Some problems are: The fastest magnetically levetated trains so far are about 300 MPH = miles per hour, while orbital speed in low Earth orbit is over 17,000 MPH. If t = 2500 seconds and a = 1/3 g = 10 feet per second per second: v = at = 10 times 2500 = 25,000 feet per second, which is about orbital speed. The vacuum tube needs to be s = 1/2 at squared = 5 time 62,500,000 = 312,500,000 feet long. Accelerating at ten g is deadly for humans, plus requires enormous amounts of energy. This reduces the tube length to a bit over ten million feet which is still too long. The vacuum tube needs to allow the space craft to exit at at least 200,000 feet to avoid white hot due to air friction at this speed. Also large amounts of air rush into the vacuum tube when it is opened to allow the space craft to exit. A large cost is needed to pump the tube to high vacuum for the next launch. Major technological advances are needed, however mag lev may work for the first 1000 feet and uses a second stage rocket to get the rest of the altitude and speed.   Neil

It will take conciderable energy to lift several tons to GEO, but we should get more than 1/2 back, when we lower it again.   Neil

Reeling mostly at GEO does have some advantages, but the disadvantages are infrastructure must be installed at GEO before we can add strengthening threads. Actually I think we need to also have reeling capability at the anchor ship and the counterweight, if we have it at GEO. For through traffic, reels at GEO require some sort of transfer device. None of this is a big deal for the third and fourth elevator. We will be in a big rush to complete elevator two as elevator one may fail before two is completed.
Suppose we have 75,000 kilometers of tapered cable for the GEO to anchor ship leg. We need at least that much for GEO to counterweight. In both cases the big end of the taper is at GEO. Let's suppose the big end of both has a mass of 400 grams per kilometer; the mid point is 200 grams per kilometer and the ends 100 grams per kilometer. Total mass is 150,000 times 200 grams = 30,000,000 grams = 30,000 kilograms = 30 tons that we need to launch to GEO plus whatever infrastructure is needed. We obviously need lots of solar panels and a storage device such as super capacitors, as the peak demand of the reel motors is much more than the solar panels produce. If we wind nearly all the ribbon onto the reel at the anchor ship and/or the counterweight, the 100 to 200 gram per kilometer ribbon is on the reels, so we are using the 200 to 400 grams per kilometer ribbon temporarily. Let's assume a breaking strength of two ton; one ton can be lifted safely. If we move most of the ribbon to the GEO reels, now the 200 to 400 gram ribbon is on the GEO reels. We can now lift only 1/2 ton, less, as I don't think it is linear.   Neil

I can't do most of the math, but the concensus of those who can do the math is 30GPa is not enough for an Edwards type elevator and 60GPa to 100GPa is needed for the type you have in mind, as it is not practical to taper the ribbon. The counter weight is helpful if it is a payload going the opposite direction.
Stretch transients are important to the design whether they disapate in a twenty thousand kilometers or two hundred thousand kilometers. If it is 20,000 kilometers, it appears to me that the counter weight will remain unaffected by the stretch transients produced by the payload car, and the reverse.
In any case, reeling in or out affects the ribbon and things on it 20,000 kilometers away after about a one hour delay due to the speed of sound in the ribbon. This is counter intuitive as we have rarely experienced cables even 2 kilometers long which produce a time delay of about 2 seconds.  Neil

Some of the near Earth asteroids have 10% carbon. We can launch the CNT factory when the asteroid passes closer than the moon, and deliver the CNT to a GEO stationary base on each close pass, several times per decade. Likely less costly than a moon base since delta v for landing and liftoff is near zero. The asteroid will also have most of the other elements that the moon regloth contains in simular quantities, but CNT is an good general purpose constuction material.   Neil

To send multi-ton payloads to GEO altitude or Mars requires gigawatts for the first stage. I think this is possible, but the cost is too high. Apparently you are thinking uninsulated conductors with a 100 meter trolley. This has been done to about 5000 volts, but that high a voltage uninsulated is likely not possible at 30 kilometers altitude due to the ionization of low pressure air. You need 10,000 amps to deliver ten gigawatts at 1000 volts dc. Ionization problems are worse with 60 hertz ac and supplying that much power single phase will require major construction by the local power company.
I'd guess 30 kilometers is possible with a balloon several kilometers in diameter or perhaps one hundred balloons one kilometer in diameter. At not much higher altitude, the size and number of balloons becomes rediculous.
The gigawatts required a few minutes per day (assuming one launch per day) is also expensive. It is called use of demand and the power company has good reasons for charging a very large premium. Typically it would mean they buy 20 big methane powered turbines that they use only for your daily launch. The power line that brings you the gigawatts carries thousands of times less power in between launches, so that also requires a large capital investment, with very little revenue, if your company files bankruptsey.
You want to launch approximately vertical for the first 30 kilometers to reduce wind resistance losses, so you need about 40 kilometers of balloon supported cable instead of 100 kilometers. You likely cannot reach speeds higher than about 10,000 kilometers per hour before you run off the end of your cable. This is not nearly fast enough to reach any orbit from 30 kilometers without a second stage.
If we develop cheap super conductors that can tolerate mega amps, faster than 10,000 kilometers per hour, the use of demand and the weight of the cable from ground to balloon are greatly improved. Lots of research dollars have gone into super conductors the past 60 years, with only a little sucess, so the breakthough is not likely soon.
Taper calculations are far beyond my math talents, but I think someone can calculate taper, if you choose CNT with a GPa of 30, a maximum current (10,000 amps?) and a maximum temperature (400 degrees c?) for two aluminum condutors insulated 1/3 of their circumfrence and spaced one meter apart, with a CNT ribbon? Highest temperature will occur at 30 kilometers altitude due to the thin air carrying away very little heat. The IR loss heat must be mostly radiated. The CNT spacing ribbon needs to carry nearly all the weight. It may also be desirable to taper the aluminum conductors as they can disapate the IR losses much better at the low end.  Neil

