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[BigMacnFries]

I read a newspaper article today about trials being approved for a "space ladder." Apparantly they can drop a line from a sattalite that orbits above a set point on earth. Does anyone know much about these. Obviously there are some limitations to simply dropping down a loop of rope attached to a motor or NASA wouldn't spend millions on rockets but I can't imagine what those limits would be.

 

[blimkie]

Yes its being discussed in another thread but has grown off topic. a company called liftport group has set there dead line for 2018. They plan to drop a ribbn of carbon nano tubes from Earth's orbit down and anchor is somwhere in an ocean. Space crafts with attach to the ribbon and drive themselves to the top then use a thurster of some sort to postition them selves in orbit.

 

[JesseM]

I read a newspaper article today about trials being approved for a "space ladder." Apparantly they can drop a line from a sattalite that orbits above a set point on earth. Does anyone know much about these. Obviously there are some limitations to simply dropping down a loop of rope attached to a motor or NASA wouldn't spend millions on rockets but I can't imagine what those limits would be.

The usual name for this is a space elevator--there's a good wikipedia article about it here, and an article from NASA's site http://science.nasa.gov/headlines/y2000/ast07sep_1.htm . To build a strong enough cable they'd need a material made of long strings of carbon nanotubes--they're not nearly there yet, but there's been a lot of progress in this area. Still, even if it could work it's probably a long way away.

 

[DaveC426913]

There's an entire newsgroup with scientists who discuss the intimate details of the space elevator. I was on it for a while.

 

[blimkie]

There's an entire newsgroup with scientists who discuss the intimate details of the space elevator. I was on it for a while.

well what are your opinions of it, is it possible?

 

[BigMacnFries]

Thanks, links were excellent

 

[Cybersteve]

well what are your opinions of it, is it possible?

It's not possible yet, it's expected that with advances in material technology it may become theoretically possible.

However actually building the thing is another matter. As I understand it the plan would be to have another huge mass on another tether "above" the orbiting station. You could then let the lower tether down while letting the higher one up to keep the system in balance.
However I've no idea how you would catch the lower end of the 200 mile rope when it's swinging in the breeze!

There's also the very real prospect of something going wrong, what kind of damage could a 200 mile cable do to those beneath it should it fall!

 

[JesseM]

There's also the very real prospect of something going wrong, what kind of damage could a 200 mile cable do to those beneath it should it fall!

This problem was analyzed too, I believe most of it would burn up in the atmosphere.

 

[Cybersteve]

This problem was analyzed too, I believe most of it would burn up in the atmosphere.

It's been a while since I read up about this so I'll follow up the links you suggested above.

One thing I couldn't find out before was why the cable had to be attached to such an impossibly tall tower. Is this just to minimise the length or to avoid weather conditions?

 

[JesseM]

It's been a while since I read up about this so I'll follow up the links you suggested above.

One thing I couldn't find out before was why the cable had to be attached to such an impossibly tall tower. Is this just to minimise the length or to avoid weather conditions?

The wikipedia link says the thickness of the cable has to increase exponentially from the base to geostationary orbit, so one way to minimize the maximum thickness is:

Increasing the height of a tip of the base station, where the base of cable is attached. The exponential relationship means a small increase in base height results in a large decrease in thickness at geostationary level. Towers of up to 100 km high have been proposed. Not only would a tower of such height reduce the cable mass, it would also avoid exposure of the cable to atmospheric processes.

 

[Cybersteve]

I should have checked first!
Thanks.

 

[Chronos]

No known material has the strength to weight ratio necessary to make such a cable.

 

[JesseM]

No known material has the strength to weight ratio necessary to make such a cable.

Not true, the calculations suggest that carbon nanotubes would do the trick, although they'd have to find a way to make much longer ones than they've made so far.

 

[Chronos]

Show me the material! I'm an engineer. If I can't abuse it, I can't use it.

 

[JesseM]

Show me the material! I'm an engineer. If I can't abuse it, I can't use it.

