# The Cube

Discussion in 'General Science & Technology' started by Success_Machine, Feb 11, 2003.

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3. ### Success_MachineImpossible? I can do thatRegistered Senior Member

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A skyhook consists of a heavy mass orbiting the earth, which deploys a light tether a few thousand kilometers in length. The whole thing then rotates so that the velocity of the tip of the tether exactly cancels with the orbital velocity of the heavy central mass. The tether tip can thus enter the atmosphere at relatively low speeds and rendezvous with a cargo aircraft. If the tether is long enough there should be sufficient window to attach a payload. A longer tether also minimizes g-forces on the payload, and can almost eliminate atmospheric drag. I have designed a skyhook with a 4000-km long tether, made from carbon fiber.

The trade-off in momentum between the heavy mass (the Hub) and the payload being catapaulted from the tether tip is significant. If the Hub mass is 57,000 tonnes, then catapaulting a 50-tonne payload will lower its orbit by 5 kilometers. When the tether rotates around for a subsequent encounter with the earth's atmosphere it will dip 5 kilometers lower, maybe even crash into the ground. Either the Hub orbit must be re-boosted, or the tether retracted, before this happens.

I intend to do both. I have designed electrodynamic stationkeeping engines for the Hub, powered by 13-gigawatt ground-based beam power transmitters. These will allow the Hub to repel earth's magnetic field and boost itelf to it's original orbit, without using any propellant whatsoever. Therefore the Hub does not require any refuelling missions. However if there is a power failure then electrodynamic re-boost will not be available, and the tether itself must be retracted. It would take days to retract a tether 4000 km long, certainly hours to retract the lower 150 km so that it could 'miss' the atmosphere as it rotates.

One might say that a simple electric motor and winder should be adequate to re-wind the tether 150 km. But with a payload attached, plus the weight of the tether itself, there will be over 100 tonnes of tensile force in the tether. An electric motor would have to be geared very low to pull it in, and would work very slowly. For a skyhook that rotates every 66 minutes, it would have to rewind the tether at a rate of 300 km/hr to clear the atmosphere in the time between payload release and atmospheric re-entry. To address this problem I have designed THE CUBE .... it is a fast-retract system that augments the conventional winders.

Unlike an electric winder The Cube employs hydraulic pistons to actuate a super-sized 3-dimensional vice. The Cube expands in all three dimensions, the pistons forcing it apart. The tether criss-crosses the entire volume, so that as The Cube expands it absorbs an enormous length of tether, easily retracting 150 km of tether in under 30 minutes.

The Cube is key to the realistic implementation of a skyhook. It is possibly the most powerful machine component in the entire assembly, AND because it can offset a large change in the orbital altitude of the Hub - a much lower mass for the counterweight is possible. If The Cube can absorb 150 km of tether then it can offset 4-5 times the change in orbital altitude than the original 'dumb' mass of the counterweight.

The Cube allows us to use only 1/4th the original mass required for the counterweight. This can be launched and assembled in 1/4 the time, for 1/4 the cost. In simpler terms we can build a skyhook in ten years, instead of forty.

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7. ### Success_MachineImpossible? I can do thatRegistered Senior Member

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Yeah, they got me beat. They've got all the physics cold, and even have software with graphic interface to simulate their ideas. But I have a few better ideas than them. For example, there is no way you can rendezvous with a 'suborbital' craft at hypersonic speeds, all within a timeframe of a few seconds, which is what their idea is.... that's just suicide.

My idea is to make the tether longer to extend the rendezvous timeframe to a few minutes at least, and also to lower the rendezvous altitude to 10 km, where a conventional Boeing 747 can be used for the rendezvous. This aircraft has proven performance capabilities and a long successful service record. It is also relatively cheap, at $80 million bucks a piece. There are fleets of Boeing 747 aircraft around the world that can be brought into service any time. In contrast their hypersonic suborbital craft is still theoretical, dubious at best, would probably disintegrate or explode, or have several months between flights like the space shuttles, and cost$billions of dollars each.

My idea also includes a light unpiloted drone attached to the tether tip, with its own ramjet to help overcome the drag on the tether during re-entry, and keep the tether more-or-less straight, and at full length, so that the 10-km rendezvous altitude can be achieved in a timely manner. It will also provide maneuverability and control for the rendezvous scenario, which I think will be very difficult to achieve in under 5 minutes with just a Boeing 747 carrying a 50-tonne payload on its back.

Other than that, I'd love to get my hands on their mathematical models, but they haven't answered my emails!

8. ### JaxomTau ZeroRegistered Senior Member

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I'd still like to know the advantages of the skyhook over a stationary elevator. They require similar technologies structure-wise, but the elevator doesn't require extra dynamic technologies.