Dr. Bradly Edwards has a separate organization that is getting some Nasa funding. He said he would make an important annoucement in 2008. Generally the various groups share information, so liftport can concentrate on a nitch or two. Sometimes sponcers cause more trouble than their money can solve. Additional feasability studies may not be very useful until CNT ribbon or another very strong material is mass produced. NASA is sponcering climber competition likely in Nov 2008. Likely DoD is still trying to develop the high power lasers needed to propel the climbers, and hypersonic aircraft which could deliver a pay load to a Bolo. Orbiting a bolo may happen in 2008. Some development work is done in secret.  Neil

Hi Andreas: Does that mean you are doubtful that the oscillating ribbon can get the climber to 91,000 kilometers in one day? From 91,000 kilometers we can crash into any object in the inner solar system without any fuel or propulsion in the payload. We will need some payload delta v to make an orbit insertion or a soft landing at the destination. The space elevator may still be a good idea, even if we have cheap access to LEO and GEO. If we build a 200,000 kilometer space elevator I think we can pass Saturn's orbit in about one year/ 52,000 miles per hour with respect to Earth's surface when the payload leaves the ribbon?   Neil

Perhaps you can give us more details on your first paragraph. About 24,000 miles is the minimum length for the Brad Edwards' space elevator. We can do a bolo (soon) which is much shorter, but also much different. A bolo is a thousand miles or so of ribbon flipping end over end in low earth orbit. It is oriented so the low end travels about 16,000 miles per hour with respect to Earth's surface, so a hypersonic rocket is needed to attach the payload to the low end. Timing is very critical as the low end and high end switch places several times per hour. The payload is released near the high end about 1000 miles higher and about 3000 miles per hour faster. With stronger ribbon, which may be available soon, the bolo can flip more times per hour. Faster means the release speed is faster, perhaps fast enough to hit the Moon or Mars without any fuel or propulsion. Some payload propulsion is needed to make a soft landing at the Moon or Mars. Making the bolo longer has some advantages, but not much.
If the tips of the bolo dip as low as 65 miles altitude, air friction losses slow the bolo more than can be recovered with some solar panels near each end.   Neil

Apparently Virgin plans to fly Space ship two in 2010 to 110 kilometers altitude, sub orbital, with a payload of about 1.3 tons. The 20 ton starter ribbon needs to be lofted to about 36,000 kilometers, which may be possible with space ship 3 or some other private corporation. With bad luck on ribbon strength, the starter ribbon may weigh as much as 100 tons.  Neil

The climber on an Edwards type ribbon is travening much less than orbital speed, so graviy at 300 kilometers altitude is still about 0.99g The climber must overcome significant gravity until it reaches an altitude of about 35,000 kilometers.
Some beam spreading occurs, so a beam which has traveled a million kilometers total over many reflections requires a prohibitably large mirror at both ends, especially if the mirror aiming is typically in error by one second of arc. At visable light wavelengths, the mirors also require extreem surface precision to avoid scattering most of the light out of the beam. That is not to say the concept is entirely worthless, but we will be disappointed if we expect a big improvemement.  Neil

Banning vehicles is going too far, but we can get more use from the railroad tracks that are in place and do pilot programs with a few more no vehicle communities. Zoning our cities so most everything is available in each square kilometer (or an adjacent square kilometer) would reduce driving across town for many of us.   Neil

If your aircraft cariers ect have an average density of more than one kilogram per cubic meter = 1/1000 th the density of water, they will float below the cloud tops of Venus where the sulpheric acid clouds are found, and little solar energy is available. CNT = carbon nano tubes may permit a low enough average density to stay just above the cloud tops which I agree is ideal, except the habitats will fall if large quantities of carbon dioxide leak in. I agree linking them (with air locks) makes falling very improbable as it is very unlikely more than one would have a bad leak. Floating cities high in Earth's atmosphere have much the same problem, except the inhabitants might survive the fall of their habitat to Earth's surface.   Neil

Unless the new iceage is worse than the previous iceages, I'm possibly safe in Florida: But survivil will be difficult 1000 kilometers farther from the equator and perhaps no survivers more than 2000 kilometers farther from the Equator. Solar mirrors, in solar synchonous semi polar low Earth orbit, could ease the suffering of people who live more than 30 degrees from the Equator.   Neil

Some experts think the Sun's Energy output will increase about 1% per 100 million years, so Earth and Venus will be in trouble in about 3 billion years when the Sun produces 30% more energy. Mars will then have comfortable temperatures, but in a few billion more years (perhaps sooner) Mars will also be too hot. No need to worry, 3 billion is 3000 times 1000 times 1000 which is extreemly far in the future. The sun is not expected to change detectably in the million years, it may take to terraform Mars and part of Venus.   Neil