Podcast: Plasma Thruster Prototype

[Fraser]

SUMMARY: If you're going to fly in space, you need some kind of propulsion system. Chemical rockets can accelerate quickly, but they need a lot of heavy fuel. Ion engines are extremely fuel efficient but don't generate a lot of power, so they accelerate over months and even years. A new thrusting technology called the Helicon Double Layer Thruster could be even more efficient with its fuel. Dr. Christine Charles from the Australian National University in Canberra is the inventor.

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What do you think about this story? post your comments below.

[Avatar28]

Sooooo, basically, it's like an ion engine. But with less thrust. And less efficient. Oh, but it might be more fuel efficient.

Sorry, but what we REALLY need right now is a better high-thrust rocket, something we can use to climb up out of the earth's gravity well into orbit.

[GBendt]

hi

It would be nice to have a powerful, steady, easy-to-handle and cheap high-thrust rocket to travel into space. Those rockets we have in use are powerful, but they are powerful for a very short time only, consuming all their fuel within minutes. As soon as the fuel is spent, these rockets become useless space junk.
It would be great to have a rocket which throws its fuel through its nozzle at such a high speed that the momentum caused by this matches that of a typical high-thrust rocket! Then the fuel would be lasting longer!

In the sixtees of the last century, the US tried a high-power nuclear reactor as a power source for a rocket. The reactor was cooled by liquid hydrogen, which was heated up to several thousand centigrades. the thrust was good, I would hesitate to fly a rocket wich is propelled by a reactor close to melting, by vapourising the cooling liquid. What will happen to me when this is spent?

Ion engines have very little thrust, and they are rather complex machines. They are very fuel-efficient, are powered by electricity and thus can use solar panels. The HDLT thruster is as fuel-effecient and has a less complex design, and thus may offer some advantages over the ion engines, as soon as the thrust of the HDLT can equal that of an ion engine.

Therefore I think we should be looking forward to a better HDLT design to show up in the near future.

Regards,

Günther

[Greg]

I would not be too disappointed with the results from this early prototype. They are still studying the basic science behind how it works and are nowhere near the point of reaching its maximum potential. It should end up being quite a bit superior to ion engines and possibly a great deal superior, but will need at least 20 years of R&D to get there.
This kind of engine would not useful for blasting out of Earth's orbit from the ground, but would be quite useful if launched from orbit on a long distance mission. My hopes are on developing a space elevator to dramatically lessen ground to orbit costs. In the shorter term, commercial applications are developing now getting small payloads to orbit more cheaply than the ESA, NASA, or the RKA can currently.

[GBendt]

Greg,

We cannot hope for a space elevator. It is a dream without relation to physics. Sorry.

Regards,

Günther

[Greg]

I have some serious doubts about snafus that may come up if someone attempted to construct a space elevator. These have more to do with the effects of the atmosphere on a tether of such length (friction, lightning, winds). The more practical and immediate concerns involve constructing a tether with sufficient tensile strength not to snap which should be possible using carbon nanotubules. The physics as I understand it appears to be sound and NASA has funded a preliminary investigation (competition) and is exploring the matter. See link below for more information on this matter.

http://usgovinfo.about.com/library/weekly/aa041702a.htm

[Van Rijn]

Greg,

We cannot hope for a space elevator. It is a dream without relation to physics. Sorry.

Regards,
Günther

Greg is correct. A space elevator is well within the bounds of physics, though it requires substantial improvement in the mass production of very high tensile strength carbon nanotube based material. You might want to do a bit of research on the subject.

[GBendt]

Hi,

I did some investigations on the concept of the space elevator, and came to the conclusion that it cannot work.

As I understand it, the concept of the space elevator is based on the idea that you connect a station which is in a geosynchroneous orbit to a place on earth by means of a straight rope, tether or something like that, and that you let an elevator move up or down that rope to carry loads up to the geosynchroneous station, and back from this station to the ground. In this concept, ground station, rope, elevator and geosynchroneous station are meant to move together at a constant and common angular speed.

