[TowerTalk] spider balls

[Jim Lux]

At 08:39 AM 7/28/2004 -0400, you wrote:
> > My thought was to attract lightning to the three
>foundations, not to avoid a
> > strike.  I thought that if I were to minimize the path to
>the tower itself
> > it would help avoid a direct strike on the tower.  If I
>could get 1-2 ohms
> > to ground at each point and make an attempt to isolate the
>tower such that
> > it sees a much larger resistance to ground....
>
>This really all comes down to pretty simple physics. The
>cloud has a charge, and the earth has a charge. Unless we
>can reduce that voltage, which it is proven we can't, then
>we have a spark gap. Only the voltage gradient of the
>electric field matters. The resistance of the conductors in
>that gap really don't matter until after the strike starts,
>and then we want everything LOW impedance an all tied to a
>common point if possible. Grounding the tower, having
>intentional leakage, whatever rows your boat enjoy it. But
>the facts are other than lowering height compared to
>surrounding objects, there isn't anything we can do to
>prevent a hit.

A good example of this is the lightning rods surrounding the launch pads in 
Kourou, which has a fairly high frequency of thunderstorms (although mid 
coastal FL, where we have a big factility, may be worse).  You certainly 
don't want lightning hitting the launch vehicle sitting there, so they put 
rods on towers higher than the height of the rocket and gantry around the pad.

It helps, but isn't perfect, because they still get strikes.  And, of 
course, eventually the rocket rises into the air above the lightning rods, 
and then IT is the highest thing around, with a nice conductive trail to 
the ground.  Fortunately, by then, the strikes don't do much bad stuff, 
unlike when you are on the ground connected with umbilicals to ground 
support equipment (and people!)

There are some pictures of Kourou on the web...
http://www.planet4589.org/space/jsr/csg/csg3.html is someone's picture log

Baikonur has a similar strategy
http://members.aol.com/nodin/N1pages/N1index1.html