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Re: Orbit B

From: KellySt
To: T.L.G.vanderLinden; stevev; jim; zkulpa; hous0042; rddesign; David; 
lparker; bmansur
Subject: Re: Orbit B
Date: Wednesday, March 06, 1996 11:56PM

>From Brian

>But the ship would be reacting to the stern thrust of the earth beam months
>before it would get to the mirror, and the mirror could respond.

You've pointed out a serious problem that only this morning I even realized 
was there.  You remember how nervous the Columbia crew was that their 
tethered sattilite might bounce back at them?  Think of see a really huge 
sail falling down around you.  It would, no doubt, bend the cable/guy wires 
beyond use even if the sail didn't rip itself apart after crumbling down.

The solution is to move out of the Sol beam path, pack up your sail (and no, 
I don't know how to do that), and wait for the retro-beam to reach the 
Asimov's range.  Then you move back into the Sol beam and then into the 
retro-beam.  You deploy your sail and voila.  Just pray that the retro-beam 
doesn't get jerked off target or else you'd better have a spare sail and a 
good robotic team to put it up.  Come to think of it, you'd need a lot of 
bots just to deploy the main sail.

>Besides how
>could you keep the mirror perfectly focused with all the random fluxuations
>in the beam?  Keep the reflector tuned exactly to the sail, or know if the
>sail drifted a little due to problems, or back thrust from earth (that the
>reflector wouldn't see for months).

Random fluxuations from where?  Not in my refocusing retro-mirror design. 
 Oh no, I just realized something.  Even if you refocus the beam down into a 
smaller one via a series of mirrors, it will still have the same energy 
density distortions as the unfocuses beam.  They would just be on a smaller 
scale.  Exactly like a picture from a telescope.  Maybe the fluxuations 
won't be so bad.  Then again, maybe not.  But I don't see how we could 
correct for this.

By the way.  It is possible, as I think you noted earlier this week, that 
the mirror would cause plenty of fluxations in its own write.  After all, 
the huge reflecting surface is an ultra-thin sheet that requires tension for 
shape and force from the Sol beam for tension.  I see where fluxuations 
would compound on fluxuations.  E-GAD!  THIS IS A NIGHTMARE!

Time for a reality check.

For this entire light sail idea to work, you 100% HAVE TO assume that the 
beam from the retro-mirror remains steady AND relatively free of 
fluxuations.  The Asimov will have to take care of itself and keep its 
posistion dead in the retro-beam.  Any mistakes and you have a wrecked sail. 
 Unless that can be replaced in a matter of a few days, your crew is going 
to have to drop weights (hab shielding mass which is where half of the ship 
mass is minus fuel/RM) and hope that whatever kind of sail you get up before 
slamming into the retro-mirror  can take the extra g's of deceleration.  Not 
to mention the crew.

Question: Why does reality have to bite so hard?

>How the hell do you focus a lose flapping mesh sheet, the size of jupiter,
>while its taking 100s-1000's-? of G in acceleration?

Over long distances, you don't.  You don't even try.  See my RETRO MIRRORS 
paper mailed just an hour or so ago.  You focus on a much closer target. 
 And, as you'll see on the paper, my ideas don't bother making the sheet so 
large and they will probably be composited and linked by cables.  By the 
way, the weight of the of the Retro mirror can be in excess of 5E6 tons. 
 Don't laugh.  I calculated myself that you can, in fact launch that sucker 
with 1.1E18 W to .33c in roughly 260 days.  The Asimov, I'm assuming, weighs 
5E5 tons.  That can be accelerated to .75c (not accounting for relativity if 
I understand the equation I used) at 10m/s^2 over 260 or so days.  Here is 
the equation.  Incidentally, I got it from a previous discussion from 
November, I think.

P = Power
m =  Mass
v = velocity
c = lightspeed (3E8 m/s)

P = [mc^2/(1 - (v^2)/(c^2))^.5] - mc^2