However, a constant angular speed is not possible for bodies which are moving in a gravity field, orbiting in orbits which have different radii. This is so because the gravity force which a body feels in the gravity field of the earth varies with its distance from the earth, and therefore you need different orbiting speeds at different orbit radii to establish the very centrifugal force which is required to balance the weight the orbiting body has at this very distance of his from the earth.

And so, this idea of a space elevator cannot function.

Any additional force that is applied to the geosynchroneous station will break the delicate equilibrium which keeps the geosynchroneous station in its geosynchroneous orbit.
If you attach a rope to the geosynchroneous station and link this rope to earth, the weight of the rope will pull the geosynchroneous station down. It will fall towards earth, thus accellerate and thus leave its geosynchroneous orbit. As the orbiting station is linked by the rope to the ground and the length of the rope is fixed, the rope will force the station to crash into earth as soon as the the orbiting speed of the station changes its angular speed compared to the angular speed of the ground station.

The weight of the rope cannot be balanced by the centrifugal force that is caused by the speed of the geosynchroneous station. As you are coming closer to earth than the geosynchroneous station, the gravity potential of the earth becomes greater, and this requires a higher speed for a piece of rope if you want to balance the weight of that piece of rope by centrifugal force.
On a geosynchroneous orbit, the orbiting speed of the station must be 9424 km/h to keep it in its geosynchroneous orbit. If you have a piece of rope let´s say 16000 km above the centre of the earth, it must travel at a speed of 16283 km/h to balance the gravity caused on it by the earth. This speed value is much faster than the speed of the rope must be to maintain the required condition of a constant angular speed of that rope. The closer the rope is to earth, the higher the speed of the rope must be to create a centrifugal force that is able to compensate the weight of the rope at the distance from earth where it is.

So, it is evident that the orbiting station is not able to keep the rope up, if the rope has a weight. To make the rope apparently weightless, each part of the rope must move at a speed which is the square root of (G x mass of earth, divided by the local distance of that part of the rope from the point of gravity of earth).
e. g. :To keep a piece of a rope weightless at the height of a space shuttle orbit, this piece must orbit at the speed of the space shuttle. This is 28000 km/h, much more than the 9424 km/h at which the geosynchroneous station is travelling.

The basic condition for the space elevator, which demands to keep the rope moving at the same angular speed than does the place on the ground and the geosynchroneous station cannot be fulfilled. So you cannot place a rope or tether between geosynchroneous station and the ground: Its weight cannot be compensated by the available centrifugal force, and thus the weight of the rope will pull the geosynchroneous station out of its geosynchroneous orbit. However, if you increase the speed of any single section of the rope to a speed that compensates the weight of the rope by means of centrifugal force, this would immediately cause a break of the rope, because this speed will be a different one for every piece of rope.

Nanotubes won´t help here, even if they are ten times stronger than steel an may weigh just a few grams per kilometer. Any weight they have will end in a desaster for the geosynchroneous station. And as soon as you let the elevator move up the rope, the rope will have to carry the weight of the elevator and the weight of its load, and this load will hang on the geosynchroneous station and will pull the geosynchroneous station down.

I assume that the space elevator is an april joke devised by NASA, and NASA waits for people to realise that it is a joke. It may be horrifying for them to see that so many people do not understand the law of gravity.

Regards,

Günther

[korjik]

The individual bits of rope arent in freefall. The tension in the cabeling keeps the rope at one angular velocity. This counterweight is past the geosync distance so the centripital force puts tension on the cable. Once everything is in place it should be stable. The problem is getting every thing into space at the right speed at the right time.

[GBendt]

Hi,

if you were able to let down a rope from a satellite which is in orbit, that rope will come closer to earth and such the rope will become heavier, as the gravity force grows when the distance to earth is reduced. As the rope is attached to the satellite, the additional force caused by the growing weight of the rope will pull the satellite from its orbit.

