The electric sail (ESAIL), invented by Dr. Pekka Janhunen at the Finnish Kumpula Space Centre in 2006, produces propulsion power for a spacecraft by utilizing the solar wind. The sail features electrically charged long and thin metal tethers that interact with the solar wind. Using ultrasonic welding, the Electronics Research Laboratory at the University of Helsinki successfully produced a 1 km long ESAIL tether. Four years ago, global experts in ultrasonic welding considered it impossible to weld together such thin wires. The produced tether proves that manufacturing full size ESAIL tethers is possible. The theoretically predicted electric sail force will be measured in space during 2013.
An electric solar wind sail, a.k.a electric sail, consists of long, thin (25–50 micron) electrically conductive tethers manufactured from aluminium wires. A full-scale sail can include up to 100 tethers, each 20 kilometres long. In addition, the craft will contain a high-voltage source and an electron gun that creates a positive charge in the tethers. The electric field of the charged tethers will extend approximately 100 metres into the surrounding solar wind plasma. Charged particles from the solar wind crash into this field, creating an interaction that transfers momentum from the solar wind to the spacecraft. Compared with other methods, such as ion engines, the electric sail produces a large amount of propulsion considering its mass and power requirement. Since the sail consumes no propellant, it has in principle an unlimited operating time.
The electric sail is raising a lot of interest in space circles, but until now it has been unclear whether its most important parts, i.e. the long, thin metal tethers, can be produced.
The team at the University of Helsinki is apparently the first one in the world to use ultrasonic welding to join wires together into a tether, says the team leader, Professor Edward Hæggström from the Department of Physics.
A single metal wire is not suitable as an ESAIL tether, as micrometeoroids present everywhere in space would soon cut it. Therefore the tether must be manufactured from several wires joined together every centimetre [Image 1]. In this way, micrometeoroids can cut individual wires without breaking the entire tether.
The tether factory has so far produced ultrasonic welds for one kilometre of aluminium tether
The Electronics Research Laboratory team started studying the production problem four years ago. At the time, the view of international experts in ultrasonic welding was that joining thin wires together was not possible. However, the one-kilometre-long tether produced now, featuring 90,000 ultrasonic welds, shows that the method works and that producing long electric sail tethers is possible.
The wire is produced with a fully automated tether factory, a fine mechanical device under computer control, developed and constructed by the team itself. [Image 2]. The tether factory at the Kumpula Science Campus in Helsinki, Finland, was integrated into a modified commercial ultrasonic welding device. Ultrasonic welding is widely in the electronics industry, but normally it is used for joining a wire to a base.
We have a challenging task, as keeping thin wires repeatedly in the precisely correct position is hard, says Timo Rauhala who works in the laboratory.
Approximately three metres of tether is currently produced per hour. Its quality is verified optically with a real-time measurement that inspects the connection of every individual joint. In the future, the production speed is to be raised and the weld quality will be assured during the production process.
The products of the tether factory will soon see action in space. The first opportunity will be the ESTCube-1 satellite, an Estonian small satellite to be launched in March 2013. ESTCube-1 will deploy a 15-metre long tether in space and measure the ESAIL force it is subjected to. This is ground-breaking as, so far, the theoretically predicted electric sail force has not yet been experimentally measured.
Next in turn will be the Aalto-1 small satellite from the Aalto University, to be launched in 2014, which will deploy a 100-metre long tether.
The deployed tethers are kept straight in space by the centrifugal force, the magnitude of which is five grams in a full-scale electric sail. The wire-to-wire welds of the ESAIL tether produced at the University of Helsinki will tolerate a pull of 10 grams.
Explore further:
EU project to build Electric Solar Wind Sail
More information:
www.electric-sailing.fi/
LagomorphZero
I would have expected the charge to do this, since wires of like charges repel. I'm not sure how well centripetal force would work for this especially for such a minute weight and long distance, but the math probably works for some values.
cantdrive85
Q-Star
A dense vacuum of aetheric plasma perhaps?
cantdrive85
I asked about an electrically neutral environment, space plasma is NOT electrically neutral. Try again, Mr. Hot Gas.
Q-Star
Well if you didn't like that guess, I'll try again.
Perhaps, since the electromagnet force is considered to act across infinite distances, maybe they point the sails this way and that way until they find a difference of potential in the EM field that pushes in the direction they wish to go,,,, just because space is neutral, doesn't mean there are no fields crossing it.
