Lorentz Drive

[sevenperforce]

Was just thinking...

Magnetic field lines produce a measurable force perpendicular to the velocity vector of a charged particle. This is the Lorentz force.

As explained in this paper, care must be taken that geostationary comsats do not acquire net-positive or net-negative electrical charge, as their orbital motion through the magnetic field of the Earth can cause perturbations if they are charged, which can change their apsides.

Earth's magnetic field protects us from high-energy charged particles because they are deflected to the poles and to the Van Allen Belts via Lorentz forces.

A rail gun is fired at high velocity when the DC electron flow through the projectile is traveling perpendicularly to the magnetic field lines produced on the rails. This is perhaps the most violent exhibition of the Lorentz force.

If the magnetic field at geostationary differences is enough to perturb the motion of comsats, would it be possible to build an effectively-reactionless drive using either properly-oriented current flow across long wires, or by using counter-rotating charged wires in low earth orbit?

Obviously it is not truly reactionless; it's pushing against Earth's magnetic field. And obviously the effective thrust would be quite low because Earth's magnetic field is so weak. But it might be enough for station-keeping or spiraling between orbits, right?

I will need to break out an old textbook or two in order to work out the maths....

[hms_warrior]

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

Yes it is a valid concept.

[sevenperforce]

20 minutes ago, hms_warrior said:

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

Yes it is a valid concept.

**headdesk**

[MaverickSawyer]

1 hour ago, hms_warrior said:

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

Yes it is a valid concept.

1 hour ago, sevenperforce said:

**headdesk**

 

If this isn't an example of "great minds think alike", then I don't know what is.

[p1t1o]

This was a pure moment. I could frame this thread and put it on my wall

 

[hms_warrior]

Don't know what you guys talking at. JAXA did allready a experiment of this as a possible de-orbit "engine".

But nevermind the physics of this are all valid and even need to be considered by communication-satelites(as disturbing effects on their orbit).

Some guys need to do their homework better...

Edited April 28, 2018 by hms_warrior

[YNM]

1 minute ago, hms_warrior said:

JAXA did an experiment of this as a possible de-orbit "engine".

Reverse the whole thing, and it's a station-keeping engine !

But yeah, that's as far as you can go. I don't think you can reach escape velocity by EDT alone.

[p1t1o]

5 hours ago, hms_warrior said:

Some guys need to do their homework better...

Or glance at the 2nd post?

[Zeiss Ikon]

5 hours ago, YNM said:

I don't think you can reach escape velocity by EDT alone.

Thrust is thrust.  Apply pulses centered on, say, 200 km perigee, thereby raising apogee until the last thrust gives you an escape trajectory.  Just like using an ion engine or solar sail.  And the EDT should still work in Solar-orbit space, since the Sun's magnetic field pervades the entire Solar System (that field is a great deal weaker than Earth's, but you can go to continuous thrust once you're past Earth's magnetopause or on an escape orbit).

One practical limit on this is that you have to collect enough electrons on the negative end of the tether to offset the electron gun that keeps the positive end positive, and thus avoid building up a huge positive charge on the spacecraft (which, if large enough, may cause your electronics to malfunction).  There are electrons in the solar wind, of course; in fact, it may be more practical to collect electrons outside Earth's influence than it is inside the Van Allen belts.  The other limitation is that you need a magnetic field to react against -- get outside the Solar System and you're dependent on the galactic field, which is much, much weaker than the Sun's field (even at the edge of the Oort Cloud).

Then there's the problem of keeping the tether stretched.  In LEO, the tidal effects of Earth's gravity can do that job.  Get out in Solar orbit, and the available tides won't be enough to keep the positive end of the tether from attracting the negative end.  You'd have to make the "tether" rigid enough to resist this attraction, which adds mass to the "engine".

[YNM]

50 minutes ago, Zeiss Ikon said:

Apply pulses centered on, say, 200 km perigee, thereby raising apogee until the last thrust gives you an escape trajectory.

But ion engines don't really gives loads of acceleration...

I could only assume EDT don't give loads of acceleration as well.

[Zeiss Ikon]

8 hours ago, YNM said:

But ion engines don't really gives loads of acceleration...

I could only assume EDT don't give loads of acceleration as well.

