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Re: Engineering Newsletter

Timothy re: Kelly
Subject   : Plasma mirror

>You still haven't dealt with the problems of a drop mirror.  
>  - Given that the point of the exercise is that the mirror and ship will
>accelerate apart until they are moving apart at nearly light speed.  They
>will be getting very far apart, and the "Retro-mirror" will need to aim to
>track the decelerating ship.  By having the ships flat reflector mirror the
>size of the full power mirrors you eliminate the retro-mirror needs to focus,
>but not to keep aiming at the retreating ship.

"Very far" would be something like 2 ly:
The mirror and the ship will be furthest apart when the Asimov has
decelerated and is near TC.
It takes approximately 8 years to decelerate the Asimov. The mirror never
exceeds the speed of light and thus travels about 8 ly. It is released from
the Asimov when 6 ly from TC. So after 8 years the Asimov has decelerated
and is near TC. The mirror is then thus 2 ly further than TC and the Asimov.

So the extra distance is not so much as you would think. But of course still
the problem of aiming is there. I've no solution other than some
gyro-system. But what I don't understand is what is the difference between
aiming the beam that comes from Earth and the beam that comes from the
retro-mirror? The only thing the retro-mirror has to do is not turn, so it
does not have to aim but just not have to turn.
Do you know what the difference is between aiming the beam from Earth and
aiming from the retro-mirror?

> In trade you've added increased need for structural material, and added
>extra forward thrust from the earth beam on the back of the ship.  (I don't
>by the ship running on a tiny beam that just fits in the sail.  The ship will
>need to maneuver, and the transmitters couldn't hope to generate that much

Increased structural material, indeed, but that does not matter, because the
heavier the retro-mirror the less the doppler shifts.
I don't buy you saying that the beam may be much bigger than the mirror. If
you throw away most of the energy anyway, just by using a mirror that is too
small, than you could also use an engine that has a worthless effeciency.

Furthermore, I don't see why the ship wants to maneuver so much. It always
has to stay in the beam for most of the time.
If indeed the accuracy of the transmitters is that worse, my advice is to no
use the beaming idea.

>- The energy retuning to ship will drop off like a rock as the distance
>between ship and retro or drop mirror increases.  Given that the mirror will
>not be that smooth (it will probably be rippling) the beam will be diverging
>badly after it reflects.

How well do you think that a plasma will reflect? Probably my almost flat
mirror would do a much better job.

I really don't agree using a beam for about 10% or less. It should be at
least 50% otherwise, other systems may be more effecient.


Timothy re: Kelly
Subject   : nanoAI

>Easier?  The ship would still need to carry something like its own weight in
>matter anti-mater.  A mass of thousands to millions of tons.  Forward was
>hoping optimistically we'd be able to routinely generate and store milligrams
>to grams of antimatter.

Once again, anti-matter is just energy. The only thing that is important is
the efficiency. How well can we transfer energy to matter? My assumtion is
that this efficiency should be about 50% in 2040. If this isn't reasonable,
than indeed it is a bad choice.
My guess is that Forward also has no idea of generating 10E18 Watts of power
or a total amount of 1E26 Joules of energy!
And this is approx. the energy and power needed for most designs.

>True, but since no current A.I. system works very well, and no nano-tech
>systems work at all, I'd have a very hard time expecting them to be developed
>to that degree of reliability in 50 years.  Its out there with the "we
>discover warp drive possibilities".  Sooner or later we'll do them, or
>something like them; but they don't fit within LITs "no radical new tech"

Have you seen any (well) working plasma-mirrors yet? I really think that
nanoAI is not that exotic as you think.
The AI systems of today are capable of learning to read out loud (but not to
understand). One of the things that keeps them from doing more difficult
tasks is the amount of "neurons" (currently about 1E8 or so). This amount
depends both on fast and vast memories. There has just been designed a chip
with a build in neural-network for all kinds of purposes.
About nano-tech I know a bit less, but certainly there are significant
breaktroughs. And nano-tech has the same potential as computers had. Once
there is a beginning, growth will be exponentially.

>One thing I was considering was what we can do.  My Explorer design could
>certainly be able to carry enough fusion fuel to decelerate from 1/10th C.
> Marshal Savage mention something like a 20 to 1 fuel to ship mass ration to
>do that.  I'd like to check that, but for the moment will assume its true.

I checked it and its even better. For acceleration and deceleration a ratio
of 1:10 when the exhaust speed is about 0.088 c (Quite critical value)

For 0.2c the ratio becomes 1:100 and for 0.3c it is about 1:1100

> Obviously trying to do that at 2/10ths C would take 20 squared (400) ship
>masses of fuel.  So thats out, but at even those speeds drag is a serious
>factor.  Savages book mentions that at near light speed the (one atom per
>cubic centimeter) inter-stellar Medium could cause up to 37 milligrams of
>drag pressure per square centimeter of frontal area.  I'd like to work up the
>numbers for various interstellar densities and ship speeds; but it seems
>likely that some kind of magnetic or electrostatic, scoop or parachute could
>give us a big amount of breaking force.

3E10 cm/s * 1 atom/cm^3 = 3E10 atoms/cm^2
3E10 atoms/cm^2 * 1.67E-27 kg/atom = 5E-17 kg/cm^2

As you can see that number of Savage is completely wrong. (I did this
calculation a week ago also!)

>If we stay with a .1 to .2 C top speed ship we might be able to get a
>practical mission to some of the nearer stars.  Not Tau C, but Alpha Centuri,
>Barnard's, Rigil Kent (Rigel kent A is a G2 yellow Main star at 4.4 LY, B is
>a K6 orange-main at 4.4).  A .2 C ship could get there in a usable period of
>time.  A version of my Explorer Fusion design could get there and back.  If
>you have some reason of wanting to go there regularly.  The first ship could
>assemble automated fuel launchers in the target systems.  That would allow
>lighter ships to make the same run at higher speeds without carrying heavy
>fuel loads.  (Assuming they trusted the fuel launcher at the receiving system
>to answer their launch command.)

4.4/0.2=22 years for a complete one way trip. Not better than we are
currently planning for TC.

>So I guess what we want to do there is a question that we'd need to resolve.
> More specifically why we'ld feel we needed to do it then?  If you willing to
>wait another half century.  You could expect to have relyable equipment based
>on physics unknown to us now.  (Matter conversion?  Time space distortion?
> Such things have been seriously proposed.)

This was what I meant, and I haven't got a clue what the answer could be.
But as you said this has nothing to do with the initial goal of SD.