There are sensitive instruments onboard and additional charges could lead to uninvited problem! By the way, which source you are going to use as charge suppliers? Remember, you do not want to increase the onboard instrumentation, the complexities unnecessarily. One prefers to maximize relevant instrumentation in a very limited volume!

Remember the craft is embedded in the Solar Wind. Depending on the polarity, it will attract or repel electrons in the wind so you need a small current to maintain a constant voltage, that means it uses some power. You need to calculate what voltage is required to get a desired thrust, estimate the power needed to manitain that and compare it with other methods.

Vikash,

the charge will be limited to certain insensitive regions of the spacecraft. This idea is like https://en.wikipedia.org/wiki/Electric_sail only that it interacts with the Earth's magnetic field and not the solar wind.

George,

the solar wind does not penetrate into the magnetosphere of planets. I assume you mean the ionosphere and the van Allen belts. The region between the two van Allen belts has almost no charge carriers. So you will have a very low chance of getting fully discharged at that region.

@ Max,

Which part of a spacecraft is insensitive? As I mentioned earlier, you must avoid unnecessary payload (charge generator) onboard a satellite. The electric sail you mentioned extracts the momentum of protons which come with the solar wind (source sun). 

The bottom line is optimization. If a source charge generator + a mechanism which can control the charge distribution on the surface of spacecraft does not increases the complexities unnecessarily, it could be given a try!

Max, I had assumed you were trying to use this for launching inter-planetary probes. I still think you need to calculate the rough voltage you need, I suspect the field on any local protrusion may be so high that you will get corona disharge.

George,

there is no way you will get a corona discharge without a fluid to ionize. Remember we are in a high vacuum.

Vikash,

a Faraday cage can be used to safe guard the sensitive instruments from high electric fields generated.

Let me clearly show the value of this invention.

Lets give our space craft a p.d of 100kV with a capacitance of 10-6 Farads. Lets place it in the Sun's magnetic field which is about 0.1Telsa. Lets assume we are moving at 10kms-1, we shall experience a force of 100 Newtons.

Lets assume our space craft is 10kg. Then it will be accelerated by 10ms-2 lets assume it is accelerated at this rate in the Sun's magnetosphere for only 3 hours.

Our velocity will be a staggering 108 kms-1

This is clearly worth undertaking.

From what I know, electrodynamic tethers work by utilizing the gradient in the magnetic field. This means that the shorter the tether is, the more power you need for the same effect. A "tether" entirely within a spacecraft would need a great deal of power indeed.

As to your last post, I'm not overly familiar with this sort of thing, but I don't see how you could give a craft, esp. a 6U cubesat (Which I assume you are referring to, given the 10 kg mass), such a large charge, and if you could, it would still take a massive amount of power.

ME: Lets give our space craft a p.d of 100kV with a capacitance of 10-6 Farads.

Your capacitance is wrong by about 107, it's roughly 1pF per metre of radius for a spherical craft so 10-13F would be closer.

ME: Lets place it in the Sun's magnetic field which is about 0.1Telsa.

Max, where did you get that number? That is about right for the surface of the Sun in a sunspot! The Earth's magnetic field at the surface ranges from 25 to 65 microteslas and the Sun's field at 1AU is of the same order so your value is too high by about 105. However you said you weren't using the Sun's field, you were working inside the Earth's magnetosphere. In orbit the Earth's field must be even smaller.

I haven't calculated it fully but a rough estimate gives a force less than 10-10N for 100kV.

ME: there is no way you will get a corona discharge without a fluid to ionize. Remember we are in a high vacuum.

For any practical force you will need such a high voltage that you will ionise every atom nearby (100N needs roughly 1017V). Either the free electrons or the ions (depending on polarity) will be attracted to the craft creating a current. It may be a small current but at that voltage it still requires a significant continuous power drain.

The other drawback of course is that, other than polarity, you have no way to direct the force.

George,

your right I over estimated the average magnetic field strength of the Sun and getting close to a Sunspot will be dangerous.

Let me be realistic.

If we are using long wire tethers of 2km in length we shall have a capacitance of 10-7 Farads. Lets assume a voltage of 1MV. We shall gain a charge of 0.1C . Lets have a velocity of 10kms-1. Taking the Earth's magnetic field of 10-4 Tesla we shall experience a force of 0.1Newtons. This force is more than the one got from current solar sail technologies and ion thruster engines.

Lets assume the space craft is 100Kg it will have an acceleration of 10-3 ms-2. If this acceleration stays constant for a day we shall be moving at 86ms-1.

Yes there can be slight corona discharge but a small current can be applied from solar power to cater for the discharge but we shall try as much as possible to stay away from charged particles.

If we have about three long wires we can alter the charges in each wire to move in a given direction and for orbital adjustments.

This idea is best used to cater for orbital decays so that satellites can stay in orbit indefinitely.

