starship-design: Timothy's beamed power paper

[jimaclem@juno.com]

Timothy,
I looked at your page on beam propulsion, very interesting.  Here are
some comments 
on it.  Anyone else, please chime in!

2a The beaming station very likely needs to be build on a moving/rotating
object like a planet, moon or asteroid. Keeping the beam on track means
the whole beaming station should be able to actively steer the beam.

	Well, if you use an asteroid, or a collection of them, it seems
reasonable that any motion by them could be canceled out by simple
thrusters, either chemical, nuclear, or some variation of a solar thermal
engine.

5 Probably the smallest effect, the amount of dust may not be that much
compared to the power that is needed. The beam itself will push away a
lot of the dust so that the path is "smoothed" a bit.
	
	Probably true, however, has anyone considered the effects of
erosion on the sail by dust, etc. when the vehicle reaches an appreciable
fraction of c?  If enough damage is done to the sail, it may well suffer
a catastrophic failure, i.e., a large portion breaking off.

12b It would be preferable that the sail is at the Earth-side of the
ship, this way the ship itself is not "shined" upon (and not heated).
This also means the sail needs to push the starship which may be more
difficult than the pull (parachute) method.

	Here we run into the problems of tension vs. compression members.
 Example, take a short length of wire, does'nt matter how long.  Hang it
up and add weight to the free end.  This is tension, or pulling, stress.
Now stand the wire on end and add weight to the top.  This is
compressive, or pushing stress.  Try it if you like, but you can probably
guess that the wire in compression will fail with less weight than the
wire in tension.  The end result is that compression members, like you're
proposing, will be heavier than tension members needed to handle the same
mass of vehicle.

18 Either the beaming station is build on a heavy soil (a moon/big
asteroid) or it 'shines' also in the opposite direction of the target
system.

	Actually, since the mass of the beaming station won't be
negligible, simply placing positioning thrusters on it and using them
occasionally should keep the station in position. Why spend energy, hard
enough to come by anyway, beaming in another direction.

Jim