Exactly true.  The magnetic fields involved are pretty weak (lots of Tesla, but not many Gauss), and there's a limit to how much current you can run through the tether, because you can only collect and pump so many electrons through the system.

[sevenperforce]

The question then presents, what's the minimum realistic size for a solar-powered tether-based propulsion system large enough to overcome its own drag in LEO, and how far would you need to scale it up in order to have an infinite-persistence LEO robotug?

I am sure that if I do a little digging I will find whitepapers that have already been written about this.

[Zeiss Ikon]

Originally, the electric tether was intended to generate power independent of solar panels.  You mount an electron emitter on one end, an electron collector on the other, and the motion through Earth's magnetic field makes a current that you can tap to (for instance) charge batteries.  Down side is, you're lowering your orbit whenever the current is running.  So, someone got the idea to force the current the other direction (using a fuel cell, solar panel, or even RTG).

The longer the cable, the less current you need for a given thrust, and the better it stabilizes itself with one end pointed at Earth (which is a desirable direction to give thrust/drag more or less aligned with the equator).  It'll all be tradeoffs -- longer cable, higher mass, shorter cable, more power generation required.  Higher thrust requires longer cable and/or more power, more compact system is limited to lower thrust.

[p1t1o]

It seems difficult though. The wire experiences thrust perpendicular to its axis no? How do you turn that into useful thrust without pulling the wire out of alignment with the field? And if its long enough, its going to want to form a ring-segment aligned with the equator just by virtue of mass and gravity.

 

Edited May 1, 2018 by p1t1o

[sevenperforce]

36 minutes ago, p1t1o said:

It seems difficult though. The wire experiences thrust perpendicular to its axis no? How do you turn that into useful thrust without pulling the wire out of alignment with the field? And if its long enough, its going to want to form a ring-segment aligned with the equator just by virtue of mass and gravity.

One option would be to eschew the solid-state approach. If you're using a "fixed" (positionally; obviously it's orbiting) tether, then you run electrical current through it to achieve your "charged particle moving perpendicular to a magnetic field" state. What if, instead, you constructed a capacitor comprising two separate, helical ribbons which counter-rotate? I'd have to take a close look at the geometry, but it's a way to potentially pack a large "cable" into a fairly small volume and avoid the need for a large electrical current flow altogether. It would be essentially an electromagnetic propeller/propulsor. Perhaps a cyclorotor arrangement could be possible.

If the capacitor approach was too challenging (or if the geometry didn't close) then you could still use a counter-rotating propulsor that used an AC current to move electrical charge back and forth within the rotating frame. Again, the geometry is challenging (a bit like trying to make an overbalanced wheel), but there is a working fluid here so it should be possible.

[YNM]

54 minutes ago, p1t1o said:

How do you turn that into useful thrust without pulling the wire out of alignment with the field?

Counterweight, gravity gradient.

 

And this is why you're going to have problemS when making a high-thrust version in any means.

[Zeiss Ikon]

21 hours ago, p1t1o said:

It seems difficult though. The wire experiences thrust perpendicular to its axis no? How do you turn that into useful thrust without pulling the wire out of alignment with the field? And if its long enough, its going to want to form a ring-segment aligned with the equator just by virtue of mass and gravity.

Kerbal Space Program doesn't correctly model tides, but if you have Principia, you can build a long, flexible object in orbit (a string of twenty or so empty FL-T800 tanks should do it) and watch it stabilize -- not like a segment of an equatorial ring, but rather radial/vertical.  This is because the pull of gravity is slightly stronger on parts closer to the parent body, slightly weaker on parts further away.  This means that a body with sufficient length, initially not tumbling, and no active stabilization will always stabilize in a vertical/radial position, because one end will always get a little closer than the other, and then the tidal forces (which is what this phenomenon is called) will pull it into the radial position.

This has actually been done in orbit at least twice; once with a 20 km wire tether extended from a Shuttle cargo bay (the wire snarled and then broke about 10 km out).  Not only was the wire radial, there was enough tension to keep it pulled straight once a couple hundred meters was unreeled.

