Rotating Space Elevator Propels its Own Load

May 21, 2009 by Lisa Zyga feature
(Left) A rotating space elevator and (right) a uniform stress rotating space elevator (an RSE combined with an LSE). The rotating systems can propel “sliding climbers” up the loops using the centrifugal force, similar to how stirred coffee can rise and splash out of a cup. Image credit: Golubović and Knudsen.

(PhysOrg.com) -- The idea of the space elevator just got a little crazier. While the “traditional” concept involved using rocket propulsion or laser light pressure to propel loads up a cable anchored to Earth, a new study shows that a rotating space elevator could do away with engines or laser light pressure application completely. Instead, the unique double rotating motion of looped strings could provide a mechanism for objects to slide up the elevator cable into outer space. The space elevator could launch satellites and spacecraft with humans, and even be used to host space stations and research posts.

Ever since Konstantin Tsiolkovsky first proposed the idea of a giant string connecting the and the heavens in 1895, scientists and sci-fi writers alike have been intrigued. But it seems no one has yet suggested that a space elevator could propel its own load, without the need for internal engines, pressure, or any kind of propulsion. In a recent study funded in part by , theoretical physicist Leonardo Golubović and his graduate student Steven Knudsen at West Virginia University have explained that this concept is possible, at least in theory.

Golubović and Knudsen have introduced the Rotating Space Elevator (RSE), a rotating system of a floppy string that forms an ellipse-like shape. Unlike the traditional Linear Space Elevator (LSE) made of a single straight cable at rest, the RSE rotates in a quasi-periodic state. As the scientists explain, RSE motion is nearly a geometrical superposition of two components: its geosynchronous rotation around Earth (which has a one-day period), and the internal rotation of the string system that goes on around the axis perpendicular to the Earth (about a 10-minute period). This internal rotation of the string is especially important, as it provides a mechanism for objects to freely slide along the string, and also provides the dynamical stability to maintain the elevator shape.

“The idea came by itself,” Golubović told PhysOrg.com. “I was thinking how to make things move easily and quickly up the traditional Tsiolkovsky-type space elevators. In my kitchen, I was mixing coffee in my cup too vigorously and the centrifugal force on the rotating coffee won over gravity to make some of the coffee lift and splash out the cup. This was my ‘eureka’ that lead to adding a similar conceptual feature to the old space elevator idea, the internal rotation. Indeed, much like the coffee would lift and splash out the cup if rotated fast enough, the climbers on our Rotating Space Elevator will be lifted up by the centrifugal force winning over gravity.”

Like the LSE, the RSE is attached to the Earth to provide loading access. To initiate the double rotational motion, the string system is given an initial spin. Other than this initial spin, the RSE moves purely under the influence of inertia and gravity. In simulations, Golubović and Knudsen show how a load starting at rest near the Earth spontaneously oscillates between its starting point near Earth and a turning point in outer space (close to the top of the string). Using a specially chosen variation of the tapered elevator cross-sectional area, the scientists could ensure that the RSE string will indefinitely maintain its initial looped shape. Golubović said that, as far as he knew, this type of motion does not occur in any other areas of physics or astronomy.

“There are no known astronomical size solid (or floppy) objects that are shaped as strings,” he said. “It thus remains for humans to make them. As stated by a referee of our paper ‘No one is an expert on the ideas displayed in this paper.’ At this moment, the only judge is pure mathematics and it shows that our RSE can work, in a fascinating fashion.”

Golubović and Knudsen also proposed a slightly different form of the RSE, which combines an RSE with an LSE (an ellipse-like rotating string is attached to a linear string). This “uniform stress RSE” (USRSE) could be designed with its loop positioned above the Earth’s surface, which might have advantages for launching satellites. The scientists also show that stacking several USRSE loops could create pathways reaching deeply into outer space, and loads could cross from string to string at intersection points.

