Electric Sail Technology Status Review Pekka Janhunen Finnish Meteorological Institute, (Kumpula Space Centre) ESA/ESTEC May 19, 2008

P.Janhunen, 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

P.Janhunen, 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

P.Janhunen, 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 MPa, Density 9 g/cm 3 – 99% Al, 1% Si: Tensile strength ~300 MPa, Density 2.7 g/cm 3 ● Dense metal has better micrometeoroid tolerance?

P.Janhunen, Tether manufacture ● Prof. Edward Haeggström, Univ. Helsinki, Electronics Research Lab – Presented by Henri Seppänen

P.Janhunen, 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)

P.Janhunen, 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

P.Janhunen, Main properties of designed guns

P.Janhunen, 40 kV gun design

P.Janhunen, Electron gun summary ● 40 kV, 2 kW, 50 mA gun: Mass 3.9 kg including power supply (2 kg) and radiator (0.9 kg) ● LaB 6 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, 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.

P.Janhunen, Tether Direction Sensors ● 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, Mechanical simulations ● Numerola Ltd company, Jyväskylä, Finland, together with P. Janhunen

P.Janhunen, Orbital calculations ● University of Pisa, prof. Giovanni Mengali, Alessandro Quarta

P.Janhunen, 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

P.Janhunen, 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)

P.Janhunen, 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)

P.Janhunen, 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

P.Janhunen, 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.