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Re - Can we now build the space elevator?

In the post copied below I discussed tying together nanotubes to get arbitrarily long nanotubes. If simply tied together, the longer nanotube would be weaker but note even if many short segments were tied together the resulting decrease in strength would still be only the same fraction as when two segments were tied together. That is, if the tensional strength decreased to 60% of a single nanotube when two were tied together, the tensional strength for when 1 million nanotubes were tied together end-to-end would still be 60% of a single nanotube. Note then that the tensional strength of nanotubes is so markedly higher than other materials the strength of these tied together nanotubes would still be significantly higher than other materials. This page gives measured strengths of some common knots:

The Most Useful Rope Knots for the Average Person to Know

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Note that some strengths range down to 43% but most lie in the range of 60% to 70%.

These small diameter nanotubes would have to be tied into knots. So a means of manipulating them at the nanoscale would be required. Firstly, nanotubes at centimeter lengths have been formed, as described in the Science article "Direct Synthesis of Long Single-Walled Carbon Nanotube Strands" I cited below. Another research team has also created centimeter long nanotubes:

Extra-long carbon nanotubes set new record

20 September 2004
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A longer strand of tiny tough stuff.
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Then their lengths is not a problem for forming into knots, just their small diameters for manipulation purposes. The "Direct Synthesis of Long Single-Walled Carbon Nanotube Strands" paper notes the 20 cm long nanotubes were aligned into approx. 10-20 nm diameter ropes. The authors make a distinction between *strands* of the nanotubes which were in the 10 micron diameter range and *ropes* in the 10-20 nanometer diameter range. The key distinction is that their measurements of the strand's strengths show only a small fraction of that estimated for nanotubes. The reason is that these are not formed from nanotubes all the same length. They are formed from a conglomeration of nanotubes of varying lengths. Then the forces holding them together are just van der Waals forces, rather than the carbon-carbon bonds of nanotubes. In such a case holding a strand at both ends and pulling allows the separate nanotubes to peel apart. This is described on the page "Pulling nanotubes makes thread" I cited below. However, it is the researchers opinion that the *ropes* are formed of nanotubes formed all the same length. Then presumably their tensile strength would be the same as the individual nanotubes. Note then that devices do exist that can manipulate items at the tens of nanometers scale:

Nanotube Nanotweezers. Science, Vol. 286, No. 5447, p. 2148-2150, 10 December 1999

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Fabrication and actuation of customized nanotweezers with a 25 nm gap. Nanotechnology, 12, p. 331-335, 2001
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Three-dimensional manipulation of carbon nanotubes under a scanning electron microscope. Nanotechnology, 10, p. 244-252, 1999
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An alternative method for linking the nanotubes together would be to connect them with nanotube rings:

Ring Closure of Carbon Nanotubes. Science, Vol. 293, No. 5533, p. 1299-1301, 17 August 2001

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These are closed rings formed from one or more nanotubes. They are about 540 nm across so several of the ropes would have to be fitted into the rings. One question would be how to tighten the rings around the nanotubes once they were fitted into the rings. One possibility might be to use the piezoelectric effect of nanotubes where they can lengthen when subjected to an electric field. Then you would apply the field to the rings to widen them, place the ends of the nanotube ropes inside, then switch off the power to tighten the rings around the nanotube ropes. Note that using the rings as a means of binding the ropes together means you are using frictional effects to get the nanotubes to hold together. Then is this any better than the van der Waals forces holding the "strands" together? I believe it can be as long as you make the rings stricture tight enough. But if it is made too tight, this would cut into the nanotube ropes reducing their strength. Then the optimal degree of tightening would have to be found to maintain the greatest strength.

Interestingly, the method of knotting the nanotubes together or binding them by rings might also be applied to the strands, that is, to the case where the nanotubes are of different lengths held together by van der Waals forces. You would note the shortest length of the nanotubes composing a strand and tie knots around the strand or bind it with a rings at short enough intervals to insure that every nanotube is held tightly with a tie or knot at least once all along the length of the strand. Note then there have been several reported cases of nanotube strands or ribbons macroscopically long but much weaker than individual nanotubes because they are held together by van der Waals forces. Then this method of binding them at short intervals may provide a means of recovering close to the full strength of the individual nanotubes. Another question that would need to be answered is how binding together a group of equally long nanotubes effects the strength of the nanotubes when the binding is only going around the outer nanotubes. That is, suppose you created a string made from *individual* nanotubes bound end-to-end and measured the string's strength. Then you composed a string by using nanotube ropes that all contained the same number of nanotubes, say 100, and bound these ropes end-to-end. Would the string composed of the ropes be able to hold 100 times as much as the string composed of individual nanotubes? This is asking a somewhat different question than how knotting weakens the nanotubes. It's asking how strong a composed string will be when a binding can only go around the outer nanotubes composing the string.

