Absolutely correct! And it looks as if R&D is cheaper for Edwards' ribbon anyway. Go ribbon!

Be careful: there are 2 kinds of coriolis effects that are occuring on the SE. Ascension coriolis is the 'force' causing climber to move upwards faster as it approaches GEO. Rotational coriolis is the 'force' causing ribbon mass to lag westward as the Earth turns. I was speaking of rotational coriolis. Neither are actual forces, but really apparent movement changes due to a changing frame of perspective.

Most people envision the SE as being at right angles where it is connected to the surface of the Earth at the base station. Angling the ribbon would involve moving the counterweight slightly to the east or slightly west of the base station. Actually, the counterweight would attempting to move east of the base at all times. The reason for this is that without the force of the counterweight, the ribbon would spiral dramatically westward as it lagged behind the earth's rotation. What keeps the ribbon straight and from not spiralling is the fact that it is connected to the counterweight. The counterweight is orbiting Earth above GEO with a 24 hour rotational period. Most satellites put in this position would immediately move into a higher, ellipictal orbit. The reason the counterweight does not move into a higher, ellipictal orbit is the fact that it is being held back by the ribbon. With the counterweight pulling up on the ribbon, the ribbon is also held taut. Since the Earth and the counterweight are both pulling the ribbon eastward at the ends, the ribbon actually spoons westward in the middle. By "coriolis drift" I meant the amount of spooning that occurs.

Upon reflection, however, I am totally wrong. Neither tension nor spooning is necessarily dependant on the east-west movement of the counterweight. If the counterweight were to move eastward, it would also need to swing downward (moving into a lower and slower orbit but still with a 24 hour rotational period) in order to keep the same tension on the ribbon. Also, reeling the counterweight downwards would decrease the tension on the ribbon. This is why the required strength of the ribbon is dependant on the mass of the ribbon, and not the mass of the counterweight. Moving the counterweight westward would likely be impossible, since the counterweight is moving so fast for it's altitude it would immediatly swing back to the east. Perhaps it could be done temporarily with the pendulum method Temlakos describes.
Things to consider: whiplash effect, reeling in/out the counterweight while angling, combining oscillations with angling, combining oscillations with pendulum movement, other satellite orbits.

Be safe
-Nydoc

Looks like I was wrong about the current density, which likely lead to a lot of confusion. Zhen is talking about metallic nanotubes. When I first read that I thought it was carbon nanotubes.

Here are the direct quotes from the Edward's report :



And from Zhen Yao's paper: "Individual metallic nanotubes appear to be able to carry remarkably high current densities exceeding a gigaampere per square cm without breaking down."

Besides oscillations, another means of altering the escape trajectory would be angling the ribbon. Angled eastward, tension increases and coriolis drift decreases. Angled westward, coriolis drift increases and tension decreases. Of course, too much westward and the ribbon pulls the counterweight down. Also angling eastward is limited by the strength of the ribbon, which probably will be barely within margin.

It would certainly depend on the final width of the ribbon. I have heard estimates between 30GPa and 120GPa, but am unaware of a specific strength.

Now for a picture of nanoyarn being spun!

Note that the individual threads are not continuous tubes. They are in fact many small pieces stuck together like wet noodles. The threads get pulled off the "wall" of the forest in an up-and-down fasion which is similar to that of a stack of folded printer paper being unfolded. This would have a lower GPa than the ideal continous tube.

Hmm. I'm not quite sure what you mean. In order for a climber to pull up a new thread, the initial thread would have to support the weight of the climber and the new thread. It would essentially be self-lifting at that point and therefore unnecessary. If you are talking about reeling a new thread down from GEO, that *is* possible. However, the whole point of combining threads is to save on rocket launches. It would be better to launch a thread that's twice as wide than 2 seperate ones.

The wording in section 2.6 is a bit tricky.

I think you are correct in asserting that there is a practical height limit for a fractal truss. The actual height Smitherman gives is 3000 km, not 300 km. That kind of makes his claims a bit more suspicious. However, even a 1/4 mass reduction on the ribbon would mean savings on launch costs and maintance costs, so perhaps a smaller tower would be a good idea in conjunction with the elevator. I am trying to find a formula for ribbon mass as a function of tower height.

The formula for the cable radius I have is x(y) = x exp{(d/2p)y[(gM/R(R+y))-w^2(R+(y/2))]}
Where:
M = 5.976*10^24 kg             the Earth mass
g = 6.6732*10^-11N m2/kg2  general gravitational constant
w = 7.2921*10^-5 s^-1        the Earth angular speed
R = 6.378*10^6m                 the Earth radius  
y                                       the distance from the ground (altitude)
d = 1.3*10^3                      theoretical density of CNT
p = 150*10^9                      theoretical tensile strength of CNT

To use your maximum 300km tower, R would instead equal 6.678*10^6. The value of y for GEO is 35.786*10^6. The equation should look something like this:
x(y)= x exp{(1.3*10^3/2(150*10^9))3.5786*10^6[(6.6732*10^-11(7.35*10^22)/6.678*10^6(6.678*10^6+35.786*10^6))(-7.2921*10^-5)^2(6.678*10^6+(35.786*10^6/2))]}

Unfortunately, the batteries on my calculator are dead. =( Would anyone care to check this? There should also be a slightly decreased tensile requirement on the ribbon as it is shorter with less gravity.

