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Johnny Pneumatic
17th November 2004, 01:11 PM
Space elevator prototype climbs at MIT (http://www.space.com/astronotes/astronotes.html)

This has to be one of the best ideas for getting to space for cheap. String up a 60,000 mile long ribbon made of carbon nanotubes and have a car drive up it to an orbiting asteroid space station.

Bruce
17th November 2004, 01:49 PM
I think it's quite a bit far-fetched. Coordinating the stationary orbit of a satellite to lower the ribbon alone is hard enough, but out of curiosity, I did some calculations.

I cut a 2"x 3" piece of ribbon and weighed it: 1 g

2 in x 3 in = 6 in^2

1 g = 0.0022 lb

(0.0022lb / 3 in) * (5280 in / mi) * 60000 mi * (1 ton / 2000 lb) =

116 tons of ribbon

116/6 = 19 tons/in^2

That would have to be one damn strong ribbon to withstand 19 tons of force per square inch!

Johnny Pneumatic
17th November 2004, 02:07 PM
Originally posted by Bruce
That would have to be one damn strong ribbon to withstand 19 tons of force per square inch!

Yes it would but.....
http://www.nasa.gov/images/content/34506main_super_s1.jpg

Bruce
17th November 2004, 04:30 PM
Bet I could catch one hell of a fish with that stuff....

Xeriar
17th November 2004, 04:52 PM
Originally posted by Bruce
116/6 = 19 tons/in^2

That would have to be one damn strong ribbon to withstand 19 tons of force per square inch!

Which is 3 tons of force per square centimeter, or ~27,000 Newtons or .27 GPa.

Rob Lister
17th November 2004, 04:59 PM
Originally posted by Xeriar
Which is 3 tons of force per square centimeter, or ~27,000 Newtons or .27 GPa.

What if it ...broke? (terrorism, industrial accident, faulty engineering, errant airplane, Murphy)

Doesn't look pretty.

No sir, I don't like it.

Xeriar
17th November 2004, 05:05 PM
Originally posted by Rob Lister
What if it ...broke? (terrorism, industrial accident, faulty engineering, errant airplane, Murphy)

Doesn't look pretty.

No sir, I don't like it.

The ribbon flies off into space, and the remaining length floats gently down to Earth.

Bikewer
17th November 2004, 06:23 PM
Arthur Clarke wrote a novel based on the "space elevator" concept some time ago. Tis' an interesting idea if the technology can be developed.

Beanbag
17th November 2004, 09:27 PM
Funny how everyone seems to think hanging a ribbon from a satellite is a static condition. No one appears to take into account aerodynamic and meteoroligical effects on the hanging ribbon. You know, little things like the jet stream? You've got variable forces acting on the ribbon, which will have an ongoing, gradual influence on the orbiting platform, which will have to be countered over the long run.

Carbon anotubes? Sounds to me like they would be electrically conductive to some degree. Anyone considered the possibility of lightning strikes on your ribbon? Even if it turned out that carbon nanotubes are a poor conductor, at the level of the potentials involved, even a poor conductor becomes a significant risk. I wonder what the potential difference would be from one end of the ribbon to the other? Especially when you go from a moist air atmosphere to a nice, insulating vacuum or fairly thin gasses being bombarded with various forms of radiation and becoming ionized. If nothing else, it would be interesting to see what happens when you run a conductive line from ground to space. You might get some neat auroral effects.

I think conditions on the orbiting end would be hell. Imagine being jerked around as different forces played with the ribbon. You might find your head banging the ceiling as the jet stream made a slight meander that crossed the ribbon. Think "marble in a bucket." Oh, sure, you could build in some sort of propulsion system to stabilize the platform, but we're talking a lot of force that would be needed to counteract the twangings of a 19-ton ribbon. How much reactive mass would have to be expended, and what would it do to the system's so-called economy if you had to keep sending up replacement fuel to replenish what you've used?

Nice idea, though I suspect reality has a few bites for this scheme.

Regards;
Beanbag

Xeriar
17th November 2004, 09:31 PM
There's no 'station' hooked to the end, it's just a 100,000 km long ribbon. Yes, nanotubes are pretty good conductors - it's helpful, even.