If you stretch a cable into space the end of which is attached to a geosynchroneously orbiting satellite, the weight of the cable will pull this satellite from its geosynchroneous orbit.

If you put an additional mass ("counterweight") into an orbit which is above the geosynchroneous station´s orbit, this mass orbits slower than the geosynchroneous station, because the distance of that mass from earth is greater. If all components are linked together, this forces the station to leave its geosynchroneous orbit.

Simply spoken:
The idea that you can have additional forces acting on a satellite in orbit without this affecting the satellite´s orbit does not work.

The idea of flinging a long cable such that the centrifugal force stretches it cannot work in the gravity field of earth when this cable is requested to rotate at the angular speed of a geosynchroneously orbiting satellite. In this case, the speed of any part of the cable is slower than that of the geosynchroneously orbiting satellite. So, the centrifugal force available at this speed is too low for any section of the rope to balance the weight of the respective section of the rope, and thus cannot stretch the rope. Only at the position of the geosynchroneous orbit the centrifugal force just balances the weight of the satellite. There is no force that stretches the rope and balances its weight.

If you want things that work, you must build them in accordance to the physical laws. Otherwise you will fail. The space elevator cannot work. Celestial mechanics simply do not support it.

Regards,

Günther

[Van Rijn]

If you put an additional mass ("counterweight") into an orbit which is above the geosynchroneous station´s orbit, this mass orbits slower than the geosynchroneous station, because the distance of that mass from earth is greater. If all components are linked together, this forces the station to leave its geosynchroneous orbit.

NO! The counterweight (whether just cable or cable and a large mass used to reduce the cable length) would be moving faster than orbital velocity at that altitude. That is why it can act as a counterweight. Conceptually, you start with a satellite in geosynchronous orbit and send one cable up and one cable down at the same time to offset each other, while keeping the entire cable under tension. Only the part in geosynch is at orbital velocity. Cable below geosynch is moving slower than orbital velocity for that altitude, cable above is moving faster. You keep the whole thing in balance while reeling out the cable.

In practice, there is a bit more to it. You might find this article useful:

http://www.isr.us/Downloads/niac_pdf/chapter5.html

which is part of a larger paper on the space elevator concept:

http://www.isr.us/Downloads/niac_pdf/contents.html

[Gullible Jones]

In the sixtees of the last century, the US tried a high-power nuclear reactor as a power source for a rocket. The reactor was cooled by liquid hydrogen, which was heated up to several thousand centigrades. the thrust was good, I would hesitate to fly a rocket wich is propelled by a reactor close to melting, by vapourising the cooling liquid. What will happen to me when this is spent?

You shut down the reactor and send out a call for help. Sorry to point out the bleeding obvious, but fission reactors have control rods!

(In fact, if the reactor were well-designed, it would shut itself down when the fuel ran out. Fail-safe systems are the way to go.)\

Edit: the problem with space elevators isn't prohibitive laws of physics, it's prohibitive materials requirements. Until we can synthesize carbon nanotubes of sygnificant length in large quantities, it probably ain't going to happen. That said, we've been making huge advances in materials science of late.

[GBendt]

Hi Gullible Jones,

Of course fission reactors use control rods for the neutron flux control, but this design works fine with water-cooled fission reactors which are operated at temperatures far away from core-melting temperatures. The design of such a reactor includes emergency cooling facilities, and thermal capacity of such a reactor leaves you a couple of seconds to react effectively if the coolant is lost while your reactor is still working at full power.

But if the fission reactor is operated at as high a temperature as possible to heat up a liquid gas as effectively as possible in order to produce as much hot, high-pressure gas as possible to drive it out through a nozzle, the time left to close down the reactor by pushing in the rods as soon as the coolant is spent might be too short.

So I still would hesitate to fly a rocket which is propelled by such a design.

It is obvious that this design was never applied in spaceflight. I don´t know why, so I can only guess...

Regards,

Günther

[GBendt]

Hi van Rijn,

Thank you for the link on the article.