Or it could have something to do with plasma.
javjav
Both the solar wind and the electrical sail are charged, so the sail will experience EM force. The space environment is irrelevant.
Yarking_Dawg
Phil DePayne
javjav
that_guy
Found a half pound of 44 gauge speaker coil wire (41,600 feet/12.67KM) here for $36.
http://www.schatt...wire.htm
Let's extrapolate for fun:
100 wires x 3 threads x 20 KM length = 6000KM of wire/7.62KM per spool = 474 Spools = $17,000 dollars at consumer prices.
And all that wire would only weigh 100-200 Pounds.
Sounds pretty light and cheap, potentially.
cantdrive85
Yes, and the Earth is also a charged body in the solar plasma, and your statement still rings true.
that_guy
http://www.ebay.c...-content
So, about 100 pounds and $10,000.
@cant drive - Basically all of known space has some kind of (electro)magnetic field and charged particles flying through it. What's your point? Are you trying to make a thought experiment that doesn't apply to the known universe?
This sail exploits universal electromagnetic properties that apply anywhere we would travel that we know of - even far into the future or through intergalactic space. It seems that you completely forgot about the 'magnetism' part of electromagnetism.
Last i knew, electromagnetic fields did not suddenly stop simply because they encountered 'empty' space.
hoseppan
javjav
PPihkala
Maybe the original text was 5 G and 10 G, where G is the gravity acceleration constant, not gram as a measure of weight.
Eikka
The wires would be attracted towards the satellite body that holds the electron pump, because to charge the wires up it needs to store the opposite charge.
Without the spinning action, the sail would just collapse like a clinging shower curtain.
Eikka
There is a unit called gram-force, and a kilogram-force. It's also known as the kilopond, and it's approximately 9.8 Newtons.
GSwift7
This thing isn't being pushed by the magnetic field. It is being pushed by the momentum of the particles in the solar wind. The solar wind has a net neutral charge because it is a mixture of particles with positive, negative and neutral charge. By blocking either the positive OR the negative particles, you steal their momentum. Once again, it is physical momentum here, not the ambient magnetic field.
We have talked about this before. The solar field strength at one AU is only a few micro-gaus. That's not enough to be useful for propulsion.
I'm skeptical of this idea though. With this strategy you are only using one type of charge. You are missing out on all the opposite charge particles and the neutral ones. A solid reflective sail of the same size should be at least twice as efficient.
nonsense.
antialias_physorg
On he other hand: A soid sail would be more than twice as heavy (and also more prone to damage/harder to repair).
The sail has to be accelerated, too. So overall you're dealing with a transfer of impulse that results in a net velocity change (delta p equals m times delta v). If you halve the captured impulse but quarter the mass of your craft at the same time then you come out with double the velocity
GSwift7
The equipment that generates the charge in the ribbons and the power source for it are probably heavier than a solid thin film sail.
As for damage/reliability, it is difficult to think of anything more reliable than a solid reflective sail. The idea above relies on the electron pump and a power supply continuously functioning for the durration of use, which would be years at the very least.
I'm curious to see any kind of sail type device get a real test. We have been finding that the solar wind is much more turbulent than we thought. This may make any kind of solar wind propulsion unrealistic.
cantdrive85
You not only suffer from an inability to read and comprehend, but your preconceptions of plasma are COMPLETELY false and totally IGNORANT of the facts. I know how old dogs fail at new tricks, but you really need some new tricks.
antialias_physorg
Really depends on how big you make this stuff. Sail mass increases with the square of the radius.
The type of structure described in the article's mass increases linearly with the radius. (Holy mother of...is that the correct place to put the apostrophe? That looks weird.)
That means that no matter the additional mass there is some breakeven point where the ribbons are better.
I'm thinking microdebris. The ribbons you could retract individually if damaged and repair/replace - without losing too much thrust. For a sail you either have to completely furl it or fly out and do the repairs in space.
It's an interesting concept in any case.
GSwift7
lol.
Possibly a good point, though I have to wonder about that. I totally agree that you get an exponential mass advantage with the ribbon as your radius increases. However, shouldn't the power required to run the field also increase exponentially with radius?
By the way, small holes in a sail shouldn't matter. There's almost no force on the sail at any given point, so ripping shouldn't be an issue. If your sail is thin enough micrometeors should just make little holes and keep going.
I'm not a big fan of solar sailing in any form though.
antialias_physorg
The increase in power need should be quadratic (proportional to the 'virtual area' of the sail)
If we look at powerplants then the power is created in a volume* (nuclear, fusion, any kind of combustion - though that last one is irrelvant for space). So if you double the size you more than double the possible power output.
