Re: Laser Aperture Size

[Brian Mansur <bmansur@oc.edu>]

>From Brian

>Lee?
> 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.

>Kevin
>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.

And we can't figure out how to get the darn mirrors keep form, aim, and who 
knows what else!

>Lee
> 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.

>Kevin
>I disagree.

Brian
At this time, I disagree as well.  Both require HUGE amounts of power to 
work because both have to get up relativistic speeds to be of any use.  The 
RAIR needs 5E16W/s (if my recalculations on Kelly's paper are correct) just 
as the sail does if we want to get up  to 1/3c.  Both need to get that power 
from someplace.

> 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.

Problem is Kevin, as you pointed out above, that the doplar shift shoots the 
electric bill through as you try to get speed near c.

Another problem I may have just spotted is the that the effects of impacts 
with interstellar matter increases as you approach a light-speed.  Think of 
it.  Your apparant mass (internal energy, something like that) keeps going 
up even though you keep pushing your speed infintely closer to c.  While you 
are only going .999c (I'm making these numbers up to make a point), your 
impact with that speck of dust feels as if you were going 1000c.

 Am I making any sense?  Do I even know what I'm talking about?  I get this 
idea from word about a superfast particle known as the "Oh My God" Particle 
(although I detest the name on the basis of religious convictions). 
 Apparantly, if it were to hit Earth's atmosphere at its speed of something 
like .99999999999999999999c, it would be as if a brick were in its place. 
 And that is for a particle.  Think of what a starship would be feeling from 
every interstellar hydrogen atom at that speeds.

So while the time dialation advantage is enticing, its tradeoffs seem to 
make it impractical.  By the way, the design I'm working on for the Argosy 
easily accounts for the acceleration and cruise phases by using a dumbell 
centrifuge with hab modules for cruise phase and then stacking the hab 
modules on top of each other come time for accel/decel phase.  I know Kelly 
is skeptical of a dumbell setup for erosion reasons, but we may have a way 
of  protecting against that too.  Read about it in my Argosy paper when I 
send it to the group after break.