I will check it and see what it tells me, and see if its assumptions are correct. I shall give you my answer here, but not today. I have to do some other work first.

Kind Regards,

Günther

[GBendt]

Hi,

I read the article on the space elevator concept. Having read it, I have the impression that the author lacks an understanding of the basic concepts of physics. Perhaps a few lectures in physics would help. But this forum is not the proper place for that.

I just would like to give a few words.

The basic idea of the space elevator assumes a cable that is flung into space and stretched out up into space by the centrifugal force caused by its rotation with the earth. The idea assumes that the speed of each section of the cable depends of its local distance from earth. The idea says that if the distance is long enough, the speed will be so high that it would even exceed the speed necessary to leave the gravity field of our planet.

The essential fault of this concept is that it violates several basic physical laws. One is the law which says that you can neither generate nor destroy energy, you can obly convert one sort of energy into the other.
Well, if you are on the surface of the earth, just on the equator, you will move at a speed of 462 m/s, which gives you a kinetic energy of 107 kJ/kg. The space elevator concept now assumes that you can increase this energy simply by climbing up the rope while it rotates synchroneously with the earth. On a geosynchroneous orbit, you will move at a speed of 3075 m/s and thus your kinetic energy will be 4,7 MJ/kg. The higher you climb, the faster you get and the greater your kinetic energy becomes. But where does this increase in energy come from? The space elevator concept does not give an answer.
Instead, it proposes this wonderful energy creation properties of its design as a reason to build it, as a wonderful means to launch spacecrafts without any effort. VEry curious.

Another physical law says that the angular momentum in a system of elements is constant. As a result, the rotation speed of a rotating system slows down when parts of the system attain a higher distance from the barycenter of the system. You can see this e. g. when watching a figure scater, or a governor. As the figure scater stretches his or her arms on a pirouette, the speed of rotation decreases. It does nor rise by increasing the distance of the arms from the scater´s barycenter. I have the impression that the space elevator concept converts the law of the conservation of the angular momentum into one of the conservation of angular velocity. Such a law, however, does not exist.

If you stand on the earth, you have a constant momentum (so says Isaac Newton). If you start climbing up a rope that is rotating synchroneously with earth, the angular velocity of the rotating rope will not provide you with a higher speed the higher you get. What will deliver the force to accelerate you such the space elevator concepts assumes you will be accelerated?


From a design engineer´s point of view, the design of the space elevator concept is impossible to control and to handle. You have a 100000 km long cable of unknown mass and of unknown physical capabilities, a geosychroneus station, a massive counterweight, a climber and last, but not least, the variable paylod which is to be transported by the space elevator. The various elements of the space elevator design move at a constant angular speed with earth, but at different individual speeds within a gravity field, and each of its element must be controlled carefully to maintain the delicate balance and relative assembly of the entire structure. The whole thing is many times greater than the earth, it is difficult to control and monitor such a large device.

If you have a station in geosynchroneous orbit, this station moves at the same angular speed than does a point on the surface below it on earth. If you link the station with the ground by means of a rope, then earth, rope and orbiting station move at a constant angular speed, but only as long as no additional force disturbs the equilibrium of the system.
Any force acting on the station will break its equilibrium. If the equilibrium is disturbed, the geosynchroneous station will leave its orbit and the condition of constant angular speed is violated. Then the rope which connects station and ground will force the station down.
Such a force which disturbs the equilibrium is that of the weight of cable, climber and payload. This weight may be several tons, which pull at the geosynchroneous station. To balance the force caused by this weight, you apply a counterweight which moves at a greater distance than the geosynchroneous station, and at such a high speed that its resulting centrifugal force is able to balance the weight of rope, climber and payload. Further, it is necessary to be able to adjust the current centrifugal force in strict accordance with the current weight which this centrifugal force has to balance, as this weight may variy day by day, and in addition changes permanently, due to the changing position of climber and payload, which both are moving in the gravity field the local strength of which depends on its local distance from earth.