I don't think 'lightning bolt' is quite the term, this thing is a direct route to ground from above the atmosphere. I'll wager there will be a fairly continous stream of current into it from that mere fact alone.

Checkmite
17th November 2004, 11:51 PM
As I read it, the space end of the ribbon is tied to a geostationary satellite, and that's it. What people seem to be overlooking is the fact that our geostationary satellite is free floating, and even the slightest tug on our nanotube ribbon will send it careening out of position. Someone tried to tell me that once the ribbon is anchored, centripetal force will hold the satellite in position and keep the ribbon "taut"; however, I still fail to see how the ribbon will be completely unaffected by winds aloft.

Matabiri
18th November 2004, 01:53 AM
Originally posted by SkepticJ
Yes it would but.....
http://www.nasa.gov/images/content/34506main_super_s1.jpg

... for single, perfect nanotubes. There was a new world record set for nanotube length last year: 2 mm (http://www.azonano.com/details.asp?ArticleID=546).

Once you factor in the lattice faults (and in a 36,000 km long crystal, they are thermodynamically inevitable), nanotubes are no stronger than carbon fibre, because that's essentially what they are, just a neat configuration of it.

Edit to add: coolest thing from that article:
By altering the gas flow direction, researchers were also able to make cross connected nanotube grids that may be able to be used as nanosized circuits.

Also edit to add: who needs stainless steel, when you can do cool things with bainite (http://www.msm.cam.ac.uk/phase-trans/2002/low.temperature.bainite.html)? 2.5 GPa yield stress and plastic failure in a dirty steel.

MRC_Hans
18th November 2004, 02:10 AM
The ribbon/space elevator is not just tied to a geostationary sattelite. A counterweight is extending upwards, presumably holding the whole thing in equilibrium. I haven't seen any theorethicans tear the basic theory to schreds, so I assume the math is solid enough.

Plenty of other problems, obviously. A totally new material technology is needed, but that is a sort of barrier we have overcome many times before.

Hans

Matabiri
18th November 2004, 02:22 AM
Originally posted by MRC_Hans
The ribbon/space elevator is not just tied to a geostationary sattelite. A counterweight is extending upwards, presumably holding the whole thing in equilibrium. I haven't seen any theorethicans tear the basic theory to schreds, so I assume the math is solid enough.

Yeah - the centre of mass has to be geostationary. This is also the point where the forces are greatest.

Originally posted by Xeriar
The ribbon flies off into space, and the remaining length floats gently down to Earth.

Ideally most of it burns up in the atmosphere on re-entry.

I would be interested in knowing what standing waves might be set up, though...

69dodge
18th November 2004, 02:26 AM
Originally posted by Bruce
I think it's quite a bit far-fetched. Coordinating the stationary orbit of a satellite to lower the ribbon alone is hard enough, but out of curiosity, I did some calculations.

I cut a 2"x 3" piece of ribbon and weighed it: 1 g

2 in x 3 in = 6 in^2

1 g = 0.0022 lb

(0.0022lb / 3 in) * (5280 in / mi) * 60000 mi * (1 ton / 2000 lb) =

116 tons of ribbon

116/6 = 19 tons/in^2

That would have to be one damn strong ribbon to withstand 19 tons of force per square inch!That's a mighty short mile you got there . . . :D

Plus, you shouldn't divide at the end by the area of your piece of ribbon (2 inches times 3 inches), but rather by its cross-sectional area (2 inches times the very small thickness of the ribbon).

Xeriar
18th November 2004, 02:40 AM
I thought that was a mighty small tensile strength for the project. :-)

Redoing the math, I come up with around 260 GPa.

However, there is a good chance that the nanotube ribbon is going to be considerably lighter.

Soapy Sam
18th November 2004, 02:46 AM
The structure extends as far above the geostationary orbit as below. Then you build another two 120 degrees apart.

Then you...

Nice idea, but I'm not holding my breath. Clarke's "The Fountains of Paradise" is a good yarn though.

