Elevator to the stars...

[SGT]

The idea is that the endpoint would be a very large mass and it would be further from earth than a geosynch orbit. This would support the weight of the cable and keep it under positive tension.

In order to get a stable orbit further from Earth than a geosynch orbit, the angular velocity of the endpoint would be different from the Earth's. How would the structure be stationary with respect to the surface?
May be I am missing some point. Could the Bad Astronomer help us?

 

[scotth]

In order to get a stable orbit further from Earth than a geosynch orbit, the angular velocity of the endpoint would be different from the Earth's. How would the structure be stationary with respect to the surface?
May be I am missing some point. Could the Bad Astronomer help us?

The missing point....

The tension on the cable really makes the situation not really an orbit. You can't really consider it like an orbit.

A higher than geosynch orbit would be longer than 24 hours to complete. If the speed of the object at a higher than geosych altitude was increased to remain geostationary, it would have too much energy for that orbit and try to move further from the earth. This is what supports the weight of the cable and puts tension on it.

 

[Hellbound]

The missing point....

The tension on the cable really makes the situation not really an orbit. You can't really consider it like an orbit.

A higher than geosynch orbit would be longer than 24 hours to complete. If the speed of the object at a higher than geosych altitude was increased to remain geostationary, it would have too much energy for that orbit and try to move further from the earth. This is what supports the weight of the cable and puts tension on it.

Actualyl, thos would break the cable because of the added tension.

The biggest problem with this idea is finding a cable that can support its own weight at that length (much less a load). So far, carbon nanotube fibers (each fiber the length of the cable) are the only thing that looks capable of supporting it's own weight well enough for this type of use.

As to balance, location, etc...you would want your station in a geosynchronous orbit. It works like this:

1. Launch your satellite/station, with the cable, into a stable, geosynchronous orbit.

2. Begin lowering the cable toward Earth. At the same time, begin string an equal mass of cable out away from Earth.

3. The cable leading out away from Earth counterbalances the one lowered to Earth, thus negating the tension forces of using a higher orbit or the changes in center of gravity that would cause it to fall to a lower one.



No, I'm not really that smart, just read up on the ideas behind this recently

 

[scotth]

Actualyl, thos would break the cable because of the added tension.

The biggest problem with this idea is finding a cable that can support its own weight at that length (much less a load). So far, carbon nanotube fibers (each fiber the length of the cable) are the only thing that looks capable of supporting it's own weight well enough for this type of use.

As to balance, location, etc...you would want your station in a geosynchronous orbit. It works like this:

1. Launch your satellite/station, with the cable, into a stable, geosynchronous orbit.

2. Begin lowering the cable toward Earth. At the same time, begin string an equal mass of cable out away from Earth.

3. The cable leading out away from Earth counterbalances the one lowered to Earth, thus negating the tension forces of using a higher orbit or the changes in center of gravity that would cause it to fall to a lower one.



No, I'm not really that smart, just read up on the ideas behind this recently

You would want the cable under tension, just not so much tension as to break it.

And point by point...
1) You would not want launch the upper end of the station.... You would not want to put nearly that much mass into orbit. Bringing in a small asteroid would be far more practical.

2a) Realing out an equal mass of cable away from the earth wouldn't work, you would have to real out much more mass in cable to the outside as you would toward the earth to keep the station at its original geosych atlitude. Remember, gravity declines by the square of the distance from the gravity source, so the cable going down will have significantly more impact on the station.
2b) Much simpler to use rockets/thrusters to simply move the station a bit higher to offset the pull of the cable as it is lowered.

3) Already covered by 1 and 2.

Keep reading.

 

[Badly Shaved Monkey]

The "space elevator" has also been called a "Heavenly Fununcular". I thought it was a much older concept than Arthur C. Clarke's rendition.

I think you mean funicular. Fununcular is an adjective pertaining to boils. It may be in the back of your mind if you read Harry Potter in which "fununculus" (or "fununculous", can't remember which) is a charm that induces boils.

