Re: Laser Aperture Size

[Kevin C Houston <hous0042@maroon.tc.umn.edu>]

Lee:
> Applying basic optical diffraction theory (Rayleigh's Criteria) to determine
> the required aperature (diameter) of the laser transmitter, using 0.5um
> yellow light and have it just fill the sail 11 light years away at TC, the
> aperature would have to be 400 km. Was somebody worrying about staying in
> the beam? Better worry some more, it isn't going to be as wide as they were
> thinking!

Kevin:
The aperature can be aas wide as we like, through the use of synthetic 
aperature technology.  This is the same as was used on the magellan 
mission to Venus.  it is especially useful for masers, becuase they can 
be linked electronically, without having to be linked optically (like vis 
lasers)

> 
> As far as your mirror problem, why bother? Why not just pass TC by, use
> Lorentz Force Turning to swing around on a return course and then use the
> original beam to decelerate into the TC system from behind?
> 

Oh Goody, I see someone has been reading his interstellar filght handbook.
(no disrespect intended)  I have proposed this very Idea before you 
arrived to the group. Kelly seems to think that we'd be going to fast, 
but of course, that would just generate more force for the turn.  Anyone 
know what the galactic magnetic field strength is?

> Of course, this makes the mission longer....

If we make the turn at a high % of C, the crew won't notice it much.

> The Fresnel lens has already been discounted on the basis that it introduces
> chromatic aberrations into the beam, rendering it largely useless. A
> reflecting optical system would work better.
> 
> If you must do it with mirrors, there is no need to get complicated, just
> build a staged design (kind of like onion rings) where the main ship detachs
> large chunks of sail to function as a deceleration stage, then a slightly
> smaller chunk as an acceleration stage to leave TC, and decelerates back
> into Sol on the unreflected beam. Each stage would function as a mirror to
> propel the remaining ship and sail sections in the appropriate direction.
> This method would probably still require a Fresnel lens though.


This is the design put forth by Robert Forward, the main problem is that 
you can't get anywhere near light speed, because then the doppler shift 
from the retreating sail will reduce the energy to near zero.

> As much as I like the sail concept, I still think the RAIR concept is more
> promising. Especially if you can boost fuel into its path to increase its
> cruise velocity. I like the idea of using sails for the fuel, though.

I disagree, I think the simplest mission in terms of hab space design is 
going to be a 1g thrust to midpoint, then a 1G thrust to target.  The big 
advantage, is the time dialation for the crew.

Kevin