Is an electromagnetic sail possible?

[Daumic]

A series of articles published recently (1) (2) described an effective shielding against a magnetic field. This effect is obtained by associating two concentric shells, one made of a superconductor, the other made of a ferromagnetic material. The shielding makes it possible to mask a volume from an external magnetic field or to dissimulate a magnetic field from the external world. It can in particular move away a magnetic pole from its opposite and thus to simulate a magnetic monopole artificially.

 

Perhaps this magnetic shielding would make it possible to create an electromagnetic sail that means a system of propulsion which would use the natural magnetic fields, in particular the terrestrial magnetic field. Imagine a ring of electric current placed in this magnetic field. Lorentz forces that are exerted on the electric current cancel: no translation of the ring is possible. On the other hand, if part of this ring is taken in magnetic shielding described above, the action of the magnetic field on the electrical current in the masked section is cancelled. Lorentz forces that are exerted on the whole of the electric current do not cancel any more: the ring can be put moving.

 

Is this sort of electromagnetic sail possible?

 

(1) Gomory, F. et al. Experimental realization of a magnetic cloak. Science 335, 1466 (2012)

 

(2) Prat-Camps, J. et al. A Magnetic Wormhole. Sci. Rep. 5, 12488; doi: 10.1038/

srep12488 (2015)

[swansont]

If the current ring penetrates the shield, then it will generate a field inside the shield. It's a barrier, not a neutralization device.

[Daumic]

If the current ring penetrates the shield, then it will generate a field inside the shield. It's a barrier, not a neutralization device.

 

Yes, the shield is a barrier that insulates the inner volume from the external magnetic field.

[swansont]

 

Yes, the shield is a barrier that insulates the inner volume from the external magnetic field.

 

 

"if part of this ring is taken in magnetic shielding described above, the action of the magnetic field on the electrical current in the masked section is cancelled" implies penetration of the shield. The shield does not zero out the field generated inside. Fields inside stay inside. Fields outside stay outside.

 

Basically you are proposing a reactionless drive. The issue isn't whether it will work; it won't. The issue is identifying the flaw in the reasoning.

[Daumic]

 

Basically you are proposing a reactionless drive. The issue isn't whether it will work; it won't. The issue is identifying the flaw in the reasoning.

 

 

Correction: in my first message, I speak about the Lorentz forces. The Lorentz forces concern the action of a magnetic field on a charged particle moving. The Laplace forces concern the action of a magnetic field on an electric current. With the electromagnetic sail, we are clearly in the second case: Laplace forces.

 

The external magnetic field is generated by an external electric current, for example the electric currents in the Earth’s core for the geomagnetic field. This external magnetic field generates Laplace forces on the part of the ring electric current no protected by the shield. This is action. The ring electric current generates also a magnetic field. A part of this magnetic field remains inside the shield. The other part of this magnetic field, produced by the part of electric current outside the shield, generates Laplace forces on the electric current generating the external magnetic field, for example the electric currents in the Earth’s core. This is reaction.

[swansont]

The Lorentz forces concern the action of a magnetic field on a charged particle moving. The Laplace forces concern the action of a magnetic field on an electric current.

Basically the same thing. A current is due to moving charges. qv X B and IL X B are equivalent statements.

 

As for action and reaction, they act on different objects, so that's really not an issue.

 

You have a current-carrying conductor in the earth's field. It will feel a force. A small one. Not enough to lift the wire and whatever is generating the current. It's not at all clear what role the shield is supposed to play.

[Daumic]

 

It's not at all clear what role the shield is supposed to play.

 

 

Imagine a conducting square circuit located at the terrestrial equator. The circuit is orthogonal to the terrestrial magnetic field. The direction of the electric current is such as Laplace forces are directed towards the outside of the circuit. The forces applied to the western and eastern parts of the circuit cancel each other. The low part of the circuit is coated with the magnetic shield described in my first message and thus does not undergo Laplace force. The high part of the circuit undergoes a Laplace force that is not compensated. The shield is necessary to create a dissymmetry in Laplace forces. This dissymmetry permit the movement of the electric circuit.

