On Tue, Feb 27, 2018 at 1:51 AM, Tim <xxxxxx@little-possums.net> wrote:
On Sun, Feb 25, 2018 at 11:36:55PM -0500, Richard Aiken wrote:
> > Even for the slower tether rotations where the tip never gets slower
> > than 4 km/s or so relative to ground, its height above ground will
> > still vary a lot even during its lowest second.
>
> Why so? The hub of the system is capable of maneuvering, even in the
> real-world examples given.

It's just the nature of circular motion that the tip is under constant
acceleration.  That means near the lowest point, the tip's altitude
changes as described.

Now it might be possible for the tip to have some automated high-speed
retraction mechanism . . .

Multiple stacked take-up reels:
First reel ~5 kilometers up the cable, for kilometer-scale adjustments, made while the tip is still ~10 kilometers out from the pickup point.
Second reel ~100 meters up the cable, for decameter-scale adjustments, made while the tip is still ~1 kilometer out.
Third reel ~10 meters up the cable, for meter scale adjustments, made while the tip is still ~100 meters out.
Fourth reel ~1 meter up the cable, for centimeter scale adjustments, made as the tip closes in for contact.
 
. . . about needing either good automation or very steady nerves.

Given laser range finders, I'm fairly sure today's level of automation could easily handle something like the above.

--
Richard Aiken

"Never insult anyone by accident."  Robert A. Heinlein
"I studied the Koran a great deal. I came away from that study with the conviction there have been few religions in the world as deadly to men as Muhammed." Alexis de Tocqueville
"We know a little about a lot of things; just enough to make us dangerous." Dean Winchester (fictional monster hunter portrayed by Jensen Ackles)
"It has been my experience that a gun doesn't care who pulls its trigger." Newton Knight (as portrayed by Matthew McConaughey), to a scoffing Confederate tax collector facing the weapons held by Knight's young children and wife.