Through simulations, the scientists investigated the effects of changing the initial period of the RSE. They found that this alteration causes a phase transition, creating tension that causes the elliptical RSE shape to narrow into two linear-type space elevators that fluctuate nearly independently. Over a period of two weeks, the RSE’s angular momentum decays to zero, causing the RSE top to drift away from its initial position to a higher position around which the RSE top continues to oscillate in a chaotic fashion.

When asked how far away current technology is from actually building space elevators or rotating space elevators, Golubović was hopeful. “Our USRSE is technologically not more demanding than the traditional space elevator that can be indeed made to stand high tensile pressure by using the existing carbon nanotube composite materials,” he said. “I hope my children or grandchildren will see both the traditional and rotating space elevator one day, if not on the Earth than on other planets such as Mars, which is free of man-made space debris that can hurt space elevators.

“Moreover, some new and related ideas may come up soon to aid space elevator technology. It’s not all about materials being used but rather about structures being employed and new ideas being propagated. The very concept of ‘simple rotation’ is not so simple as commonly thought. Our RSE is a good example of this within the non-relativistic theory of inertia and gravity. Other fascinating examples involve rotating black holes in general relativity that are (theoretically) known to act as time machines. Fortunately, unlike the black holes, the non-relativistic physics of our rotating space elevator is within modern-day technology limits.”

More information: L. Golubović and S. Knudsen. “Classical and Statistical mechanics of celestial-scale spinning strings: Rotating space elevators.” Europhysics Letters, 86 (2009) 34001. doi: 10.1209/0295-5075/86/34001.

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User comments : 33

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Nik_2213
5 / 5 (5) May 21, 2009
An animation would be nice: My "mind's eye" baulked at this one...

Um, is this at all related to the proposed Martian space-elevator, which must 'wobble' like 'Galloping Gertie' bridge to avoid Phobos & Deimos ??
Myria83
5 / 5 (4) May 21, 2009
Fascinating idea...
Sean_W
3.3 / 5 (7) May 21, 2009
It will never beat yogic flying for cost efficiency.
earls
5 / 5 (1) May 21, 2009
Brilliant.
Yogaman
3.2 / 5 (9) May 21, 2009
Proof-reading?

"This was my eureka that led" not
"This was my eureka that lead"

"if not on the Earth, then on other planets" not
"if not on the Earth than on other planets"
googleplex
2.6 / 5 (5) May 21, 2009
I recall that the last space tether experiment was severed by space junk. Presumably a longer string going all the way to Clark orbit would have an even greater chance of sever. Without some defense mechanism the idea any space elevator is unfeesible due to the high level of earth orbit pollution.
superhuman
5 / 5 (5) May 21, 2009
Interesting idea although it seems rather unrealistic.

The rotation with a period of 10 minutes and the radius of rotation of 3000km (judging from the left picture) means linear speed of 2*pi*3000 =18850 km / 10min = 113000 km/h, so the middle section of the string on the left picture would have a velocity of 113000 kilometers per hour or 92 machs which sounds rather absurd, air drag would generate extreme amounts of heat which would most likely be impossible to deal with.

There's also the problem of how do we accelerate 60000 km of flexible string to such speeds.
Arkaleus
4 / 5 (5) May 21, 2009
Googleplex - The last tether experiment wasn't severed by space junk, it had too much current flowing through the cable and it either melted or was jettisoned automaticaly as a safety precaution.

This sounds fine when you don't consider the tidal forces of the moon and other planetary bodies. The question becomes, how to you adjust the ocillations of this structure so its period is properly maintained and it doesn't come crashing down in a marvellous heap, or flings it's passengers into a trajectory that re-enters the atmosphere?

It sounds like trying to spin a hula hoop while riding a unicycle on a DJ's turntable.
thales
3.5 / 5 (2) May 21, 2009
Interesting idea although it seems rather unrealistic.

