The Elevator Which Rises Into Space...

[emudude]

With the 2009 Space Elevator Games coming up, I've been thinking about a few of the problems presented with using carbon nanotubes as the material for the cable/ribbon which will be used as the material for the tether. An Italian researcher recently calculated that carbon nanotube-based tethers would not work as even a single carbon atom out of place would reduce the tensile strength by around 30%. As defects on the atomic scale are seemingly inevitable, this would rule out carbon nanotubes as an effective tether material. A space elevator must be able to supply at least 62 gigapascals of tension in order to function; a single nanotube structure can supply 100 GPa, but a structure made of nanotubes jumbled together would supply less than one GPa...and this is all with no defects. What I'm curious about is whether we could use the superconducting properties of carbon nanotubes to create a "powered cable" (think electromagnets) which has an induced tensile strength when turned on which would be sufficient to carry cargo...food for thought.

 

[tampaDreamer]

If we did that, a power failure would lead to a falling tether.. kind of a big risk.

 

[emudude]

Yes, I've thought of that...but considering the ability to provide redundant power systems (beamed power from the ground and/or space if it is legal and not considered a death ray), and also that power to the cable would be coming from space from a constant supply of solar power (at least for a few billion years ), or even from a nuclear reactor, the only forseeable loss of power would be from debris severing the tether...which would cause the tether to fall anyway. If carbon nanotubes are indeed infeasible by themselves, this approach may be our only hope... as long as this is done over the ocean and humans are not the cargo (I believe the amount of radiation we would be exposed to through our long trip into the Van Allen belts would require prohibitively heavy shielding anyway), I think the risks are acceptable.

 

[neilsox]

~A quick check in Google, suggest that there was a flurry of articles in
2001 about super conducting carbon nano tubes, mostly colder than 20 degrees k. My guess is a CNT tether that is a better electrical conductor than copper, is far in our future, if ever = about the same year as gigawatt-hour cold fusion.
Keeping the space elevator colder than 20 degrees k may be easy in the Oort cloud, and beyond, but not practical near Earth's orbit.~
Nanotubes hint at room temperature superconductivity ~New Scientist~
19:00 28 November 2001 by Adrian Cho
Tiny tubes of carbon may conduct electricity without any resistance, at temperatures stretching up past the boiling point of water. The tubes would be the first superconductors to work at room temperature.

Guo-meng Zhao and Yong Sheng Wang of the University of Houston in Texas found subtle signs of superconductivity. It wasn't zero resistance, but it's the closest anyone's got so far. "I think all the experimental results are consistent with superconductivity," Zhao says. "But I cannot rule out other explanations."

At the moment no superconductor will work above about 130 kelvin (-143°C). But if a material could carry current with no resistance at room temperature, no energy would be lost as heat, meaning faster, lower-power electronics. And electricity could be carried long distances with 100 per cent efficiency.

Nanotube bundles
Zhao and Wang studied the effects of magnetic fields on hollow fibres of carbon known as "multiwall carbon nanotubes". Each nanotube is typically a millionth of a metre long, several billionths of a metre in diameter and with walls a few atoms thick. The nanotubes cling together in oblong bundles about a millimetre in length.

The researchers did not see zero resistance in their bundles. They think this is because the connections between the tiny tubes never become superconducting. But they did see more subtle signs of superconductivity within the tubes themselves.

For example, when the researchers put a magnetic field across a bundle at temperatures up to 400 kelvin (127°C), the bundle generated its own weak, opposing magnetic field. Such a reaction can be a sign of superconductivity.

And when the team cooled the bundles from even higher temperatures then turned the external field off, they stayed magnetised. A current running around within the tubes could generate this lingering field if there wasn't any resistance to make it fade away.

While each effect could have a more prosaic explanation, they varied in similar ways as the temperature of the bundles changed. The correlation suggests superconductivity was responsible, Zhao and Wang argue in a paper to be published in Philosophical Magazine B.

Dominating effect
However, their argument doesn't convince Paul Grant, a physicist with the Electric Power Research Institute in Palo Alto, California. "Generally, superconductivity is such a dominating effect that when it occurs it just shouts out at you," Grant says. "It doesn't appear in these indirect ways."

Superconductivity theories do not forbid the phenomenon at very high temperatures, says Sasha Alexandrov, a theoretical physicist at Loughborough University in the UK.
~Copper or aluminum conductors in a very long tether add too much weight, unless the current is very small and thus not useful to propel the payload. Neil~