[p1t1o]

45 minutes ago, Zeiss Ikon said:

Kerbal Space Program doesn't correctly model tides, but if you have Principia, you can build a long, flexible object in orbit (a string of twenty or so empty FL-T800 tanks should do it) and watch it stabilize -- not like a segment of an equatorial ring, but rather radial/vertical.  This is because the pull of gravity is slightly stronger on parts closer to the parent body, slightly weaker on parts further away.  This means that a body with sufficient length, initially not tumbling, and no active stabilization will always stabilize in a vertical/radial position, because one end will always get a little closer than the other, and then the tidal forces (which is what this phenomenon is called) will pull it into the radial position.

This has actually been done in orbit at least twice; once with a 20 km wire tether extended from a Shuttle cargo bay (the wire snarled and then broke about 10 km out).  Not only was the wire radial, there was enough tension to keep it pulled straight once a couple hundred meters was unreeled.

Do you know, I have no idea what I was thinking there, I know about tidal forces...brain fart.

I think I read something somewhere about one failure mode of a snapped space elevator cable has the cable whip itself into an equatorial attitude but thats a whole other kettle of fish and I must have made a wrong connection.

[Zeiss Ikon]

Failures of a geosynchronous tether are complex.  How it fails depends strongly on where it fails.  If the cable comes loose from the counterweight but is still attached at the bottom, you get the "equatorial firestorm", with the cable wrapping all the way around Earth.  If it breaks loose at the bottom, it will lift up (due to release of the anchoring tension) and drag through the atmosphere, or even pull right out of the atmosphere (a couple hundred km vs. the 35000+ km length of the entire cable), then remain vertical due to tidal forces as it orbits.

If there's a failure condition where the (nearly complete) cable manages to wind up horizontal, it's a strictly temporary condition; horizontal is not a stable position for any slender body in any orbit.

[sevenperforce]

12 hours ago, Zeiss Ikon said:

Failures of a geosynchronous tether are complex.  How it fails depends strongly on where it fails.  If the cable comes loose from the counterweight but is still attached at the bottom, you get the "equatorial firestorm", with the cable wrapping all the way around Earth. 

IIRC, concerns of an equatorial firestorm are greatly exaggerated. The cable would be very very lightweight and so it would either float down gently or it would burn up too high to cause problems.

[Zeiss Ikon]

The cable can't be that light weight.  Even if it's the size of a party ribbon at the ground, it'll be tens of meters wide and thick at GEO station, just to hold up the weight of the cable below.  In fact, the thickness ratio (anchor to GEO) is one of the determining factors on whether a space elevator is even possible.  If you use a material as weak as mere glass fiber, you can't make the cable thick enough to hold itself up on Earth.  Carbon fiber does better, but even with carbon nanotube fibers, you'll still have thousands of tons of cable above the atmosphere by the time you can lift a useful load.  The "float down gently" will turn into "thousands of kilometers of giant meteorite" by the time GEO station enters atmosphere.

[sevenperforce]

9 minutes ago, Zeiss Ikon said:

The cable can't be that light weight.  Even if it's the size of a party ribbon at the ground, it'll be tens of meters wide and thick at GEO station, just to hold up the weight of the cable below.  In fact, the thickness ratio (anchor to GEO) is one of the determining factors on whether a space elevator is even possible.  If you use a material as weak as mere glass fiber, you can't make the cable thick enough to hold itself up on Earth.  Carbon fiber does better, but even with carbon nanotube fibers, you'll still have thousands of tons of cable above the atmosphere by the time you can lift a useful load.  The "float down gently" will turn into "thousands of kilometers of giant meteorite" by the time GEO station enters atmosphere.

Well, not lightweight in an aggregate, but the ribbon itself has a very low density. And that's the stat that matters.

If the cable comes loose at the bottom but is still attached to the counterweight, the counterweight takes off into space at a nice steady clip, pulling the ribbon up, up, and away. 

If the cable breaks at or near the counterweight, the ribbon falls to Earth under its own weight faster than the Earth's attachment point can "wind" it up. Most of it will burn up in the atmosphere; the remainder is so low-density that it will flutter to the ground harmlessly.

[p1t1o]

Many people quote the raw gravitational potential energy of a space elevator cable, and compare it to nuclear weapons, but [if it did fall all the way to the ground] the energy would be released over a period of many minutes and over a very large area, rather than in a handful of microseconds in a volume the size of a football.