The rotation with a period of 10 minutes and the radius of rotation of 3000km (judging from the left picture) means linear speed of 2*pi*3000 =18850 km / 10min = 113000 km/h, so the middle section of the string on the left picture would have a velocity of 113000 kilometers per hour or 92 machs which sounds rather absurd, air drag would generate extreme amounts of heat which would most likely be impossible to deal with.

There's also the problem of how do we accelerate 60000 km of flexible string to such speeds.


Thanks for putting some numbers together, although I haven't checked them. I would imagine that enclosing the whole thing in another tube would allow the space to be vacuumed out to eliminate air drag.
dirk_bruere
4.2 / 5 (5) May 21, 2009
Anyone considered a simple pulley system? Admittedly, it would require a double tether, but all the power problems disappear.
weewilly
5 / 5 (2) May 21, 2009
I am all for thinking outside of the box but in this case is there a box to begin with?
Yogaman
5 / 5 (2) May 21, 2009
Full text with more diagrams is currently online at http://www.iop.or...001.html

From it we can calculate that the illustrated USRSE (right figure of this physorg report) with 200 mile (322 km) radius and approx 6.25-minute period travels "only" 1256/6.25 = 12000 mph (19300 km/h) or about 2.7 miles/second (4,35 km/s) at its widest point about 80 miles above the earth. This is what the author says makes his USRSE better than a plain-old left-figure RSE which provides similar auto-lift capability but with impractically higher materials stresses.

But even if the USRSE really can be built, its fast, wide rotation will make dodging LEO GPS, surveillance, communications and resource-monitoring satellites more difficult, I suspect. And let's also not overlook the vastly increased impact on migratory birds.

dirk: this is similar to a pulley; the load goes up one side and down the other returning to its origin 6.25 minutes after leaving in the USRSE example. Mind your step please.
Yogaman
5 / 5 (1) May 21, 2009
Oops. should be 3.3 miles/second or 5,36 km/s.
docknowledge
5 / 5 (1) May 21, 2009
I recall that the last space tether experiment was severed by space junk. Presumably a longer string going all the way to Clark orbit would have an even greater chance of sever. Without some defense mechanism the idea any space elevator is unfeesible due to the high level of earth orbit pollution.


A little inexpensive research doesn't hurt, so that one day it may be possible. And while a space elevator on Earth may be subject to the slings and arrows of fortune, one on Mars, for example would not.

Another point. For decades now, NASA and private companies have been justifying rockets primarily on the basis of "lower cost per pound to orbit". I don't know what percent hardware costs are of missions, now, but they were rapidly shrinking in the 1980s. E.g., a good part of the shuttle cost is software. (40%?) As robotics improve, and miniaturization progresses, the importance of launch weight costs goes down. I.e., by the time space elevators become practical, they may not make very much difference.
mongander
5 / 5 (1) May 21, 2009
I never could understand the old string theory,
much less a looped one.
sender
5 / 5 (1) May 22, 2009
Old idea called a space fountain and it doesnt require large cables, everything wireless and still self powered.

http://en.wikiped...fountain
Noumenon
4.9 / 5 (48) May 22, 2009
Will never happen, pointless.

I like how the article drops needless physics catch phrases ,... 'superposition', 'quasi-periodic', 'phase transition', and then they mention black holes for some reason.

Thales, you would need a tube to surround the tube, then another to surround that tube, etc.
DGBEACH
3 / 5 (2) May 22, 2009
I recall that the last space tether experiment was severed by space junk. Presumably a longer string going all the way to Clark orbit would have an even greater chance of sever. Without some defense mechanism the idea any space elevator is unfeesible due to the high level of earth orbit pollution.
And what an interesting addition a kilometers-long piece of this would be to the soup of trash above us.
E_L_Earnhardt
3 / 5 (2) May 22, 2009
Spend your OWN money on this thing, not mine! We already HAVE one Saturn. This could make another!
GSwift7
3.7 / 5 (3) May 22, 2009
Very creative. For people having a hard time visualizing, perhaps think of it like a rodeo rope trick. You hold the top of the rope and spin the loop at the bottom. The middle bulges out and draws the loop at the bottom up off the ground.

