Posted on 02/15/2006 10:24:11 AM PST by Neville72
In January, LiftPort team members deployed a mile-long tether with the help of three large balloons in the Arizona desert (N Aung/LiftPort Group)Related Articles A slim cable for a space elevator has been built stretching a mile into the sky, enabling robots to scrabble some way up and down the line.
LiftPort Group, a private US company on a quest to build a space elevator by April 2018, stretched the strong carbon ribbon 1 mile (1.6 km) into the sky from the Arizona desert outside Phoenix in January tests, it announced on Monday.
The company's lofty objective will sound familiar to followers of NASA's Centennial Challenges programme. The desired outcome is a 62,000-mile (99,779 km) tether that robotic lifters – powered by laser beams from Earth – can climb, ferrying cargo, satellites and eventually people into space.
The recent test followed a September 2005 demonstration in which LiftPort's robots climbed 300 metres of ribbon tethered to the Earth and pulled taut by a large balloon. This time around, the company tested an improved cable pulled aloft by three balloons.
Rock solid To make the cable, researchers sandwiched three carbon-fibre composite strings between four sheets of fibreglass tape, creating a mile-long cable about 5 centimetres wide and no thicker than about six sheets of paper.
"For this one, the real critical test was making a string strong enough," says Michael Laine, president of LiftPort. "We made a cable that was stationed by the balloons at a mile high for 6 hours…it was rock solid."
A platform linking the balloons and the tether was successfully launched and held in place during the test. LiftPort calls the platform HALE, High Altitude Long Endurance, and plans to market it for aerial observation and communication purposes.
But the test was not completely without problems.
The company's battery-operated robotic lifters were designed to climb up and down the entire length of the ribbon but only made it about 460 m above ground. Laine told New Scientist that the robots had worked properly during preparatory tests and his team is still analysing the problem.
Carbon nanotubes In March, LiftPort hopes to set up a HALE system in Utah's Mars Desert Research Station and maintain it for three weeks. Then, later in the spring, Laine says he wants to test a 2-mile (3.2-km) tether with robots scaling to at least half way up.
Laine aims to produce a functioning space elevator by 2018 – a date his company chose in 2003 based on a NASA Institute for Advanced Concepts study, which said an elevator could be built in 15 years. "This is a baby step, but it's part of the process," he says of LiftPort's recent test.
The idea is to build the actual elevator's ribbon from ultra-strong carbon nanotube composites and to have solar-powered lifters carry 100 tonnes of cargo into space once a week, 50 times a year.
Beams and climbers Laine sits on the board of the California-based Spaceward Foundation, which partnered with NASA to put on two space-elevator-related competitions that were the first of the agency's Centennial Challenges programme – the Tether Challenge and the Beam Power Challenge.
The first is designed to test the strength of lightweight tethers while the beam challenge tests the climbing ability and weight-bearing capability of robots scaling a cable. Laine’s team is not competing in the NASA challenges so there is no conflict of interest.
In October 2005, none of the competition entrants performed well enough to claim the twin $50,000 purses. But the challenges are scheduled to take place again in August 2006 with $150,000 top prizes. Nineteen teams have signed up for the beam power challenge so far and three will compete in the tether challenge.
Ben Shelef, founder of the Spaceward Foundation, hopes the competitions will drum up interest and drive technological innovation. He told New Scientist he is pleased to hear of LiftPort's successful test. "A journey of a thousand miles starts with a single step," he says.
Correct. So you accelerate it and cause a tension on the cable, sizing the weight (mass) to equal the mass of the ribbon plus the mass of the payload. When balanced, you should be able to disconnect the ribbon from the ground (not that you would).
You CAN put the counterweight higher than geosynchronous orbit. Yes, a free flying object could not sit in a geosynchronous orbit above that height, but in this the tether (cable, ribbon, whatever) is restraining it. It is essentially JUST LIKE twirling a weight around on a string - except that it also has gravity pulling it towards earth, which reduced the tension necessary to keep it in that "orbit" (orbit not really being the right term for an object at the end of a cable.
Seriously, guys, people have sat down and worked out all the equations, static and dynamic that would affect such a system, and in theory it could work. See the link I posted for an example of such. With the right tools and knowledge this system, including having a car climb the elevator, can, and I'm sure has been, modeled on a computer, showing exactly what the behaviour of the system would be. If you want to examine the equations and figure out exactly what incorrect assumptions they've made or factors they've neglected, please do. Otherwise I'm assuming that it's the critics here who do not have a complete grasp of the dynamics of this system.
Gasp!
You mean that doesn't happen to the rest of you?
Incorrect. The velocity of the objects at a higher altitude would have to be greater to maintain it's position relative to the ground. This increase in velocity would normally send the object out of stable orbit. However this would not be a normal satellite. The cable would be preventing it from leaving its stable orbit. This is a Geostationary orbit at an altitude that would not be possible without it being tethered to the ground. The tension on the cable would be determined by the mass of the satellite and it's altitude. Because of it's velocity, it would always be trying to move away from the earth.
You would set it as needed depending on how much weight you wanted to lift, and how much tension needed in the cable to resist bending from the horizontal forces imposed on it by accelerating the payload as it moved up the cable. Those horizontal forces can be controlled by the speed at which a given payload is elevated. Lighter payloads could lifted faster than heavier ones, while still maintaining the same horizontal acceleration forces.
