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dk54321
2004-Jan-12, 02:46 AM
Could someone explain how this space elevator concept is supposed to
work? As I understand it, you have a geostationary satellite tethered
to a point on the ground. Then you have an elevator that climbs the
cable. I understand the bit about monomolecular wire to make a very
long, light, and strong tether, but I don't understand where the
energy to raise a payload to orbital velocity comes from.

If the tether were rigid, it would come from the rotational inertia
of the earth, but this would also make it a very tall, heavy and
expensive Tower of Babel. If the tether is flexible, as the elevator
climbs, it decelerates the satellite, which gradually comes to a
lower, faster orbit, and snap goes the tether. The only way I see to
avoid this is to have orbit-maintaining thrusters on the satellite.
(Maybe a continuous low thrust from an ion engine.)

So, where is the economy? The bulk of the energy required for orbit
is not spent in raising a satellite to orbital altitude, but in the
delta-V to accelerate it to orbital velocity. Are you saving energy
by not having to reach the higher velocities required at lower
altitudes, the slower trip to the top, the lower thrust needed to
maintain the orbit, or some combination of these?

Littlemews
2004-Jan-12, 03:00 AM
See : http://science.howstuffworks.com/space-elevator.htm
I think someone already post it in this forum before(not the site), take sometime to search for it ^^

jkmccrann
2005-Nov-01, 11:28 AM
I have my doubts about whether a space elevator will ever be built, mainly because I think it would be an endless black hole monetarily and I don't see any private firm taking up the risk to build one. Who would insure such a monstrous development and how would they price that insurance?

No insurance, it won't get built, and it's plain just too expensive.

eburacum45
2005-Nov-01, 01:47 PM
If you move mass up the elevator, all you are doing is adding mass to one side of the elevator ; to counterbalance this you could ever fire thrusters as you suggest, or move a counterweight outwards along the part of the elevator which is further out than geostationary orbit.

Yes, it still takes energy to get to orbit; but the advantage is, you don't have to lift the fuel off the ground in order to do so. If you use an electric lifting motor powered by photovoltaic cells (as per many of the designs on the table) then you need no fuel on board the craft at all.

Ken G
2005-Nov-01, 03:01 PM
If you move mass up the elevator, all you are doing is adding mass to one side of the elevator ; to counterbalance this you could ever fire thrusters as you suggest, or move a counterweight outwards along the part of the elevator which is further out than geostationary orbit.
.
This overlooks the issue of where the angular momentum comes from. You still need a thruster of some kind to generate the torque. At the end of the day, you have X satellites of Y mass in orbit, so you need Z energy. The energy savings must be that you are not fighting through atmospheric drag, and you are not spewing a bunch of wasted fuel all over the place. Basically, a rocket is not energy-limited, it is momentum-limited. And a lot of the momentum flux goes into supporting against gravity, and the rest generates lift. That's wasteful. But putting thrusters on the elevator couldn't be too easy either-- that's a lot of fuel weight to get up there. It must be kind of like a Saturn rocket where the last stage never fires, it is used to maintain the orbit. Think of it as an "economy of scale" argument.

But here's an argument to ponder as well-- if something went wrong and the orbit decayed, you'd have a wire flying all over the place slicing things up!

Ken G
2005-Nov-01, 04:07 PM
After looking at the above link, it appears that a different concept will be used-- the tether is attached to the Earth, so the angular momentum does come from the Earth itself. As long as the cable is longer than the geostationary length (and not much longer, or the stress is too high), the Earth will pull the space elevator and give it any angular momentum it needs.
Note the tether need not be "rigid", it merely needs to be able to sustain a stress along the direction of the cable. You only have to get it to the geostationary point to begin with, and then you can just use lift along the cable to get the mass up. I missed the entire point, which is to get all your rotational energy from the Earth-- all you need mechanically is a force slightly greater than gravity. In other words, you or I could easily climb into space if we had enough time!

eburacum45
2005-Nov-01, 05:56 PM
Strangely enough, the elevator doesn't have to be attached to the Earth; some designs have it terminating at the top of the atmosphere, perhaps attached to a large balloon. I don't think you would transfer much angular momentum through a balloon.

devilmech
2005-Nov-01, 06:56 PM
http://en.wikipedia.org/wiki/Space_elevator gives a nice overview of several different concepts, from composition to the logistical problems. Even has a little bit of the math behind it.

