- To: undisclosed-recipients:;
http://www.sff.net/people/geoffrey.landis/STL.htp
>>>>BRIN: I have heard that there are enough warheads in the arsenals of
the United States and the former Soviet
Union that, if we beat them all into plowshares and used them all for
Orion ships, that we could send a mass
equivalent to the United States Navy to Mars.
LANDIS: What are some of the other possibilities for ways that we might
get to the stars with real technology that
we know today?
BRIN: All sorts of possibilities have been discussed. At the opposite
end of the spectrum from the image of a
fiery antimatter rocket, was the idea of sending a little "Starwisp"
spacecraft streaking out of the solar system.
This was originally Bob Forward's notion; a broad, very light sail that
takes a focused microwave beam to drive
this little one-ounce spacecraft. The microwave beam sends it hurtling
across the starscape.
FORWARD: One of the newest ideas is one that Dana Andrews, Bob Zubrin,
and Geoffrey Landis have
proposed, particle-beam propulsion. The basic idea is to have a particle
beam generator, stuck to an asteroid
(because you can't use it on the Earth, the atmosphere gets in the way,
and once it gets firing it has a lot of
recoil, so you have to put it on something heavy). So you take an
asteroid and you build your particle beam
generator, and you beam both positive particles and negative particles
out into space--
BRIN: You make your vehicle a hoop.
FORWARD: Right, a hoop. And you put current through it to make a strong
magnetic field, and when the
charged particles come they hit the magnetic field, and they give the
magnetic field a push, and it gives the wire
a push, and the wire gives the spacecraft a push, and so that's the way
you get up to speed.
BRIN: This is a variant on the idea of sending a microwave
beam--Forward's Starwisp--or of hitting a solar sail
with a laser.
FORWARD: Beamed power propulsion.
BRIN: What all three of these ideas have in common is that you can send
a ship out that doesn't have to carry its
own energy, doesn't have to carry its own fuel. Because the biggest
problem in approaching the speed of light is
that you not only have to accelerate your own ship, but you have to
accelerate the fuel that you're going to use
for later acceleration. So people have been swinging over to this idea
that the best way to reach the stars is to
have a home base shoot power out to you, whether by particle beams,
lasers, or microwaves.
FORWARD: I think that, after years of study, it's now very obvious that,
if you want to go to the stars, don't use
rockets. You have to use something else. Beamed power is one way.
The beauty about this engine is that, unlike some of the ideas I've had
where you push it with lasers or
microwaves, is that when you enter the target solar system, you can use
it as a drag brake against the solar
wind to slow down and come to a stop, without doing anything fancy
except re-energizing the magnetic loop.
POST: In fact, any kind of interstellar craft can use a magnetic sail to
brake. So we're talking about hybrids, good
ideas that combine two other good ideas.
The ancestor of the magnetic sail was the interstellar ramjet of Robert
Bussard. Many people played with that
concept, which uses interstellar hydrogen as freely-available fuel, but
the scoop to collect the hydrogen seems
to produce more drag than thrust. James Stephens of JPL tried to patent
the magsail first, under the name
'loopsat.' When I worked for Dana Andrews on Boeing's 1981 survey of
advanced propulsion, I tried to hybridize
huge superconducting loops with ion drives, and considered trajectories
through the Earth's magnetotail, the
Jovian magnetosphere, and the Io Flux tube. Bob Zubrin deserves credit
as father of the magsail--he derived the
essential equations--but the idea has many grandfathers, and clearly
Bussard is the great-grandfather.