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Al Globus wrote:

> 10-20 cm ropes of single walled carbon nanotubes have now been built.

20cm is good. However, I would be more impressed if it was 1 cm
thick.

> "Direct Synthesis of Long Single-Walled Carbon Nanotube Strands," H.W.
> Zhu, C. L. Xu, D. H. Wu, B. Q Wei, R. Vajtai, and P.M. Ajayan, Science
> Volume 296, pages 884-886, 3 may 2002 reports synthesis of 10-20 cm
> ropes of single walled carbon nanotubes. Stress testing indicates a
> Young's modulus of 77 GPascal. The theoretical Young's modulus of a
> single carbon naontube is about 1000 GPascal.
>
> This is a big step towards a space elevator, although we still need a
> eight orders of magnitude in length and perhaps a factor 10 in strength.
>
> Can anyone give a good explanation of Young's modulus?

Young's modulus is how much a material stretches when you tug
on it. In other words how stiff the material is. Rubber has a
very low young's modulus; steel has a very high one.

To measure it, you take a uniform sample of the material of
known cross-sectional area (the more material cross-section
you have the more material there is to fight any tug, and
the less deflection you will get) and known length. You apply a
known force and measure the extension. You then divide it
through by the crossectional area and divide by the ratio of
the lengths to give a standardised number which has units of
pressure (Pascal).

Young's modulus is mostly unrelated to ultimate strength;
which is probably the most important variable for a space
elevator; however it will effect the speed of sound
in the rope- the higher the Young's modulus, the faster,
and that may have effects on the response to environmental
stresses; and it may affect the deployment strategy.

> Space tourism could be our ticket to the stars. Save your pennies,
> suborbital flights for $100,000 may start in 2005! See
> http://www.spaceadventures.com/suborbital for details.
>
> Al Globus
> CSC at NASA Ames Research Center
> http://www.nas.nasa.gov/~globus/home.html
>

--
- Ian Woollard (ian.woollard@...)

"Is a planetary surface the right place for an expanding
technological civilization?"
- Gerard O'Neill