It is further evident that this counterweight must move at the same angular speed than any part of the space elevator, as otherwise it would pull at the geosynchroneous station such that this looses its equilibrium. Once the condition of constant angular speed is lost, the whole apparatus of masses and long cables becomes instable, and I don´t see a viable means to re-establish stability of the design once it is lost.

A centrifugal force is proportional to the mass of the affected body, proportional to the square of its speed, and is inversely proportional to the radius of the orbit of the body. Due to the demand for constant angular speed, we can calculate the speed of the body, which at constant angular speed is a linear function of the respective radius, and thus we can estimate which centrifugal force can be produced by a body of a given mass and at a given radius.

The paper on the space elevator concept assumes a cable weight of 16 tons. If we want to compensate that weight by the centrifugal force of a counterweight, this counterweight will create a centrifugal force of 0,44 N per kg of its mass, when moving at a speed of 6150 m/s, and at a radius of 85500 km.
The centrifugal force will rise to 0,88 N per kg of counterweight mass if the speed is raised to 12300 m/s, at a radius of 171000 km.
The force will rise to 1,78 N per kg of counterweight mass if the speed is raised to 24600 m/s, at a radius of 340000 km.
Thus, to establish a centrifugal force which can balance the 16 tons of cable weight pulling at the geosynchroneos station, you need a counterweight of 356727 kg in the first case, one of 178363 kg in the second case and one of 89181 kg in the third case. And there is no means to adjust the centrifugal force induced by the counterweight such that it can instantaneously be adapted to balance any current change in the load. You can´t change the mass easily, and you must not change the speed at the given radius, as otherwise you would violate the imperative demand to maintain the constant angular speed.

The largest mass that we were yet able to bring to a speed of 11000 m/s was the Cassinis spacecraft. We have no rocket engine powerful enough to bring a mass of 178363 kg to a speed of 12300 m/s. And it is obvious that you can´t get this speed just by "letting out" the cable with a mass attached to it, into space.


The longest nanotubes yet created have a length of 1 mm, and we currently can only produce a few grams at a time. Thus we are far from the 100000 km long nanotubes that we need for the space elevator. As long as we do not know how strong the rope will finally be, and how great its rigidity, stiffness, mass and weight will be, we have no basis to do a functional design.

I asked myself how my physics professors would react if I would offer them the papers on the space elevator concept.
If their sense of humour is underdeveloped, they might ask me sternly to return my certificate in physical engineering. Perhaps they might take it for a delayed april joke, just as I did it. Perhaps they would wallow on the carpets in their offices, bursting with exhilaration.
I really don´t know.

It is late now. Time to go to bed.

Regards,

Günther

[phunk]

Wow that was a long post. You answered yourself though, conservation of angular momentum. The energy given to the rising elevator comes from the rotation of the earth itself. Launch enough mass and the day will get longer. But that's alot, nothing we have to worry about unless we plan on launching mountains.

Also, the counterweight can be carried up the elevator itself in small pieces. You only need an initial counterweight large enough to hold a small cable with a small payload, then you send climbers from the ground to add more cable and to add more counterweight. We can certainly get a 16 ton cable to geosync in 1 launch. A few more launches to send up counterweight and stabilizing thrusters & fuel, then you unspool the cable downwards and upwards at the same time, keeping the center of gravity at geosync. It would require a very light thrust over a long period, maybe an ion engine craft pulling each end of the cable and one to keep the station at geosync. As soon as one end reached the ground, you can attach it, send more counterweight outwards from the geosynch to tension the cable, and then start loading the counterweight and sending up additional cable from the ground.

You don't need to perfectly balance the weight of the cable either, you want to have a heavier counterweight so there is tension in the cable. Then when you add a climber, it doesn't suddenly unbalance the system.

The nanotube length problem is certainly something that will need to be overcome before an elevator can really be planned. There's no reason to believe that we won't come up with a way to mass produce long nanotubes though.

