# Re: starship-design: Timothy's beamed power paper

```Hello Jim,

>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.

What I didn't include in the beaming-page was the reason for using a planet
or asteroid. I believe the main goal was to have a sufficient mass, so that
the reverse momentum of the beaming station would not push itself away at a
significant rate.
I never really did a calculation to figure out what the effect would be on a
planet or asteroid.
Now that I did, I'm a bit shocked, we're going to use about 1E26 Joule of
total energy. With that energy you can give a mass similar to Earth a
velocity of a few meters per second.

>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.

really satisfying solution for shielding. We have some solutions, but they
are either crude or speculative.

>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.

True, the parachute design would likely be easier and lighter. The mean
reason for pointing out the above problem was that you'd realize that we
needed some extra shielding.

>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.