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From: davidson@sdcsvax.UUCP (Greg Davidson)
Newsgroups: net.space
Subject: Re: Fuels for rocket propulsion
Message-ID: <1096@sdcsvax.UUCP>
Date: Thu, 12-Sep-85 03:09:59 EDT
Article-I.D.: sdcsvax.1096
Posted: Thu Sep 12 03:09:59 1985
Date-Received: Wed, 18-Sep-85 03:33:19 EDT
References: <384@decwrl.UUCP>
Reply-To: davidson@sdcsvax.UUCP (Greg Davidson)
Distribution: net
Organization: EECS Dept. U.C. San Diego
Lines: 50
Keywords: light sails,black holes,spacecraft propulsion
Summary: lightsails not so bad & mini-black hole drive


Let me add a couple of things to Mike Augeri's account of spacecraft
propulsion methods (I won't say rocket, since not all the methods he
talked about use rockets).  First of all, he mentioned that light sails
are too slow for interstellar propulsion, and aren't suitable for
manned spacecraft.  Not so!  Although their acceleration might well be
very low for massive manned spacecraft, they have the best performance
for interstellar misions of all the systems he mentioned.

Light sails have such excellent performance because they don't have
to carry their fuel.  With solar pumped lasers to keep them going, a
manned light sail spacecraft can reach destinations 40 light years
away, and reach a cruising speed of 1/3 light speed.  Large manned
spacecraft might have very low accelerations, but could reach cruising
speed after several months or a few years.  Unmanned untralight probes
using light or microwave sails can have accelerations of 1000's of
g's.  Robert Forward has done much of the work on this and used the
concept in his recent novel Rocheworld.

Second topic.  There is another method for obtaining total conversion
of matter to energy, and not in the unusable forms of gamma rays and
neutrinos.  It does not need any dangerous and hard to make
anti-matter.  It needs a less dangerous, but MUCH harder to make
mini-black hole.  I don't know who first thought this up, but Clarke
used this idea in Imperial Earth.  You use a heavily charged black hole
so you can hold onto it.  You can dribble matter into it in such a way
that most of the energy of the matter is turned to energy through
friction.  Only a tiny bit gets in past the event horizon.

Assuming that there is nothing wrong with current black hole theory, I
see no scientific barrier to this form of transportation.  If we can't
find any such black hole, we will eventually be able to make one (see
next paragraph).  And given one, I understand that you can make more
fairly easily.  Clarke envisaged using two very long (space borne)
opposed mass drivers to create mini-black holes.  Does anyone know how
feasible this is?

My background assumption is that in a few hundred years (maybe much
sooner) we will be able to construct true von Neuman machines; that is,
self reproducing automatic factories.  Given space based von Neuman
machines using solar energy and asteroidal matter I see no barrier to
attacking really huge construction projects.  So its really not a
problem if you want a solar sail thousands of kilometers wide, with a
bank of lasers big enough to drive it.  Its also not a problem to
construct mass drivers tens of thousands of kilometers long.  Either
of the above methods of propulsion should give us the stars!

Ad astra,

_Greg Davidson			Virtual Infinity Systems, San Diego


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