MIRRORS (Round Two)

[Brian Mansur <bmansur@oc.edu>]

Brian 4:00 PM CT 3/5/96

Okay.  My first draft for the Mars Hybrid (now renamed Argosy Class) has now 
officially been hulled.  The course correction problems needed avoid the Sol 
beam during decel phase has necessitated that I go back and rework my sail 
and retro mirror design.

I now have two options for the Argosy design's sail.  The first is born of 
the classic concentric mirror concept for the light-sail design by Friedman 
(or whatever the guy's name is).  Basically, the diverging beam is now 
refocused by the retro mirror (thanks Tim for the term retro) onto the 
Asimov's smaller sail.  As noted in the light sail design, there is some 
pressure on the back of the Asimov's own sail.  But the newly focused, 
redirected beam from the retro mirror wins out the push thus giving net 
decel.

(Sound of modest applause at an already well known design having saved the 
day).

Note: We still have to figure out how to correct the retro-mirror's course 
and still keep its shape.  Let's face it, our starships will wander a bit 
and who knows if our maser array might wander off target over the light 
years.

Not wanting to settle for such a simple, though slightly inefficient design 
(what am I, nuts?),  I though of another.  Note that I don't pretend that 
this is a better idea, but I do think we should examine  as many 
possibilities as we can.  Who knows what other ideas this might spark.

Okay, for decel phase, we split the Asimov's sail into two parts (or into a 
washer-like sail with a hole)  to let the maser beam fire between/through.

        Beam
     From Sol
              !
              !
              !
 --------------     ---------------
\    \                /  /
         \           \            /         /
                    \  \        /  /
             A
     
     Asimov = A

     !              !
     !              !
     !              !
Beams From Retro Mirror

The flat horizontal  lines represent the sail and the slants are the 
connecting cables.  A cable will probably stretch between the two sails and 
is not shown for covienince.  By the way, if this ASCII art is not comming 
through, I'm genuinely sorry.  Somehow fonts seem to be different from 
person to person.

Now there would still be some drag inefficiencies on any lines connecting 
the two sails and on the Asimov if we tether it between both sails but that 
might be converted to electricity for whatever uses.  To further reduce this 
drag, I thought about connecting two starship components to the sails, one 
for each sail component and tethering the two sails.  If we do this, I don't 
see how we could make a more than less structurally flimsy circular sail 
while keeping a reasonably reflective shape.

Course corrections are a detail we are still working on.  I'm beginning to 
think that the tugs that I've envisioned elsewhere might be replaced by the 
ion engine of the Asimov.  It could be fired at angles although the exhaust 
would probably be into the sail.  More thought needed.

I noticed that for this sail to work, the diameter would be greater than 
Jupiter's to let the beam through.  ARRGGHH!  On the other hand it doesn't 
have to be as precisely shaped as the retro mirror.  Still not sure how to 
correct retro-mirror's course and keep its shape.  I'm trying to figure if 
there is a way to break it down into components that will reflect at the 
slight angle needed to hit the new rig.

My motivation for thinking up the two sail design was to take give us a way 
of canceling the angular propulsion that the retro mirror and ship would 
experience with my sidesteping of the beam idea while still giving us little 
drag from Sol's maser (that's a long sentence).  I'm beginning to think that 
putting a Fresnel lens midway to TC would help solve the beam divergance 
problem and keep our sail at something closer to moon size.

I also toyed with the idea of having a one-way mirror/sail for the Asimov. 
 I don't know how light we'd have o get a plastic lens for this.  Obviously 
can't use glass.  By the way.  Can we actually make a one way mirror with 
plastics?  If so, how thin?  Also, just to be clear, do microwaves bounce of 
hand mirrors better or worse than wire meshes?

Okay, there's my reworking of the mirror problem.  Let's see what comments 
it gets.