Electric Sail
Technology Status Review
Pekka Janhunen
Finnish Meteorological Institute,
(Kumpula Space Centre)
ESA/ESTEC
May 19, 2008
P.Janhunen, www.electric-sailing.com
Contents
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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
P.Janhunen, www.electric-sailing.com
Tether material & tech selection
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Initial material & technology study was made by Prof.
S.-P. Hannula et al. at Helsinki Univ. Tech.
Technology options covered:
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Laser-cut tether from metal sheet (efficiency? quality?)
Metal-clad fibres (CTE? radiation?)
Wire-wire bonding
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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
P.Janhunen, www.electric-sailing.com
Wire metal selection
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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:
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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?
P.Janhunen, www.electric-sailing.com
Tether manufacture
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Prof. Edward Haeggström, Univ. Helsinki, Electronics
Research Lab
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Presented by Henri Seppänen
P.Janhunen, www.electric-sailing.com
Tether reels
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Preparatory work by Lutz Richter, DLR-Bremen
Baseline plan
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Spinning reel, maybe with capstains
Outreeling only, or reeling both in and out
Ordinary or magnetic bearing
Other ideas also considered
Plan for proceeding
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TRL 4 level work can commence when at least few metre
piece of tether is available (either final-type or mockup, this
is TBD)
P.Janhunen, www.electric-sailing.com
Electron gun
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Prof. Mikhail Zavyalov, Pavel Tujrujkanov, E.N.
Evlanov, Space Research Institute IKI, Moscow
Three new designs produced, based on IKI heritage
hardware:
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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
P.Janhunen, www.electric-sailing.com
Main properties of designed guns
P.Janhunen, www.electric-sailing.com
40 kV gun design
P.Janhunen, www.electric-sailing.com
Electron gun summary
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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 <4 kg mass. Could have more than
one gun for redundancy.
P.Janhunen, www.electric-sailing.com
Tether Direction Sensors
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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.
P.Janhunen, www.electric-sailing.com
Tether Direction Sensors
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TRL 3 analysis done, basically
Modest-sized cameras enough
unless >10-15 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
P.Janhunen, www.electric-sailing.com
Mechanical simulations
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Numerola Ltd company, Jyväskylä, Finland,
together with P. Janhunen
P.Janhunen, www.electric-sailing.com
Orbital calculations
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University of Pisa, prof. Giovanni Mengali,
Alessandro Quarta
P.Janhunen, www.electric-sailing.com
Integration of components
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General approach
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Spinup strategy
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Spinup rockets
Siamese Twins
Placement of reels
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Design whole s/c around electric sail
Add electric sail to existing s/c design
At outer edge of s/c disk
At deployable booms at ends of solar panel arrays
High voltage path design (grounding plan)
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Whole s/c at high positive potential
Only reels and electron gun at high positive potential
P.Janhunen, www.electric-sailing.com
Control
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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:
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Reel in and out (needs reliable reeling of partly
damaged tether or thicker monofilament base tether)
Reel out only (must have enough spare tether)
P.Janhunen, www.electric-sailing.com
Flight algorithm
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Inputs (partly redundant):
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Output commands:
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Pointing direction of each tether (direction sensor)
Spacecraft potential (electron detector)
DC current flowing in each tether
Thrust (accelerometer)
Overall thrust (electron gun current and voltage)
Individual tether potentials (potentiometers)
Tether length fine-tuning (reel motors)
Running in parallel:
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S/C body spin state control so that it conforms with
tethers (star sensor and ACS)
P.Janhunen, www.electric-sailing.com
Technical Status Summary
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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
P.Janhunen, www.electric-sailing.com
Demonstration goals
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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.