Sensors

• TRL 3 analysis done, basically
• Modest-sized cameras enough unless gt10-15 AU
  distance
• May have to mat-finish wires to create diffuse
  reflectance
• Seeing root of tether enough to determine its
  direction
• Seeing the tip would be good as tether breakage
  alarm

13
Mechanical simulations

• Numerola Ltd company, Jyväskylä, Finland,
  together with P. Janhunen

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Orbital calculations

• University of Pisa, prof. Giovanni Mengali,
  Alessandro Quarta

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Integration of components

• General approach
• Design whole s/c around electric sail
• Add electric sail to existing s/c design
• Spinup strategy
• Spinup rockets
• Siamese Twins
• Placement of reels
• At outer edge of s/c disk
• At deployable booms at ends of solar panel arrays
• High voltage path design (grounding plan)
• Whole s/c at high positive potential
• Only reels and electron gun at high positive
  potential

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Control

• Tethers have two degrees of freedom in spinplane
  and perpendicular to spinplane
• Thus we need two controls potential (controls
  solar wind force) and length (controls angular
  speed)
• Length fine-tuning strategies
• Reel in and out (needs reliable reeling of partly
  damaged tether or thicker monofilament base
  tether)
• Reel out only (must have enough spare tether)

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Flight algorithm

• Inputs (partly redundant)
• Pointing direction of each tether (direction
  sensor)
• Spacecraft potential (electron detector)
• DC current flowing in each tether
• Thrust (accelerometer)
• Output commands
• Overall thrust (electron gun current and voltage)
• Individual tether potentials (potentiometers)
• Tether length fine-tuning (reel motors)
• Running in parallel
• S/C body spin state control so that it conforms
  with tethers (star sensor and ACS)

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Technical Status Summary

• Tether manufacture Progressing well, required
  before test mission can fly
• Tether reels No serious problems seen, but must
  be done to demonstrate reeling of final-type
  tether
• Electron gun Straightforward (could use spare
  cathodes/guns for redundancy)
• Tether direction sensors Should be
  straightforward
• Dynamic tether simulations No problems seen, but
  should be done more comprehensively still
• Orbital calculations OK
• Overall design OK

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Demonstration goals

• Reel to reel tether production (10 m, 100 m, 1
  km, 10 km) with quality control
• Reliable reeling of the tether
• After these, one can make decision to build test
  mission. Technological development risk remaining
  after this is small.