Ken G
2005-Nov-01, 09:26 PM
Strangely enough, the elevator doesn't have to be attached to the Earth; some designs have it terminating at the top of the atmosphere, perhaps attached to a large balloon. I don't think you would transfer much angular momentum through a balloon.
That must be a very different concept, because the key to whole idea is drawing on the Earth's angular momentum. Then you only need to concentrate on vertical lift, the sideways orbital motion is provided automatically. I agree that a balloon at the base would need to drag on the atmosphere to get the same effect, and that doesn't seem very effective. And how do you lift it up to the balloon anyway? What's the advantage of that design?

gbaikie
2005-Nov-02, 03:32 AM
Could someone explain how this space elevator concept is supposed to
work? As I understand it, you have a geostationary satellite tethered
to a point on the ground. Then you have an elevator that climbs the
cable. I understand the bit about monomolecular wire to make a very
long, light, and strong tether, but I don't understand where the
energy to raise a payload to orbital velocity comes from.

If the tether were rigid, it would come from the rotational inertia
of the earth, but this would also make it a very tall, heavy and
expensive Tower of Babel. If the tether is flexible, as the elevator
climbs, it decelerates the satellite, which gradually comes to a
lower, faster orbit, and snap goes the tether. The only way I see to
avoid this is to have orbit-maintaining thrusters on the satellite.
(Maybe a continuous low thrust from an ion engine.)

So, where is the economy? The bulk of the energy required for orbit
is not spent in raising a satellite to orbital altitude, but in the
delta-V to accelerate it to orbital velocity. Are you saving energy
by not having to reach the higher velocities required at lower
altitudes, the slower trip to the top, the lower thrust needed to
maintain the orbit, or some combination of these?

Even if a space elevator were made of super strong material which was very low weight in relations to it's strength, the thousands of miles of cable by itself will have a massive weight. Whatever thing that will ride this cable will add a insignificant fraction to the total overall weight of the structure. Let's say you had a "car" and payload which weighed 20 tons if you accelerate it at 1 gee you have effectively made the car weight 40 tons. Whereas the cable itself could weigh thousands or millions of tons.

One way to say this is the cable is constantly accelerating it's weight and vehicle which using this cable is adding to the acceleration needed [or adding to it's weight]. So the main question is how can you build this structure lift it's own weight. The only way this can be done is by tapering the material. For instance, take a steel rod say 2" in diameter. Even if the steel has high tensional strength if it's 30 mile long it won't hold it's weight if hanged in mid-air [via some "skyhook"], but if this steel rod is tapered say going from 1/4" diameter to 2" in diameter, then it's possible for very a high tensional strength steel cable to hold it's own weight and the addition weight that the 1/4" steel rod can lift [tons of weight]. So the weight of car is significant, in that if you taper this structure down to a hair diameter, then the payload is limited to whatever this thin material can support. So the main problem is building anything light and strong enough to support it's own weight- if you can do that, the rest is fairly easy.
Now as far as where the energy comes from. These cars or "lifter" are going to actually provide to energy to lift vehicle. They can be electric motors which can get electical power transmitted to them in some fashion (from the space elevator structure or perhaps beamed to them) or there are other ways other than using normal electrical motors.

Ok, you still might be a bit puzzled by this. The main thing you might be missing is that rockets lifting payload into orbit are inefficent in terms of energy. The inefficency is similar to having car needing to carry all the fuel it needs to travel across world- 24000 miles. Your car would different beast if it needed to carry 48 times as much fuel as it currently does- it would look more like semi instead a sedan.

eburacum45
2005-Nov-02, 08:26 AM
That must be a very different concept, because the key to whole idea is drawing on the Earth's angular momentum.
No. it is the same concept; remember a stationary balloon at the top of the atmosphere will rotate with the Earth, so has the same angular momentum as an object on the ground (give or take a smidgen).
The reason some designs are connected to balloons or tall towers is that the elevator hangs down from geostationary orbit; so it has to support its own weight, and the upper part of the elevator has to be strong enough to do so.
If you can make it shorter by a few kilometers that is a little less weight that has to be carried by the upper parts of the tether; and because of the effects of wind and weather (including electrical storms) the part of the elevator which is in the atmosphere has to be particularly robust.
So the best way to lighten the load, or to reduce the stress on the upper part of the elevator, is to attach the elevator above the effects of the atmosphere; this could be acheived by a massive floating balloon platform, reached by a second elevator which is supported by the balloon not by the space tether.
Or in another set of designs, the elevator could attach to a rigid surface structure which is as high as possible (Arthur Clarke described his fictional elevator as reaching down to an equally fictional mountain); Dani Eder suggested a massive self-supporting concrete and carbon buckyfibre structure fifteen kilometers tall.
If this structure (whether rigid or balloon-supported) is as high as possible it will lift goods above the worst effects of the atmosphere without adding weight onto the bottom of the elevator.

