How does Space Elevator work?

[rogerfroud]

Am I right in thinking this is a 'string' at the equator with a weight in geostationary orbit with a transport climbing the string?<br />Obviously it takes energy to climb the string and presumably there has to be a tangential acceleration as you climb the string. Where does this come from? Presumably the climber provides both these forces else the weight would be slowed down & enter a lower orbit.<br />The problem I have is that there doesn't seem to be any net energy saving if all of these forces come from the climber, only in the rate at which the energy is released. Is that true?<br />I'm an Engineer not a scientist so it's the practical things that I'm curious about.<br />

 

[bitbanger]

It's a momentum transfer from the earth to the payload. The payload gets angular velocity, while the earth looses. Actually the counterweight attached to the tether is past GEO in order to maintain tension on the tether. It will tend to slow down too as payloads climb, so some thrust will probably be required there. It can be a low thrust and efficient engine though.<br /><br />A rotating tether is probably a better choice in the short term though.

 

[nacnud]

The tether is kept up by the Earths rotation, the payload has to use energy to climb the tether but this is more efficent than using rockets. The payload does pull the teather down a bit at it climbs but the rotation of the Earth soon pulls the teather straight again.<br /><br />

 

[barrykirk]

BitBanger is correct.<br /><br />A tether is probably a lot easier to design and build than a space elevator. He also pointed to the best, IMO, web site on the subject.<br /><br />The problem with a rocket, is that it has to lift it's own energy supply in the form of fuel. That fuel weighs a lot, which further increases the amount of energy required.<br /><br />Also, a rocket has to get to orbit fast because all of the time that it is at suborbital speed, it is losing energy to gravity. At launch gravity is bleeding off energy at 1 G. As it gets closer to orbital velocity, that energy bleed tapers off to zero.<br /><br />A typical rocket will have less than 5% of it's liftoff weight as payload.<br /><br />A climber on a space elevator can leave it's power source on the ground. It doesn't have to climb quickly. It doesn't lose energy because it climbs slowly.<br /><br />They can be powered by any number of sources.<br /><br />1) Solar.<br />2) Power beamed from the ground<br />3) Nuke ( Not likely from political considerations)<br />4) Running power lines up the elevator cable.<br />5) Technically you could use fuel cells, but it wouldn't be much more efficient than a rocket.<br /><br />Tether's don't need climbers!!!<br /><br />But you need to send a rocket on a suborbital path to catch them. Depending on the tether, that suborbital path could take significantly less energy than required to go all the way to orbit.<br /><br />With the right tether, Spaceship One if properly modified could have caught a tether and gone all the way to orbit. Spaceship One also had sufficient thermal protection that it could return from orbit with a tether assist!<br />

 

[barrykirk]

Why is a tether only a short term solution?<br /><br />Assume we have CNT tech that is more than ample for a space elevator.<br /><br />Well than space tethers would be practical at that point.<br /><br />Let's look at the typical scenerio 50 years from now. A business man needs to travel to a space factory in GEO.<br /><br />Scenerio 1 with Space elevator.<br /><br />1) He flies from NYC to the equator.<br />2) He gets onboard a climber and 6 days later, he arrives in GEO. Assume that the factorys are clustered around GEO.<br /><br />Return to earth is the reverse.<br /><br />Scenerio 2 with tethers. Assume an orbital inclination of 35 degrees.<br /><br />1) He gets into something looking like spaceship one and takes off from JFK airport.<br />2) Launches to tether in LEO. He is in LEO in 2 hours after leaving JFK.<br /><br />3) Catches another tether to GEO.<br /><br />4) Gets captured by another tether at GEO.<br /><br />He is at GEO in less than 3 days.<br /><br />Tether might be the long term and the short term solution.

 

[chonner]

Space Elevators arn't really designed for humans because they take so long. As you would travel you would spend alot of time in the Van Allen Belt (lots of radiation) which wouldn't be very good for you. This problem also affects solar panels as they are degraded by radiation. So tethers are probablly much better for human transport. <br /><br />Where the space elevator really comes into their own is transporting things like water and big bulky structures to GEO. The first elevator designs all are able to carry around 20tons of cargo upwards about every 4 days. I'm not sure how much it would cost to accelerate 20tons to suborbital velocity to reach a tether so i'm not sure how that would compare. Some estimates have said that maby 3rd or 4th gen elavators may be able to carry as much as 200tons skywards every day. How that compares to a rocket/theter system i don't know.<br /><br />For information check out the Liftport forums at www.liftport.com, they got alot of info on there.<br /><br />Liftport are a company planning to build a space elevator. A couple of days ago they did a test with a robot lifter climbing a 1000ft cable suspended by a helium baloon. It all went very well and the next stage is to progress up gradually to a height of 1 mile with helium baloons. (approx 5000ft, i think, not to good with imperial measurements ) <br /><br />Chonner