 Daniel,

lets use the above example.

The energy to charge the spacecraft will be charge times voltage which becomes 0.1C times 1MV which becomes 100kj. I don't think this is much energy since it can be provided by solar panels and is far less than the one consumed by ion thruster engines.

In this case I have assumed a constant velocity, but as the spacecraft speeds up we get a higher value of velocity which in turn produces a bigger Lorenz force on the spacecraft.

100kj of electric energy has given us a kinetic energy of 369,800j.

You can clearly see that you input little and get much that's the most valuable part in this idea.

ME: If we are using long wire tethers of 2km in length we shall have a capacitance of 10-7 Farads.

Your capacitance is still wrong Max, the capacitance of a complete sphere of 2km diameter is just 0.11uF. There's a table linked below, the final entry applies for the long wire. I haven't worked out the resulting value but I think you need to revisit that and check the diameter you assumed for the tether wire.

ME: If we have about three long wires we can alter the charges in each wire to move in a given direction and for orbital adjustments.

The direction of the force is perpendicular to the magnetic field and the present velocity, changing the charges would not alter that.

ME: we shall try as much as possible to stay away from charged particles.

The voltage will ionise them. Find out the gas density and work out the area of your tethers, then work out the volume swept out are the craft velocity in a second. Multiply by density to get the total mass and divide by the particle mass to get the number of ions per second. Multiply by charge to get the continuous current you need to supply and by voltage to get the power.

Your problem is that the capacitance (hence charge and force) will, to first order, depend on the tether surface area, but so will the discharge current.

https://en.wikipedia.org/wiki/Capacitance#Capacitance_of_simple_systems

https://en.wikipedia.org/wiki/Capacitance#Self-capacitance

George thanks for those links.

The best design would be of a circular disc stretched out by the centrifugal force of the spinning craft. To have a capacitance of 10-9 Farads it should be about 15 meters in radius. This will also produce a descent motion competitive with most modes of propulsion like ion thruster engines.

If it's moving with its edge the number of charged particles it will collide with to create a discharge will be minimal. It's best when it moves perpendicular to the Earth's surface.

We can use reaction wheels to spin the spacecraft in any given direction for orbital adjustments especially attitude control since it will be acting like a current loop in a magnetic field and this will change the effect of the Earth's magnetic field on it. .

ME: To have a capacitance of 10-9 Farads it should be about 15 meters in radius.

That sounds much more realistic.

ME: If it's moving with its edge the number of charged particles it will collide with to create a discharge will be minimal.

Right, and the simple drag will be minimal too.

ME: It's best when it moves perpendicular to the Earth's surface.

That never happens for an orbit, in fact it will be close to parallel for a typical low eccentricity orbit.

ME: ... acting like a current loop in a magnetic field ...

Yes, and the direction of the current is the direction of motion of the craft. For a circular equatorial orbit, the force could only be directed north or south. That's the biggest drawback. In particular, you can't use the force to accelerate the craft to combat orbital decay.

George,

now we have a spacecraft that moves with it's leading edge hence it minimizes electric discharge and orbital decay thanks to control moment gyros we can easily change the attitude of our spacecraft.

I don't think your right about the force direction of North to South, it is towards or away from the Earth's surface. Using the right hand rule the force is always out of the magnetic field.

We can use this to raise and lower  the orbit of a spacecraft and for deorbiting.

Imagine we are in an equatorial orbit moving in the West to East direction and we give our spacecraft a positive charge this will increase it's orbital height. If we want to move to a lower orbit or deorbit the spacecraft we shall give it a negative charge.

This brings about an efficient, simple and cheap solution to keep artificial satellites in orbit without using propellant that will eventually run out and render an artificial satellite out of service.

Lets assume in the near future we can get to a safe distance from a magnetar we shall achieve velocities of time travel.

.

ME: I don't think your right about the force in the North to South direction, it should be towards or away from the Earth's surface. Using the right hand rule the force is always out of the magnetic field.

Oops, you're right, that's a careless slip on my part.

ME: Imagine we are in an equatorial orbit moving in the West to East direction and we give our spacecraft a positive charge it will increase in it's orbital height

Not quite, you don't have a force acting along the orbit so it wouldn't change the orbital energy. The effect would only be to alter the eccentricity slightly if applied for a short time. To accumulate the effect you'd have to switch between positive and negative for half an orbit each.

The thrust your numbers would generate would be about 1% of what you would get from an ion thruster, and you still have to consider how you would generate the voltage. the answer is that you have to fire charged particles away from the craft. That could be an electron gun to give the craft a positive charge but needs to be a positive ion engine to make it negative. In other words, you need an ion thruster to create the charge, and that device will give much more thrust that the charge interaction.

The good thing about the thruster is that you can point it in any direction, and that allows you to boost the craft's speed. The Dawn craft added about 10km/s using its ion thruster.

