Transport, the near future

[Peter Fairbrother]

Steve Schear wrote:

> 
> My preference is the space elevator.  In simple terms, the space elevator
> is a ribbon with one end attached to the Earth's surface and the other end
> in space beyond geosynchronous orbit (35,800 km altitude). The competing
> forces of gravity at the lower end, and outward centripetal acceleration at
> the farther end, keep the ribbon under tension and stationary over a single
> position on Earth. This ribbon, once deployed, can be ascended by
> mechanical means to Earth orbit. If a climber proceeds to the far end of
> the ribbon and releases, it would have sufficient energy to escape from
> Earth's gravity and travel to the Moon, Mars, Venus and the asteroids.
> 
> http://www.highliftsystems.com/
> 
> 
> "Reality must take precedence over public relations, for nature cannot be
> fooled."
> -- Richard P. Feynman

It's a nice idea, but it needs a tensile-strength-to-mass ratio equivalent
to holding a girl and her mother up by a single thread of her 10 denier
stockings. Not easy to achieve. You'd need carbon nanotubes or the like, and
at the moment we can't build it. You also need 45,000 km or so of tether.
Expensive. Huge investment, fragile. Unrealistic, imo.

Rotating tethers on the other hand can use hi-test fishing line. Really, no
kidding.  You only need a few hundred km, or at most a few thousand km, of
tether. Cheap.

There are two types, landing takeup and hypersonic takeup. They work a bit
like this (here goes a try at some ascii art...)


               [] orbiting mass-->
                \
                 \ rotating tether
                  \
                   \
                  <-\      space
            --------------------
                           atmosphere
           ________________________
                           earth
                   

(on this scale a space elevator cable would be roughly six feet long)

The tether, whose centre of gravity is in a fairly low orbit, dips it's end
into the earth's atmosphere every so often. Hypersonic takeup tethers catch
a 'plane flying at hypersonic speeds in the upper atmosphere, and landing
takeup tethers reach the surface. The energy/momentum is replaced by sending
current through the tether as it passes through the Earth's magnetic field.

Hypersonic takeup tethers are better studied, even the rendezvous techniques
apparently work, and can use fishing line except for the short length that
enters the upper atmosphere (it would melt). They use a mesh-like tether
structure to avoid catastrophic damage from meteorites etc (a patented, but
IMO obvious, idea).

Landing tethers sort of cast the line a bit ahead, like a fisherman; it hits
the ground, is tied on to the spaceship (good knots!) and then the line and
the spaceship are dragged up. No-one really has studied them much (except
me, and I'm not telling yet), but the strength (and length) of line needed
is _much_ (order of mag+) less than a space elevator. And you don't need a
hypersonic 'plane.

You can also fling things away from the tether when they're going away from
the Earth. Can get any (reasonable) speed you like.


-- 
Peter Fairbrother