Space sailing soon: A one-kilometer-long electric sail tether produced

Jan 08, 2013

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 to the spacecraft. Compared with other methods, such as , the electric sail produces a large amount of propulsion considering its mass and power requirement. Since the sail consumes no , 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 produced at the University of Helsinki will tolerate a pull of 10 grams.

Explore further: Google exec makes record skydive from edge of space

More information: www.electric-sailing.fi/

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LagomorphZero
not rated yet Jan 08, 2013
"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."

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
1.5 / 5 (16) Jan 08, 2013
How in the world could an electric sail work in an electrically neutral space environment?
Q-Star
3 / 5 (6) Jan 08, 2013
How in the world could an electric sail work in an electrically neutral space environment?


A dense vacuum of aetheric plasma perhaps?
cantdrive85
1.4 / 5 (10) Jan 08, 2013
How in the world could an electric sail work in an electrically neutral space environment?


A dense vacuum of aetheric plasma perhaps?

I asked about an electrically neutral environment, space plasma is NOT electrically neutral. Try again, Mr. Hot Gas.
Q-Star
3 / 5 (7) Jan 08, 2013
How in the world could an electric sail work in an electrically neutral space environment?


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
5 / 5 (4) Jan 08, 2013
How in the world could an electric sail work in an electrically neutral space environment?

Both the solar wind and the electrical sail are charged, so the sail will experience EM force. The space environment is irrelevant.
Yarking_Dawg
4 / 5 (4) Jan 08, 2013
The solar wind is actually a plasma, electrically charged, that's why the Earth's magnetic field protects us from it. The electric sail gets a push from the reflecting positively charges ions in the wind back toward the sun, and gets a slight pull from the negatively charged ions and free electrons that fly through the electric field they create. In theory, it should be far more efficient that a photonic solar sail since electrons and hydrogen ions have significant mass.
Phil DePayne
3 / 5 (8) Jan 08, 2013
This is real science, since data produced by these experiments will resolve a hitherto unmeasured property of nature. Not fraud science, as one might learn from the trolls online.
javjav
not rated yet Jan 08, 2013
In theory, it should be far more efficient that a photonic solar sail
It may worth it to complement both of them, adding a photo-voltaic sail, also able to produce electricity to reinforce the EM field of the electrical sail and to provide energy for on-board devices.
that_guy
5 / 5 (1) Jan 08, 2013
Did some research. 25-50 microns is appx 44-46 AWG gauge.

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
1 / 5 (7) Jan 08, 2013
How in the world could an electric sail work in an electrically neutral space environment?

Both the solar wind and the electrical sail are charged, so the sail will experience EM force. The space environment is irrelevant.

Yes, and the Earth is also a charged body in the solar plasma, and your statement still rings true.
that_guy
not rated yet Jan 08, 2013
Edit 194k (~60 KM) feet of 45 gauge wire (2 pounds) is $92
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
not rated yet Jan 09, 2013
However, solar wind has its limitations. Therefore the electric solar wind sail will stay in the solar system.
javjav
5 / 5 (1) Jan 09, 2013
However, solar wind has its limitations. Therefore the electric solar wind sail will stay in the solar system.
I agree, but it could be used as fuel for an ion engine later: Use both engines in hybrid mode near the Sun (it sounds to be efficient if the ship produces an EM field shared for both), then use the magnetic fields from planets passing away (specially Jupiter), and from that point start recycling the sail as a fuel for the ion engine (the article says the sail is made of Aluminium microfibers, an excellent material to produce Aluminium ions for the ion engine). Also bring a mini nuclear reactor & plutonium to produce electricity.
PPihkala
1 / 5 (1) Jan 09, 2013
"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."

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
not rated yet Jan 09, 2013
I would have expected the charge to do this, since wires of like charges repel.


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
5 / 5 (1) Jan 09, 2013
Maybe the original text was 5 G and 10 G, where G is the gravity acceleration constant, not gram as a measure of weight.


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
2.3 / 5 (6) Jan 10, 2013
How in the world could an electric sail work in an electrically neutral space environment?


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.

Yes, and the Earth is also a charged body in the solar plasma


nonsense.
antialias_physorg
5 / 5 (3) Jan 10, 2013
A solid reflective sail of the same size should be at least twice as efficient.

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
2.6 / 5 (5) Jan 10, 2013
On he other hand: A soid sail would be more than twice as heavy (and also more prone to damage/harder to repair).


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
1.3 / 5 (12) Jan 10, 2013
Not to Swift,
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
5 / 5 (3) Jan 10, 2013
The equipment that generates the charge in the ribbons and the power source for it are probably heavier than a solid thin film sail.

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.