 

[nexium]

All the posts are good information. The following supplements information you can find at www.liftport.com A string will work, but a meter wide (very thin) ribbon is proposed for better traction for the climber and to reduce failure probability due to micrometeorite impact. I think several lasers of about one megawatt each are needed to assure fast climbing on all but the cloudiest days. The efficiency is low but it beats rockets by a cost factor of ten or more. Transients will move perhaps 500 kilometers per hour, up and down the tether, so the climber may be able to ride a transient like a surf board, reducing the energy needs to climb. This will drain energy from the transient which might otherwise become dangerously strong. At 500 kilometers per hour (average speed) the climber will take 200 hours to reach the far end of the tether, so we don't want the climber to go much slower. The factory may be near the far end of the tether (replacing some of the ballast) as the gravity near the far end is about 1g = Earth gravity, I think. Would you believe 0.05g?<br />The energy savings is because the climber carries little or no fuel, while a rocket has to lift all the fuel it will need for the entire mission. It may be practical to use a transient to snap the payload off the far end of the elevator (or rotating tether) as fast as 10 kilometers per second, allowing the payload to coast as far as Jupiter where it can do a gravity assist maneuver = sling shot maneuver which will take the payload almost anywhere in the solar system with only miminal power for mid course corrections and maneuvering at the destination and for the sling shot maneuver. I don't think sufficient energy can be sent up an electrically conducting tether unless superconducting becomes practical. Neil

 

[chonner]

Reading the forums on the liftport website all the guys there who are seriously into the whole eleavtor concept have done the required calculations came with gravity at the end of a 100,000km elevator of about 0.05g, or about 1/3 of the g on the moon.<br /><br />0.05g probably makes it easier to move around, but i think it would still have the same biolgical effects as microgravity (muscle/bone atrophy) so might not be completely pratical for a long term manned station at the end.<br /><br />I asked the guys over there and they said the required legnth of the elevator to achieve 1g would be something like 2.3million km, about 10 times further out than the moon. Slightly impracticle i think.<br /><br />Oh and Liftport seems to be making some good progress, their carbon nanotube factory as just started some early production runs. Not to long and we might actually some prototype elevator cable. <br /><br />Chonner<br />

 

[nexium]

I'm glad to read that they are making nano tubes. I suggest a short rotating tether with a climber, so they can practice making and dealing with transients and testing the laser propulsion, and the tread lying and tether repair. If they can't make it work in low Earth orbit, they are in trouble powering the climbers and elevators all the way to Geo orbit. It is down hill from geo to the end, but eventually they will want to run an elevator from the end toward geo orbit.<br />The practice tether could be mostly kevar.<br />Could that 0.05 g be Earth's gravity 100,000 Kilometers above Earth's surface which is more than cancelled by the centripical "gravity"? Neil

 

[chonner]

Regarding the 0.05g, i believe it is that due to the centripital action not earths gravity.<br /><br />Looking back over the stuff the guys said on the liftport forums, they say that for an object that remains stationary over one point on the earths surface( like the elevator would), outward g (centripital action) scales linearly by 0.05g every 100,000km beyond geo you go. (don't quote me on that though, they could be wrong and i havn't checked it)<br />As geo is already at about 30,000km, the g at the end of a 100,000km elevator would be actually about 0.03g<br /><br />Of course centrapital actions is goverened by your orbital velocity so if you were to move faster around the earth but still at the same altitude you would experience greater g.<br /><br />I'm not that great at physics as i'm only 15, but thats my understanding of it.<br /><br />Chonner

 

[barrykirk]

As for climbers on tethers, the tether guys are already looking into this on some of there tether models.

 

[romesmite]

What I would like to know is, will the space elevator start on earth? If so then how does it manage to not sway from the turbulance in the upper reaches of the stratosphere?

 

[nexium]

The elevator ribbon will sway perhaps a kilometer due to wind loading when the tension is low, and significantly even at highest allowable tension. It will swing sort of like a pendulum but very slowly because of the enormous length. The sway will be important only when the tether is close to the maximum allowable tension, I think, but it will generate mild transients that travel to geo orbit, perhaps farther. Neil