Problems with a Saturnian trans-atmospheric skyhook
TSC
out some issues surrounding something that I'd like to put into a SF
computer adventure game that I'm working on.
Consider your run-of-the mill orbital skyhook. It extends up into
space perpendicular to the surface of the planet that it orbits.
Simple enough. Let's suppose that this skyhook is in orbit around
Saturn. Now, if we were to lower the orbit of the skyhook so that its
lower tip extends into the atmosphere of Saturn, it's going to
experience quite a bit of atmospheric drag. Eventually, its orbit will
decay and it will fall into the atmosphere, with the net result that
we've just thrown a very large amount of money into a gas giant. Not
good. The best solution is to keep the skyhook from ever entering the
atmosphere.
As it turns out, though, that's exactly what we want to do in order to
get at all the very useful He3 that you find in the atmosphere. So the
other option is to use some sort of propulsion system to counteract
the effects of the drag. Fortunately for us, Saturn has a rather
powerful magnetosphere, which we can use to our advantage. Since the
skyhook is nothing but a tether, it should be possible to use it as
its own electrodynamic tether propulsion system: you pump electricity
into it, and it raises itself out of the atmosphere. This should allow
it to offset the atmospheric drag and the loss of orbital velocity
caused by raising large amounts of He3 and other gases into orbit. And
where does the power for the electrodynamic propulsion come from? We
could burn some of that He3 in a fusion reactor, but we're looking to
sell that elsewhere. Instead, why not put a number of other tethers in
a higher orbit (not extending into the atmosphere) to generate
electricity by passing through the magnetosphere and then beam it to
our skyhook?
That's the plan, anyway. Here are the problems that I have:
First, I'm not sure that the electrodynamic propulsion system would
provide thrust in the proper direction to counteract atmospheric drag
and loss of momentum due to the constant pumping of gases up the
skyhook. As I understand it, to raise an orbit, you want to thrust in
the direction of your orbit; the tether propulsion system - again, as
I understand it - will thrust in the direction that the tether is
oriented. In this case, that would be perpendicular to the direction
of travel. Is this correct? And, if it is, would this be sufficient to
prevent orbital degradation or do I need to thrust in the direction of
travel?
Second, I'm not sure just how much drag and momentum loss we're
talking about, here. Would there be too much of either (or both) for
the propulsion system, assuming it works, to counteract? What would
the power requirements be? Would the electrodynamic power generation
tethers be up to the task?
Third, if everything else can be made to work, would the fact that the
drag on the skyhook is applied to one end only mean that the thing
would tip over? Would it just lean and then become stable at an angle?
If it would be a problem, is there anything that can be done to
counteract it?
Fourth, just what is a reasonable orbital period and speed for such a
setup? Ideally, the orbital period would match the rotation of Saturn
at the equator (about 614 minutes), but I'm not sure that such an
orbit is possible. If the orbital period is longer than the period of
Saturn's rotation, would that mean that the atmosphere is blowing in
the direction of the skyhook's travel? If so, how would this affect
its orbit?
And if you can think of any other problems with the setup, I'd
appreciate a heads-up.
Thanks a lot, guys.
j...@google.com
j...@google.com
eripe
Shawn Wilson
> Consider your run-of-the mill orbital skyhook. It extends up into
> space perpendicular to the surface of the planet that it orbits.
> Simple enough. Let's suppose that this skyhook is in orbit around
> Saturn. Now, if we were to lower the orbit of the skyhook so that its
> lower tip extends into the atmosphere of Saturn, it's going to
> experience quite a bit of atmospheric drag.
You missed a critical skyhook point- the center of mass is in
GEOSYNCRONOUS ORBIT. No (well, not much) drag.
Yes, otherwise it will fall down, go boom. (or at least fall down,
sizzle, rain ash)
Tim Little
> why not put a number of other tethers in a higher orbit (not
> extending into the atmosphere) to generate electricity by passing
> through the magnetosphere and then beam it to our skyhook?
Mainly because the energy generated comes from their kinetic energy,
thus lowering their orbit. That can't last for long.
> the tether propulsion system - again, as I understand it - will
> thrust in the direction that the tether is oriented.
Perpendicular to both tether orientation and magnetic field, actually,
which is exactly what you want.
> Second, I'm not sure just how much drag and momentum loss we're
> talking about, here.
As much or as little as you want, depending how deep in the atmosphere
you extend the end of the tether. Erosion and heating of the end of
the tether would also be an issue.
> Would there be too much of either (or both) for the propulsion
> system, assuming it works, to counteract?
In theory, enough current (and therefore power) would be able to
produce arbitrarily large forces. In practice there are numerous
material limitations and low plasma density with which to interact.
There are probably also as yet unknown factors.
> What would the power requirements be?
In theory, just enough to balance the power dissipation in atmosphere.
In practice somewhat more as there will be various forms of loss and
probably internal dissipation of various kinds.
> Third, if everything else can be made to work, would the fact that
> the drag on the skyhook is applied to one end only mean that the
> thing would tip over?
Yes, though balanced to an extent by tidal torque. One complication
is that a forward-tilted electrodynamic propulsive tether will have a
downward component of acceleration. So it would probably have to be
in a "forced orbit" slightly faster than the usual speed to counteract
that.
Another possibility might be to deliberately have the tip ahead of the
bulk of the tether, and more vertical than the rest. Then the EM
force on the bulk is "upward and forward", while the tip is more
strongly "forward". This extra forward component of force counteracts
the drag near the tip. I am fairly sure there could be an equilibrium
like that, but I'd be extremely surprised if it were a stable one.
Active monitoring and geometry maintenance would be required.
Such an active system would be required in any event, due to
variations in plasma density, magnetic fields and tether oscillations.
> Fourth, just what is a reasonable orbital period and speed for such
> a setup? Ideally, the orbital period would match the rotation of
> Saturn at the equator (about 614 minutes), but I'm not sure that
> such an orbit is possible.
With *extremely* strong materials, it would be. Certainly not for
anything we can realistically extrapolate though. The gravitational
stress from such a long tether near Saturn would certainly snap it.
The orbital period would in practice have to be shorter than Saturn's
rotation. Orbital speed in low orbit is about 25 km/s while the
rotational speed at the equator is about 10 km/s. With reasonable
materials that orbital speed could not be reduced much below 20 km/s.
So there will necessarily be at least around 10 km/s difference
between tip and atmosphere.
- Tim
Greg Goss
>Third, if everything else can be made to work, would the fact that the
>drag on the skyhook is applied to one end only mean that the thing
>would tip over? Would it just lean and then become stable at an angle?
>If it would be a problem, is there anything that can be done to
>counteract it?
>
>Fourth, just what is a reasonable orbital period and speed for such a
>setup? Ideally, the orbital period would match the rotation of Saturn
>at the equator (about 614 minutes), but I'm not sure that such an
>orbit is possible. If the orbital period is longer than the period of
>Saturn's rotation, would that mean that the atmosphere is blowing in
>the direction of the skyhook's travel? If so, how would this affect
>its orbit?
If your center of gravity is higher than syncronous, then your
atmosphere is indeed pushing your skyhook. That push could counteract
the momentum lost when you pump gases up the hook and fling them (in
some suitable pressure container) towards the customers.
Tide is pulling your hook back towards vertical. If the atmosphere is
pushing it off true, then it's stable at the point where the drag
counteracts the tide.
I don't have the energy to check the moons. Make sure that there are
no moons below the upper edge of your skyhook. KSR had a skyhook
dodging a moon in her Mars series, but that never seemed reasonable to
me.
--
Tomorrow is today already.
Greg Goss, 1989-01-27