Bob Clark

************************************************************ Newsgroups: sci.astro, sci.physics, sci.space.policy, sci.materials From: snipped-for-privacy@yahoo.com (Robert Clark) Date: 29 Aug 2004 08:01:49 -0700 Local: Sun,Aug 29 2004 11:01 am Subject: Can we now build the space elevator?

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=AD=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D From: Robert Clark ( snipped-for-privacy@yahoo.com) Subject: Re: beanstalks (was Re: Metallic hydrogen ...) Newsgroups: sci.physics, sci.astro, sci.space.policy, sci.materials, sci.energy Date: 2004-06-09 02:06:53 PST

h= snipped-for-privacy@spsystems.net (Henry Spencer) wrote in message ...

... > Given that the nanotubes themselves are far thinner than even a one-micron > ribbon, any material technology that ties them together into bulky > materials should work just as well for such ribbons, with some adjustment > in the details of manufacturing. Even such a ribbon *is* a bulky > material, when the fibers involved are nanotubes. > ...

Tie? Hmmm. Do you think it might work to tie the ends together of the 20 centimeter long nanotubes already produced? Looking up some links on knots, the knotted ropes always have less strength than the single, unbroken ropes. I confirmed this by testing on sewing thread. Still it might be interesting to find out how strong they are compared to single nanotubes.

Bob Clark =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=AD=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

Testing with thread confirmed that a break always occurred where two strands were tied together. However, to estimate the strength of a single strand of thread, I wrapped two ends around my fingers and found that the break occurred in the middle of the thread, not where I was holding the thread. My guess was that the softness of my fingers prevented the thread from breaking at the attachment point (where I was holding it.) I confirmed this by holding one end by a pair of pliers and the other end with my fingers. The break occurred where the pliers held the thread. However, when I put a soft cloth between the thread and the pliers, the break occurred in the middle of the thread, as when I was holding both ends with my fingers. I imagine this must actually be a common way of testing tensile strength. That is, you don't want to attach the strand or rope to something that will make the rope break at the attachment point. This would give an invalid measure of the rope strength. You want it to break somewhere in the middle. I therefore suggest connecting together the already produced 20 centimeter long nanotubes with a soft material or by whatever means used to insure nanotubes don't break at the attachment point during tensile strength testing. This will allow the full strength of the nanotubes to be maintained even when they are connected together. What will need to be investigated is what soft material will also be light enough so as not to cancel out the weight savings of using the nanotubes. Note that this soft material might be heavier than the nanotube material but because it only has to be used at the connections it can be quite small so quite conceivable may only add minimally to total weight. It still needs to be confirmed that the macroscopic sized nanotubes really are as strong as the nanotubes tested on the microscale. This report showed that 20 centimeter interwoven strands were significantly weaker than the tested individual microscale nanotubes:

Direct Synthesis of Long Single-Walled Carbon Nanotube Strands. Science, Vol 296, Issue 5569, 884-886 , 3 May 2002

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However, the theory is that this is because there were many single nanotubes connected together by weaker van der Waals forces rather than the stronger carbon-carbon molecular bonds that prevail in individual nanotubes. This is explained here:

Pulling nanotubes makes thread

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What still needs to be tested is the strength of the *individual* nanotubes that make up these 20 centimeter long strands.

This article describes a group that proposes that competively offered prizes could make possible the technologies required for the space elevator by 2010:

Space elevator contest proposed. 'Elevator:2010' aimed at encouraging technology development.

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Bob Clark

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Reply to
Robert Clark
Loading thread data ...

POOR SCALING IDEA ???

KNOTS TO YOU

Yes, there is some data on the strength reduction of macroscopic ropes being tied into knots.

The question is "Who would assume that the physics of knots scale down to NANOtubes.......?????"

Not I.

You can't tie a good knot into a single strand of graphite fiber or glasss fiber.

WHY ? ?

