On Sun, Feb 25, 2018 at 07:47:34PM -0500, Richard Aiken wrote:
> On Sat, Feb 24, 2018 at 10:50 AM, shadow at shadowgard.com (via tml list) <
> xxxxxx@simplelists.com> wrote:
>
> (big snip)
>
> > Usually set up so that the low end is stationary (only momentarily)
> > at some height above the ground.
> >
>
> (little snip)
>
>
> > Requires steady nerves or good automation to dock on either end. :-)
>
> If the first is true, then I don't see why the second has to be true.

The end of the tether is constantly accelerating.  Proposals where the
tether tip is momentarily stationary typically have accelerations on
the order of 10 gees.

From the point of view of someone hovering in atmosphere: 3 seconds
before rendezvous, the tether tip is still half a kilometre away and
moving at about the speed of sound toward them.  About one second
before rendezvous, the sonic boom reaches them while it's about fifty
metres away and still shooting straight for them at about 100 miles
per hour.  It slows to more "human" speeds (say 10 m/s) only for a
fifth of a second.  If the payload isn't right there, at the right
place and time to latch on, the best case is that the tether merely
misses and shoots back into space (with another sonic boom on the way
out).

More practical proposals have the tether end still moving
hypersonically (say, 4 km/s) at the pickup, because current tether
materials can't support the 7+ km/s rotation required for a stationary
pickup.

- Tim