What I find worrying is how as soon as anyone proposes an ambitious structure these days, someone immediately pops up with the terrorism question. Yes, a terrorist can put a car in front of a 200mph train. He can drop rocks off a freeway bridge. He can poison water supplies. Should we therefore do without such things?

Let's not let the cavemen win, people.

Matabiri
18th November 2004, 02:56 AM
Originally posted by Soapy Sam
What I find worrying is how as soon as anyone proposes an ambitious structure these days, someone immediately pops up with the terrorism question. Yes, a terrorist can put a car in front of a 200mph train. He can drop rocks off a freeway bridge. He can poison water supplies. Should we therefore do without such things?

Be pretty silly for terrorists to blow a space elevator up... not when they could use it to drop rocks from orbit.

Bruce
18th November 2004, 06:52 AM
Originally posted by 69dodge
That's a mighty short mile you got there . . . :D

.....errr.....I have small feet. :D

Johnny Pneumatic
18th November 2004, 07:12 AM
Originally posted by Matabiri
Once you factor in the lattice faults (and in a 36,000 km long crystal, they are thermodynamically inevitable), nanotubes are no stronger than carbon fibre, because that's essentially what they are, just a neat configuration of it.

Well, we might not even attempt to make a space elevator until nanites are made that will build it without the carbon lattice flaws.
You are correct, they aren't any stronger than carbon can be because that's what they are. What they are though is a molecule of nothing but sp2 bonds which are stronger than diamond's sp3s. I think the graph I posted is incorrect also; didn't notice it until I had posted it. I think it has their strength at about 1/3 of what it really is.

Matabiri
18th November 2004, 07:38 AM
Originally posted by SkepticJ
Well, we might not even attempt to make a space elevator until nanites are made that will build it without the carbon lattice flaws.

Doesn't matter; the flaws will appear anyway after a time. A perfect structure is thermodynamically unstable.

I know you're crazy about nanotubes, but the most interesting potential uses for them involve the electronic structure and the way you can contain other atoms inside them, rather than the physical engineering capabilities.

Johnny Pneumatic
18th November 2004, 07:54 AM
Originally posted by Matabiri
Doesn't matter; the flaws will appear anyway after a time. A perfect structure is thermodynamically unstable.

I know you're crazy about nanotubes, but the most interesting potential uses for them involve the electronic structure and the way you can contain other atoms inside them, rather than the physical engineering capabilities.

Yeah those are neat to but there's just something neat about super strong buildings. I'll be morose for who knows how long now.:( Could you explain this thermodynamic instability? Is it basically entropy? One problem a cnt ribbon would have is O1 in the upper atmosphere. Not sure what kind of damage it'd cause but oxidation from these reactive atoms cause damage to craft in low earth orbit over time.

Matabiri
18th November 2004, 08:36 AM
Originally posted by SkepticJ
Could you explain this thermodynamic instability? Is it basically entropy?

Basically, yeah. The configurational entropy of a crystal is derived from an expression of the form

(X ln(X) + (1-X) ln(1-X))

where X is the fractional concentration of the base element (in this case carbon), and (1-X) is the fractional concentration of impurities, including vacancies (missing atoms). A crystal is thermodynamically stable at local minima in this curve.

If you differentiate this equation, you find that at the points X=1 (pure crystal) and X=0 (pure impurity), the gradient is always infinite - that is, these points are never stable. You can engineer the relevant energies to make the stable point very close to X=1, but never actually force it to be at the pure crystal.

This mean that small crystals with few overall atoms can be made perfectly, but as the number of atoms increases it becomes far more likely that stacking errors will form, with a likelihood which is a function of

exp(-Q/kT)

where Q is the energy required to remove an atom from the lattice, k is Boltzmann's constant, and T is the temperature. Even if Q is very high, with 5x10^22 atoms in every gram of nanotube, the chance of having vacancies approaches 1 very rapidly as you reach macroscale lengths.

(More on this at http://www.matter.org.uk/matscicdrom/manual/po.html - the discussion is for metals, but applies to all crystals.)

Matabiri
18th November 2004, 08:39 AM
For super-buildings, have a look at the Taipei tower:

Apparently the 800-tonne weight hanging around the 88th floor (to provide earthquake stability) is quite a sight.