If you calculate the strain of a knot in a single filament (mostly simple physical geometry is all you need), the strain is typically on the order of 50% to 100%.

The failure strain of glass fiber might be as high as 10%, and the failure strain of a graphite fiber will be well under 10%.

So, before the knot is tied, the fiber is broken. The failure strain is exceeded.

Fishermen know that you can tie knots in reasonably ductile polymeric monofilament line and the stretchiness of the line is important for the knot to work.

That is that the monofilament has enough elongation at failure that the knot does not cause brittle failure upon being tied.

You have a KNOTY problem.

You seem to have great personal tenacity in this technical issue pf | "Space Elevator".

Keep on trying. But not tying.

Robert Clark wrote:

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Reply to
jbuch

Probably not needed. I experimented with taking two strands of kevlar string (maybe 100 fibers) intermingling the fibers over 1/3 of their length (1m) and gluing the two strands together with silicone adhesive. The result was almost as strong as the kevlar.

Reply to
Ian Stirling

Their stiffness may indeed make bending individual tubes them into knots difficult. However, Fig. 1 in the "Direct Synthesis of Long Single-Walled Carbon Nanotube Strands" paper shows the much thicker

*strands* (approx. 30 micron diameter) can be bent into knots. However, this may be due to their being not as stiff or strong due to the van der Waals bonds between the nanotubes here. There have been reports though of nanotubes being formed into coils:

Nanocoils spring into place.

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From the diameters of the coils I would say these are more likely ropes of nanotubes rather than individual nanotubes, but this would be sufficient for our purposes. Another posibility is to induce the bending in the tubes while they are being formed. This research shows quite sharp bends can be formed in this way:

NANOTECH ADVANCE MAKES CARBON NANOTUBES MORE USEFUL. "San Diego, CA, April 11, 2005 -- Researchers at UCSD have made carbon nanotubes bent in sharp predetermined angles, a technical advance that could lead to use of the long, thin cylinders of carbon as tiny springs, tips for atomic force microscopes, smaller electrical connectors in integrated circuits, and in many other nanotechnology applications."

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Bob Clark

Reply to
Robert Clark

However, nanotubes seem to buckle, rather than break. So, it's likely possible that they could sustain being knotted. You'd need bloody small knitting needles though, and I doubt that the strength would be as high. I suspect that it's pretty irrelevant if you can align long fibers in a composite rope, if the substrate is at all elastic, even if it doesn't grip the long fibers well, it should work.

Reply to
Ian Stirling

The essential point is still true. Scaling Laws.

One canot just grab a macroscopic correlation - like the one on knots - and project it down to the nano-scale without some honest thinking.

There is often a lack of honest thinking in the "Space Elevator" cheering section. But, the desire is so high, that it is to be expected that all straws of hope will be grasped.

Macroscopic ropes are fairly strong even if the fibers are discontinuous and there is no "glue". But there is a reason for this.

Reply to
jbuch

This page shows sharp bends can be made even in individual nanotubes. See the images here:

Carbon nanotubes.

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These bends were induced by a device called the NanoManipulator:

NanoManipulator=99 DP-100/200

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Bob Clark

Robert Clark wrote:

Reply to
Robert Clark

This article from doing actual measurements found a highest strength of

63 GPa:

Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load. SCIENCE, VOL 287, p. 637-640, 28 JANUARY 2000

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However, in some of the cases the attachments broke before the nanotubes. It is a little unclear if they are including in their reported cases the nanotubes for which the attachment broke first:

"A deposit at least 100 nm square was typically made at each MWCNT/ AFM tip interface and was usually a strong enough attachment to allow the loading and breaking of MWCNTs before the attachment failed (20-22). For the MWCNTs in the current sample set, about half would become detached at one of the deposit sites during the tensile-loading experiment. We report here the results from the successful mounting, tensile loading, and breaking of 19 MWCNTs." p=2E 637.

In any case the force required to break the nanotube itself in the cases where the attachment broke first would be unknown. Therefore it is possible some nanotubes have strength higher than 63 GPa.

And this report showed measured tensile strengths up to 150 GPa:

Direct mechanical measurement of the tensile strength and elastic modulus of multiwalled carbon nanotubes. B=2EG. Demczyk et al. Materials Science and Engineering, A334 (2002), 174, 173-178.

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They also showed examples of inducing sharp bends in individual nanotubes which suggests the possibility of tying the tubes into knots.