They are able to make carbon nanotubes in small amounts, if that's what you're asking. It used to be that they could only make them a few nanometers in length, but recently they discovered a new technique which allowed them to make nanotubes up to 4 cm long, and according to the researchers there's no ultimate limit on the length of nanotubes that could potentially be produced using this technique. See the article here:

http://www.trnmag.com/Stories/2004/110304/Nanotubes_lengthen_to_centimeters_Brief_110304.html

 

[Chronos]

Still a bit short to tether it to an Earth orbit. Nanotubes are too brittle... at least by current standards... agreed?

 

[HallsofIvy]

Arthur Clark wrote a science fiction novel, "The Fountains of Paradise" about this idea- he called it a "space elevator".

HOWEVER, you can't have one end in "stationary orbit" around the earth! The point of stationary orbit is that for an object in such an orbit, the force necessary to keep the object in orbit at that speed is precisely the gravitational force at that height. If a little below, gravity pulls you down. If a little higher, gravity is not enough to keep you from going higher still.

But every part of the "ladder" hanging between the satellite and Earth would have gravity pulling it down- a satellite in stationary orbit can't offset that- there is no upward force on it to do that. What you would need is a satellite well above stationary orbit so that the total "upward pull" (i.e. gravity is not enough to maintain orbit) of the part above stationary orbit will offset the downward pull of gravity on the part below. Since gravity falls off as 1/r² the satellite "anchoring" the ladder would have to be many times "stationary orbit" away.

 

[russ_watters]

Since gravity falls off as 1/r² the satellite "anchoring" the ladder would have to be many times "stationary orbit" away.

Well, either that or it would just have to be a little further away and enormous.

 

[turbo]

Well, either that or it would just have to be a little further away and enormous.

It's goin to have to be enormous anyway, isn't it? After all, it has to provide enough tension on the tether to support the full weight of the tether, the weight of the climbing craft (elevator), AND the weight of payload, plus the inertia involved in accelerating the craft and payload.

Maybe it would be easier to construct a very large rail-gun near the equator, facing east and keep the entire launch mechanism anchored to the ground. It might be possible to put up small payloads at regular intervals and assemble things in near-Earth orbit, instead of launching big chemical-fueled rockets every few months.

 

[JesseM]

Still a bit short to tether it to an Earth orbit. Nanotubes are too brittle... at least by current standards... agreed?

Brittle? I hadn't heard that brittleness was a problem, do you have a source? I thought the problem was just being able to make sufficiently long nanotubes, and since that new technique increased the maximum length by a factor of 10 million or so, I don't think it's so implausible that with people continuing to pour money into this (carbon nanotubes have many more practical applications besides space elevators) the maximum length will continue to increase rapidly.

 

[pervect]

I ought to re-read the space elevator proposal again, but it looks like it's moved. Anyway, from what I can recall, the problem with production of nanotubes wasn't length. The problem was that you needed single walled tubes (not nested cylinders), and that current production methods that produced these sorts of tubes came out in a tangled mess, not suitable for making cables.

Chemical means of straightening the tangles out out apparently do exist, but these methods have the "brittleness" problem previously alluded to. (Yes, those chemicals are as bad for the nanotubes as they are for your hair - joke!). The brittleness problem was not fundamental, though.

I gathered that the idea always has been that the nano-tubes would be glued together, and that with reaonably well spaced joints (I don't recall the exact spacing, it was on the order of inches or fractions of an inch, not feet) the glue would not contribute significantly to the weight of the cable. I never quite understood the details of this, but the authors seemed convinced that this was true and that the formulation of a suitable "glue" wasn't a problem.

While I couldn't confirm all of these details quickly (they are from memory which is alas all too fallible), I did see a note that there was an ongoing competition to produce strong naontube material, and another one to address another issue, the power beaming question. This was at:

http://www.spaceref.com/news/viewpr.html?pid=17887

 

[russ_watters]

It's goin to have to be enormous anyway, isn't it?

Yeah, either way. Let's make an order-of-magnitude estimate: if the material for the tether weighs .1 lb/ft on average, that's 11 million pounds. And that's just for the part from geostationary orbit down.