(*with the exception of solar and fuel cells which go with the area. With them we'd have a 1:1 relation between bigger sail and bigger powerplant)
But I really don't have the figures to do the weight calculations to make any kind of guess whether the breakeven point would come with hundreds of meters or megakilometers squared area.
I'm also thinking maneuverability. Realigning a sail sounds tougher than realigning ribbons (but that's a weak argument because 'sudden maneuvers' is not something that is needed in space)
We'll know soon enough. The test is this year.
GSwift7
GSwift7
I prefer "Not so Swift" though. My 7th grade history teacher came up with that one, and he was kinda cool, so I've kinda made it my own.
cantdrive85
That is not the case, this is not about a physical sail collecting the wind as on the sea. The object creates an electric field that reaches tens of meters beyond the length of the wires, it is the electric field which "catches" the wind, not the sail itself.
cantdrive85
http://www.electr...ling.fi/
A solar powered electron gun (few hundred watts) will power the above example.
TheGhostofOtto1923
cantdrive85
What I was implying there is that just as this Esail becomes part of the solar circuit in the above example, so too is the Earth part of that same solar circuit. The implication should be obvious, being what they are discussing is charged bodies within a radial electric field.
RealScience
The field must be strong enough to reflect most protons because any that slip through contribute no momentum. A large sail (relative to how far it will pull in electrons) reflects almost as many protons as it absorbs electrons, so in the ideal case it would gain twice the momentum of a proton for only ejecting a single electron of the same energy.
However given the proton energy distribution and edge effects one would be lucky to get close to half that, so it basically equates to an ion drive where the ions are harvested for free rather than having to be carried along.
For a short mission this doesn't matter, but for a long mission this provides unlimited fuel.
barakn
Is this the same fictional radial electric field driving particles of opposite charge in the same direction that we discussed before? You've had days to study this and figure out why it doesn't work. The fact that you're still bringing it up suggests you're too stupid to figure it out.
cantdrive85
Particles of sufficient energy will go as they may, cosmic rays come to mind.
GSwift7
lol. Cantdrive, you're a moron. The sun does have a magnetic field, but it is orders of magnitude weaker than you think. We have measured it. It is only a few micro-gaus around Earth. Not to mention the fact that it is AC and changes direction about once every 11 years. Funny how the solar wind doesn't change direction or even slow down when that happens. And since you also claim that the planetary orbits are electric, I guess they should all change direction and go retrograde too.
GSwift7
There is no longer any debate about the electric universe theories. Numerous and detailed observations and measurements have been done, both remotely with telescopes and directly with spacecraft. The nature of the magnetic field around the sun is not a mystery. We watch it and measure it every day. Probes like STEREO send us gigabytes of data about it every few hours. There's no secret current flowing through the solar system. If there was, we would be detecting it. We started examining the EM nature of the solar system way back in the 60's. This isn't an ignored or forgotten part of science. This has been studied in great detail. Just because you haven't read the material doesn't mean that it's not there for anyone to see.
The way it really works is actually rather beautiful. You should check it out sometime.
antialias_physorg
With the slight drawback that it works less well the further away from the sun you get.
But the unlimited fuel argument is certainly a big one when you consider long term missions like deflecting asteroids or just moving some closer to Earth for study/exploitation/refitting.
barakn
RealScience
The tether's specs give a force-to-sail-mass ratio comparable to what a 1-micron-thick polymer membrane mirror solar sail would give - I was expecting something quite a bit higher.
GSwift7
Yeah, I did some further reading as well. The biggest negative I see with an e-sail is that you can't control the thrust vector very much. As long as you're headed away from the sun that's fine, but if you want to get back it's a problem. The wiki says that you can only get the thrust vector 30 degrees off axis from directly away from the sun.
A reflective film sail can be used for both outbound and return trips. An e-sail is no good for a return trip.
cantdrive85
Actually, contrary to your claims, I said the electron flow would be radial about the Sun, with most of the influx at the equator. There will also be a polar flow ("cosmic rays" in these diagrams) as well, which can be seen in these two diagrams provided by NASA.
http://ulysses.jp...s_lg.gif
http://ulysses.jp...iral.gif
We also won't get into the elephant in the room that much of what Ulysses discovered was a "surprise" and didn't fit the currently accepted models of the Sun's EM field.
