Date: Tue, 03 May 2005 15:18:53 -0400
To: WSFA members <WSFAlist at WSFA.org>
From: "Mike B." <omni at omniphile.com>
Subject: [WSFA] Re: Bring a lunch for this elevator ride...
Reply-To: WSFA members <WSFAlist at WSFA.org>

At 01:05 PM 5/3/05 -0400, Paul Haggerty wrote:
>Mike B. wrote:

>>That's amazing.  I didn't know they were so close to actually building a
>>beanstalk.  I hope this doesn't turn out to be a scam.

>For more information on space applications of tethers check out:
>http://www.tethers.com/

Beanstalks and tethers are two different ideas.  They rely on similar
technological advances (really strong cables), but the physics and use
methods are different.

A beanstalk is a cable from the ground up to a counterweight that's farther
out than geosynchronous orbit (which is a bit over 22,000 miles up).  Once
it's in place you climb up it and let go to be in orbit (depending on where
you let go, some adjustment will have to be made to keep your altitude).

A tether system for launch is usually thought of as a long cable that is
rotating as it orbits at low altitude (low like maybe 200-700 or so miles
up)...think of a wagon wheel rolling around the planet, lift it up to
orbit, make it really big, then delete all but the hub and two opposite
spokes...that's a momentum transfer tether.  The site you pointed out talks
about them, but for orbital transfer, rather than launch up out of the
atmosphere.  For launches the idea is to fly up and dock with one end while
it's at its lowest point in the upper atmosphere, then ride it up until it
is at the highest point, well out into space, and let go.  You will have
picked up energy from the tether, which will have slowed down an equivalent
amount.  The tether is "spun up" again by running currents through it to
cause magnetic fields which interact with those of the Earth (like the
armature of a big motor), stealing momentum from the Earth's rotation and
slowing the planet by a tiny amount to get the tether back up to speed for
the launch.  You can also adjust the tether's speed by catching an incoming
payload and slowing it to de-orbit speed by catching an up going one, or
using the magnetic field trick again.

The main idea in both methods is to get to and from space without having to
throw away large masses of on-board propellant.  Both require really strong
cables, but the beanstalk needs the strongest, or so I've read.  There are
SF stories that include both of these methods.

-- Mike B.
--
You can name your own salary here.  I call mine Fred.