CurtC
18th November 2004, 09:22 AM
Above the atmosphere, damage would also be done by micrometeors. To me, this seems more significant than oxidation damage and bond breaking due to entropy.

Matabiri
18th November 2004, 09:25 AM
Originally posted by CurtC
Above the atmosphere, damage would also be done by micrometeors. To me, this seems more significant than oxidation damage and bond breaking due to entropy.

Pah! An engineering difficulty, not a physical limitation.

Johnny Pneumatic
18th November 2004, 09:29 AM
Originally posted by CurtC
Above the atmosphere, damage would also be done by micrometeors. To me, this seems more significant than oxidation damage and bond breaking due to entropy.

Very very good point. At meteoric speeds a pencil eraser size rock hitting something is like a powerful pipe bomb going off. KE=1/2 times M times V squared.

Johnny Pneumatic
18th November 2004, 09:45 AM
Originally posted by Matabiri
For super-buildings, have a look at the Taipei tower:

Apparently the 800-tonne weight hanging around the 88th floor (to provide earthquake stability) is quite a sight.

Looks kind of like one of my favorite plants, bamboo. I think a slightly conical building a few miles high made from cnt composite members, polycarbonate/glass/aerogel windows etc. would be much better. Why? Because the frequency that the building would vibrate at would be much less than the ground shaking in the strongest earthquake, making it immune to these due both to its super strength members, clading and it just can't shake to pieces like much shorter buildings. I'm still not seeing the entropy problem though. Any ordered matter will fall apart eventually won't it? Why is atomic flaws happening after many years a problem for this space ribbon? Please forgive my lack of complex material science knowledge.

Matabiri
18th November 2004, 10:09 AM
Originally posted by SkepticJ
Any ordered matter will fall apart eventually won't it? Why is atomic flaws happening after many years a problem for this space ribbon? Please forgive my lack of complex material science knowledge.

This isn't a long time-scale thing in an entropy-always-increases way. Basically, when an ordered phase forms, it does so because the free energy of that phase is lower than the phase it's forming from (this is, essentially, the energy of crystallisation in freezing, for example). The phase will configure itself to lower its overall free energy, given the temperature, the pressure, and the chemical composition*. The free energy is calculated from two functions - the enthalpy (http://www.brainyencyclopedia.com/encyclopedia/e/en/enthalpy.html), and the entropy (http://www.brainyencyclopedia.com/encyclopedia/e/en/entropy.html). The entropy is a configurational energy term.

Essentially, it is always energetically desirable, due to the entropy, to have a few defects in an ordered structure. The equilibrium concentration of those defects can be calculated, and it is likely that thermal equilibrium (i.e. some defects) would be reached from a perfect original crystal over a timescale of few years, even for a very stable structure such as carbon nanotubes.

It's probable, though, that if manufacture can be made cheap enough, and nanotubes can be made long enough, that they'll make stronger composites than carbon fibre - but they won't be orders of magnitude stronger, due to these missing carbon atoms. Your graph compares a theoretically perfect macromolecule to an engineering material - it's no wonder the apparent difference is so striking.

Hope that helps - unless you're familiar with the thermodynamic basics I've quite possibly flubbed explaining it well, though.

(* This is a simplification; there are metastable phases, for example, and transformations can be thermodynamically desirable but kinetically impossible if the temperature is dropped sharply (in glasses, for example), but it'll do for now.)

Bruce
18th November 2004, 10:42 AM
Originally posted by CurtC
Above the atmosphere, damage would also be done by micrometeors. To me, this seems more significant than oxidation damage and bond breaking due to entropy.

You should see what nanometeors can do! ;)

TillEulenspiegel
18th November 2004, 10:44 AM
I'm' not a materials scientist, but I suspect that rather then a monolithic structure , something like the ribbon would be a composite with different materials complimenting the others characteristics. I.E. Stretchability, torsional flexibility, high tensile strength, weight Etc.

As an interesting footnote rather then see the conductivity of carbon fiber or nanotubes as a negative , the ribbon could be used as a generator using the electrical potential difference between top and bottom and also the path thru charged space. Good old Maxwell.