Heck of a laser, Fetus. How about a nuclear plant on the ground for launching with that, and then once in orbit, a nuclear engine for planet-planet.

Actually, I'd want all these ideas at once...

9. ### ElectricFetusSanity going, going, goneValued Senior Member

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Here Jaxom this gots the basics on just about every possible form of space propulsion: http://www.islandone.org/APC/
I must have recomended this site like 20 times now!

Hey Success_Machine
I orgional though you were crazy but now that you showed us your idea in detail I can see you are a good guy with big plans: I hope for the best for you. Still why is it called THE CUBE???

10. ### Success_MachineImpossible? I can do thatRegistered Senior Member

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A skyhook uses a shorter tether than a space elevator. My estimate for a skyhook is a 4000 km tether, whereas a space elevator needs 100,000 km tether. A shorter tether is more feasible than a long one because of strength issues. Materials are not yet available for a space elevator, but a skyhook just might be possible with adequate quality control technologies employed during manufacture to ensure maximum tensile strength and uniform elasticity are built into every inch of its length.

The tether itself is the main technological issue. A simple string won't do. Even a tether with a tapered thickness isn't good enough. Why? Because of the risk of micrometeorite impacts which could sever the line. During atmospheric entry the portion of tether that encounters atmospheric drag should be a tight cylinder (like a conventional rope). But the majority of the tether that is always above the atmosphere, more than 3800 km length, is exposed to vacuum continuously, and to space debris impacts.

The entire length of the tether must have a cross-sectional area that varies - thick near the root, and thin near the tip. But in space even a 4 cm-thick tether can be completely severed by a 5-cm space rock. The solution is to fashion the tether into a hollow tube at least 2-3 meters in diameter. That way even a 30-cm space rock can pass through it and compromise only a fraction of its cross-sectional area. The tether would remain functional, at least long enough to complete any launch of a payload that is in progress.

The tube tether must be repaired as well. That means removing severed fibers from the tether and replacing them with new ones. The most obvious way of doing this is to retract the tether, and perhaps change it out with a new one during a shuttle re-supply mission. But we could instead fashion the tether so that it has load-bearing nodes spaced every 400 km along its length. That way at most only a 400 km long fiber would have to be replaced. A robotic climber could do that, or a partial tether retract with repairs carried out with robotic arms could be accomplished. What's more, we don't want 400 km-long fibers hanging loose causing havoc, so quite frequently there should be non-load-bearing nodes built into the tube tether. These would essentially hold the severed fibers in place, allowing perhaps only a few kilometers to dangle around.

This is complicated, but there are so few people working on the problem. If you look at the space shuttle, which is considered to be the most complex machine ever built, you will find thousands of people dedicating their careers to making it work. If you throw those kind of resources at something as seemingly complex as a skyhook you will probably see it work successfully.

But back to the space elevator versus skyhook debate....

What if you wanted to build a space elevator on the moon? Or Venus? These worlds have rotation periods ranging from a month to a year. There isn't enough rotation to keep a space elevator up, since a space elevator requires a geosynchronous position, AND rotation to keep the tether taut. Also, a space elevator does not deliver a payload to orbital velocity *unless* it is released above geosynchronous altitude. And with a space elevator it would take a few weeks at least to get there. A skyhook delivers a payload to orbit in a few hours. It's sole purpose is to get the payloads above the atmosphere. Once there it can employ super-efficient electrodynamic tethers or ion engines which cannot be used to generate high thrust for launch. Those super-efficient engines can take the payload anywhere, to geosynchronous or beyond, nearly as efficiently as a space elevator.

But the main problem with a space elevator is tether strength. There are a few possible candidates for a skyhook, but there are none currently available for a space elevator. The only one being proposed is carbon nanotubes, and a recent announcement by Chinese researchers who managed to draw 30 cm-long nanotube strands, were only able to do so by simple entanglement of filaments. The resulting strand was over a million times weaker than the constituent fibers. There was no chemical bonding taking place between the nanotubes that would create superstrong tether.

As for THE CUBE, it is so-named because it is shaped like a cube, and remains cube-shaped even when it changes size.

Last edited: Feb 18, 2003
11. ### JaxomTau ZeroRegistered Senior Member

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Agreed, that's a nice site, but it doesn't compare the two. Look at highliftsystems.com and you'll see the difference in data. The only big question mark on their mind is how lightning, atmosphere, and sprite interaction will affect durability. I just see the tethers as adding guidance and propulsion issues into the mix, but what added benefits does it give?

Success, your general idea of having the structure do the big part of the necessary lifting is a good addition to the skyhook. But you have to sell me the skyhook over the elevator first.

Edit: answered me while I was asking. Good points, especially about other worlds not having the rotation needed.