Ken G
2005-Nov-02, 02:05 PM
So the best way to lighten the load, or to reduce the stress on the upper part of the elevator, is to attach the elevator above the effects of the atmosphere; this could be acheived by a massive floating balloon platform, reached by a second elevator which is supported by the balloon not by the space tether.
.
These are engineering considerations, and important ones, but what I am pointing out is that the fundamental concept must draw on the Earth's rotation to be at all interesting or practical. If you want to tether it to a balloon, you'll still need the balloon to drag on the Earth's atmosphere and thereby communicate the necessary angular momentum. I'm not saying that's impractical, maybe it works fine, I'm just saying if you need the drag, then you are not "above the effects of the atmosphere". I would point out that upper atmospheric winds would factor in, and you may have a new definition of "geosynchronous" orbit-- windosynchronous. But then you can't tether the second elevator to the ground. In all, the balloon idea does not seem to escape the basic problems.

neilzero
2005-Nov-22, 04:33 AM
The space elevator described at www.liftport.com seems to be much the same as the Dr. Edwards elevator. The CNT = carbon nano tubes needs to be hundreds of times stronger than Kevlar for the starter ribbon to have a mass of 20 tons and the finished ribbon 200 tons. The climbers = lifters or elevator car will be only 5% to 10% of the total ribbon mass, so the total movement is rather small. Perhaps more important, nothing happens a few miles up the ribbon when a climber starts up the ribbon. The ribbon stretches locally due to the weight which is increased by the climber accellerating up the ribbon. The stretch transient travels up the ribbon at perhaps 500 kilometers per hour, so the climber could outrun it's own stretch transcent.
If my 500 kilometer per hour estimate is average, it is 200 hours before the far end starts to fall toward Earth. By then the climber is likely past GEO altitude, and is now pushing the ribbon away from Earth which partially cancels the pulling it was doing the first 36,000 kilometers. If the climbers stop at or before GEO altitude or leave the ribbon for destinations elsewhere in the solar system instead of parking at the far end as a counter-weight, there are acumulating problems keeping the ribbon from falling toward Earth. Over periods of weeks, it will require careful management of the transcients traveling on the ribbon. Over compensating can mean the ribbon is tryng too hard to move away from Earth which may break the ribbon due to excessive tension. This is an unlikely problem, unless telemetering gives bad data and/or humans make a bad judgement call or micro meteorites damages the ribbon just before a strong transcient arrives. Please embelish, refute and/or comment as I may have this wrong. Neil

LurchGS
2005-Nov-23, 05:03 AM
Eventually, some form of elevator will be built. economies of scale dictate this - per pound, using an elevator is hundreds - if not thousands - of times cheaper than rocket. Not only does a rocket need to carry ejectible fuel mix, a large percentage of that ejecta is wasted energy. This is not the case for a load going up the elevator - very little waste energy.

Kaptain K
2005-Nov-23, 01:04 PM
The problem, as I see it, with the space elevator is that every single satelite, spent booster and bit of space junk has to be removed from orbit to allow the elevator to be built!

GOURDHEAD
2005-Nov-23, 02:59 PM
The problem, as I see it, with the space elevator is that every single satellite, spent booster and bit of space junk has to be removed from orbit to allow the elevator to be built! Or some sort of collision avoidance provided to protect the space elevator cable/ribbon.

Has anyone discovered a site that describes CNTs with sufficient tensile strength to make a space elevator feasible?

Ken G
2005-Nov-23, 03:56 PM
The problem, as I see it, with the space elevator is that every single satelite, spent booster and bit of space junk has to be removed from orbit to allow the elevator to be built!
Interesting point, although I think the situation is helped by the fact that I'd expect everything in the equatorial plane to be in geosynchronous orbit anyway. Here's what I wonder-- how bad of a problem is the precession of the Earth's equatorial plane? It might start exciting some nasty north-south waves in the CNT fairly quickly. As I see it, the amplitude of the oscillation would increase 360/26,000 = 0.013 degrees every year. Geosynchronous satallites must already have this wobble problem, I don't know if they do orbital corrections or just tolerate it. The space elevator would have to make orbital corrections all the time to avoid the oscillations, that might require a lot of fuel. Then again, if you can just lift fuel up when you need it...

