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March 09, 2010, 02:29:03 PM *
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Wayne Kinne
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« on: May 21, 2009, 06:57:42 PM »

Perhaps this has been covered before, but why not loop the ribbon so cars need not to be powered. Just lock them to the moving ribbon, power it from space and from the ground. One side of the ribbon is going up while the other side is going down. Slowly break rollers on the cars so they slowly pick up speed so there is no sudden jerk on lift-off. Maybe counter weights could come down to make lifting real heavy loads easier. I know it would require a ribbon twice as long but it would rid the cars of needing to be powered.

I'm not an engineer, just wondering.

wkinne
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neil
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« Reply #1 on: May 22, 2009, 05:12:11 AM »

The main problem is even optimistic specs for CNT = carbon nano tubes are not strong enough. We can't taper the ribbon in a loop, more than slightly. We need to taper for CNT with a GPa of less than about 1000. We are hoping GPa = 60 ribbon will be available soon. Another problem is the ribbon stretches making the high tension side of the loop longer and thus likely to tangle unless skillfully restrained.
A pair of space elevators may be practical with the climber switching perhaps hourly to the ribbon with a stretch transient moving the correct direction. A long ago thread supposed that standing waves could be induced in the ribbon to move the climbers rapidly, but no one seemed sure that was workable.   Neil
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A_M_Swallow
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« Reply #2 on: May 22, 2009, 02:16:09 PM »

It is about time we built a model space elevator that uses an oscillating ribbon to raise the climber.  It only needs to be room size.  The ribbon could be piano wire or guitar string.
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Andrew Swallow
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« Reply #3 on: May 22, 2009, 03:21:15 PM »

I suspect that this looping ribbon thread was triggered by this article.  An interesting new type of elevator.
http://nextbigfuture.com/2009/05/improving-space-elevators-by-having.html
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Andrew Swallow
neil
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« Reply #4 on: May 31, 2009, 04:18:27 AM »

Hi A_M: I looked at the next future link, but most was unclear to me. Unless energy is added frequently, by accelerating the loop with a pulley at the Earth surface end, it is a perpetual motion machine = too good to be true. To over come gravity, air resistance and drag up the lifter and payload, requires the up going string to be pushed = compression. Small scale, a string like a long ago speedometer cable could do that, small scale = less than one kilometer is my guess. The pushed string would get very hot in Earth's atmosphere at more than about 1000 kilometers per hour. I suppose the string would decompress above earth's atmosphere increasing the speed slightly. Near the highest altitude, the push is changed to horizontal allowing a small portion of orbital speed in the desired direction to serve as the first stage to orbit. Re-entry of the string in the down portion, stretches the string increasing the speed, perhaps to the point that active or ablative cooling is necessary. Did I get any of that correct?   Neil
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A_M_Swallow
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« Reply #5 on: May 31, 2009, 06:16:28 AM »

A big motor and winch on the Liftport to power the cable will be needed.  I have not looked into friction on this particular version.

The ribbon can be pulled down and the movement allowed to travel all the way around the ribbon, producing an upward pull.  I do not know if this works without a counter weight.
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Bob Munck
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« Reply #6 on: June 04, 2009, 07:29:17 PM »

Unless energy is added frequently, by accelerating the loop with a pulley at the Earth surface end, it is a perpetual motion machine = too good to be true.
My solution was to add energy with a solar-powered station at some distance above geosynchronous orbit on the upward-bound leg of the loop.  It would in effect be climbing the ribbon (toward GEO) continuously and just at the appropriate speed to keep it motionless. A neat design, IMHO; solar cells, motors, rollers convert solar energy into mechanical energy in one simple step.

In my sketchy design, the anchor end was going through a big pulley at ground level, but was not being accelerated.  That allowed the pulley to consist of a film of compressed air with nothing actually touching the ribbon. The other end of the loop, far beyond GEO, didn't need a pulley or anything at all; the loop could just hang there, pulled outward by centrifugal force. I also described fairly tricky mechanisms for getting payloads on and off the ribbon at GEO and ground-level; not an easy task if the ribbon is moving at all fast.
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neil
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« Reply #7 on: June 06, 2009, 12:18:25 PM »

Hi Bob, It is great to see you posting again. That seems workable, except the ribbon can be only slightly tapered. I'm thinking parts of the ribbon can be thin if we avoid jerk and other loading when these are in the atmosphere, near your air pully or the climber which is accelerating the stronger portions of the ribbon. I especially like the air pulley whose radius is adjustable to take up the variable slack and help modify transients. Would it not also be practical to accelerate at least a little at GEO altitude = 36,000 kilometers? Multiple stationary climbers between the far end and GEO should be practical to spread the stress and provide redundancy, especially if the far end is at 144,000 miles or a bit more.  Neil
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Bob Munck
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« Reply #8 on: June 06, 2009, 07:45:56 PM »

That seems workable, except the ribbon can be only slightly tapered.
No, no.  No taper at all. My design is a single loop, maybe 300,000 km of ribbon that all goes all the way around. So there's no part of it that's never under the maximum strain, and it can't be tapered.  This means it needs a very strong ribbon, maybe (probably) too strong to be possible.

Nonetheless, I occasionally consider gathering together all the parts of the Moving Ribbon design that I did in the spring of 2005 and making it into a coherent document. It had some neat ideas, including the perpetually-climbing power supply and a big rotating ring at GEO between the upward and downward parts of the loop. The hardest part, for which I never had a solution I was satisfied with, was getting on the ribbon at ground-level. You really have to accelerate to the speed of the ribbon (1000 kph) before grabbing onto it, and it's going straight up.

The moving ribbon seemed to me to be the only usable way to get people to GEO. At that speed it's only a 36-hour trip, so you get through the radiation belts with all due haste.
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A_M_Swallow
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« Reply #9 on: June 08, 2009, 04:19:02 AM »

Plenty of jet engines can reach 1000 kph, it is less than the speed of sound.

The looping ribbon is presumably being powered from the Liftport.  Add a tall tower to the Liftport and an electrically powered vehicle could reach 1000 kph.  After the cargo/passengers have been clamped to the ribbon the vehicle releases the capsule and returns to the ground.

Height of the tower for 3 g acceleration
v2 = u2 + 2 a s
giving s = (v2 - u2) / ( 2 a) = ((1000*1000/(60*60))^2 - 0)/(2 * 3 * 9.81)
= 1311 meters tall

That is too tall for a ship so the first stage vehicle would have to climb a land based tower or the side of a mountain.
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Andrew Swallow
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