Space Elevators

I wrote
>> What about the rotavator or slingshot?

Dan Wylie-Sears replied
> Probably better for reality than a Beanstalk, but
>it's harder to do back-of-the-envelope shoot-the-bull
>about them: what kind of precision do you need, and
>what happens if you don't have it?

Forward (of TUI) said that you have anywhere between 1/2 minute to 3+
minutes to match the tether tip if you're in an approaching spaceplane.
The tether tip has a reel and some sideways swing capability. As the
tip drops down to the bottom of its arc, it reels out to meet the
rendesvous point early; as it reaches the bottom of its swing, it reels
back in to keep the tip in the right spot and relatively motionless,
and then it reels back out as the rotation brings it away.
Note also that anything in the geomagnetic field gets free energy from
an electrodynamic exchange.
Forward pointed out that with tether pumping of a rotavator with reels
in the center and the end points can change its orbital inclination
without ED or reaction mass. It can also raise its perigee by trading
apogee height (and the other way around) without reaction mass, just by
pumping the tether. Add in electrodynamics, and the tether is very
versatile.

The tip is either something like Mackenzie's catcher with all the
necessary rendesvous telemetry, or going up to the lower end of the
Hypersonic Skyhook station, it's more like a warship's helo pad.
As a helo approaches the stern of the ship, it drops a line with a
hook. Along the edges of the pad are cables: the ends of 2 cables
running fore/aft along the sides ride in slots on the front and back of
the pad, while the cables running from side to side slide in slots at
the port/starboard edges of the pad. When the hook drops down to the
deck, the cables slide in their endslots to close in on the center of
the pad like a tic-tac-toe grid, trapping the hook, and the cable is
reeled in while the helo applies constant lift to keep it taut until
the arresting gear on the helicopter is trapped solidly on the deck.
Similarly, warplanes massing 20+ tons approach a carrier at 120+ kts to
trap for a landing, sometimes in zero visibility, at night, in rolling
seas that make a 110,000 ton ship heave about. They rely on their
systems for the airmen's lives and their ships when missing a trap can
cost a plane, ship, a battle or the nation that loses the battle.
Argentina lost the Falklands partly because their A-4 Skyhawks couldn't
get enough wind over the bows to carry any effective payload far enough
to attack the British carriers. The first strike in that war was by
British Vulcan bombers crossing thousands of NM of ocean at night,
using radio locators to triangulate the runways at Port Stanley. The
plane never even broke the horizon of the airport; they went from low
altitude up in a pop-up and release attack which lob/tossed the
(unguided iron) bombs more than 4 miles. The center of the stick of
bombs hit just off center of the crossing of the runways, preventing
the Argentines from using the airport.

Missiles are relied on to hit a small target in a big ocean, when the
target is trying to evade/avoid it, and throwing ECM and
countermeasures to throw it off and defensive firepower to kill it
before it kills the ship. You might be appalled at how effective these
things are -both offensive and defensive. A missile can cross hundreds
of miles of ocean, and accurately hit a specific ship in an enemy
formation directly in the superstructure where the ship's bridge/CIC is
located.
The British/Euro Goalkeeper CIWS antimissile/anti aircraft 30mm gun
mount has sucessfully hit incoming 157mm artillery rounds on target
ranges. Take it up with the US Navy if you doubt the effectiveness of
their Aegis robotic ship/weapon/task group organization and weapons.

For a spaceplane meeting a pad dangled at the end of a hypersonic
skyhook, both the plane and the station want to meet the other, and all
the effort weapons and ships expend hitting or avoiding each other is
applied to helping the plane land on the deck. If it misses, the plane
drops back down (like it normally would after dropping its passengers
off and being dropped from the deck at the end of a flight) and lands,
much like the Space Ship One did. Note that the plane never gets up to
orbital height or velocity, whether it traps sucessfully or not (any of
the X-Prize contenders would work).

For bigger applications like tossing a manned capsule from high Earth
orbit into a trajectory to Mars and being caught there, it's a lot
trickier, since a missed catch sends the crew out on a cometary
trajectory... Yes, it's a lot to consider back-of-the-envelope here,
but what can be said beyond generalities and theoretical mathematics
about a Beanstalk?

In any case, I made my point that you shouldn't fail to consider and
study these other tether applications just because the Beanstalk is
sexier... I like the end of Clarke's book, where the Earth is girdled
by a Geosynchronous station going completely around the planet, with
multiple radial spokes of Beanstalks going down to the planet. I also
like the image of the solar system spotted in many orbits with huge
rotating tethers which catch and toss payloads around the system for
free.
What's the interplanetary transfer velocity of a payload tossed from a
tether tip spinning for 4 Gees, during an 18 hour half-rotation period?

In particular, look over the mathematics and extrapolations in
Mackenzies "Bootstrapping..." paper. It makes sense and helps to
understand in small-scale terms how these things work.
Bootstrapping space Communities
http://www.ari.net/moon/forum/mp/mp-4/bootstrap.html

Tethers Unlimited, Inc.
http://www.tethers.com/

Hypersonic Skyhook
http://www.affordablespaceflight.com/step3.html