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<Two bits worth
Brian here again with a very small amount of time to throw in a slight
variant on the magsail idea. I get the feeling that no one really thinks a
magsail/ramscoop brake can be made to work. But if it can, I suggest the
following.
Looking back on LIT's (Seger's contribution I think) on Relativistic
Particle Beam propulsion (RPBP), it is noted that a low .036 g acceleration
was deemed possible by Zubrin for a magsail RPB driven starship weighing
1000 tons at the cost of 11 GW/ton.
Now what I have noticed in our discussions is that the particle beam
accelerators are a lot like stationary ion rockets that spew a particle
stream that hits a magnetic sail/scoop on the starship tht either fuels the
ship or imparts momentum with the particles' collection/collision or both.
Since we have always (I think) assumed that we could make an ion accelerator
powerful enough to produce a 1g thrust, I wonder why we couldn't just rig
one of these accelerator/ion rockets with a magsail, launch it ahead of the
manned ship via RPB propulsion methods and use it as a kind of portable RBP
source to stop the primary starship near Tau Ceti.
This would be analagous to stopping a ship by putting it in the exhaust of
another rocket. In this case, the exhaust is really a particle beam that
can reach hundreds of AUs.
I make a couple of obvious assumuptions here (some I'm probably not seeing).
For one, I acknowledge that we will need many Terawatts to push this
decelerator rocket and all of the fuel it will need to carry. Since we are
already assuming 1E18W for some other ideas, I assume it here. I also
assume that the decelerator is completely automated for at least some part
of the duration of the starship deceleration phase.
By the way, the decelerator could be manned for the first part of its
mission. If we are willing to wait a while, we could launch it at a slower
speed than the prime starship and it could begin pounding the faster moving
Asimov with ions while still a ways off. The decelerator would speed up some
while slowing the Asimov down and the two ships could be brought to equal
speed near each other. Note that the heavier we can make the decelerator,
the better because the further the decelerator is from the Asimov, the
harder it will be to hit it with a RPB.
Once the decelerator and Asimov are at equal speeds, they could dock and let
the decelerator's crew transfer to the Asimov, leaving the decelerator on
automatic. The decelerator could then continue its job.
For getting back home, we could bring along another ion accerator like the
one used to stop us. It could be used as an ion rocket for in system
shuttling (this is basically Kelly's Explorer design). Again, the ion
rockets we've discused so far sound powerful enough to mean that we need
take only one. Since we've kicked the idea of a launcher in Tau Ceti around
a lot in previous discussions I'll stop here.
I'm not sure that my mail is getting through to everyone so I apologize. If
it is and this is old material to everyone, well, nuts. I thought it was
different enough to be worth posting. I await comments.