George,

you won't have to change the charges per half orbit, one given charge will always point in one direction, always away from the Earth's surface for a given charge and always  towards the Earth's surface for the opposite sign. Try to simulate it again.

Lets assume we have a 15 meter radius spacecraft in a circular disc form, we shall have a capacitance of 10-9 Farads, if we have a pd of 10MV , at 10kms-1 we shall produce a force of 0.01 Newtons, this is about 40% the thrust of ion thruster engines.https://en.wikipedia.org/wiki/Ion_thruster using 25mN as the lower bond shown in the link.

A similar device to a  Van de Graaff generator can be used by converting electrical energy from solar panels to mechanical energy  then to electric potential energy. After charging is done one of the charged bodies is discharged using a current leaving a one sign charge on the spacecraft. This left charged body can be given a small current to cater for any discharge if available.

The biggest problem with ion engines is that the propellant will get exhausted and the spacecraft will go out of service in orbit.

ME: you won't have to change the charges per half orbit, one given charge will always point in one direction, always away from the Earth's surface for a given charge and always towards the Earth's surface for the opposite sign.

Correct, and that's the problem.

ME: Try to simulate it again.

You try, I've done lots of this. Apply a constant outward force and see what difference it makes to a Keplerian orbit of constant energy.

ME: A similar device to a Van de Graaff generator can be used by converting electrical energy from solar panels to mechanical energy then to electric potential energy.

Well that's hopelessly inefficient but a solid state high voltage generator would do the job. However, whatever method you use, that only separates the charges within the craft. You are forgetting you have no "ground" on a spacecraft so to create any potential at all in the way you want, you have to eject the charge from the craft.

Thank you George.

We now all agree that a spacecraft can use this technology for propulsion. Like all other propulsion techniques it has it's drawbacks, if everything required can be put in place it would be wise to test it.

What makes it beat other forms of propulsion like ion engines is being able to acquire more kinetic energy than the electrical energy consumed, simple to impliment and also not depending on propellant that eventually gets exhausted.

ME: We now all agree that a spacecraft can use this technology for propulsion. 

No we don't. Have you not noticed that I emphasised that the force was always perpendicular to the velocity? That means it never propels the craft, it can only change its direction without increasing its speed, it never changes the kinetic energy.

ME: What makes it beat other forms of propulsion like ion engines ...

What you are forgetting/ignoring is that you have to expel charged ions or electrons from the craft in order to give it a charge. In other words, you have to use an ion engine anyway!

George,

this propulsion technique is able to increase the gravitational potential energy of a satellite i.e It can increase and lower the orbital heights of satellites. It can also deorbit a satellite. A decrease in the gravitational potential energy of a satellite produces an increase in it's kinetic energy. So one can say this technique can change the kinetic energy of a spacecraft. The fact that we have a constant velocity that changes direction means it has an acceleration which can only be given by a propulsion system. This proves that this is a propulsion system.

There are other ways to give it charge without necessarily expelling ions as you mentioned. Let me give you an example, a van de Graaf and solid state generator produce charge by making one part positively charged and the other part negatively charge. We don't have to eject for example the negative part we can pass a current through it to neutralize it and leave the overall system with a positive charge.

Everything shows that this propulsion technique deserves a test.

ME: this propulsion technique is able to increase the gravitational potential energy of a satellite

No, the force is always perpendicular to the velocity so the energy doesn't change.

ME: i.e It can increase and lower the orbital heights of satellites.

It would initially push the path away from a circular orbit (for example), increasing the potential energy but decreasing the kinetic energy since more distant orbits are slower. The total energy would not change. The extra upward motion would overshoot so it would reach a maximum (apogee) and then fall back going inside the radius of the circular orbit. In other words, it would change the eccentricity but not the total orbital energy.

ME: It can also deorbit a satellite.

Yes, by increasing the eccentricity, you could decrease the perigee so the craft would dip into the atmosphere, but that's all. There's no point adding a system to do that when you can use the existing propulsion system.

ME: A decrease in the gravitational potential energy of a satellite produces an increase in it's kinetic energy.

Exactly, and vice versa. The total doesn't change because the force is perpendicular to the velocity.

ME: The fact that we have a constant velocity

You don't have a constant velocity, you just (correctly) said that reducing the potential energy increases the kinetic (and vice versa).

ME: We don't have to eject for example the negative part we can pass a current through it to neutralize it.

Don't be silly, charge is conserved. The Earth's magnetic field would act on both the positive and negative electrostatic charges which would mean the forces cancel out.

As a separate exercise, you could create an on-board current which would itself interact with the Earth's field but you could do that without the high voltage generator, it's a different proposal.

ME: Everything shows that this propulsion technique deserves a test.

It is already well known. Ion thrusters generate a space charge of a few kV which stops them working, so methods have to be included to get rid of it.