By the way, if you put "fununcular" into Google, it helpfully suggests;

"Did you mean: funicular"

Rather excellently, one of only 4 hits is a broken link that shares your confusion;

"In Lisbon a trip to the Porto Institute is a must. It's at the
top of the fununcular just outside downtown."

That's all for now from the wild world of lexicography.

 

[Hellbound]

You would want the cable under tension, just not so much tension as to break it.

And point by point...
1) You would not want launch the upper end of the station.... You would not want to put nearly that much mass into orbit. Bringing in a small asteroid would be far more practical.

2a) Realing out an equal mass of cable away from the earth wouldn't work, you would have to real out much more mass in cable to the outside as you would toward the earth to keep the station at its original geosych atlitude. Remember, gravity declines by the square of the distance from the gravity source, so the cable going down will have significantly more impact on the station.
2b) Much simpler to use rockets/thrusters to simply move the station a bit higher to offset the pull of the cable as it is lowered.

3) Already covered by 1 and 2.

Keep reading.

Alright, then.

Point 1. Doesn't matter, just start with the upper end. It would be cheaper getting an asteroid, but I'd imagine it would be done piece by pice taken into orbit. the reason being you'd want a technological structure there of some sort, and you either have to orbit the pieces or orbit the gear to manufacture it from the asteroid. Either way, not really that relevant; cheapest is best.

Point 2a. Um, the cable reeled out is not to counteract gravity; it's to maintain the center of mass. This has nothing to do with gravitational force. It's about centrepital and centrifugal forces. Gravity only becomes a factor if center of mass changes and the upper end gets pulled out of geosynch orbit. You are right, though, that they would not be equal mass. The outgoing cable could be shorter due to the increase in centrifugal force; or your handy asteroid could be attached to a predetermined length of cable (but this would require propulsion to maintain position until enough cabel was reeled out to balance the forces towards and away).

Point 2b. And how high would you have to move it? Even lightweight cables end up with outrageous weights at these lengths. The calculations using single-filament carbon nanotube weaves leave little room for ANY extra force on the cable, if any sort of useable load rating is expected. Remember, any tension you add to the cable does not counteract the tension weight, but adds to the forces acting on the cable. You're pulling on both ends of the rope instead of one.

Point 3. This is where I explained that the cable is not to counteract gravitational force, but center of mass/center of gravity. Poor terminology on my part. Although the terms are interchangeable on Earth, center of mass would be the correct one to use here. The outgoing cable also provides a few other functions:

1. It is moving at a faster rate, the outgoing cable can actually be used as a booster to accelerate spacecraft.
2. The length of the cable from satellite to the outgoing end provides an excellent generator if a conductive material is run along it. Wire moving through Earth's magnetic field=electricity.
3. The counterbalance also provides a safety net. If the cable to ground breaks, it's mass will pull the station away from Earth.

I'll post some sources when I get home and find the relevant books.

Edited to add some web links:

NASA's description, with geosynchronous transfer station
Another NASA site, specifically stating a "large counterbalance beyond geosynchronous"
And finally, the Phase 1 and Phase 2 reports from NASA's Institute for Advanced Studies.

 

[scotth]

Alright, then.

Point 1. Doesn't matter, just start with the upper end. It would be cheaper getting an asteroid, but I'd imagine it would be done piece by pice taken into orbit. the reason being you'd want a technological structure there of some sort, and you either have to orbit the pieces or orbit the gear to manufacture it from the asteroid. Either way, not really that relevant; cheapest is best.

Point 2a. Um, the cable reeled out is not to counteract gravity; it's to maintain the center of mass. This has nothing to do with gravitational force. It's about centrepital and centrifugal forces. Gravity only becomes a factor if center of mass changes and the upper end gets pulled out of geosynch orbit. You are right, though, that they would not be equal mass. The outgoing cable could be shorter due to the increase in centrifugal force; or your handy asteroid could be attached to a predetermined length of cable (but this would require propulsion to maintain position until enough cabel was reeled out to balance the forces towards and away).

Point 2b. And how high would you have to move it? Even lightweight cables end up with outrageous weights at these lengths. The calculations using single-filament carbon nanotube weaves leave little room for ANY extra force on the cable, if any sort of useable load rating is expected. Remember, any tension you add to the cable does not counteract the tension weight, but adds to the forces acting on the cable. You're pulling on both ends of the rope instead of one.