RealScience
The tethers could really have an advantage when they can be carbon nanotubes (or graphene sheet) rather than aluminum. We don't have the technology yet, but a half-theoretical-strength CNT tether could give a 2+ orders of magnitude better force to mass ratio, and thus some serious (~0.01G) acceleration.
GSwift7
A reflective solar sail's limit is around 60 degrees off axis from the sun versus 30 for an EM sail.
This is hugely significant if you want to do a return trip from somewhere like Mars.
Being able to get past the 50 degree angle is the cut-off point for being able to do it. You need to have more thrust slowing you down than you have pushing you away from the sun.
With an EM field sail you have 70% of your thrust still pushing you out at max sail angle of 30 degrees. With the reflective sail at 60 degrees tilt, you get 40% of your thrust pushing you away from the sun. The other 60% can slow you down so you start to fall back sunward.
RealScience
(I have not studied solar wind sails until this article, so I could be missing something obvious.)
As long as the tethers are close enough for their fields to overlap well (e.g., a net of tethers or numerous fine tethers), a well-designed solar wind sail should act as a mirror for protons coming from the sun. The dynamics of this should (to me at least) strongly resemble the dynamics of a light sail that is a mirror for photons coming from the sun. In either case the angle of the sail to the sun, rather than the sail type, should determine the thrust vector.
As long as the tethers are close enough together for the field to appear 'flat' to the protons, and as long as the diameter is large enough that edge effect are minor, why would a solar wind sail only be able to tack at half maximum angle of a solar light sail?
GSwift7
Ah, good question.
A magnetic field will always propogate out spherically. Each of the antanea is making a spherical field. By spreading them out far enough you can get a flattened 'surface' but it's not really flat.
It's actually an oblate spheroid with a curved 'front' surface. Imagine a really fat pancake, like the shape of an M&M candy. Once you turn it past a certain point, part of the 'front' of the surface falls into the shaddow of the rest of it, so you lose the reflection from that part.
A flat reflective sail does not cast it's own shadow onto itself at steep angles.
GSwift7
Also, the back side has the same charge as the front, so when you reach the angle that starts to cast a shaddow on part of the front, you also get a push in the opposite direction from the 'back' side of the field.
So you max out on your ability to slow down at about 30%, while the other 70% is speeding you up. (speeding up and slowing down is a loose term. Orbital angular momentum is actually what is working here)
You can't actually 'tack' with a solar sail. The only way to go downstream is to slow your orbit (decrease angular momentum) and let yourself fall back towards the sun.
RealScience
Which suggests that constant-voltage tethers that are more densely packed in the center is not ideal. A branching net with higher conductor density towards the edge would be an improvement. A higher voltage in an outer ring, and rings of lower and lower voltage further in would be even more of an improvement. MRI folks should have this well worked out for magnetic fields flat across the bulk of an area.
(I agree on 'tacking' - with no Venturi effect one cannot 'pull' towards the sun, so if your radial velocity exceeds escape velocity you are in for a long ride).
cantdrive85
"Although the direction of the thrust is basically away from the Sun, the direction can be varied within some limits by inclining the sail. Tacking towards the Sun is therefore also possible."
http://www.electr...ling.fi/
Oops, wrong again!
JRi
Or it would just form a sphere, like the hair of young children when electrically charged by friction by playing on a sofa.
http://www.youtub...1e0AaGGg
RealScience
It's not using the protons to pull you toward the sun. It's reflecting the protons to reduce angular momentum and then letting the sun pull the craft inward.
That's not the same as a sailboat tacking in which the sail is set at an angle that causes the wind to pull the boat into the wind (and alternating angles so that the average is straight into the wind).
Hence the bit of discussion in the posts above on whether 'tacking' was a a good term for it.
Now if you find details on what E-sail has to say about how much the sail can be inclined to the sun or the shape of the field, that would be interesting.
Tausch
yyz
RealScience
GSwift7
I'm not sure who wrote that part of the web page web page, but they are wrong.
With a max off-axis thrust vector of 30 degrees, you won't be able to tack back down stream. In the FAQ section of the we site you linked to, they say the following:
that statement is true, so I can't imagine why they said you can tack back towards the sun with this thing. Nobody on any other site about esails agrees with that. Not anybody reputable.
I'll look and see if I can find anything about it at JPL or somethhing.
Tausch
You rate the thanks I have for the good commentary the participants have contributed here with one?
GSwift7
Well, I can't find anything specific about it, probably because it's so hairbrained wrong.