As for damage/reliability, it is difficult to think of anything more reliable than a solid reflective sail

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
2.7 / 5 (7) Jan 10, 2013
Not to Swift,
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


lol.

That means that no matter the additional mass there is some breakeven point where the ribbons are better


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
5 / 5 (2) Jan 10, 2013
However, shouldn't the power required to run the field also increase exponentially?

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
2.6 / 5 (5) Jan 10, 2013
Yeah, there's an economy of scale with power plants on Earth, but you are limited here. With a mission time probably more than a decade, solar or RTG are about your only practical choices.
GSwift7
2.6 / 5 (5) Jan 10, 2013
cantdrive, I'm not the best at spelling and/or grammar, so I know I'm calling the kettle black here, but it should be "Not Too Swift" (two o's in too).

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
1.8 / 5 (5) Jan 10, 2013
A solid reflective sail of the same size should be at least twice as efficient.


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
1.8 / 5 (5) Jan 10, 2013
"A 20-km long electric sail wire weighs only a few hundred grams and fits in a small reel, but when opened in space and connected to the spacecraft's electron gun, it can produce several square kilometre effective solar wind sail area which is capable of extracting about 10 millinewton force from the solar wind. For example, by equipping a 1000 kg spacecraft with 100 such wires, one may produce acceleration of about 1 mm/s^2. After acting for one year, this acceleration would produce a significant final speed of 30 km/s. Smaller payloads could be moved quite fast in space using the electric sail, a Pluto flyby could occur in less than five years, for example."

http://www.electr...ling.fi/

A solar powered electron gun (few hundred watts) will power the above example.
TheGhostofOtto1923
1.8 / 5 (5) Jan 10, 2013
How in the world could an electric sail work in an electrically neutral space environment?

Both the solar wind and the electrical sail are charged, so the sail will experience EM force. The space environment is irrelevant.

Yes, and the Earth is also a charged body in the solar plasma, and your statement still rings true.
So, are you implying that if this sail does work then your theories are disproven? Because it sounds like this is what you are implying.
A solid reflective sail of the same size should be at least twice as efficient.
This would have more mass and be harder to manage, more expensive, and technologically farther in the future.
cantdrive85
1.5 / 5 (8) Jan 10, 2013
So, are you implying that if this sail does work then your theories are disproven? Because it sounds like this is what you are implying.


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
5 / 5 (4) Jan 10, 2013
While the solar wind is overall electrically neutral, the solar wind protons carry far more momentum than the electrons do. So a sail that reflects the protons and gains twice their momentum but absorbs electrons and shoots them out again will gain more momentum than one that absorbs all particles.

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
3 / 5 (4) Jan 10, 2013
The implication should be obvious, being what they are discussing is charged bodies within a radial electric field. -cantdrive85

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
1.6 / 5 (7) Jan 11, 2013
The implication should be obvious, being what they are discussing is charged bodies within a radial electric field. -cantdrive85

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.

Particles of sufficient energy will go as they may, cosmic rays come to mind.
GSwift7
2.3 / 5 (9) Jan 11, 2013
The implication should be obvious, being what they are discussing is charged bodies within a radial electric field. -cantdrive85

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.

Particles of sufficient energy will go as they may, cosmic rays come to mind


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
3 / 5 (8) Jan 11, 2013
cantdrive, ignoring basic facts and fundamentals, despite having been told about them numerous times, is either a sign of delusion or low mental function. Or both.

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
5 / 5 (2) Jan 11, 2013
For a short mission this doesn't matter, but for a long mission this provides unlimited fuel.

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
5 / 5 (5) Jan 11, 2013
In another thread cantdrive85 claimed that there was an equatorial outflow of electrons and protons but at the solar poles there was an influx of electrons. I pointed out that Ulysses data showed an outflowing solar wind of both electrons and protons over the poles, and it is twice as fast as the equatorial solar wind. The data doesn't jive with his world and he doesn't really understand it, so he ignores it.
RealScience
5 / 5 (2) Jan 11, 2013
I was surprised to read (under 'more information' that the solar wind pressure is 5000 times weaker the radiation pressure. Their tethers are very light per effective field area, but they therefore have to be 5000x lighter than a solar sail to achieve comparable effectiveness.
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
2.3 / 5 (3) Jan 11, 2013
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


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
1.5 / 5 (8) Jan 11, 2013
In another thread cantdrive85 claimed that there was an equatorial outflow of electrons and protons but at the solar poles there was an influx of electrons. I pointed out that Ulysses data showed an outflowing solar wind of both electrons and protons over the poles, and it is twice as fast as the equatorial solar wind. The data doesn't jive with his world and he doesn't really understand it, so he ignores it.