Point 3. This is where I explained that the cable is not to counteract gravitational force, but center of mass/center of gravity. Poor terminology on my part. Although the terms are interchangeable on Earth, center of mass would be the correct one to use here. The outgoing cable also provides a few other functions:

1. It is moving at a faster rate, the outgoing cable can actually be used as a booster to accelerate spacecraft.
2. The length of the cable from satellite to the outgoing end provides an excellent generator if a conductive material is run along it. Wire moving through Earth's magnetic field=electricity.
3. The counterbalance also provides a safety net. If the cable to ground breaks, it's mass will pull the station away from Earth.

I'll post some sources when I get home and find the relevant books.

2a) It isn't a simple "center of mass" issue anymore, you will have an object that is long enough that the gravitational field experienced at each end will be radically different. Reeling out an equal mass of cable each way isn't gonna do the trick. Also, this cable is going to be quite expensive, and now you would have to make twice as much of it.

2b) You move it as high as necessary. The more massive the station, the less you have to move it. And as far as the amount of tension, you will need to have at the very minimum as much tension as the heaviest load you ever expect to lift (including the elevator), but I suspect that amount of tension would pale in comparison to the amount you would want to keep jetstream winds from moving the cable around significantly.

3) The reason you want to change the center of mass is to counteract gravity, why else would you be doing it? No other reason that to support the weight of the descending cable due to gravity.

3-1) Not really, if let a potential spacecraft climb the outer cable at any useful speed, it will cause the cable to fall behind the station. Conservation of momentum and all. Thrust would have to be applied to the spacecraft to prevent this. If you want to launch a spacecraft, just release it from upper station and rocket away.
3-2) The cables would most likely be conductive on their own. Electricity could be generated without the extra cable. But the amount electricity that could be produced would probably be an insignificant contribution to the economics of a project like this.
3-3) You have exactly the same safety by moving the station out. If the cable breaks, the station and all the cable above the break would stay safely away from earth.

 

[Hellbound]

2a) It isn't a simple "center of mass" issue anymore, you will have an object that is long enough that the gravitational field experienced at each end will be radically different. Reeling out an equal mass of cable each way isn't gonna do the trick. Also, this cable is going to be quite expensive, and now you would have to make twice as much of it.

As you can see in my last post, I said that equal masses were not necessarily needed...one could use a counterweight (such as a small asteroid). In any case, you'd use additional cable to move the station out, so either way you add more cable. No difference there. And the gravity is not an issue. If the center of mass is at geosynchronous orbit, then by definition the gravitational forces are equalized. That's the whole point of a geosynch CoM.

2b) You move it as high as necessary. The more massive the station, the less you have to move it. And as far as the amount of tension, you will need to have at the very minimum as much tension as the heaviest load you ever expect to lift (including the elevator), but I suspect that amount of tension would pale in comparison to the amount you would want to keep jetstream winds from moving the cable around significantly.

The mass of the station and cable (and the fact that it is attached at the bottem) act against movement by winds. Also, and I might be wrong here, but as I understand it there are not jet stream winds at the equator (where a space elevator would have to be built).

3) The reason you want to change the center of mass is to counteract gravity, why else would you be doing it? No other reason that to support the weight of the descending cable due to gravity.

Even moving the station out does the same thing I suggested, as I reflect on it. No more tension would be added, but you are still moving the CoM of the station/cable combo to geosynch orbit. As long as the CoM is in geosynch, gravity is counteracted.

3-1) Not really, if let a potential spacecraft climb the outer cable at any useful speed, it will cause the cable to fall behind the station. Conservation of momentum and all. Thrust would have to be applied to the spacecraft to prevent this. If you want to launch a spacecraft, just release it from upper station and rocket away.

I'll give you this one .

3-2) The cables would most likely be conductive on their own. Electricity could be generated without the extra cable. But the amount electricity that could be produced would probably be an insignificant contribution to the economics of a project like this.

