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starship-design: Anti-antimatter

First let me sound a quick note of technical confusion... Timothy's 
response to my post on laser-plasma accelerators came through to my mail 
system as a series of random characters.  I assumed it happened to 
everyone, but since Rex just quoted Timothy's response to me in his last 
post I suppose the problem was more localized (I have to Telnet down to 
UCLA to read my mail, although I'm not sure how that would mess things 
up so badly in this isolated case; all Timothy's other posts come 
through fine.)  Could someone please re-post Timothy's 1/30 Re: 
Relativistic Electric Thrusters so I could try to read it?  Thanks!

	As for the ongoing discussion on Relativistic Thrusters, I'm 
slowly getting up to speed. I checked out Timothy's Web Page and saw why 
a fusion spacecraft should limit the fuel speed to 0.1c; I didn't check 
the numbers, but it sounds reasonable.
	So for self-powered ships, that seems to leave relativistic 
engines to the regime where you can transfer your ship mass to energy 
with a near-unity efficiency.  The most obvious way to do this is with 
antimatter, of course, but here I'm a bit skeptical.  Even if we're 
assuming some pretty amazing technological advances, I find it hard to 
imagine storing huge quantities of anti-matter on a ship.  I don't know 
if this has been discussed already, but I would guess there would be 
some sort of theoretical minimum matter/antimatter ratio, just from 
containment considerations.  Anyone want to tackle that one?
	So, keeping this in mind, here are another two ideas for 
"antimatter-type" engines; ways to convert mass to pure energy--without 
using antimatter.  These are nearly as speculative as large-scale 
antimatter containment, but I'd still argue that they're more probable.

	The first is probably the most easily shot down: power the ship 
from an enormous relativistic or near-relativistic flywheel.  There's a 
significant amount of research into flywheels for electric cars, but 
there are many big disadvantages:  they have to be kept in vacuum, they 
need to always point in the same direction, and they need to be small 
enough to fit in a car.  All three of these problems, of course, aren't 
big deals for an interstellar spacecraft.  The big limiting factor is 
the acceleration that the rim can handle before flying apart, but for a 
given acceleration limit--with a hollow cylindrical flywheel--an 
arbitrarily high Energy/Mass ratio is possible just by making the 
flywheel larger.  To be specific, the Energy/Mass (J/Kg) of the system 
is simply half the acceleration of the rim (m/s^2) times the radius of 
the wheel (m).
	Current flywheels are made out of graphite and can handle rim 
accelerations greater than 10^8 m/s^2.  The actual limits are not public 
knowledge, as far as I can tell; too much competition between the people 
who make these things.  So already, for a 200 meter radius flywheel, 
we're talking about an energy storage of 10^10 J/Kg.  We'd probably need 
at least 10^14 J/Kg to make a decent spaceship.  Making up four orders 
of magnitude as well as structurally weakening the flywheel by making it 
hollow seems quite a long-shot.  But two possibilities that are at least 
credible are fullerene and diamond flywheels.  I'm not sure which would 
be stronger; Diamond is a 3-D structure while buckytubes are 2-D, but 
the crystal structure of a diamond magnifies small imperfections.  
Perhaps a braid of buckytubes would be the best; on the other hand, if 
nanotechnology allows the creation of a *perfect* diamond flywheel, it's 
hard to see how anything could beat it.
	Of course, a 200 meter flywheel with 10^14 J/Kg means the rim is 
traveling at 10^7 m/s.  Any bigger/faster and you get relativity playing 
a big role.  Probably some problems inherent in that situation... but it 
might make a great gravity-wave generator!

	Idea #2:  Catalyze nucleon decay using captured magnetic 
monopoles.  Okay, okay... at least antimatter has been proven to exist, 
right?  Granted, magnetic monopoles are still hypothetical particles, 
but on the plus side every single Grand Unified Theory predicts that 
they should exist.  Basically, if there IS a grand unified theory, then 
magnetic monopoles are viable and stable particles.  Unfortunately, most 
of the GUT's predict HUGE masses; sometimes as big as 10^19 GeV!  If 
correct, this means we're not going to make a monopole in an 
accelerator: they're only going to be left over from the Big Bang, and 
we're going to have to find one and catch it.
	As I mentioned, another predicted feature of monopoles is that 
they should catalyze proton and neutron decay.  The great thing about 
this is that it doesn't destroy the monopole; it's still waiting around 
to decay more nucleons.  One can easily imagine what a great power 
source this would be; feed a monopole protons and neutrons and it 
continually converts them into pure energy.  You'd probably need quite a 
few monopoles to make a decent spaceship engine, but it'd be a whole lot 
less than the needed amount of antimatter.

	Well, this was a pretty big tangent off the subject here, but 
hopefully it'll spark some ideas...