Lightning is natures attempt to equalize the potential between earth and sky. Since the difference can be anywhere from 10's to 100's of millions of volts the ability to couple such a wide range would require a significant engineering effort. If I'm not too addled the current "sink" would also produce a neutral or charge free zone around the structure and actually help prevent lightning discharges.

Since some have discussed entropy and nano technology it's seems relevant to mention one of the latest efforts in material science is self-healing structures. I'm not sure where the idea rests today but the approaches seem novel and realizable.

http://www.netcomposites.com/news.asp?2377

Johnny Pneumatic
18th November 2004, 11:42 AM
Originally posted by Matabiri
This isn't a long time-scale thing in an entropy-always-increases way. Basically, when an ordered phase forms, it does so because the free energy of that phase is lower than the phase it's forming from (this is, essentially, the energy of crystallisation in freezing, for example). The phase will configure itself to lower its overall free energy, given the temperature, the pressure, and the chemical composition*. The free energy is calculated from two functions - the enthalpy (http://www.brainyencyclopedia.com/encyclopedia/e/en/enthalpy.html), and the entropy (http://www.brainyencyclopedia.com/encyclopedia/e/en/entropy.html). The entropy is a configurational energy term.

Essentially, it is always energetically desirable, due to the entropy, to have a few defects in an ordered structure. The equilibrium concentration of those defects can be calculated, and it is likely that thermal equilibrium (i.e. some defects) would be reached from a perfect original crystal over a timescale of few years, even for a very stable structure such as carbon nanotubes.

It's probable, though, that if manufacture can be made cheap enough, and nanotubes can be made long enough, that they'll make stronger composites than carbon fibre - but they won't be orders of magnitude stronger, due to these missing carbon atoms. Your graph compares a theoretically perfect macromolecule to an engineering material - it's no wonder the apparent difference is so striking.

Hope that helps - unless you're familiar with the thermodynamic basics I've quite possibly flubbed explaining it well, though.

It'd have been nice if NASA.gov would have talked about this before they got my hopes up a few years back.:( Nasa is generally a bastion of not blowing the significance of things out of proportion and blowing smoke so I came to trust them. Thought the uber material was just a few decades away. Garrrrrr the trust is tainted and I'm p*ssed!:mad:

Bruce
18th November 2004, 12:27 PM
Originally posted by SkepticJ
It'd have been nice if NASA.gov would have talked about this before they got my hopes up a few years back.:( Nasa is generally a bastion of not blowing the significance of things out of proportion and blowing smoke so I came to trust them. Thought the uber material was just a few decades away. Garrrrrr the trust is tainted and I'm p*ssed!:mad:

There, there, SkepticJ. NASA is not trying to decieve you personally. The world of research is a constant game of making your investors and grant providers believe that the next huge money-making breakthrough is only a breath away. :D ;)

TillEulenspiegel
18th November 2004, 12:40 PM
Bruce in re Your sig line about Your pride in Your offspring, just remember.
It's better to be a parent then obscure.

Bruce
18th November 2004, 12:44 PM
Originally posted by TillEulenspiegel
Bruce in re Your sig line about Your pride in Your offspring, just remember.
It's better to be a parent then obscure.

huh?

Johnny Pneumatic
18th November 2004, 12:52 PM
Originally posted by Bruce
huh?

Apparent than obscure. Get it?:hit:

Johnny Pneumatic
18th November 2004, 12:59 PM
Originally posted by Bruce
There, there, SkepticJ. NASA is not trying to decieve you personally. The world of research is a constant game of making your investors and grant providers believe that the next huge money-making breakthrough is only a breath away. :D ;)

But when you've spent hundreds of hours drawing, working math problems, writing down and daydreaming new ideas under the false impression that this material will allow them to actually work you can understand the magnitude of my let down. Damn.

patnray
18th November 2004, 01:17 PM
See Electrodynamic Tethers in Space in the Aug 2004 Scientific American for an interesting discussion of this topic. Unfortunately, the SciAm website no longer allows you to view articles without paying for them...

Xeriar
18th November 2004, 01:30 PM
Question for the materials people here...

Are carbon strings possible? Basically C=C=C=C=...