Angular momentum works on the square of the radius, so with only 30% thrust max slowing you down and the other 70% pushing you out, you're going to head outbound like a bat out of hell compared to your ability to tack back inwards. The physics of this are really basic.
GSwift7
the lite account is a griefing bot that someone set up a long time ago. It's just an automated 1 giver. Don't worry about it. I think it's programmed to keep people's overall rating below 3.
In fact, don't bother rating anyone, and don't worry about your rating. It doesn't mean anything. I just use the ratings page on my recent activity to see when people might have responded to my comments, so I can answer questions and such.
cantdrive85
How can one "tack" towards the Sun?
By inclining the sail one can produce a thrust component which either brakes or accelerates the spacecraft in its orbit around the Sun, depending on which way one inclines it. If braking, one spirals inwards, if accelerating, one spirals outwards. This works as long as the sail's thrust is not so high that the radial component overcomes Sun's gravity. In practice, in the inner solar system one can then travel both inward and outward with the electric sail with travel times that range from months to some years, i.e. not faster than with traditional methods, but using no fuel.
Tassie Mike
Wires that long would want to be stationary,, movement in relationship to the Earth's magnetic field generates electricity -- Huh? How fast does a fuse blow?
GSwift7
Yeah, I saw that, but that's wrong. Good example of why you can't believe everything you read on the internet.
I already explained in detail why that's wrong. If you didn't understand, I'll be happy to answer any questions you might have. Which part are you confused about?
GSwift7
A vector works like a right angle triangle. If your thrust is 30 degrees off axis then the bulk of your thrust is pushing you away from the sun. It doesn't matter how much you slow down your orbit in that case, you still aren't going to get going back towards the sun. You might try to slow your orbit to zero and then cut off the field, but you'll still pick up a lot of outward momentum while you are trying to slow your orbit. I suppose it would be okay if you have a lot of time, but it seems pointless to use an esail in stead of a simple reflective sail if you want to make two-way trips.
RealScience
GSwift makes a good argument that a bulging electric field (such as from a constant-voltage disk) can't work at an angle that generates net inward orbital change, and the angle cited in papers supports this. It might be possible to sculpt the field enough to reach the critical angle (90-degree deflection of protons), but I haven't seen anything on E-sail's web site (or patent US 7641151) on flattening the field.
(There are mentions of spiraling inward, so I might have missed something and would be happy to have a technical reference pointed out.)
RealScience
But there is a relationship between angular velocity and depth in a gravity well.
Unless the protons are deflected more than some critical angle one loses more from outward thrust than one gains from reduced angular momentum. While one can increase eccentricity and decrease perihelion, one's semi-major axis still increases.
(At first glance the critical angle for a circular orbit would be 90-degree deflection).
Still one could produce an orbit where perihelion allowed gravity assist from a planet for braking.
RealScience
But for eccentric orbits the critical angle varies, approaching a minimum of 2*arcsine(1/√3)= ~70° deflection (35° flat-sail angle) near perihelion of a long ellipse.
If the sail can exceed that angle, then one can brake into a more eccentric orbit even at the expense of increasing total energy, and then get some serious braking in denser in the denser solar wind near perihelion.
Plus there's the option of changing one's orbit to get gravitational assist from the inner planets for the braking.
Not exactly nimble on an inward journey, but workable in a pinch.
GSwift7
Yes, and/or friction braking in an atmosphere, but that severely limits the things you can do and lengthens mission time in most cases.
Yeah, and the relationship is exponential.
The web site that cantdrive linked to just doesn't make any sense at all. They are directly contradicting themselves. I'm guessing that whoever wrote the page looked up info on traditional solar sails and made the incorrect assumption that it works the same for esails as it does for reflective sails. You can tell that the site is light on content. They were probably trying to fill empty space with fluff borrowed from other sources and didn't realize they were grabbing something that was wrong for their page. Probably written by a web author rather than a physics guy. That's not unusual.
RealScience
Cantdrive's link had a sub-link for a technical paper. Good analysis of the force, and it is not a flattened field; wires are so sparse that it isn't even a bulging disk field, so even the THRUST angle can only be steered a few tens of degrees.
But only the conclusion mentions spiraling inward, and (in contrast to the rigor in the body), in casual language with no supporting math or links. Therefore I am pretty skeptical that the conclusion was written by someone who understands the effect of thrust angles on orbital dynamics.
So 'braking' to go inward would indeed take a flattened field (more mass) or else involve planets for gravity assists (or air braking for thrill seekers!).
Tausch
http://www.electr...dia.html