[swansont]

Imagine a conducting square circuit located at the terrestrial equator. The circuit is orthogonal to the terrestrial magnetic field. The direction of the electric current is such as Laplace forces are directed towards the outside of the circuit. The forces applied to the western and eastern parts of the circuit cancel each other. The low part of the circuit is coated with the magnetic shield described in my first message and thus does not undergo Laplace force. The high part of the circuit undergoes a Laplace force that is not compensated. The shield is necessary to create a dissymmetry in Laplace forces. This dissymmetry permit the movement of the electric circuit.

What about the effect of the field induced in the shield by the current?

[Daumic]

What about the effect of the field induced in the shield by the current?

 

According to the articles cited in my first message, the magnetic field generated by the part of ring current covered by the shield remains confined in the shield. This confinement is obtained by the currents induced in the superconductive shell of the shield that create an opposed magnetic field.

[swansont]

According to the articles cited in my first message, the magnetic field generated by the part of ring current covered by the shield remains confined in the shield. This confinement is obtained by the currents induced in the superconductive shell of the shield that create an opposed magnetic field.

Yes. And when you induce a field in a shield it extends outside of it. Inside you have canceled a field, but outside you have not.

[Daumic]

Yes. And when you induce a field in a shield it extends outside of it. Inside you have canceled a field, but outside you have not.

 

The article “A Magnetic Wormhole” cited in my first message describes an experiment where the poles of a magnetic field are separated by a magnetic shield. The magnetic field connecting the two poles is hidden inside the shield. This experiment shows that the magnetic shield is an efficient barrier for an inner magnetic field.

[swansont]

 

The article “A Magnetic Wormhole” cited in my first message describes an experiment where the poles of a magnetic field are separated by a magnetic shield. The magnetic field connecting the two poles is hidden inside the shield. This experiment shows that the magnetic shield is an efficient barrier for an inner magnetic field.

 

 

That's a different geometry than what I think you're describing. Perhaps a diagram would help clarify this, because as I interpret it, you are asking the shield to do something it can't do.

 

You won't get propulsion any greater than if you had a simple current-carrying loop in the earth's field.

[Daumic]

 

 

That's a different geometry than what I think you're describing. Perhaps a diagram would help clarify this, because as I interpret it, you are asking the shield to do something it can't do.

 

 

The following image summarizes my idea. The electric conductor is red. The magnetic shield is blue.

 

[Sensei]

What I advice everybody talking about magnetism is buying compass array device, like below one. I just took photo:

It's 2D version (there are also 3D version were magnetic arrows can spin in either axis, not just one axis)

 

 

You could place permanent/electro magnet on the one side, then some material, which you examine to shield against magnetic field created by magnet, and on other side compass array.

Compare results obtained with and without shielding material between them.

[Daumic]

The electromagnetic sail as space launcher

 

If this electromagnetic sail is possible, I see it as a space launcher. The terrestrial magnetic field extends to several thousands kilometers from ground. It could thus provide a support to an electromagnetic sail until in orbit.

 

To obtain an effective sail, the material constituting the conducting ring must carry the electric current most intense possible. The best choice for this material is a superconductor. To ensure that this superconductor can be used for an electromagnetic sail, the minimal condition is Laplace force that is applied to it higher than its weight. Following simple calculation shows that two criteria are significant to satisfy this condition: the critical current density that can carry the superconductor and its density.

 

Criteria of the superconductor for an electromagnetic sail

 

Let a conducting square circuit located at the terrestrial equator. The circuit is orthogonal to the terrestrial magnetic field. The direction of the electric current is such as Laplace forces are directed towards the outside of the circuit. The forces applied to the western and eastern parts of the circuit cancel each other. The low part of the circuit is coated with the magnetic shielding described in my first message and thus does not undergo Laplace force. The high part of the circuit undergoes a Laplace force that is not compensated. What should be this Laplace force to support the weight of the whole circuit?

 

F = I. B. L

 

With F: Laplace force applied to the higher part of the circuit,

I: intensity of the electric current,

B: horizontal component of the terrestrial magnetic field,

L: length of the higher part of the circuit.

 

P = 4. d. v. g

 

With P: weight of the whole circuit,

d: density of the circuit material,

v: volume of the higher part of the circuit,

g: acceleration of terrestrial gravity.