Rather than four or three bonds, if you have only two it would seem the tensile strength would go up even more, even if you're basically left with atomic string.

Johnny Pneumatic
18th November 2004, 01:40 PM
Spin baby spin (http://www.sciam.com/article.cfm?articleID=00069535-2EF9-1052-AEF983414B7F0000&sc=I100322)
I found the name of Clark's book with space elevators, The Fountains of Paradise.

Bruce
18th November 2004, 02:25 PM
Originally posted by SkepticJ
But when you've spent hundreds of hours drawing, working math problems, writing down and daydreaming new ideas under the false impression that this material will allow them to actually work you can understand the magnitude of my let down. Damn.

(Sigh) You must still be a student. There's a nice Dilbert comic that addresses the moment when your dreams and ideas come crashing down and you become a mindless corporate zombie managed by hopeless idiots. Your day will come. Until then, keep daydreaming and coming up with ideas. Maybe you'll be able to think of a way to save us from ourselves before you join our leagues. ;)

Bruce
18th November 2004, 02:27 PM
Originally posted by SkepticJ
Apparent than obscure. Get it?:hit:

Joke like that work better when spoken allowed. :D

Johnny Pneumatic
18th November 2004, 05:33 PM
I found a Power Point presentation on stresses and defects in nanotubes, but I don't think it has really anything to do with what Matabiri is talking about. Here you go (http://www.eng.fsu.edu/~chandra/Presentations/SrishCsit17oct.ppt#1)

There, there, SkepticJ. NASA is not trying to decieve you personally. The world of research is a constant game of making your investors and grant providers believe that the next huge money-making breakthrough is only a breath away.-Bruce

I'm assuming from you smilieys that you're kidding. Or are a lot of scientists really that unscrupulous?

So these missing carbon atoms, where do they go? They just fall off or what, or do they not go anywhere but the atomic structure changes slightly at that point opening up the small void?

Johnny Pneumatic
18th November 2004, 07:01 PM
If Matabiri's math is right then carbon nanotubes aren't even close to the strength needed to make a space elevator ribbon. If that's the case why is time and money being wasted on this then? 260 GPa strength give or take would be needed for it to work, and a heafty safety margin above this would be good. So is Nasa and many people at many colleges wasting money on an immpossible dream or Matabiri doesn't know what he's talking about, or both? I'm in a state of bafflement here, somebody help me out.:confused: Estimates that I'm finding for perfect(which have been said already to be unstable and thus not able to be super strong) nanotubes range from about 600 GPa-5.5TPa mostly around 1TPa, so the graph I posted is way off, but may have been up to date when it was new. I don't know. I'm not even training to be a scientist. Graphic Arts is my current path, but science will always be my first love.:) I might not be back until Monday, cheers.

Bruce
18th November 2004, 07:07 PM
Originally posted by SkepticJ
Or are a lot of scientists really that unscrupulous?

You need to work on your terminology, SkepticJ. Instead of "unscrupulous", use words like "resourceful", "convincing", "inspirational", "hopeful", and "promising."

Have you ever written a grant before or given a presentation for potential investors? Do you really think anyone is going to write you a check if you included any of the negative sounding replies you've heard on this post.

Most of the people here are intelligent enough to know that the criticisms of the ribbon idea are only trying to point out the potential obstacles involved, not to shoot it down and write it off as unworthy of further work. People that are writing checks for millions of dollars usually don't see it that way. In fact, if I had 1 million dollars and I read this post, there is no way I would fund this research.

If you have an idea and you want to get funding for it, you have to believe in the potential of what you're researching and convince them that you can make it work, especially if you're talking to investors. Investors want that money back, and then some.

Bruce
18th November 2004, 07:26 PM
Originally posted by SkepticJ
If that's the case why is time and money being wasted on this then?

I was writing my previous reply when you wrote this one. The above comment really stings. I don't think you understand how research works. Researchers explore new ideas, concepts, and inventions for the love of science, and because what they are doing has never been done before. Do you know how hard it is to get funding for research in today's world?

Nanotubes were only discovered about 10 years ago. We still don't know how to make them affordably, let alone make them into something that can be sold and make a profit. Investors don't often understand how slowly science moves. They want to see returns on their investment almost immediately. So who's left to get the money from? The government, of course.

