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September 18, 2008

Solar Electric Sail Developments and Plans


A simplified picture of the electric sail. An actual system would have 50 to 100 or more 20 kilometer wires. 100 kg spaceships could be accelerated to final speeds of 40-100 km/second. [Further refinement can enable a 800km/s top speed - the top solar wind speed]

This site has identified the solar electric sail as one of the top ten near term space developments that would have the most impact in increasing capabilities in space.

The preparation of components for an actual deployment in space of an electric sail is proceeding. There was an electric sail workshop by ESA ESTEC (European Space Agency) on May 19, 2008

A powerpoint where progress on electric sail work was presented.

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

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.

Commercial Uses of E-sails
Electric Sail is a propellantless non-impulsive propulsion method, suitable for small and medium payloads
● Electric Sail does not produce much thrust inside the magnetosphere, i.e. at Earth orbit
● Water mining and transporting from asteroids, for producing chemical propellant, is a way to use the E-sail to the utility of any space activity

Asteroid mining schemes
● Water
– Mine water at ice-containing asteroid (KY-26 ?)
– Transfer to Earth orbit by E-sailer
– Water customers at LEO, GTO or MEO:
● ISS
● Electrolysis spacecraft (Orbital Transfer Service for satellites)
● Platinum group metals:
– Challenge: mining
– Transfer by E-sailer to Earth reentry
– PGMs are rare on Earth (differentiated planet), needed as catalysts (fuel cells + other “green” tech's)
● Structural materials (bricks, stones, basic trusses)

How to mine water
● Straightforward way: Dig out material one piece at a time. Put piece into container, close the lid and heat. Container fills with vapour. Open pipe
into cold trap where let H2O condense.
● Another way: Enclose whole asteroid in gold covered bag so that it gets heated. Install pipe to a cold (white) bag where ice condenses. Might be feasible for small asteroid such as KY-26.
Benefit: insensitive to type of asteroid material.

Getting to Earth orbit from asteroid
● E-sailer used to get payload to Earth-Moon system rendezvous
● Lunar capture used to kill incoming delta-v (up to 1.5 km/s) ==> get into high elliptic orbit (stable for ~1 year)
● Use aerobrakings to lower apogee (using solar panels, like Mars missions do) until at GTO or LEO
● E-sailer can detach before Moon ==> no need to fly with E-sail through near-Earth region

Mining platinum group metals
● Many benefits and one big challenge
● Benefits:
– Easy to store in space during E-sail transportation
– Easy to sell once dropped to Earth
– Precious enough (> 10,000 eur/kg)
– Guaranteed, growing market (automotive industry)
● Challenge:
– Mining (enrichment) at asteroid is probably not simple
– Can be done, since can be done at Earth; but at what initial cost?

E-sail logistics chain. How to use that capability?
● Cheaper launch to GEO, MEO and make space operations beyond LEO cheaper

Is E-sail required for asteroid mining ?
● If icy asteroids exist nearby, water can be fetched by electrolysis rockets without losing too much on the way. But E-sail is more lightweight than any
electrolysis rocket.
● “Dry” ores reasonable to fetch by electrolysis rocket only if water is also mined nearby. E-sail is not dependent on any fuel supply.
● E-sail has better thrust/power ratio than ion engines, plus needs no propellant

Tether Material and Technology selection was made
Technology options covered:
-Laser-cut tether from metal sheet (efficiency? quality?)
-Metal-clad fibres (CTE? radiation?)
-Wire-wire bonding
---Laser welding
---Ultrasonic welding [This was chosen, others are fallback]
---Soldering (temperature range? CTE?)
---Glueing (reliability? CTE?)
---Wrap wire (not done at 20 um scale?)

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

Tether Reels
Baseline plan
-Spinning reel, maybe with capstains
-Outreeling only, or reeling both in and out
-Ordinary or magnetic bearing
-Other ideas also considered
TRL 4 level work can commence when at least few metre piece of tether is available.





Electron Gun
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

-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

Tether Direction Sensors
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.
-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

Integration of Components
-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

Control
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)


FURTHER READING
Comparison of solar electric sail to dandelion seed and tethering several e-sails together to move heavier objects.

The base design is for one hundred 20km wires that would generate 0.1-0.2 N thrust which gives 1-2 mm/s^2 acceleration to a 100 kg spacecraft. In one year this acceleration changes the velocity vector of the spacecraft by 30-60 km/s which is already an excellent achievement.

One can increase the thrust by increasing the number of tethers [no limit], their length [100 km with current material] and the power of the electron gun. In addition, it may be possible to use part of the electric power for radio frequency modulation of the electron beam, which may give a possibility to heat the electron population which is trapped in the potential well of the tethers. The heating expands the electron cloud, in other words the Debye length increases. Then the electric field of the tether penetrates a longer distance into the surrounding solar wind plasma so that the effective sail area of the tether increases, which increases the thrust. Modelling the electron heating is challenging, but testing it in space would be straightforward. For this reason, an electric sail test mission should be built as soon as possible. After becoming familiar with electric sail technology, one could increase its thrust perhaps even hundredfold, that is, to some tens of newtons, by using these techniques. Progress of material physics into industrial production of tethers made of carbon nanowires could possibly increase the upper limit of the thrust even further. With faster acceleration and more force the solar electric sail could reach the top speed of the solar wind at 800 km/s. [1,720,000 mph)]

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