Re: Non-synchronous elevators

"Hop David" <hopspageHATESSPAaMmM@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:431E4600.8060407@xxxxxxxxxxxxxxxxxxxxxxxx
>
>
> Logan Kearsley wrote:
> > I have been considering the problem of getting a space elevator to work
on
> > worlds without a synchronous orbit around them or a convenient Lagrange
> > point to make use of. Like Venus, Mercury, or the Galilean moons. What I
> > have come up with ought to work for making elevators much easier on
> > high-gravity worlds like Earth, too.
> >
> > The first thing that comes to mind (apart from a rotovator, anyway) is
just
> > a simple moving skyhook, a cable that does not quite touch the ground,
and,
> > being in orbit but not in a synchronous orbit, moves relative to the
ground.
> > You can put the center of gravity in whatever orbit you want, though, so
you
> > can tune the groundspeed to whatever is most convenient. This, however,
has
> > stability problems, as, if there's an atmosphere, such as on Venus or
Earth,
> > the cable will experience significant drag, and, additionally, since it
> > isn't anchored to the ground, it can't steal rotational energy from the
> > planet, and so every time something goes up it the cable will lose
altitude.
>
> If you're going to have to move to catch the tether, you might as well
> make the "catcher's mit" above the atmosphere. The angular velocity of
> the tether could match the angular velocity the shuttle has at the top
> of a suborbital hop. So their relative velocity would be zero at the
> moment tether catches shuttle.

Yup. Depends on how much effort you want to make to catch the cable, though,
both in energy expenditure and coordination/timing.
It's certainly worth it to build one like that, that would pick up
suborbital hops, as it's still significantly less work than launching all by
rocket power even if it doesn't quite live up to conventional stationary
elevator. But if you can, maybe you want to go even lower, so that airplanes
could catch a ride, or even balloons (OK, maybe that's a stretch, but it's
possible).
Of course, if there's no atmosphere to worry about, like on Mercury or the
Galilean moons, a surface pickup and a suborbital hop pickup are almost the
same thing.

> The tether likely will have photovoltaic cells or a nuclear power
> source. And it'll probably be moving through a magnetic field. If it
> runs a current one direction it'd lower itself, another direction would
> boost it. Thus it could compensate for angular momentum lost (or gained)
> from catching shuttles.

Right. A point I made in the next paragraph that you snipped. If the bottom
of the tether is outside of the atmosphere, that'll be pretty much all that
the magnetic propulsion system would have to compensate for, along with a
few adjustments to correct potential tidal perturbations and such, but if
it's going down into the atmosphere, there'll be a continuous need for
magnetic propulsion to counteract drag.

> If it catches both incoming and outgoing cargo it would be gaining or
> losing angular momentum depending on whether it's sending stuff up or
down.

And thus the magnetic propulsion system could be used to generate power as
well.

-l.
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