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
5 / 5 (2) Jan 11, 2013
@GSwift7 - A light sail isn't much better at tacking - there's no Venturi effect to help there, either.

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
2.6 / 5 (5) Jan 11, 2013
@GSwift7 - A light sail isn't much better at tacking - there's no Venturi effect to help there, either


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
5 / 5 (2) Jan 11, 2013
@GS - I'm curious as to why.
(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
3.3 / 5 (7) Jan 12, 2013
@GS - I'm curious as to why

I have not studied solar wind sails until this article, so I could be missing something obvious


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
3 / 5 (6) Jan 12, 2013
continued:

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
5 / 5 (2) Jan 12, 2013
Thank you for pointing out that I was missing something obvious - that the electric from a constant-voltage disk is not flat away from the edges (it does not hug the disk). I certainly should have seen that myself.

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
1.7 / 5 (6) Jan 12, 2013
This from the the E-sail website;
"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
not rated yet Jan 13, 2013
I would have expected the charge to do this, since wires of like charges repel.


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.


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
5 / 5 (2) Jan 13, 2013
@cantdrive - How does that make any of the discussion above wrong?
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
1.8 / 5 (5) Jan 13, 2013
In the desert called commentary this is an oasis. The best part is everyone knows the participants of such an exemplary exchange of understanding and learning - dialogue - that includes readers fortunate enough to have found such a source for their thirst. Thks.
yyz
5 / 5 (2) Jan 13, 2013
"Electric sail, photonic sail and deorbiting applications of the freely guided photonic blade" by Dr Janhunen has recently been posted on arXiv: http://arxiv.org/abs/1301.2100
RealScience
5 / 5 (1) Jan 13, 2013
@yyz - Thanks, interesting paper!
GSwift7
2.3 / 5 (3) Jan 13, 2013
cantdrive:

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:

When a tether is "horizontal" (i.e. in the ecliptic plane), however, it is pushed partly sideways perpendicular to itself. Averaged over all tethers, the thrust vector then has an angle which is approximately halfway between purely radial (solar wind) direction and the normal direction of the spin plane. Hence, if one inclines the sail e.g. 60 degrees, the thrust vector gets turned about 30 degrees off radial


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
1 / 5 (2) Jan 13, 2013
@lite
You rate the thanks I have for the good commentary the participants have contributed here with one?
GSwift7
3 / 5 (4) Jan 13, 2013
continued:

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
2.3 / 5 (3) Jan 13, 2013
@lite

You rate the thanks I have for the good commentary the participants have contributed here with one?


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
1.7 / 5 (6) Jan 14, 2013
This from the same "hair-brained" website;

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
1 / 5 (3) Jan 14, 2013
How in the world could an electric sail work in an electrically neutral space environment?


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
3 / 5 (4) Jan 14, 2013
cantdrive:

This works as long as the sail's thrust is not so high that the radial component overcomes Sun's gravity


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
3 / 5 (4) Jan 14, 2013
oops, the part I quoted is the part they got right. The part where they said you can go back towards the sun is wrong.

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
5 / 5 (2) Jan 14, 2013
@cantdrive85 - Angling a sail is clear, and one can angle a light sail sufficiently to get a thrust vector that slows one's radial velocity more than it pushes one outward. (Whether or not this is 'tacking' is a matter of semantics; you cannot 'pull' toward the sun the way sailboat tacking would, so if you are beyond the sun's escape velocity you are in for a long ride.)

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
5 / 5 (2) Jan 14, 2013
@GSwift -Slowing angular momentum and letting the sun's gravity do its job looked reasonable at first to me, too.
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
5 / 5 (1) Jan 16, 2013
@Gswift - I worked out the critical angle. It is indeed 90-degree deflection (45-degree sail if the sail is flat like a mirror) for a circular orbit. At that angle one adds potential energy as fast as one sheds orbital velocity energy.

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
2.3 / 5 (3) Jan 16, 2013
Still one could produce an orbit where perihelion allowed gravity assist from a planet for braking


Yes, and/or friction braking in an atmosphere, but that severely limits the things you can do and lengthens mission time in most cases.

But there is a relationship between angular velocity and depth in a gravity well


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
5 / 5 (1) Jan 16, 2013
@Gswift - Friction braking in an atmosphere with those tethers attached would be like a dandelion seed fluff caught in a flame if you went in too far, and even ultra-thin atmosphere would put a lot of stress on them (and reel them in to brake would add complexity.

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
1 / 5 (3) Jan 20, 2013
E sail videos and animations. Of course all the additional links as well. Many thanks for the links to the technical papers provided in this thread commentary as well.
http://www.electr...dia.html