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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
 >> >accelerate apart until they are moving apart at nearly light speed.
 >> >will be getting very far apart, and the "Retro-mirror" will need to aim
 >> >track the decelerating ship.  By having the ships flat reflector mirror
 >> >size of the full power mirrors you eliminate the retro-mirror needs to
 >> >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
 >> 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
 >> So the extra distance is not so much as you would think. But of course
 >> 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
 >> 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
 >> 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?

Yes, the beam from earth can be maintained by a massive infrastructure of
people carefully tuning it to keep it in the track to T.C.  They also can
make up for accuracy problems by making a bigger beam.  Transmitting a
hundred or a thousand times as much power as needed to compensate for all the
waste that misses the ship.  Thats not very efficient (but thats not a
problem), and will cost a lot more (which might be a problem); but it could
still work.  (Actually you would need to do that anyway in order to give the
ship some room to maneuver around in the beam.)  

The retro-sail on the other hand is a weak flimsy structure of huge
dimensions, which has to keep a pinpoint aim of nearly all its reflected
energy at a distance of light years.  A neat trick given its basically a huge
unsupported sheet of foil being blown along by the beam.  Oh, and it would
have to keep a pinpoint aim on a moving object up to two light years away.
 So it would be 2 years out of date in its understanding of where the ship
was, and was going.  

And no you can't just not turn.  First, given the load on it from the beam,
its certain it will being twisted by beam asysmetries.  Secound the ship
can't be in direct line between the mirror and the beam, so their will be a
changing angular offset.

 >> > 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
 >> >by the ship running on a tiny beam that just fits in the sail.  The ship
 >> >need to maneuver, and the transmitters couldn't hope to generate that
 >> >accuracy.)
 >> Increased structural material, indeed, but that does not matter, because
 >> heavier the retro-mirror the less the Doppler shifts.

Also the more power you need to drive it and the ship up to speed, and more
power needs for its maneuvering systems to turn it to keep aimed at the ship.
 Besides you couldn't possibly add enough mass to a structure hundreds to
thousands of kilometers across to keep it optically flat!

 >> I don't buy you saying that the beam may be much bigger than the mirror.
 >> you throw away most of the energy anyway, just by using a mirror that is
 >> small, than you could also use an engine that has a worthless efficiency.

The beam must be much bigger then the sail, and wasting energy isn't a
critical problem.  The purpose of the beam is to drive the ship.  As long as
enough of the beam gets to the ship to do that, the system can work.  (I.E.
the it can get the ship where it wants to go.)  Efficiency is a cost concern,
but would not effect the success of the project.  After all you don't have to
move the transmitters.  The ships engines however have to be pretty efficient
in order to get the job done (and not melt the ship in the process).  We're
nowhere near being able to design a starship that can "affordably" get back
and forth to Tau Ceti.

 >> Furthermore, I don't see why the ship wants to maneuver so much. It
 >> 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
 >> use the beaming idea.

I expect it will need to maneuver around any interstellar rocks or other such
junk. (Hitting a 4 kilometer comet at relativistic speeds is hard on the
hull!!)  One advantage of the beamed concept is that the unreflected energy
acts as a radar searchlight to show whats ahead of the ship.

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

The plasma reflector will be right next to the ship (surrounded by it
actually).  So couldn't possibly miss the ship, and doesn't need to keep a
tightly aimed beam.  The retro-mirror on the other hand will be up to 2 light
years from the ship, and need to reflect a perfectly beam that converges
inward toward a smaller drive mirror on the ship.  The later is a much harder

 >> 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 efficient.

Do you mean you don't want to use a beam system thats that inefficient?
 Efficiency in catching the beam is a luxury we can worry about later.  Right
now we're just trying to devise a system that could possibly get us there at
all!  The choice isn't going wastefully vs going efficiency, but going
wastefully or not going at all!


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

I have no idea how you plan to convert energy directly to anti-matter.
 Certainly thats not how we make it now.  Even if we could generate the anti
matter, how would you store and move amounts on that scale safely?  And of
course how do you refuel for the return trip?

Your probably right about the E18+ power being a 'show stopper' thou. I can't
think of any reasonable scenario that would have us able to put that kind of
power on line, in space, in about 50 years.

 >> >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
 >> >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
 >> >parameters
 >> 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
 >> 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
 >> with a build in neural-network for all kinds of purposes.
 >> About nano-tech I know a bit less, but certainly there are significant
 >> breakthroughs. And nano-tech has the same potential as computers had.
 >> there is a beginning, growth will be exponentially.

Your definitely more optimistic about nano and A.I. than I.  We after all can
and do reflect radio and microwave off ionized gas all the time.  We have no
Nano systems, and are making painfully slow progress in A.I.  (A.I. first
learned to read aloud over ten years ago.)  So I do think they are unlikely
to be mature enough in 50 years to help us much.