Current plans for the SE have located it off the coast of South America, so a tower might need something like the Japanese megafloat. As long as waves are not a problem, you wouldn't have to worry about earthquakes either.

If a suitable ribbon material cannot be found, then perhaps an electromagnetic launch rail would be the best bet for energy efficient space access.

-Nydoc

Found the paper: here.
Section 2.3 talks about the mass savings. Section 3.3.2 shows design of the tower. Smitherman also lists observation platforms, solar power recievers and LEO satellite replacement as potential uses of towers.

You are correct. This is another reason why I think a tower is non-feasible in the near future. However, there are no material limitations, so we may eventually consider building one in light of it's benefits.

Sure thing. The mass savings are not at the bottom of the ribbon, they are at the top. The the massive portion of the ribbon at GEO is necessary in order to support all of mass below it. Any given point below GEO has to support the mass below it as well. If the lowest point on the ribbon were to be elevated by 3000km, it would mean savings along the entire ribbon. The entire ribbon could be narrowed with 150 times less mass and still support the force necessary for a 20 ton payload.

-Nydoc

I think that an arial platform would be non-feasible, given it's complexity. But as long as we're on the subject of non-feasible designs, I thougt I'd throw one out there. This is similar to the arial platform, but more along the lines of a 3000km high tower. Designs and PBO fibers required to build towers of this height do exist, only there is no current demand for towers of this height. The main benefit of building such a tower would be the reduction in mass required to build the elevator. Previously on this thread, that benefit has been called into question:

Not true! You forget that the width of the ribbon is exponentially related to it's length. I site the August 2000 NASA paper Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium by D.V. Smitherman, Jr. (sorry no link, I found it in the members only section of the yahoo groups SE site). In the paper, it was found that for materials of the same strength-to-density ratio, compression structures are less massive than tensile structures. The paper states that a 3000km tower with an attached GEO tether would be 150 TIMES less massive than a tensile-only structure! Other benefits of a tower include:

• No wind requirements on the ribbon
• No lightning insulation requirements on the ribbon
• No required shielding of the ribbon from atomic oxygen
• Dramatically reduced chance of meteor collision on the ribbon (but not the tower)
• Lower cable loading on the ribbon, as cars will move faster with no atomospheric resistance
• The tower can be used for communications arrays
• The tower can be used for studies of the high atmoshpere
• The tower can be used to test long sections of ribbon and climbers
• The tower could be a tourist attraction

Additionally, when considering that the ribbon is required to have a high tensile-strength/density ratio, it's possible we could maintain the same volume while reducing the mass. With lowered density, the required tensile strength would be lowered as well. Perhaps even a self-lifting elevator would then be possible.

The main problem with a tower is that it would collide with LEO satellites and space debri. That is why I say it is non-feasible. The tower idea may have to wait until tensile-only SEs can be used to clean up LEO and replace the satellites. Other problems are earthquakes (for land-based towers) and terrorist attacks. Do you think we could resolve this?
 
-Nydoc

Here's more pics. The first is a still from the animation where you see it built. Part of that includes a cut-away that shows a radial climbing mechanism and then it takes off pretty quick like a rocket. Not sure what the force shield effect is supposed to represent.. The second image is the world view. You can even zoom out and see the cable from space.

After digging around, I found this article under the publications section of that site. It's about a different system called a MIXER. The MIXER has an input power level of 366kW and a current of 11.5 amps on the tether. It is built to withstands voltages in excess of 20kV. In order to work, however, that it rotates through Earth's magnetic field, so I don't think this kind of power generation is applicable to the SE, which is geostationary. I'm also given the understanding that formation of plasma along the tether is an issue, but I don't know if that would apply to the SE either.

-Nydoc

The Van Allen Belts are 200km above the surface at closest points. The troposphere, stratosphere and mesosphere combined are only 80km thick. The belts do penetrate the thermosphere and the exosphere but over 99.9999% of our atmosphere is below their level. I highly doubt reducing the density of the belts would have any effect on the amount of radiation reaching earth. Considering the belts ARE radiation, removing them may even lower the amount of radiation we get. They don't provide a barrier either, since it is Earth's magnetosphere and the atmosphere which are shielding us from radiation. Some people even speculate that the radiation belts are not natural, but are a by-product of US or Russian nuclear testing.