Retro sailing difficulties

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

To: all

I think another problem with a retro reflective sail is that it puts an 
effective upper limit on acceleration and speed of the ship.  I'm not 
sure what that limit is, but it must be substantially below 1 G accel, 
and far below C at turnaround.  The whole idea with an externally fueled 
(either mass or photonic) ship was to keep gravity constant and get close 
to C.

But observe what happens near turnaround if you do this with a retro 
reflective mirror.

At turnaround, the ship and mirror are moving at .99 of C.  Ship and 
mirror are detached.  outer sail begins to accelerate even more.  
the outer ring sail begins to get inefficient _twice_.  Once, when it sees 
the maser from earth redshifted, and twice, when the ship sees the 
reflected beam redshifted _again_.  

At the limit, it begins to look like Steve's two mirror puzzle.  We'd get 
almost nothing out of the mirror as it asymtotically approached the speed 
of light.

The plasma mirror avoids these difficulties.  Since the mirror is 
constantly being renewed at the ship's speed, the primary reflection gets 
more and more efficient as the ship slows down.

Also, this allows us to use the plasma mirror as reaction mass at the 
same time we're using it for retro reflection.  plasma exhaust may not be 
the most efficient in the universe, but it's the best I can find if you 
want to get near C.


How to aim a maser up the butt of the ship from twelve light years away:

Simple really, just reflect the maser off the main sail with the focus at 
one end of the core.  a second mirror there re-reflects the maser up the 
central core.  (Foreward shows in his design a way to make diseparate 
beams enter into a colimator, and emerge as one coherent beam.)

How to support a large sail structure hanging off the ship.

it supports itself, because it is a parabolic shape, the reflection off 
of the innermost part of the main sail imparts almost no perpendicular 
force component.  the beam that reflects off the outermost part of the 
main sail imparts a larger perpendicular (to the ships direction of 
flight) component and thus keeps the sail "inflated".

How to make the plasma go out one end only:

a transparent (to whatever energy beam we use) plate on the back end 
would allow the energy in, but prevent the the plasma from escaping


no furling or unfurling of the main sail is required, the whole thing can 
be controlled by the secondary mirror.  if it is retracted, then the 
maser bounces off the main sail, imparting thrust to the ship.  Without a 
second mirror to bounce off of, the ship accelerates away from Sol.
To begin slow-down, (can't even call it turnaround anymore, things sure 
have changed.) the secondary mirror is extended to intercept the 
reflected beam and send it into the core.  The beam hits a wall of 
plasma, bounces off, and hits an absorber.  (it can't be bounced back, 
because then it would just hit the plasma again, and be going in the 
wrong direction -- back toward Sol) 

The absorbed energy is then used to power ship's systems and further 
accelerate the plasma.  

if anyone can figure out how to bounce the maser beam back toward TC 
after it hits the plasma, I'll gladly scrap the accelerating-the-plasma 
with-the-converted-energy idea.  which would save us from having to use 
microwaves, and help out with our heat load.

Kevin

P.S. to Steve.  I just got your analysis of the design as i was about to 
send this.  seems reasonably well laid out. BTW, what is the ISBN of 
Taylor and Wheeler's book?  that will help me to order it.  It will be a 
week or so before i can do any detailed work on it.