[Van Rijn]

I read the article on the space elevator concept. Having read it, I have the impression that the author lacks an understanding of the basic concepts of physics.

I have the impression that somebody lacks an understanding of physics, but it isn't the author of the article.

The essential fault of this concept is that it violates several basic physical laws. One is the law which says that you can neither generate nor destroy energy, you can obly convert one sort of energy into the other.
Well, if you are on the surface of the earth, just on the equator, you will move at a speed of 462 m/s, which gives you a kinetic energy of 107 kJ/kg. The space elevator concept now assumes that you can increase this energy simply by climbing up the rope while it rotates synchroneously with the earth. On a geosynchroneous orbit, you will move at a speed of 3075 m/s and thus your kinetic energy will be 4,7 MJ/kg. The higher you climb, the faster you get and the greater your kinetic energy becomes. But where does this increase in energy come from?

From the earth's angular momentum, of course. It doesn't come free or violate physical law. Flinging mass from the end of such a cable would slow the earth's rotation, but the earth is so massive that it wouldn't cause a significant change.

Another physical law says that the angular momentum in a system of elements is constant. [snip]
As the figure scater stretches his or her arms on a pirouette, the speed of rotation decreases.

You're ignoring relative mass. Instead of extending arms, have the figure skater spin while unreeling a fishing line with a very small ball on the end. Note the velocity of the ball as it is extended. Yes, total angular momentum is conserved, but that part of the system will end up with a much higher velocity as it is unreeled.

From a design engineer´s point of view, the design of the space elevator concept is impossible to control and to handle. [snip]The various elements of the space elevator design move at a constant angular speed with earth, but at different individual speeds within a gravity field, and each of its element must be controlled carefully to maintain the delicate balance and relative assembly of the entire structure.

This is an engineering argument, not a physics argument, so I won't go into this in much detail. The cable is kept under tension by the counterweight and is anchored at the earth end. As long as it is kept within design limits, changes of mass of elevator cars are not going to suddenly cause the whole thing to collapse, and the structure is certainly not on a knife-edge of stability.

The longest nanotubes yet created have a length of 1 mm, and we currently can only produce a few grams at a time.

This is way off. They're up to at least several centimeters, and can make mats of the stuff.

Thus we are far from the 100000 km long nanotubes that we need for the space elevator.

They don't need to be 100,000 km long. A composite material based on carbon nanotubes with the required strength to weight ratio would be quite sufficient. As I and others have already said, this area requires significant improvement. However, we have seen significant improvement, and there is no reason to think we won't see these improvements in the next twenty years or so.

As long as we do not know how strong the rope will finally be, and how great its rigidity, stiffness, mass and weight will be, we have no basis to do a functional design.

We know the tensile strength to weight requirements and it has been demonstrated that carbon nanotubes, in principle, can meet these requirements. Design details will have to wait, but we can say quite a bit already.

I asked myself how my physics professors would react if I would offer them the papers on the space elevator concept.
If their sense of humour is underdeveloped, they might ask me sternly to return my certificate in physical engineering. Perhaps they might take it for a delayed april joke, just as I did it. Perhaps they would wallow on the carpets in their offices, bursting with exhilaration.
I really don´t know.

I think you might be surprised at their reaction, but I would suggest you be very careful with your comments. While we can't build one today, and there are valid questions of practicality, the basic physics is well established. This isn't an idea promoted by a few woo-woos, but has been discussed by many very intelligent scientists and engineers for decades. We knew the requirements, and until carbon nanotubes, we knew that no material could meet that requirement. Now, it is no longer a physics question but an engineering one.

[GBendt]

Hi,

To anwser van Rijn and Phunk:

If we consider the climber moving up the space elevator´s rope, the climber has a horizontal speed which grows as the climber climbs. It grows from an initial "ground speed" of 462 m/s up to 3075 m/s at the geosynchroneously orbiting station.