Most government workers that are signing the checks don't understand science either, but their not trying to make a profit, so the money tends to go to whoever can give the most impressive sales pitch. Having a ribbon dangling from space that a robot can climb sounds like a cool idea. It's no wonder they got money from NASA.

Are they really out to build that ribbon? They don't have to be. All they have to do is continue to work towards it and other scientific achievments are likely to blossom from it.

I'll give you a great example. Tha LASER. The laser was invented in the 1940's. During that time period, every aspect of science that researchers pitched to the government for funding ended with the question, "Yeah, but how can we make it into a weapon?" No descent scientist wants to turn all of his inventions and discoveries into a weapon, but that was the only way to get funding at the time, scrupulous or not. The scientists at the time worked toward building a laser weapon to appease the government, but along the way, other uses were found for it. It took about 40 years for lasers to be developed into bar code scanners and motion sensors, and another 10 years to develop into affordable cd's, DVD's, and hand-held laser pointers. Right now, you can buy a laser cat toy at Petco for \$2.

Think about it. If you had gone to the government in 1940 and asked for \$50 million dollars to research monochromatic light with the intention of making a \$2 cat toy, do you think we would have DVD players today? NOBODY would have predicted that monochromatic light could be used for data storage or even a fraction of the things we use lasers for today.

Don't get discouraged, SkepticJ. You live in a world full of crazy people that don't think like scientists. Good things can come out of telling a few white lies, or red and green lies for that matter. That's how things work outside of the school walls, my friend. ;)

Matabiri
19th November 2004, 01:09 AM
Originally posted by SkepticJ
It'd have been nice if NASA.gov would have talked about this before they got my hopes up a few years back.:( Nasa is generally a bastion of not blowing the significance of things out of proportion and blowing smoke so I came to trust them. Thought the uber material was just a few decades away. Garrrrrr the trust is tainted and I'm p*ssed!:mad:

Bruce has already answered this far more eloquently than I could, but then he's probably written far more grant proposals than I have.

Nasa is in the business, at least in part, of stimulating the public imagination - and using this to obtain funding for the more mundane. My personal grudge against them is that they killed the breakthrough propulsion physics (http://www.grc.nasa.gov/WWW/bpp/) group two years ago. It wasn't even costing them that much money.

Personally, I'm convinced the space elevator is viable. The physics is sound, it just requires some funky engineering and clever materials. But if it was easy, we'd have done it already, right?

For the carbon chain, I've asked this myself. Bruce can probably give you more details, but as I understand it, having that many double bonds in a row makes the chain highly reactive, and thus prone to bond with other things, breaking the double bonds.

Please don't give up on scientists. We just have to deal with politicians to get money.

Matabiri
19th November 2004, 01:22 AM
Originally posted by SkepticJ
Estimates that I'm finding for perfect(which have been said already to be unstable and thus not able to be super strong) nanotubes range from about 600 GPa-5.5TPa mostly around 1TPa, so the graph I posted is way off, but may have been up to date when it was new.

The presentation you found also makes the distinction between E (Young's modulus, ~1 TPa) and strength (yield stress, ~150 GPa). E is the stiffness, the ratio of stress to strain during elastic deformation. The yield stress is the stress required for permanent deformation - or fracture, in the case of nanotubes.

Anders
19th November 2004, 04:19 AM
Originally posted by Bruce
I think it's quite a bit far-fetched. Coordinating the stationary orbit of a satellite to lower the ribbon alone is hard enough, but out of curiosity, I did some calculations.

I cut a 2"x 3" piece of ribbon and weighed it: 1 g

2 in x 3 in = 6 in^2

1 g = 0.0022 lb

(0.0022lb / 3 in) * (5280 in / mi) * 60000 mi * (1 ton / 2000 lb) =

116 tons of ribbon

116/6 = 19 tons/in^2

That would have to be one damn strong ribbon to withstand 19 tons of force per square inch!
We have to take inaccount that at high alltitudes the gravity would have lesser impact. Additionally, the device would be hanging in the satalite, also that distributing the weight.