 

It is necessary that: F > P

 

I. B. L > 4. d. v. g

 

With s: section of the circuit,

j: density of the electric current.

 

j. s. B. L > 4. d. g. s. L

 

j. B > 4. d. g

 

Finally, to obtain a Laplace force higher than the weight, it is necessary that:

 

j > 4. d. g/B

 

The current density that the superconductor must carry to be used in an electromagnetic sail depends on its density and the ratio g/B.

 

First criterion: density of the superconductor

 

Among all known superconductors, it seems to me that the magnesium diboride shows the most interesting characteristics: a low density of 2.57 g/cm3 and a critical current density of 10000 A/mm2 (3). MgB2 seems to satisfy the minimal condition to constitute an electromagnetic sail. For example, in a horizontal magnetic field of 40 µT, the density of current necessary to ensure that the Laplace force equalizes the weight of a square circuit of MgB2 is 2520 A/mm2, level much lower than the critical current density of material.

 

To obtain a complete electromagnetic sail, it remains to add the mass of the magnetic shielding and especially the mass of the cooling system because the magnesium diboride becomes superconductive only below 40 K.

 

Second criterion: the ratio g/B

 

The Internet site (4) calculates the parameters of the terrestrial magnetic field anywhere on the terrestrial sphere and in altitude. It shows that the horizontal component of the terrestrial magnetic field, the only usable for a space launching, is maximum at the equator.

 

The acceleration of terrestrial gravity g also varies according to altitude. The following mathematical formula (5) calculates g:

 

g(h) = g0/(1 + (2h/R) + (h2/R2))

 

With g0: acceleration of terrestrial gravity to altitude 0,

g(h): acceleration of terrestrial gravity to altitude h,

h: altitude,

R: terrestrial radius.

 

The following table shows the variation of the magnetic field and terrestrial gravity according to altitude:

 

 

The data of this table are shown in the following graph:

 

 

These data show that the magnetic field decreases more quickly than the acceleration of terrestrial gravity when altitude increases. The g/B ratio thus increases with altitude. This ratio determines the effectiveness of the electromagnetic sail. The propulsion force of the electromagnetic sail decreases with altitude. As the electric current in the superconductor cannot exceed the critical current, there is a limit altitude beyond that the sail ceases working.

 

(3) http://iopscience.iop.org/article/10.1209/epl/i2002-00479-1/meta;jsessionid=E3DÇ22DC2891E013CCDD261AE163683.c1

 

(4) http://www.geomag.bgs.ac.uk/data_service/models_compass/igrf_form.shtml

 

(5) http://e.m.c.2.free.fr/poids-and-gravitation.htm

 

[Enthalpy]

Your drawing makes the proposal clearer.

 

Unfortunately, the magnetic tube won't shield the effect of the current that flows inside. In this case, the ferromagnetic material increases the induction where the material is, but doesn't reduce it elsewhere.

 

So as seen from Earth and its magnetic field, the current loop is still closed. It creates a torque (used on some satellites) but no net force to a first approximation.

 

In that configuration, thinking at the magnetic field created by the current loop is simpler than thinking at the effect of the tube on the geomagnetic field.

 

The only net force comes from the variation of the geomagnetic field with the position, but this effect is minute. I had checked it. It can't compensate the atmospheric drag at 800km even with strong permanent magnets reoriented over the orbit.

[Daumic]

Your drawing makes the proposal clearer.

 

Unfortunately, the magnetic tube won't shield the effect of the current that flows inside. In this case, the ferromagnetic material increases the induction where the material is, but doesn't reduce it elsewhere.

 

So as seen from Earth and its magnetic field, the current loop is still closed. It creates a torque (used on some satellites) but no net force to a first approximation.

 

In that configuration, thinking at the magnetic field created by the current loop is simpler than thinking at the effect of the tube on the geomagnetic field.

 

The only net force comes from the variation of the geomagnetic field with the position, but this effect is minute. I had checked it. It can't compensate the atmospheric drag at 800km even with strong permanent magnets reoriented over the orbit.

 

The magnetic shield described in the two articles cited in my first message is made of two layers: a layer made of ferromagnetic material, a layer made of superconductive material. It is true that the ferromagnetic layer increases the magnetic induction inside the shield. But the superconductive layer creates an opposed magnetic induction so the shield stops the magnetic field generated by the ring current.