Now someone who is fluent in astrodynamics correct me if I'm wrong, but if you loft a giant winch (we're talking roghly the size of a destroyer) into geostationary orbit, you can then simply unroll it and the tether will drop while the winch/counterweight rise. This would be unstable due to the different orbital periods at different altitudes, so you'd need giant "thrusters" to keep the system aligned while unrolling it.

But hey - I think we can find the money in the federal budget to loft a destroyer into geostationary orbit.

 

[JesseM]

Yeah, either way. Let's make an order-of-magnitude estimate: if the material for the tether weighs .1 lb/ft on average, that's 11 million pounds. And that's just for the part from geostationary orbit down.

The tether is not a constant width, it changes width as a function of altitude to minimize the tension. As for weight, the wikipedia page says:

Bradley C. Edwards, Director of Research for the Institute for Scientific Research (ISR), based in Fairmont, West Virginia, is a leading authority on the space elevator concept. His designs contrast with previous designs by presenting a plausible scheme showing how a space elevator could be built in little more than a decade, rather than the far future.

He proposes that a single hairlike 20 short ton (18 metric ton) 'seed' cable be deployed in the traditional way, giving a very lightweight elevator with very little lifting capacity.

Then, progressively heavier cables would be pulled up from the ground along it, repeatedly strengthening it until the elevator reaches the required mass and strength. This is much the same technique used to build suspension bridges.

Although 20 short tons for a seed cable may sound like a lot, it would actually be very lightweight — the proposed average mass is about 0.2 kilogram per kilometer. Conventional copper telephone wires running to consumer homes weigh about 4 kg/km.

Bradley Edwards has a book which goes into the engineering details of his space elevator proposal if you're interested, titled . And this site has a collection of references, some available online:

http://www.spaceelevator.com/docs/

Included are two articles written by Bradley Edwards for NASA:

http://www.spaceelevator.com/docs/472Edwards.pdf
http://www.spaceelevator.com/docs/521Edwards.pdf

 

[Garth]

Arthur Clark wrote a science fiction novel, "The Fountains of Paradise" about this idea- he called it a "space elevator".

Arthur Clarke writes about the 'Space Elevator' http://www.clarkefoundation.org/news/092405.php .

Garth

 

[turbo]

tether = conductor

Arthur Clarke writes about the 'Space Elevator' http://www.clarkefoundation.org/news/092405.php .

Garth

Thanks for the link. Clarke usually has some good "what if" ideas, however I think that he should have taken this one a bit further. What happens when you extend a conductive ribbon radially through the EM field surrounding our planet? This tether might be more useful as a source of electric power than as a means for getting things into orbit, assuming that it could handle the electrical load without vaporizing.

 

[B. Elliott]

Thanks for the link. Clarke usually has some good "what if" ideas, however I think that he should have taken this one a bit further. What happens when you extend a conductive ribbon radially through the EM field surrounding our planet? This tether might be more useful as a source of electric power than as a means for getting things into orbit, assuming that it could handle the electrical load without vaporizing.

or on a smaller scale, to power satellites...http://science.nasa.gov/newhome/headlines/ast22jan99_1.htm

 

[DaveC426913]

HOWEVER, you can't have one end in "stationary orbit" around the earth! The point of stationary orbit is that for an object in such an orbit, the force necessary to keep the object in orbit at that speed is precisely the gravitational force at that height.

Correct. Geostat is only the midpoint of the cable. The counterbalance is more cable reeled out the other side. This end has to be much longer, since gravity is correspondingly weaker. I forget, but it's, like, 3 or 4x longer.

The cool bonus of this is that it provides a free space launching system. Your elevator car is switched for a space-faring rocket, you lower it to the tip (which is now, like 120,000 miles from Earth, and moving much faster than orbital speed), and you simply let go. You get a free boost for interplanetary travel.

AND we get a free high-speed docking too. Ships don't have to slow down to achieve orbit, They just attach to the cable, which is moving at faster than orbital speed.

THAT's why space elevators are going to open up the solar system.

 

[DaveC426913]

But hey - I think we can find the money in the federal budget to loft a destroyer into geostationary orbit.

See, you don't have to 'loft' it. All you need is a space elevator and then you can ship the parts up there almost free! All we need to do is to build a space elevator!