Actually, even if they did work, they wouldn't solve any critical problems
for us.  Just improve effecency and affordability.

 >> >One thing I was considering was what we can do.  My Explorer design
 >> >certainly be able to carry enough fusion fuel to decelerate from 1/10th
 >> > 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
 >> I checked it and its even better. For acceleration and deceleration a
 >> 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

Hum... I just ran some numbers through the LIT Delta V program using the
specific impulse of Bussards Fusion engines.  I got about the same numbers at
..3c but I though I got slightly better numbers at .2c.  But I don't have the
numbers with me.   Then again, I'm not even sure the LIT program nows to add
the need to accelerate the fuel mass with the ship (I certainly hope it does,
I'll have to test it.)

 >> > Obviously trying to do that at 2/10ths C would take 20 squared (400)
 >> >masses of fuel.  So thats out, but at even those speeds drag is a
 >> >factor.  Savages book mentions that at near light speed the (one atom
 >> >cubic centimeter) inter-stellar Medium could cause up to 37 milligrams
 >> >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
 >> >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!)

Yeah, I dug up some numbers from my old web page (see above) that showed he
was off the wall too.  (Well at relativistic speeds time dilation might help
his numbers)  Looks like no magnetic drag chutes.

 >> >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
 >> >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.
 >> >you have some reason of wanting to go there regularly.  The first ship
 >> >assemble automated fuel launchers in the target systems.  That would
 >> >lighter ships to make the same run at higher speeds without carrying
 >> >fuel loads.  (Assuming they trusted the fuel launcher at the receiving
 >> >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.

But, it would require far simpler systems, and a fraction of the power.  A
comparatively simple fusion powered ship with fuel launchers in sol could
make a round trip.  If it builds a big fuel launcher in the target system it
could even cut the return trip time down.

Like I said above, I ran some numbers off using the delta-v program on the
LIT site, and got a 60 to 1 fuel to ship mass ration for a .2c  Delta v,
using the Bussard fusion drive motors.  Thats not an impossible number (bad,
but not impossible).  So I think such a ship could be built by 2050.  So we
could get to the nearer stars.  At .2c we could get to the 4.5 to 5ish ly
stars in 22-25 years.  Which should be quick enough to be do able (thou its
pushing it!), thou the crew will probably die durring the 25 year return

On the return flight the ship would load up with enough fuel to boost it back
up to speed.  It couldn't possibly carry enough fuel to accelerate, and
decelerate!   So it will have to use fuel launched by the Sol fuel launchers
to power its deceleration boost.  (Better home the folks back home still like
you! ;) )   

But then again, if we bring enough gear to build a fuel launcher at the
target star system.  The ship could boost for home with little or no fuel on
board.  Its speed wouldn't be limited to the maximum speed it can get out of
the stored fuel its carring, or that it can carry the deceleration fuel to
stop from.  Accelerating with near empty fuel tanks the ship will weight a
tiny fraction of the max weight its engines and structure were designed for.
 The crew could keep the extra for redundancy (the ship will be getting old
by then), or they could strip some or all of the systems off to lighten the
ship even further.  Allowing the crew to boost at higher G's in the
acceleration track, order to get to higher speeds.  

Remember, the limitation on accelerating in a pre-launched fuel stream.  Is
how far out you can get before the fuel gets so spread out that you can't
scoop up enough fuel to run the engines.  I.E. how accuratly can you launcher
put the fuel?  A striped down ship could not only accelerate faster, getting
to higher speeds within a given distence from the fuel launcher.  It could
also get by on less fuel, allowing it to still keep up thrust farther out
from the fuel launcher then its heavyer brother.  If the folks back home
really like you.  They will have upgraded their fuel launcher in your
absence.  Allowing you to get a usable fuel density at far farther out from

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

Yeah, but it seems to be an issue we need to resolve.  As is, a .2 C ship
with the ability to go faster with fuel launchers at both ends, seems
reasonable by 2050.  A near light speed ship seems out of the question.  The
microwave sail idea requires completely absurd amounts of power, and we can't
seem to find a practical way to stop the ship.

So until we change the physics we have to work with (or learn a much better
way to manufacture and store anti-matter) we are limited to slower, shorter
range fights.  But flights that would be technically (and financially) far
less chalenging.  Does anyone disagree?


Where is David?  I keep geting undeliverable mail mesages when I ship to his
office.  Has anyone heard from him lately?

Opps, I forgot to look up the thrust to weight ratio of the Bussard plasma
engines.  If their too low, the ship wouldn't be able to carry enough fuel.