Okay, given the materials needed, the main point for Earth usage would be a faster velocity for escape. Current guesses are 3 days to LEO I believe, which is better than current waits to orbit. Yes, you'd need either addition propulsion, but that's much easier at that starting point than at ground.

They both have the impact issues to deal with. I liked your idea of sectioning it to allow replacement if that's necessary.

Either way, the creation would be an improvement on conventional launches.

Last edited: Feb 18, 2003
12. ### Success_MachineImpossible? I can do thatRegistered Senior Member

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The Cube.... it is like a huge bulging bicep muscle, flexing. Engineered machismo!

13. ### JaxomTau ZeroRegistered Senior Member

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Resistance is futile, I guess...

14. ### ElectricFetusSanity going, going, goneValued Senior Member

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I think what is needed now is as a test of concept idea to get people to start forking out the money. What is needed now is a Tug boat: a small (2000KG max) satellite that uses Electrodynamic propulsion to grab cargo (Other Satellites) in low unstable orbit (>125Km) and move them to a higher orbits. (200-1000Km) Though this would at best reduces launch cost by 10% the Tug Boat could also grab space junk and drop it off at very low unstable orbits so that the space junk will burn up in earth’s atmosphere.

15. ### JaxomTau ZeroRegistered Senior Member

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This is a damn fine idea! Given that we have way too much debris right now in higher orbits, and I believe that some of it may have contributed to Columbia's destruction, what a great way to solve both research and trash removal.

16. ### ElectricFetusSanity going, going, goneValued Senior Member

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Last edited: Feb 18, 2003
17. ### JaxomTau ZeroRegistered Senior Member

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A geostationary object, such as the space elevator, wouldn't generate a current right, because the field rotates with the Earth?

I had know the tether experiment had problems but still worked, but I didn't know they found it worked better than planned. Great for Earth satellites, but not those of the Moon and Mars.

The idea of attaching or rolling out a tether to slowly drag a satellite out of orbit is fine, but it's not the big satellites that are the problem, it's all the pieces of satellites...the dilemma would be finding them, determining a suitable approach, and how to connect. Metal is easy, but what of plastics and such. A grappling arm? But the big advantage of the "junkhook" approach is that you can make the effect a quick one. Where drag could take years, during which the satellite could get hit and contribute to the problem, the "junkhook" will grab it and send it straight down into the unstable orbit.

I had read years ago that there was some "critical number" of pieces in orbit, banging into each other and making more pieces, that upon hitting it nothing can survive, and we have basically a danger zone trapping us. Anyone know about this, or how close we are?

18. ### ElectricFetusSanity going, going, goneValued Senior Member

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Last edited: Feb 18, 2003
19. ### JaxomTau ZeroRegistered Senior Member

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Would such a thing be constantly rotating, or begin rotating only after it positions itslef with a target? If always rotating, how do you use the EM to move around? If not, how much and with what would you begin the rotation?

I had envisioned a central piece at the center of rotation which would do the moving around, although I was thinking more conventional propellants or ion driven.

What limits would this have as far as small junk? Maybe in addition you could collect smaller pieces in one container via a physical/nonphysical net, and then when loaded begin the spinning to release towards the Earth.

Also, for the radioactive objects, you could target the sun instead of earth. Doesn't have to be a quick route, as long as it's accurate.

20. ### ElectricFetusSanity going, going, goneValued Senior Member

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It does not rotate! It sticks its tether strait down toward the earth and pushes against the earth's magnetic field for thrust. Perhaps I should make a diagram...

Here is some info on how it works:

21. ### JaxomTau ZeroRegistered Senior Member

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I get it...I was still thinking along the idea of using testing of skyhook techniques in removing debris. We're talking two different methods to acheive the same result.

Would this tug be able to move junk very quickly, or are we talking ion drive type additive forces over long times? That's why a catch-fling device in my mind would be more effective, because it could in one motion set the junk on its way quickly, and move on.

22. ### ElectricFetusSanity going, going, goneValued Senior Member

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"Johnson of Marshall's Advanced Systems and Technologies Office predicts that a 10 km (6 mi), 10 kilowatt tether system could boost a 1,000 kg (2,200 lb) satellite as much as 400 to 540 km in one day, depending on the orbit and other conditions."

We could say between 60degree and 0 latitude: a week to get up to a target and a week to bring it down. Also the Tug could bring space junk and old satellites to ISS or the Space Shuttle for repairs and/or proper disposal. Also if the space junk is too small for the Pneumatic mechanical tentacles a bag could be used with a double closing motorized neck as a kind of throat and mouth assemble for grabbing junk and swallowing it into a large bag. This way lots of small junk could be stored in a large bag to be brought to ISS, Space shuttle or very low orbit to burn up.

Last edited: Feb 18, 2003
23. ### GiftedWorld WandererRegistered Senior Member

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How long would it take to knock a suitable asteriod into orbit?