 

This effect of the shield on the inner magnetic field is not the more important. The more important effect of the shield is the neutralization of the outer geomagnetic field on the part of ring current covered by the shield. This neutralization, made by the superconductive layer of the shield, permits the dissymmetry of the Laplace forces applied on the ring current and the movement of the whole sail.

[Enthalpy]

I believe to understand your argument. Unfortunately, it is complicated both to strengthen and to refute. Its main weakness is that the geomagnetic field creates a force on the shields too, not only on the wire, and this force isn't obvious to evaluate.

 

So if you permit, I'l go deeper in the argument about the magnetic field created far away - say, at Earth's center where currents are said to create the geomagnetic field - by the spacecraft. I know this isn't what you hope, but its' simpler and gives results. I may try to check the forces by the geomagnetic field on the shields too, but only later and maybe.

 

Where varied materials are present, this one remains usable:

rot(B)=µJ and the path integral of B is µ times the total current inside the path.

 

Now, I evaluate at a good distance the fraction of B created by the wire section enclosed in the shield.

Because both the ferromagnetic and superconducting shields end, no current can flow in them parallel to the wire. Consequently, the sum of the currents within a circle centered on the wire is the same as for the wire alone, and so is the path integral of B. When this part of the cricuit has cylindrical symmetry, so has the induction B, which is the same with and without the shields.

 

The spacecraft's induction interacts with the currents in Earth's core to produce the force which the shields don't change.

[Daumic]

An energy source is necessary

The electric current in a superconductor circulates without loss of energy. Can one imagine that an electromagnetic sail can go in space without energy source other than that necessary to the starting of the electric current? In fact, not, an energy source is necessary for the electromagnetic sail because its displacement in the terrestrial magnetic field generates an electric counter voltage in the superconductor that cancels the electric current. An energy source is necessary to maintain the electric current in the electromagnetic sail during space launching.

 

The Laplace force in the electromagnetic sail is:

 

F = I. B. L

 

With F: Laplace force applied to the higher part of the circuit,

I: intensity of the electric current,

B: horizontal component of the terrestrial magnetic field,

L: length of the higher part of the circuit.

If the circuit moves at the speed v, it will appear a counter voltage:

U = B. L. v

 

With U: counter voltage applied to the higher part of the circuit,

v: speed of the circuit in the magnetic field.

 

The power p needed to maintain the electric current is:

 

p = I. U

 

This electric power can be still expressed by:

 

p = (F / B. L ). ( B. L. v ) = F. v

The needed electric power is exactly the upward mechanical power.

[Enthalpy]

Fun.

 

But - and that's intriguing with electromagnetism - the effect on the conductor doesn't depend on the induction B at the conductor: it depends on the induction within the loop. Whether you write it d(flux)/dt or as a rotational, what matters is the induction B through the surface limited by the conductor loop.

 

In your example, the shield changes the induction at the conductor, fine. But it doesn't change the flux through the loop. Take a square loop for simplicity and a symmetric shield around the wire:

• The shield soaks some flux and channels it
• Half that flux would have passed through the loop without the shield and half outside
• With the shield, half that flux passes through the loop and half outside
• So the flux hasn't changed and the induced voltage neither.
• No effect by the shield on the usefully consumed electricity, nor consequently on the resultant force.

Sorry I again answered in a different way than you maybe hoped. Thinking in terms of loop voltage here is easier than checking where along the loop the voltage is induced.

 

This isn't an intuitive part of electromagnetism. EM is tricky indeed. The A field, the vector potential, would be more physical just here because its value at the conductor affects the induced voltage. Problem: we measure the induction B, not the potential A, and the materials respond to B rather, for instance in their saturation.

 

The vector potential A is often useful, especially to compute induced voltages in electromagnetic compatibility. Sometimes the shape of the conductors make infinite integrals if you try to evaluate a flux and a resulting voltage, while Biot&Savart give easily an answer from dA/dt.

 

----------

 

I know a similar example of induction at versus within the conductor.

