From: Ross A. Finlayson (raf@tiki-lounge.com)
Date: Tue Mar 07 2000 - 09:29:06 MST
Well, something would have to float at a sufficiently high orbit and have enough
mass to stay there while acting as an elevator pulley anchor for things from the
ground. It would have to reinforce its place in orbit or gradually be pulled to
a lower orbit by lifting things from the ground. It should reach to ground at
the highest feasible altitude, or even perhaps simply hang skyhooks into the
stratosphere for ballons to be hooked. Anyways, back to propulsion, it would
need to have some form of reaction to offset lifting power. I would suggest
solar sails tacking against sunshine to do so. Then, once something is lifted
that high, if it needed to go into a higher orbit the required reaction mass
would be much less by that point.
If something that bitg fell out of orbit, it would be bad for those where it
landed, so among other things the cable would have to have some critical tensile
strength and release upon any unbalancing mass.
Fun.
Jeff Davis wrote:
> Ad Astropians,
>
> Re the tower-jump-to-orbit maneuver, or the minimum altitude for
> zero-rocket-assist orbital insertion:
>
> First,
>
> Spike Jones (spike66@ibm.net)
> Wed, 01 Mar 2000 20:16:31 -0800 wrote:
>
> >If a skyhook existed, one could climb sufficiently
> >high on it, then let go of the cable and fall into orbit. I just did a
> quick back of the envelope calc
> >on that and it looks like if you climb one Earth diameter up the cable and
> let go, one would fall
> >into a minimal orbit.
>
> Then,
> Darin Sunley
> rsunley@escape.ca commented:
>
> >It was my impression that if one wanted a reasonably circular orbit, the
> only altitude one could
> >jump into orbit from is the altitude of goesynchronous orbit. With any
> lower altitude, you need a
> >lot more sideways velocity for a circular orbit then sitting at the top of
> a tower that tall gives
> >you.
>
> I emailed Darin a little note, to wit:
>
> >Spike didn't say that the orbit was circular. <snip>... the orbit is
> elliptical.
> >
> >You're right, that a jump from a geosynchronous-orbit-tall tower will give
> you a circular orbit. >In fact, all you have to do is raise your feet off
> the platform, cause, hey, you and the platform
> >are already in orbit. No jumping to it.
>
> Then spike replied (to Darin, not to me):
>
> >That's a good question, one that I want to work on. If one went up about
> an earth diameter on a
> >cable and let go, what would be the shape of that orbit? Also, I have
> sharpened my calcs a bit
> >and found that the whole question of climbing a cable and letting go to
> get into a minimal orbit
> >might be wrong: I found a flaw in my reasoning today.
>
> So I puzzled over it for a week, and found no flaw in spike's reasoning,
> or, to be more precise, no flaw in his conclusion. Quite elegant, actually.
>
> According to Kepler and Newton all planetary orbits are elliptical (a
> circle being a special case of an ellipse), with the center of the orbited
> planet located at one of the foci. If one is considering orbits around the
> earth, then the earth is at one focus. If one then imagines an earth-sized
> sphere adjacent to the earth, its center defines a second focus, and the
> furthest out point on the earth-sized sphere--the point diametrically
> opposite the point of contact between the earth-sized sphere and the real
> earth--can be chosen to define one point--the extreme end--of an ellipse.
> Then, by symmetry the other end of the ellipse just grazes the atmosphere
> at the far side of the real earth. Now, by Newton and Kepler, this ellipse
> is a valid earth orbit, and the condition of it grazing the atmosphere of
> the real earth makes it the minimum viable orbit for the condition of
> climbing the tower and just letting go. Thus, one comes up with the
> earth-diameter high tower that spike derived. Stepping off the tower at
> that height corresponds to entering the orbit at the apogee. Just grazing
> the earth at the other end is the perigee. Nifty.
>
> Best, Jeff Davis
>
> "Everything's hard till you know how to do it."
> Ray Charles
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