As for adjusting to various tidal forces, the system would be fairly robust I believe. Any length change should drive a rotation rate change to conserve momentum.

Obviously not good under Earth conditions, but on a body with less gravity, and preferably a less dense object too, so that the ratio of the body's radius over the gravitational force is smaller, and without an atmosphere, it could be practical. There are bodies in our solar system that match those criteria. Mars meets the criteria of being less dense and less massive than Earth, and much less atmosphere. However I have a feeling you really need a small moon or asteroid sized object with a good rate of spin for this idea to be workable in the near future. Our own moon is out because it's tidally locked with earth, so there's no good gosync orbit, as are many other bodies with a good size/atmosphere match.

Nice thinking though.
Ant
3 / 5 (2) May 22, 2009
ridiculus probably some university 3rd year project again. Mind you some people think the indian rope trick is feasable.
dan42day
3 / 5 (2) May 23, 2009
I kind of like dirk's simple pulley system.In addition to allowing the lifting force to be applied at the ground level, with a few minor complications, it could provide redundancy for safety and re cabling.
vlam67
3 / 5 (2) May 23, 2009
Until we can get terrorism under control, religious nutcases will make the costs of providing security to such megastructures way more than what is required to keep them running.
Doschx
5 / 5 (1) May 23, 2009
Anyone considered a simple pulley system? Admittedly, it would require a double tether, but all the power problems disappear.


I kind of like dirk's simple pulley system.In addition to allowing the lifting force to be applied at the ground level, with a few minor complications, it could provide redundancy for safety and re cabling.


unfortunately it can't be that simple, though we all wish it could. The string in the pulley would need to support it's own weight, as well as it's payload, and when we're talking in kilometers, the string's weight itself can no longer be negligible. As yet, there's no substance with a tensile strength high enough to overcome the tension that would be put upon it, thus the evolution of the space elevators, space fountains, and all related material sciences. Could probably thank the space elevator for a good bit of our research efforts into carbon nanotubes though.

IF this spinning idea is feasible, which, given the coffee cup example seems a bit unlikely in my mind (coffee goes up the sides because centrifugal force induces a higher pressure on the outer liquid and since there's constrained volume in all directions but up, the liquid goes up. The coffee cup has to meet and exceed the pressures put on it and again, when we're talking in the kilometers range i think it all breaks down) but if it could work, I think the best location would be directly on top of either the north or south axis of rotation (north pole, though not magnetic north lol) so that whatever structure that induces the rotation can be augmented to some degree by earth's own rotation.

sudler2008
5 / 5 (1) May 24, 2009
Anyone considered a simple pulley system? Admittedly, it would require a double tether, but all the power problems disappear.


Agreed. The space elevator should follow a simple "cable car" system similar to the cable cars operating in San Francisco. A ribbon would be looped between Earth and a GEO positioned satellite. The ribbon loop would be rotated by a set of powerful motors located on Earth. Space elevators would simply grip the ribbon and be carried up by the ribbon (or down on return trips). This cable car method allows engineers to maintain control over the ribbon's speed and direction. If the line ever needs to be slowed, accelerated, stopped or reversed, the speed of ribbon can be changed on demand. The method would also be less expensive than laser powered LSEs, since the configuration would no longer need a climber on the elevator, lasers to power the elevator, tracking systems to keep the laser(s) trained on the elevator, or solar panels which are powered by the laser(s). In short, the cable car method would be a simple grip and release system.
smiffy
5 / 5 (1) May 24, 2009
I went off the idea of space elevators when I realised that they were a very expensive, use-once, lightning-conductor.