Kaptain K
2005-Nov-23, 06:52 PM
...I'd expect everything in the equatorial plane to be in geosynchronous orbit anyway.
Except that everything that is not orbiting in the equatorial plane crosses the equatorial plane twice every orbit!

Van Rijn
2005-Nov-23, 11:44 PM
Except that everything that is not orbiting in the equatorial plane crosses the equatorial plane twice every orbit!

Sure, but the horizontal cross-section of the elevator is very small. Certainly there would need to be some traffic control so that the orbits don't intersect the cable, but there's no technical reason it couldn't be done.

Ken G
2005-Nov-24, 12:02 AM
Except that everything that is not orbiting in the equatorial plane crosses the equatorial plane twice every orbit!
Yes, but you can simply not build it in the path of a satellite-- the only ones that would be hard to avoid are the ones in the equatorial plane (although there is probably some precession effects in satellite orbits, I'm not really sure of the details of what's happening to them up there.) Does anybody know what is done about wobble due to the precession of the Earth's axis? Just not worried about, or corrected for?

Senor Molinero
2005-Nov-24, 03:05 AM
There's another thing. What effect does the moon have on geostationary satellites? They are 1/10th of the way there. If the moon can affect the tides here on Earth, surely something as small as a satellite or cable platform would want to stretch towards the moon as it whipped past every day and a bit.

Ken G
2005-Nov-24, 06:42 AM
Yes that's quite a good point, especially when also lined up with the Sun. I would think that would be a nasty stretch on the fiber!

Kaptain K
2005-Nov-27, 01:13 PM
Sure, but the horizontal cross-section of the elevator is very small. Certainly there would need to be some traffic control so that the orbits don't intersect the cable, but there's no technical reason it couldn't be done.
You're joking, right? There are, literally, tens (if not hundreds) of thousands of bits of space junk in orbit. And that's just the ones that we can track! The Space Shuttle goes up for a week and comes back with pits from contact with orbiting debris. If you stretch a cable through all of circum-terran space, it will get hit often.

Elyk
2005-Nov-28, 11:47 AM
The problem, as I see it, with the space elevator is that every single satelite, spent booster and bit of space junk has to be removed from orbit to allow the elevator to be built!
Even a small piece of metal or rock could cause huge problems when it hit any part of the elevator, probably would break the cable.

Van Rijn
2005-Nov-28, 12:40 PM
You're joking, right? There are, literally, tens (if not hundreds) of thousands of bits of space junk in orbit. And that's just the ones that we can track! The Space Shuttle goes up for a week and comes back with pits from contact with orbiting debris. If you stretch a cable through all of circum-terran space, it will get hit often.

No, I'm not joking, and yes, I am aware of the debris issue. Most of that material is in LEO. In LEO, the cable would probably have debris protection, and the cable design would certainly have redundancy, perhaps the hoytether (http://www.tethers.com/Hoytether.html) design - which is specifically designed to prevent catastrophic failure due to micrometeorite/debris impact. Along with that, I expect that there would be some garbage collection effort before the the cable is put in place (and garbage collection is likely to happen eventually anyway).

After the cable is in place, there would be regular inspections and careful traffic control. It is by no means an unsolvable problem.

It comes down to this: If you can put a many thousand mile long cable in space, you certainly can go to a little effort to protect it.

GOURDHEAD
2005-Nov-28, 03:17 PM
It comes down to this: If you can put a many thousand mile long cable in space, you certainly can go to a little effort to protect it.
When you do the arithmetic to compute the CNT tensile strength to mass ratio you will understand the constraints on protection and seek a different system for lifting objects into Earth orbits.

Van Rijn
2005-Nov-29, 02:38 AM
When you do the arithmetic to compute the CNT tensile strength to mass ratio you will understand the constraints on protection and seek a different system for lifting objects into Earth orbits.

I understand there are constraints on non-structural mass. I also understand that we don't have the tensile strength in bulk carbon nanotube material yet. So? What are your specific issues?

GOURDHEAD
2005-Nov-29, 02:45 PM
So? What are your specific issues?
http://physicsweb.org/articles/world/11/1/9 and http://www.pa.msu.edu/cmp/csc/nanotube.html are portals into the world of CNTs. Casually browsing through some of the links I have not seen data supporting theoretical values that imply tensile strengths that allow the construction of space elevators in an earthlike environment; therefore, it seems imprudent to be concerned about protecting something you don't how to build. Money and ingenuity are better spent perfecting interstellar systems based on photon and particle beams as power sources for both launch and interstellar transportation systems.