Here's a link to a site dedicated to the space elevator.
http://www.elevator2010.org/site/primer.html
There's a link in the upper right hand corner entitled "intro movie". Click on that and you'll get a 2 min. video on the topic
Interesting.
I must admit, I'm still not exactly clear on the dynamics of one of these systems. To be exact, I'm still not completely clear on how it behaves as the elevator is climbing and once it stops.
Incorrect. The velocity of the objects at a higher altitude would have to be greater to maintain it's position relative to the ground. This increase in velocity would normally send the object out of stable orbit. However this would not be a normal satellite. The cable would be preventing it from leaving its stable orbit. This is a Geostationary orbit at an altitude that would not be possible without it being tethered to the ground. The tension on the cable would be determined by the mass of the satellite and it's altitude. Because of it's velocity, it would always be trying to move away from the earth.
You would set it as needed depending on how much weight you wanted to lift, and how much tension needed in the cable to resist bending from the horizontal forces imposed on it by accelerating the payload as it moved up the cable. Those horizontal forces can be controlled by the speed at which a given payload is elevated. Lighter payloads could lifted faster than heavier ones, while still maintaining the same horizontal acceleration forces.
Sorry.
Just double checked my math, and that should read 36.5 million tonnes.
Also, this example cable will mass about 200kg/m, so a 100 ton payload would shift the center of mass by about 500m, or 1/120,000 of the total.
Finally, as someone else mentioned earlier (and as Kim Stanley Robinson wrote about in his Mars trilogy) the best way to get the required material into orbit would be to start with an asteroid, which will probably mass somewhere on the order of 1 billion tonnes. Again, very able to withstand a little weather or asymmetry.
For this theory to work, your 'cable' has to be rigid. Sorry, the dynamics will kill this. I think those that say they have looked at the equations have made too many simplifying assumptions. This is balonium.
If that thing falls, I think it could hit my house.
Your one of few here who have given this any serious thought, but you fail to mention that the satellites you describe are fairly 'rigid' bodied. This 'cable', no matter how strong, to be practically 'unreeled', will have flex properties. It also has mass as you describe, and orbit dynamics of large flex structures, as well as mag field interaction (if it is conductive at all), will force all kinds of dynamics besides gravity on it.
This is one of many ideas, but its not the winner. I will bet on it.
I do have some questions.
First, I assumed the the center of mass would have to be beyond GS Orbit, to provide tension and force to oppose the forces involved in lifting the load. With the center mass directly at GS, this obviously isn't the case.
What then would be used to oppose the forces of lifting the payload? Or is the shear mass of the assembly enough to resist movement by any force imposed from lifting the pay load?
Once the payload was up to is traveling speed along the elevator, there would be no acceleration thus no thrusting force, but there would have to be acceleration to get it up to speed. Also, I would think moving the load through the atmosphere would have a slight effect as aerodynamic forces work against the load as it moved up. Wouldn't that over time, after thousands of lifts, tend to pull the assembly out of perfect GS orbit? And then, wouldn't an equal amount of force, be needed to move it back? If so, how may lifts could be performed before making a correction?
Or can the acceleration of the payload be done slow enough to prevent that? If so, how long would it take to get a load up?
At what point does the load become self motivating, if you will. I would think once past geosynchronous, it would be on its own being "flung" out along the elevator by the orbital velocity of the assembly. By the time it reached the end, the velocity would be well over standard geosynchronous velocity. If you wanted to stop it before it went off the end, how would you do that? That's a lot of energy to convert, absorb or dissipate.
Hope I'm not bugging you too much.
It seems you could use the breaking action beyond its center of mass to provide any correcting thrust needed to counteract the force used to accelerate the load up to speed. If that's the case, you could use as much acceleration as you wanted to get the load up to speed, and use the breaking at the other end to compensate for it.
Does that sound halfway plausible? Would you even want or need to stop the load anyway?
How do they propose to deal with coriolis forces?
Haven't seen a word about it and it is the deal-breaker of the entire concept.
It's in geosynchronous orbit.
How will Coriolis Effect be a factor?
Please elaborate.
As you say, it is geosynchronous, by definition, because it is tethered.
If the outer space end is 69,000 miles out, it will have an orbital (call it "horizontal") velocity of about 41,000 miles per hour. (Well above escape velocity, by the way, if the tether ever breaks!)
A load starting up the tether from the surface of the Earth starts out having a maximum horizontal velocity of about 1,000 miles per hour.
The load must accelerate horizontally to achieve a horizontal velocity proportional to its distance up the tether.
There is no way the tether can impart any horizontal acceleration (to counteract this coriolis force) except by bending like a drawn bowstring.
If the load has not reached synchronous velocity by the time it reaches the upper end of the tether, it will simply wrap the tether around the Earth like a line on a fishing reel.
In short, a rocket assist is probably what's needed, which defeats the entire purpose of the "space elevator" concept.
Lift that to geosychronous orbit!
According to the link in post #185, the ribbon cable will only weigh 1000 tons. A 100 ton rising payload is gonna bend that sucker like a banana.
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