Maybe on the Moon, for lifting Tritium.
kevinf
5 / 5 (1) May 26, 2009
Cable cars to the stars....That's the answer. Telsile strength, my tensile.....Yes, the cable needs to support the weight of itself AND the car. But the car as a proportion to the total weight of the cable is negligible. If the cable is strong enough to hold itself, it will hold a car, and possibly several.
yOnsa
1 / 5 (1) May 27, 2009
I can not for the life of me understand how this could work. I need an animation or a better explanation. So the rope is spinning at mach 92, how the hell does anyone get on or off? how does is stay in one spot while the earth is spinning? i am not convinced this is practical. and after we build this giant rope string, who's going to get it spinning? any ideas?
GSwift7
1 / 5 (1) May 28, 2009
[quote]I think the best location would be directly on top of either the north or south axis of rotation [/quote]



That won't work. The top of a space elevator needs to be in geosyncronous orbit above its connecting point. You can't have a geosyncronous orbit anywhere except the equator. It must spin with the earth. You have to use orbital inertia to hold the top of the elevator up. That's one of the problems with space elevator concepts on earth. Not only do you need to creat a cable strong enough to hold its own weight, but also withstand the mementum of the counterweight used to hold it up. The numbers add up to astronomical quantities with the physical properties of Earth. It's just never going to be practical here, even if the materials are invented that can take the loads. The execution of its construction would nearly require a miracle by the hand of god. The only way to get a cable to stretch from the earth to geosyncronous orbit is by taking the whole thing up in a rocket then unwind it and let the bottom end fall back to earth while simultaneously extending your counterweight into a higher orbit to balance the pull on the bottom end. The bottom end would have to be loose as it enters the atmosphere and would be exposed to high speed winds in the middle layers of the atmosphere (10-15 km, where the jet stream dominates). Rapid changes in wind speed and direction would be a nightmare to try to compensate for on the spacebound end of the cable. You would have to deal with constantly changing, unbelievable amounts of force. The jet stream has wind speeds typically 100 kph and can get up to nearly 400 kph. Various layers of the atmosphere will pull in different direction in unpredictable ways as well. Can you imagine what the bottom end of the cable would be doing when the middle of the cable is being pulled sideways like that? Imagine holding a long piece of string while several people gently push and pull on the middle of the string in different directions. Both ends of the string are going to move closer and farther away from eachother, and each time you change directions from lengthening to shortening, there's a jerking force on the entire length of the string. You would need a cable that could support wieghts much higher than just the static operational loads, and certainly you would need expansion/contraction joints all along the length. Even a very thin atmosphere like the one on Mars probably makes the process nearly impossible. The problem of providing power to get payloads up and down the cable is minor compared to the other engineering challenges.
ShotmanMaslo
1 / 5 (1) May 28, 2009
here are some good ideas:
http://en.wikiped...nch_loop
EM launch:
http://en.wikiped...s_driver
KBK
not rated yet May 28, 2009


It sounds like trying to spin a hula hoop while riding a unicycle on a DJ's turntable.


I was really drunk at the bar one night one night..and... well after that attempt.... they threw me out by opening all the doors - with my face.

I never did find my pants.

~~~~~~~~~~~~~~~~~~~
Read Arthur C. Clarke's 'The Fountains of Paradise' for an awesome Sci-fi story about a space elevator.
smiffy
not rated yet May 29, 2009
Gswift7:
My TV-gleaned Geography tells me that Jet Streams are a feature of the mid-latitudes (40-60 deg) and that the winds are usually light to non-existant (the doldrums) around the equator where the elevator would be anchored. Though equatorial thunderstorms are hardly unknown.
Doschx
not rated yet Jul 02, 2009
That won't work. The top of a space elevator needs to be in geosyncronous orbit above its connecting point.


Haha you're absolutely right about that Gswift7. Answer: ride a magnetic tornado like i saw in another article... or pray... or scrap the elevator idea completely. Wind messes everything up, so what should we do? create a vacuum tube to orbit and just fire shit out of a railgun at escape velocity in it. lol


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