# Re: Retro sailing difficulties

```To: Kevin

>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_.

I hadn't thought of that second shift...

>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.

But in the end it is also easier to decelerate: E=0.5 m v^2
The difference between v=1001 and v=1000 is dE=2001 while
the difference between v=101 and v=100 is dE=201.

Of course this same thing happens during acceleration: Only then more energy
is needed. And that is not only because of the relativistic effects!

>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.

Yes but don't forget that the plasma gives also a serious redshift during
the reflection.
Also there is a finite time between replenishing the plasma and the
reflection of light on that same plasma. This means the Asimov has
decelerated a bit and that means the second redshift. It may be small, but
it happens many many many times!
Besides that, a lot of extra mass is needed to replenish the plasma. This
extra mass has to be decelerated too! (Of course it get less during
replenishing).
Although I haven't done any calculations, I'm sure that a lot of mass is
needed and my physics intuition tells me that the whole thing ends up
needing the same energies as the dragon-fly.

>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.

If you use it as reaction-mass, that means that you need energy that comes
from the Asimov, which it doesn't have! Also making the plasma move faster
gives an even worse redshift.
(If you think you can use the power of the maserbeam to accelerate the
reaction-mass, then you are wrong)

>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.)

I don't get this, could you make an ASCII drawing?

>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)

That won't be bad, because we can use it again! (Of course more redshifted)

>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.

ISBN 0-7167-2327-1

Timothy

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