I would say providing they can make large enough mesh, they could build it.

But security would be a problem, how would we restrict airspace around the tube, what happens if the satalite moves a bit, or an earthquake move the foundation.

But it was built, we could have low-gravity labs for undergraduates! Heaven for chemists and molecular biologists!

Bikewer
19th November 2004, 06:02 AM
Although carbon nanotubes may well offer great promise for such constructions in the future, I submit we already have a material of such enormous tensile potential that no further research need be done:

Johnny Pneumatic
19th November 2004, 06:35 PM
Originally posted by Matabiri
The presentation you found also makes the distinction between E (Young's modulus, ~1 TPa) and strength (yield stress, ~150 GPa). E is the stiffness, the ratio of stress to strain during elastic deformation. The yield stress is the stress required for permanent deformation - or fracture, in the case of nanotubes.

So, after factoring in the point flaws what does the strength drop to? 60somethingGigaPascals?

Bruce, sorry I must have forgotten my aphorism.[slaps head] "Fund everything, because you never know where discoveries will come from."-quoting self

Bruce
19th November 2004, 08:28 PM
Originally posted by SkepticJ

Bruce, sorry I must have forgotten my aphorism.[slaps head] "Fund everything, because you never know where discoveries will come from."-quoting self

That's the spirit! Now, can I borrow \$20 for a case of Guiness? I have discovered some of my best ideas on the bottom of a Guiness bottle. (hic) :alc:

Matabiri
20th November 2004, 04:11 AM
Originally posted by SkepticJ
So, after factoring in the point flaws what does the strength drop to? 60somethingGigaPascals?

Bruce, sorry I must have forgotten my aphorism.[slaps head] "Fund everything, because you never know where discoveries will come from."-quoting self

The tests in the presentation you posted go up to 60GPa, and they quote 150GPa. But these are for short nanotubes. My instinct tells me, in the absence of real data at the scales and stresses, that you'd need Weibull (http://www.weibull.com/LifeDataWeb/the_weibull_distribution.htm) models for failure. These aren't normally used for composites, as they fail in very energy absorbing ways, but the energies stored elastically at ~260 GPa are going to be far in excess of what can be easily contained.

Johnny Pneumatic
21st November 2004, 12:18 PM
Originally posted by Matabiri
Nasa is in the business, at least in part, of stimulating the public imagination - and using this to obtain funding for the more mundane. My personal grudge against them is that they killed the breakthrough propulsion physics (http://www.grc.nasa.gov/WWW/bpp/) group two years ago. It wasn't even costing them that much money.

So that's what happened to that project. I remember reading about it in Popular Science about four or five years ago.

Originally posted by Matabiri The tests in the presentation you posted go up to 60GPa, and they quote 150GPa. But these are for short nanotubes. My instinct tells me, in the absence of real data at the scales and stresses, that you'd need Weibull models for failure. These aren't normally used for composites, as they fail in very energy absorbing ways, but the energies stored elastically at ~260 GPa are going to be far in excess of what can be easily contained.

Are you talking about multi walled or single wall nanotubes? The multi walled ones do fail at around 60GPa, but the single wall ones are supposed to be much stronger; around 200GPa if I'm not mistaken. Something needs to be discovered that can bond with them to make a composite; nanotubes are really slippery.

Matabiri
22nd November 2004, 02:16 AM
Originally posted by SkepticJ
Are you talking about multi walled or single wall nanotubes? The multi walled ones do fail at around 60GPa, but the single wall ones are supposed to be much stronger; around 200GPa if I'm not mistaken. Something needs to be discovered that can bond with them to make a composite; nanotubes are really slippery.

... and the chemistry of the bond will reduce the strength too.

Regarding the strengths, you've probably done much more reading up on nanotubes than I have - I just have issues with the reality of "perfect" molecules on the scales required.

Johnny Pneumatic
22nd November 2004, 11:05 AM
Originally posted by Bruce
That's the spirit! Now, can I borrow \$20 for a case of Guiness? I have discovered some of my best ideas on the bottom of a Guiness bottle. (hic) :alc:

Such as? I must have evidence before I fund your "research".