As we look at the situation, the climber is climbing up a vertical rope. A rope can only transfer force the vector of which is directed exacly along the axis of the rope.

But while the climber climbs up vertically, it must be also be accelerated horizontially.To do this, you need a force with a horizontal vector. Such a force cannot be provided by the vertical rope, as the forces along the rope don´t have a horizontal component. So, how is this force generated?

I read opinions which say that this force is provided by the earth´s angular momentum.

This concept would be working fine if the climber were climbing up a tower which is firmly connected to the earth. Such a tower behaves as a component of the earth, it is stiff and can carry horizontally acting forces and torque.

But a rope can´t do this, as a rope can´t transfer torque. Its properties are confined to carry forces which act along the axis of the rope, and with the space elevator, this axis is a strictly vertical one.

Thus, the energy to accelerate the climber horizontially cannot be provided by the earth´angular momentum, as long as you apply a rope. The energy to accelerate the climber to an orbiting speed has to be provided by the space elevator.

The beautiful concept of carrying things into space mainly by using the energy of the earth´s angular momentum does not work with this design.

Regards,

Günther

[phunk]

That's not true at all, a rope can absolutely provide force perpendicular to itself. Here's a simple thought experiment (try it if you really need to). Hang a weight from a rope and swing it (a pendulum). Now stick your arm in the path of the swinging rope. By your logic, the rope can't push your arm horizontally. A space elevator is just like that pendulum, only upside down.

[Van Rijn]

Hi,
I read opinions which say that this force is provided by the earth´s angular momentum.

This concept would be working fine if the climber were climbing up a tower which is firmly connected to the earth. Such a tower behaves as a component of the earth, it is stiff and can carry horizontally acting forces and torque.

As previously mentioned, the cable would be firmly connected to the earth and would be under significant tension. It most certainly can apply force perpendicular to itself.

A climber or elevator car will cause oscillations in the cable. Similar issues come up all the time in bridge design. Space elevator oscillations are discussed here in section 10.8:


http://www.isr.us/Downloads/niac_pdf/chapter10.html

The beautiful concept of carrying things into space mainly by using the energy of the earth´s angular momentum does not work with this design.

You might want to stop saying that.

[grewwalk]

Hi everybody, it's been a while since I had joined in on a thread.
..ahem..

GBent is correct on what he is saying, but it is incomplete concerning common centers-of-gravity and can be corrected.

Nasa and the Italian Space Agency have already done tests of tethered objects above and below a common obital plane (TSS-1R on STS-75 just basic info on these, the rest is from memory). Their main goal was to conduct tests on electrodynamic stuff and required (basically) an upright length of line. Consisting of two craft, they reeled a tether to extend each other out. One to an outer obit, one to an inner orbit. Their center of gravity stayed at the same spot.

So at least that part has been done before, just not on a large scale. If what GBent was saying could not be overcome, then this experiment could not have been accomplished.

As for payloads going up the cable, this will cause the cable to swing behind to some extent. But then the spin will cause the cable 'whip' back out again. Then again, there would probably be thrusters for stationkeeping out there (loads done incorrectly could also cause the cable to completely fall back, essentially winding around earth.)

-theres my 2 cents...thanks-

[phunk]

The cable couldn't lift a load heavy enough to wind itself around the earth, not by a longshot. At most it will swing like a very long pendulum, a few degrees out of vertical in either direction. All you have to do to counter the swing is send up the next load out of phase with the pendulum's swing and it cancels it out. Another problem to worry about is vibrations in the elevator, since it essentially will behave like a giant guitar string. Again, careful timing can prevent vibrations from accumulating. Ion thrusters at the counterweight and any intermediate stations (like geosync) can also make corrections on the fly.

[Astrogirl]

Ion thrusters are improving all the time.

The European Space Agency and the Australian National University have successfully tested a new design of spacecraft ion engine that dramatically improves performance over present thrusters and marks a major step forward in space propulsion capability. Source: http://www.physorg.com/news9786.html