 

Take a voice coil motor - you know, like in a loudspeaker. But this one is built with a short coil in a long uniform magnetic field, for instance because the iron poles saturate. You expect a force B*I*L, be the induction B uniform or not. But as the force produces work, you expect a counter-electromotive force which needs a flux variation. The flux varies indeed, despite the induction is constant at the conductor, because the induction varies far from the conductor, at the magnet or pole pieces within the loop. Every flux that passes from one pole to the other at any height element isn't in the pole any more beyond that height element, since the flux is conserved.

[Daumic]

 

In your example, the shield changes the induction at the conductor, fine. But it doesn't change the flux through the loop.

 

 

For me, the problem is here: the fact that a part of the loop is covered by the shield forbids any argument about a flux through the loop. The concept of magnetic flux through a conducting loop is valuable only in the case where the magnetic induction acts on the whole length of the loop.

[Moontanman]

In what universe is what is being discussed here a sail?

 

https://en.wikipedia.org/wiki/Magnetic_sail

 

https://en.wikipedia.org/wiki/Electric_sail

 

I am rather fond of this one

 

http://engineering.dartmouth.edu/~d76205x/research/shielding/docs/Winglee_00.pdf

 

http://science.nasa.gov/science-news/science-at-nasa/2000/ast04oct_1/

 

Am I missing something important here?

Edited April 14, 2016 by Moontanman

[Daumic]

In what universe is what is being discussed here a sail?

 

https://en.wikipedia.org/wiki/Magnetic_sail

 

https://en.wikipedia.org/wiki/Electric_sail

 

I am rather fond of this one

 

http://engineering.dartmouth.edu/~d76205x/research/shielding/docs/Winglee_00.pdf

 

http://science.nasa.gov/science-news/science-at-nasa/2000/ast04oct_1/

 

Am I missing something important here?

 

These documents are interesting but each one uses a principle of propulsion different of that which I propose.

 

https://en.wikipedia...i/Magnetic_sail

The magnetic sail or magsail is a proposed method of spacecraft propulsion which would use a static magnetic field to deflect charged particles radiated by the Sun.

 

https://en.wikipedia...i/Electric_sail

The electric sail (also called electric solar wind sail or e-sail) is a proposed form of spacecraft propulsion using the dynamic pressure of the solar wind as a source of thrust.

 

http://engineering.d.../Winglee_00.pdf

http://science.nasa....000/ast04oct_1/

The mini-magnetospheric plasma propulsion uses the interaction between solar wind with magnetic bubble generated by injection of plasma in the magnetic field of a coil.

 

The electromagnetic sail that I propose use the Laplace force applied to an electric current by the geomagnetic field. The concept nearest of this idea is the electrodynamic tether: https://en.wikipedia.org/wiki/Electrodynamic_tether

[swansont]

For me, the problem is here: the fact that a part of the loop is covered by the shield forbids any argument about a flux through the loop. The concept of magnetic flux through a conducting loop is valuable only in the case where the magnetic induction acts on the whole length of the loop.

F = IL x B for any length of wire. No loop required.

[Enthalpy]

"Sail" is already used for other modes of propulsion, but personally I don't care.

 

Computing by the flux through the loop holds in many cases. It suffices that the current is conserved over the turn, and then magnetic material doesn't change it. This hold in DC, as well as when the circuit is clearly smaller than a quarter wavelength. It would become uninteresting at higher frequency, when for instance a radiowave induces current in a reception antenna.

 

If you consider an electric motor or generator, they do have magnetic material and are computed by d(flux)/dt. In fact, it is highly desired (and achieved) that the machine's flux does not pass at the conductors, since the varying induction there would induce eddy currents in the copper, creating big losses. The conductor slots are designed to keep the flux and the induction away from the copper. Same for a transformer.

 

With equations: https://en.wikipedia.org/wiki/Maxwell%27s_equations

rot(E) = -d(B)/dt

holds always, whatever the materials and shapes, and so does

voltage = -d(flux)/dt

whether there is a metal around the loop or not, whether the flux passes through a core or not.

 

In some cases, for instance a straight antenna, the path closing a voltage loop isn't clear so this law becomes less useful. But in DC or at frequencies low enough, current paths are closed, and integrating the voltage along them is generally the fertile approach.