The amount of electricity generated could be enough to power the station itself, at least. One of the major concerns with this concept was creating a cable that was able to withstand the electrical stresses. The shuttle tried an electrical experiment like this, with a MUCH shorter cable; the cable broke rather quickly due to added stresses from the amount of electrical energy being generated. Of course, this is a side issue and not really pivotal to the main idea. Now, I can't give exact numbers right now, and the shuttle experiemnt was at a lower orbit/higher speed, so I am not positive that the numbers will be comparable; however, given the much larger than expected generation from the earlier experiment, I don't think this can simply be written off.

3-3) You have exactly the same safety by moving the station out. If the cable breaks, the station and all the cable above the break would stay safely away from earth.

True, it would accomplish the same thing, but it seems to me the more amount of mass you have at geosynch the more stable the orbit will be.

In any case, I'll go with NASA's studies and reccomendations (no offense intended). Have any sources or links to studies showing a method similar to what you're describing?

 

[Hellbound]

scotth:

You know, we need to get someone with more physics in their background to come in here and help us with this.

The theory I proposed is one that I know is feasible, because it's the geenral idea that's been passed around about the concept (and the same general procedure NASA has been studying). However, it seems that moving the station out should work as well, although I'm not sure if there are issues I don't know with that scenario. It would seem to me that there must be some issue with it, or NASA would have looked at that for a solution to the space elevator concept.

Anyway, we need someone who can run the math for us Til then, I think we're just arguing to argue, more or less. I'll try to do some more looking, see what I can find. I'm stuck with dial-up at home now, so I don't do much web-hunting there . I'll get what I can, btu if you can find anything I'd love to see it. Or we can try to get one of our resident physics people (isn't Tez a physics major?) to see if they can help us out

 

[scotth]

As you can see in my last post, I said that equal masses were not necessarily needed...one could use a counterweight (such as a small asteroid). In any case, you'd use additional cable to move the station out, so either way you add more cable. No difference there. And the gravity is not an issue. If the center of mass is at geosynchronous orbit, then by definition the gravitational forces are equalized. That's the whole point of a geosynch CoM.

You can only use the CoM approximation (and it is an approximation) when the object is small in length compared to the gravitational field gradient.

The mass of the station and cable (and the fact that it is attached at the bottem) act against movement by winds. Also, and I might be wrong here, but as I understand it there are not jet stream winds at the equator (where a space elevator would have to be built).

To keep the system from bowing in the winds, the cable needs to be under some tension. Even without jetstream winds there, there will be some winds/weather to endure.

Even moving the station out does the same thing I suggested, as I reflect on it. No more tension would be added, but you are still moving the CoM of the station/cable combo to geosynch orbit. As long as the CoM is in geosynch, gravity is counteracted.

The amount of electricity generated could be enough to power the station itself, at least. One of the major concerns with this concept was creating a cable that was able to withstand the electrical stresses. The shuttle tried an electrical experiment like this, with a MUCH shorter cable; the cable broke rather quickly due to added stresses from the amount of electrical energy being generated. Of course, this is a side issue and not really pivotal to the main idea. Now, I can't give exact numbers right now, and the shuttle experiemnt was at a lower orbit/higher speed, so I am not positive that the numbers will be comparable; however, given the much larger than expected generation from the earlier experiment, I don't think this can simply be written off.

The amount energy need to run the station itself is well below a figure that would be meaningful in considering the economics of the project.

True, it would accomplish the same thing, but it seems to me the more amount of mass you have at geosynch the more stable the orbit will be.

In any case, I'll go with NASA's studies and reccomendations (no offense intended). Have any sources or links to studies showing a method similar to what you're describing?

I'd like to see the studies you are referring to. I would have thought that I've seen the same ones. Could it be that we are pulling somewhat different pictures from the information?

 

[Hellbound]

scott:

I posted the links to the NASA information in one of my earlier posts...hmmm...3rd one back from here. Look at the bottem of the post. There are two links to popular descriptions at NASA's home page, and a linkt ot he Phase 1 and Phase 2 feasibility studies done by NASA's Institute for Advanced Research.