In fact the strength may actually be *better* than that quoted. Both of these studies were done on multiwalled tubes since they are larger and it's easier to make attachments with them. In the earlier study in Science, the authors from SEM imaging noted that it was actually the outer single-walled nanotube that broke first therefore it was carrying the load. This would make sense from the way the attachments were formed which could only form a bond with the outer surface of the multiwalled tube. Therefore the numbers quoted were for the strength of this outer single-walled nanotube using as thickness only that of this single-walled nanotube. However, in the later study in Materials Science and Engineering, the authors believed the attachments were made to all the layers of the multi-layered nanotube. Therefore they took the cross-sectional area to be the cross-section of the multi-walled tube viewed as a filled-in

*disk*. This is a key fact because it means the strength of this multiwalled tube is actually higher than the quoted value because it is actually hollow between the layers. The authors could not calculate the more accurate cross-sectional area because of the uncertainty in the number and size of the single layers within the multi-walled tube. This means the specific strength, the strength to density, will also be *higher* because the actual density is found by looking at the individual layers and this will be the same as that of a single-walled nanotube.

Bob Clark

Reply to
Robert Clark

I found this site that says tethered balloons are used up to 15,000 ft., 5 km:

TETHERED AEROSTAT RADAR SYSTEM.

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This page explains the tether made of Kevlar has embedded electrical conductors within it to send power up to the balloon, up to 80 kW, and metallic braids within it to safely conduct lightning to ground:

What Is a Tether?

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This would be a good place to get actual performance data on tethers kilometers long to see how well their strength to density parameters hold up at kilometer lengths.

Some reports have studied the feasibility of even longer tethers. This report recommends tethered balloons for astronomical research at up to

12 km high, 40,000 ft:

POST: A polar stratospheric telescope for the Antarctic. Publications Astronomical Society of Australia, vol. 13, no. 1, p.

48-59.
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[full text]

And this study recommends tethered balloon astronomy platforms at

65,000 feet, 20 km:

Very high altitude tethered balloon feasibility study. AIAA Lighter-Than-Air Systems Technology Conference, 11th, Clearwater Beach, FL, May 15-18, 1995, Technical Papers (A95-30317 07-01), Washington, DC, American Institute of Aeronautics and Astronautics,

1995, p. 46-51.
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This article is not available for free, but I gather from the page freely available online that the authors believe the tether can be made of Kevlar to reach this altitude.

Bob Clark

Robert Clark wrote:

=3D=3D=3D=3D=3D=3D=AD=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

=3D=3D=3D=3D=3D=3D=AD=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

Reply to
Robert Clark

The URL's for those links should be:

POST: A polar stratospheric telescope for the Antarctic. Publications Astronomical Society of Australia, vol. 13, no. 1, p.

48-59.
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[full text]

Very high altitude tethered balloon feasibility study. AIAA Lighter-Than-Air Systems Technology Conference, 11th, Clearwater Beach, FL, May 15-18, 1995, Technical Papers (A95-30317 07-01), Washington, DC, American Institute of Aeronautics and Astronautics,

1995, p. 46-51.
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Here are some other refs:

Very high altitude tethered balloon parametric sensitivity study. Surjit S. Badesha, Anthony J. Euler, and Larry D. Schroder (TCOM, Columbia, MD) AIAA-1996-579 Aerospace Sciences Meeting and Exhibit, 34th, Reno, NV, Jan. 15-18,

1996
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Very high altitude tethered balloon trajectory simulation. Surjit S. Badesha, Anthony J. Euler, and Larry D. Schroder (TCOM, Columbia, MD) AIAA-1996-3440 AIAA Atmospheric Flight Mechanics Conference, San Diego, CA, July 29-31, 1996, Technical Papers (A96-35084 09-08)
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Dynamic Simulation of High Altitude Tethered Balloon System Subject to Thunderstorm Windfield. S. Badesha and J. Bunn, Johns Hopkins University Applied Physics Laboratory, Laurel, MD AIAA-2002-4614 AIAA Atmospheric Flight Mechanics Conference and Exhibit, Monterey, California, Aug. 5-8, 2002
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Bob Clark

Reply to
Robert Clark

Ian Stirling wrote: [snip]

Yes of course. For suitable values of "almost." Socks

Reply to
Puppet_Sock

Reply to
Ian Stirling

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