# Retro sailing difficulties

```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.

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