hovering kite assemblages
Kragen Javier Sitaker
kragen at canonical.org
Sat Jun 11 15:12:58 EDT 2011
On Tue, Jun 07, 2011 at 11:02:38PM -0400, Kragen Javier Sitaker wrote:
> Radio round-trip times between the kites can provide very accurate
> measurements of how far apart they are, compensating for the
> unavoidable variability caused by the elasticity of the string.
Also, optical round-trip times.
> It seems likely that high strength-to-weight strings would be
> advantageous for this --- say, fishing-line nylon, prestretched
> polyethylene, or Kevlar, rather than just plain cotton. Non-porosity
> would also be an advantage.
Further investigation of string materials led me to the "Specific Strength"
Wikipedia page: <http://en.wikipedia.org/wiki/Free_breaking_length>
Nylon is only about 7km, polypropylene is about 9, balsa wood is about 53,
carbon fiber composites are 80, glass fiber is 133, basalt fiber is 182, carbon
fiber is 250, Kevlar is 256, and ultra-high molecular weight polyethylene
(Spectra) is 369km. Steels vary from 2km to 20km. Materials with higher
specific strengths, ideally at least an order of magnitude greater than the
actual string length needed, would be better in this application.
Vulnerability to photodegradation or fatigue would be serious drawbacks.
Spectra UHMWPE coated with carbon black would probably be adequate.
> It wouldn’t be necessary for the kites to fly low enough to be visible
> from the ground, and they could be made almost entirely of transparent
> materials with a tiny radar cross-section, and like owls, they
> wouldn’t need to make any sound while flying.
Carrying a small corner reflector made of aluminum foil, on the other hand,
could make them extremely visible on radar and thus reduce the risk that they
will be damaged by aircraft. I've previously done calculations that showed
that the risk of such a collision is quite low, but it might be useful under
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