OTS: Tie Me Up, Tie Me Down

Someone else has to do the kink in SF thread, sorry.

Rockets are surprisingly efficient within their realms
(which is to say, mass ratios less than, say, five) but they are
not the only way to fling things into deepest space. Take the
tether.

If it is just rotating in space, it's called a rotovator
and it acts like a momentum bank, where incoming cargo can be slowed
down or redirected, while outgoing cargo can be lobbed around the
system, within the contraintst of the velocity at which the roto-
vator is spinning and in which plane.

As far as I can tell, the King of the Rotovators in SF
was the late Charles Sheffield and even he didn't use them that
much. They have the slight drawback that a = v^2/r, which means
that as the necessary delta vees get larger, either the cable
becomes inconveniently long or the accelerations inconveniently
high. It's a nice idea for low v applications but you wouldn't
want to place a package near Saturn in under a year with one.

If it is dipping into the upper atmosphere of a world,
low mass ratio rockets could in theory rise up to be snagged
out of the air by the tip of the cable (Needless to say, this
requires some nice guidence and I will let someone else try
this out first).

I do not at this time recall any examples of this in SF.

The most famous application of cables is fix one end to
a planet and use it as a vertical bridge. I think Sheffield managed
to be the first person to use this in SF (Although Clarke was close
on his heels) in "Skystalk". Often the planet at the bottom is Earht
or an Earthlike planet, even though the demands on such a cable
would be impressive, but such cables are even more useful on
smaller bodies: Jerome Pearson (one of the suprisingly large
number of people to independently invent the idea of an orbital
beanstalk) designed a 6.8 tonne Lunar Beanstalk that would be able
to deliver or lift 200 kg from the Lunar surface.

I once calculated that a tether about as long as a
Lunar tether would suffice to allow material to be lifted
off Ceres and flicked across the system to Earth/Moon L4.

Most of the examples I can think of here are either
on Earth (JUMPING OFF THE PLANET, FOUNTAINS OF PARADISE) or
on an Earth-sized world (DREAD EMPIRE'S FALL). As far as
I know, nobody uses asteroid-based tethers.




--
http://www.cic.gc.ca/english/immigrate/
http://www.livejournal.com/users/james_nicoll
.

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