I haven't read throught them completely, but enough to get a brief idea. I do know that they specifically mention using a counterweight. The question then seems to be is the counterweight necessary? It might simply be convenience to keep the station at a geosynch orbit. Now that I think about it, it might be a convenience factor. At a higher orbit or lower orbit ships attempting to dock with the station would speed up/slow down with respect to the station. So it may be feasibility.

Oh, as to ship launch which was referred to earlier, one could use the same electromagnetics used to pull things up the cable to launch things off the end. A slight boost to the side from the ship (to prevent the cable from slowing) and a large boost down the rails...might be useful. The mass of the system is so large, as well, that I don't know if slowdown would be an appreciable factor (at least in the short term, if this was intended to be a regular use then yes..in any case, I can see that it's impracticle).

As to electricity generation, again, it might be much more than my top-of-the-head ideas (or less). I do recall the space shuttle test, I'll see if I can find info on it. I don't kjnow the calculations to do to find the exact figures for electrical generation from orbiting objects...I would imagine it is less at geosynchronous, but not sure how much less.

In any case, the more i think about it there really isn't much difference int he dynamics of the two methods, but it does seem that keeping the station at a geosynchronous location would ease docking and launching and other operations. I'll have to look into it more. My satudies to date haven't been on the whole realm of possibilities, but simply on the possibilities that are being seriously considered now. In other words, I looked at what they propose to do now and why, but I have not looked at the exact reasons they wanted to do it this way.

 

[scotth]

I think I just found our disconnect.

I am pretty much ignoring the work station, that probably would be put as exactly the geosych altitude for convenience. It would be (in comparision to the rest fo the project) very low mass, and rather trivial to get into place.

The hard part is getting the real end point into place, and the cable between it and the ground. That real end point will need to be quite massive, and somewhat higher than geosynch orbit.

After the getting the end point in place and cabled to the ground, adding stops along the way would be gravy.

 

[Ipecac]

I think you mean funicular. Fununcular is an adjective pertaining to boils. It may be in the back of your mind if you read Harry Potter in which "fununculus" (or "fununculous", can't remember which) is a charm that induces boils.

By the way, if you put "fununcular" into Google, it helpfully suggests;

"Did you mean: funicular"

Rather excellently, one of only 4 hits is a broken link that shares your confusion;

"In Lisbon a trip to the Porto Institute is a must. It's at the
top of the fununcular just outside downtown."

That's all for now from the wild world of lexicography.

Thanks, BSM! I blame Harry Potter.

Although I do like the idea of Heavenly Boils.

 

[Hellbound]

I think I just found our disconnect.

I am pretty much ignoring the work station, that probably would be put as exactly the geosych altitude for convenience. It would be (in comparision to the rest fo the project) very low mass, and rather trivial to get into place.

The hard part is getting the real end point into place, and the cable between it and the ground. That real end point will need to be quite massive, and somewhat higher than geosynch orbit.

After the getting the end point in place and cabled to the ground, adding stops along the way would be gravy.

Yep, I think we are on the same track now...I assumed you were talking about the station being at the very end

IN any case, I think the main point is how to set it up. Most theories I've seen start at the station in geosynch so it will remain over the same point on Earth where you want to attach the cable. If you started at the end point (higher altitude) you would need quite a bit of constant propulsion until your cable reached Earth, in order to maintain position.

The ideas I've seen are to start with the station and string out cable and counterweight in opposite directions. You don't necessarily need equal cable out the back, though, that was simply the generalization to illustrate the point. Using part of your idea, take the asteroid and the station and put them in geosynch orbit together. Then, as you string cable down you push the asteroid out (obviously, less distance on the asteroid than on the downward cable) so that the CoM maintains the geosynch orbit.

I think we're on the same sheet of music now (well, seems we were always on the same sheet, just looking at different movements )

 

[scotth]

The easiest way I can think to do would be bring in the end point (large rock/asteroid) and put in geosynch orbit.

Bring the cable to the rock.

Start lowering the cable, and gradually move the rock outward as necessary.

Once the cable is complete and secure, move the parts of the station up the cable and into place.