George,

I don't see anything silly in proposing that we generate positive and negative charges and neutralize the negative using a current to leave a positive charge. This is because having two equal magnitude opposite sign charges will produce no net force.

It is possible to give the spacecraft an increase in it's orbital energy by spinning the charges perpendicular to the direction of orbit. It has to be done in a way that a positive charge for example is span through 1800 towards the Earth's surface at high speed and the other 1800 spin away from the Earth's surface is done slowly.

ME: I don't see anything silly in proposing that we generate positive and negative charges and neutralize the negative using a current to leave a positive charge.

The only way to "neutralize the negative" is to add positive charge, you are back to square one.

ME: This is because having two equal magnitude opposite sign charges will produce no net force.

Exactly. The only way to give the craft a negative charge is to eject positive charge from it (or vice versa of course). That needs an ion thruster to fire away positive charges (or an electron gun to leave a positive charge).

ME: It is possible to give the spacecraft an increase in it's orbital energy ...

Not by the means you are proposing. The force on the craft is always perpendicular to its velocity and the power (rate of change of energy) is the 'dot product' of the force and velocity vectors which is always zero.

ME: .. by spinning the charges perpendicular to the direction of orbit.

The craft is charged, the charge is moving in the direction of motion of the craft and at 10's of km/s. There's nothing you can do about that. Any spin you give the craft would still create forces that average to zero over a rotation.

George,

what if we connect the negatively charged body to an anode connected to a battery, will the body get neutralized?

Otherwise we agree that using this system,

we can deorbit a satellite.

we can increase the eccentricity of our orbit.

we can increase the gravitational potential energy but at constant total orbital energy.

we can increase the kinetic energy by reducing the gravitational  potential energy.

I think we have tried to discuss the feasibility of this spacecraft propulsion technique. I hope it makes sense to people and it's tested.

ME: what if we connect the negatively charged body to an anode connected to a battery, will the body get neutralized?

No, you just have an internal potential difference. To get a net force on the craft, you have to create an imbalance between the number of positive and negative charges so you either need an ion thruster to expel positive charge and give the craft a negative charge,or an electron gun to give it a positive charge.

ME: we can increase the eccentricity of our orbit.

Yes, or decrease but by many orders of magnitude less than an ion thruster for the same voltage.

ME: we can increase the gravitational potential energy but at constant total orbital energy. we can increase the kinetic energy by reducing the gravitational  potential energy.

No. The Virial Theorem means that the mean over an orbit is fixed, if the PE is -2, the KE is +1. For an elliptical orbit, the PE and KE vary between perigee and apogee but the total is constant.

ME: I think we have tried to discuss the feasibility of this spacecraft propulsion technique

Yes, I think you've done that quite well.

George, I think you are wrong about the energy. The energy changes if you have an external force. the Virial Theorem is valid for a gravity only situation. Derived from the Equation of Motion in a two-body system. But that equation has to be updated with the force that you introduce.

Let's assume we have a circular orbit and perpendicular to its velocity vector a force (within the orbit plane), of course this will change the orbit's energy. The force will result in a radial accelertion, so what happens is that the radius is increased, i.e. the potential energy is increased (it is usually assumed as negative with 0 being the value for infinity, so an increase in radius, increases the potential energy), because E_pot = -mu/r . So yes the potential energy is increased, whereas the velocity (because the force acts perpendicular to it) is not touched by the acceleration. Assuming small changes because of a small interval, you can say that r is increased whereas v is unchanged. So there is an increase of total energy.  You will receive a formula for this if you use an updated version of the equations of motion (note, bold means vector, ' is time derivate:

original:

r'' = -mu r/r3

this becomes

r'' = -mu r/r3 + a_thrust

Anyway I still don't understand why you don't induct a voltage in the spacecraft, because it basically a conductor moving through a magnetic field, is it not?

Volker, think about the path that results. The radius is increasing therefore the velocity is no longer perpendicular to the line through the craft to the centre of the Earth. The applied force therefore has a component which is backwards along the orbit, (as does the gravitational "force" of course) which reduces the kinetic energy. Note that athrust is actually a vector, you can easily find that from the velocity, the field strength and the charge-to-mass ratio.

VM: Anyway I still don't understand why you don't induct a voltage in the spacecraft, because it basically a conductor moving through a magnetic field, is it not?

I was just addressing what Max had proposed, not suggesting alternatives.

I haven't read all the answers, so sorry if repeated someone's.

It seems that there's too much charged stuff on the orbit nearby to a satellite. And it will immediately compensate the charge of a satellite.So if one launches a charged satellite - this is useless.

If one somehow drops charge on the fly, then impact force might arise due anisotropy in dropped charges velocities opposite to isotropic speed direction distribution of incoming charges. But nothing here about the magnetic field.