The sun has allowed us to evolve and held us close with its gravity. Now let it send us on our way with its electromagnetic energy. Currently, we know of no other technology by which we can begin to explore the MW in the near future.

IsaacKuo
2005-Nov-30, 02:24 AM
Photon and particle beam propulsion is NOT technologies for near term interstellar exploration. The power levels required are many orders of magnitude beyond the state of the art for high power lasers and particle beam emitters.

However, a related concept, "particle puff propulsion", can be used for a relatively economical interstellar flyby mission at around .25c using current technology.

In this case, the ultimate energy source isn't the Sun but rather the stellar processes which originally created the Solar System's source material (i.e. mostly uranium 238, lithium 6, and deuterium). In a sense, it's not Sun power at all, but rather Earth power--the power of Earth's gravity well and geologic processes to concentrate useful uranium ore and hold onto hydrogen (including useful amounts of deuterium).

Van Rijn
2005-Nov-30, 10:12 PM
http://physicsweb.org/articles/world/11/1/9 and http://www.pa.msu.edu/cmp/csc/nanotube.html are portals into the world of CNTs. Casually browsing through some of the links I have not seen data supporting theoretical values that imply tensile strengths that allow the construction of space elevators in an earthlike environment; therefore, it seems imprudent to be concerned about protecting something you don't how to build.


From This NASA website: (http://science.nasa.gov/headlines/y2000/ast07sep_1.htm)



Fiber materials such as graphite, alumina, and quartz have exhibited tensile strengths greater than 20 GPa (Giga-Pascals, a unit of measurement for tensile strength) during laboratory testing for cable tethers. The desired strength for the space elevator is about 62 GPa. Carbon nanotubes have exceeded all other materials and appear to have a theoretical strength far above the desired range for space elevator structures. "The development of carbon nanotubes shows real promise," said Smitherman. "They're lightweight materials that are 100 times stronger than steel."

Carbon nanotubes are the reason for the interest in the space elevator concept. Before that, it was a completely theoretical concept. Yes, it will be some time before we can produce bulk material with the desired strength, but it certainly is physically possible.



Money and ingenuity are better spent perfecting interstellar systems based on photon and particle beams as power sources for both launch and interstellar transportation systems.


Laser launch from the earth has its own problems, economically and technologically. Beam powered starships would require a vast space infrastructure. Let's get into space first, then we can worry about starships.

Frank2
2005-Nov-30, 11:35 PM
Casually browsing through some of the links I have not seen data supporting theoretical values that imply tensile strengths that allow the construction of space elevators in an earthlike environment; therefore, it seems imprudent to be concerned about protecting something you don't how to build. Check out this article (http://www.liftport.com/papers/2005Nov_LP-Ribbon_Mass.pdf) to see how close they are to having the needed material. True, they are not there yet so it may never be doable but those who support the idea feel their time is well spent figuring out all the other problems and trusting that the material to build the ribbon will be available when they are ready.

GOURDHEAD
2005-Dec-02, 04:34 PM
Check out this article (http://www.liftport.com/papers/2005Nov_LP-Ribbon_Mass.pdf) to see how close they are to having the needed material. True, they are not there yet so it may never be doable but those who support the idea feel their time is well spent figuring out all the other problems and trusting that the material to build the ribbon will be available when they are ready.Note that little attention was given to ribbon assembly details. The article, without saying so, argued from the premise that the ribbon was continuous from the Earth anchor point to the counterbalancing mass such that no splicing processes were included. Also no mention was made of the process required to make threads from the short fibers nor how the threads are combined to construct the ribbons. What these processes do to the ultimate tensile strength of the completed ribbon is avoided.

It seems reasonable to assume that the ribbon will not be continuous and that it will require splicing, protective coating, and some special surface treatment for the areas in contact with lift modules (ribbon grabbers). Each of these will increase the total weight of the ribbon, whereas the spinning and weaving described above will likely compromise the CNT tensile strength,i.e., the tensile strength of the ribbon is less than the tensile strength of the sum of the threads, the tensile strength of the threads is less than the sum of the tensile strength of the sum of the fibers.

Despite the gloom and doom of this input, I am at heart an optimist and hope the SE advocates can demonstrate that my thinking is faulty. CNTs have great promise and, I believe, will be widely used in space exploration, so I heartily support their development and improvement.