Re: Planetery Defense Batteries

devans@uneb.edu wrote:

> Also, what is the 'down' angle to the horizon of an Earth-style
planet, say at
> 100 feet above the surface, assuming a really tall PDB? Five mi? Seven
mi? Top
> of Everest without other mountains to worry about? Top of Mons
Olympus? Near as
> much as the 30 degrees required for going to 240?

Laying myself open to corrections by better mathematicians, the
following are what
I reckon the total arcs of fire (horizon-to-horizon) would be for the
specified
altitudes on an Earth sized planet :

Planetary Radius 6500km

Altitude (m)	Arc	       Horizon (km)

0		     180.00	   0.00
10		    180.20	  11.40
50		    180.45	  25.50
100		   180.64	 36.06
250		   181.01	 57.01
500		   181.42	 80.62
1000		  182.01       114.02
2000		  182.84       161.26
5000		  184.49       255.00
8000		  185.68       322.59
10000		 186.35       360.69
1000000        239.85	    3741.66

The above all assume a perfectly spherical planet with a smooth surface.

So, to get to four FT fire arcs we need to be 1000km above the planet's
surface, a
fairly substantial orbital altitude ! Ignoring atmospheric effects,
surface
obstructions etc we can pretty conclusively say that a Planetary Defence
Battery
has a 180 degree fire-arc.

Tony