Another post in another thread, suggested drop from 18,000 miles as a low energy way from space elevator to LEO. 4000 kilometers or less will work, but requires more energy to get into LEO. If the rocket fails to ignite, you burn up in Earth's atmosphere.  Neil

By any method, it will take all day, if not longer to get to LEO = low Earth orbit, for typical travelers, just as it does take almost that long to fly to another city by airline. If the space elevator takes 8 hours to climb to an altitude of 4000 kilometers where it is dropped off the ribbon to get enough speed for LEO, you will be in the desired orbit about 9 hours after the elevator starts up the ribbon. Considering the space elevator is much less costly, I think it will be very competitive for both passengers and cargo to LEO and GEO orbit. Admittedly 200 hours to the far end of the ribbon is an ordeal, which may attract few human travelers. Does anyone think the elevator can average more than 500 kilometers per hour safely?   Neil

These platforms would be mobile, which would allow the elevator, with sufficient warning, to avoid orbiting satellites and debris by moving the anchor end of the cable back and forth about 1 km, pulling the ribbon out of the path of an oncoming object. While debris and other objects down to 10 cm in diameter are currently tracked, objects with diameters as small as 1 cm are a potential threat to the elevator. As a consequence, the current elevator system design includes a high-sensitivity ground-based radar facility to track all objects in low-Earth orbit that are at least 1 cm wide [see illustration, "Watching the Skies"]. A system like this was designed for the International Space Station but never implemented.

Eliminating erosion from atomic oxygen at altitudes of 100 to 800 km would be the job of thin metal coatings ~Each of the two starter ribbons has surface area of 200 billion square centimeters = 20 million square meters. If the metal coating averages one nano meter thick, the total metal coating is  equivent to two centimeters by one meter by one meter = about 50 kilograms. Will each micro ribbon (layed by a climber) require a metal coating? We can omit the metal coating for 99% of the 100,000 kilometers, but that may mean trouble if the ribbon brakes a few hundred kilometers above the anchor and the bottom portion is lost twice. Is even a 1000 nano meter metal coating realistic? Will the metal coating adhre to the CNT reliably over a 500 degree c temperature changes and being squeezed daily by climber rollers?~ applied to the cable. Radiation damage would be mitigated by using carbon nanotubes and plastic polymer materials that are inherently radiation resistant ~it seems to me that each change of a carbon nucleus weakens the nano tube significantly as it will be a flaw in the almost perfect molecule/crystal~.

To avoid problems with cable oscillations induced by tidal forces, my ribbon design calls for a natural resonant period—7.2 hours ~the transient needs to propagate 5000 kilometers (average)per hour if the effectve length is 36,000 kilometers/ that seems much too fast for a very thin ribbon, mostly under light tension/The outer portion is about 64,000 kilometers long and will have oscillations during micro ribbon laying. My guess is the oscillations can be useful~   that does not resonate with the 24-hour periods of the moon and sun. Any oscillations that do occur would be damped by the mobile anchor station ~likely effective the bottom thousand kilometers, but my guess is other measures are needed for 99% of the ribbon. Please correct any errors I have made.   Neil~.

~The following by Dr. Brad Edwards deserve some comments. My comments are enclosed by~ : The space elevator concept is an old one—Russian scientist Konstantin Tsiolkovsky proposed the basic concept more than a century ago. The idea resurfaced in the 1960s, but at the time there was no material in existence strong enough for the cable. To support its own weight as well as the weight of climbers, the cable has to be built out of something that is incredibly light and yet so strong that it makes steel seem like soft-serve ice cream. The space elevator faded back into the realm of sci-fi.

Then, in 1991, Japanese researcher Sumio Iijima discovered carbon nanotubes. These are long, narrow, cylindrical molecules; the cylinder walls are made of carbon atoms, and the tube is about 1 nanometer~one billion required side by side to make one meter wide, one trillion to make the ribbon 1000 nano meters thick~ in diameter.

In theory, at least, carbon-nanotube-based materials have the potential to be 100 times as strong as steel, at one-sixth the density. ~I suppose they might be that light, if you average in the void in the center of each tube/Are these voids high vacuum, the way we typically make CNT?~ This strength is three times as great as what is needed for the space elevator. The most recent experiments have produced 4-centimeter-long pieces of carbon-nanotube materials that have 70 times the strength of steel. Outside the lab, bulk carbon-nanotube composite fibers have already been made in kilometer-long lengths, but these composite fibers do not yet have the strength needed for a space elevator cable.

However, we think we know how to get there. There are two methods being examined at academic institutions and at my company, Carbon Designs Inc., in Dallas. The first approach is to use long composite fibers, which are about as strong as steel and have a composition of 3 percent carbon nanotubes, the rest being a common plastic polymer. By improving the ability of the carbon-nanotube wall to adhere to other molecules and increasing the ratio of nanotubes to plastic in the fiber to 50 percent, it should be possible to produce fibers strong enough for the space elevator cable.

The second approach is to make the cable out of spun carbon-nanotube fibers. Here, long nanotubes would be twisted together like conventional thread. This method has the potential to produce extremely strong material that could meet the demands of the space elevator. Both processes could be proved in the next few years.

With a suitable material on the horizon, the next question is the design of the cable itself. Prior to 2000, in both science fiction and the scant technical literature, the space elevator was a massive system—with huge cables 10 meters in diameter or inhabited towers more than a kilometer across. These systems also required snagging asteroids to use as the counterweight at the end of the elevator. Suffice it to say, it's all well beyond our current engineering capabilities—mechanical, electrical, material, and otherwise.

 

IN MY STUDY, I sought a design that could be built soon and could annually lift 1500 tons ~that is only 30 tons per week~ or 10 times as much mass as the United States now launches into space in a typical year. In 2000, I received a grant from NASA's Institute for Advanced Concepts to begin a new study on space elevators. The study formed the basis of a book I coauthored with Eric A. Westling, The Space Elevator: A Revolutionary Earth-to-Space Transportation System (Spageo Inc., 2002). Work continued at the Institute for Scientific Research Inc., in Fairmont, W. Va., and now at Carbon Design. The result is a preliminary design for a simplified, cheaper, and lightweight elevator.

This design calls for a ribbon instead of a round cable. The flexible ribbon, just 1 meter wide and thinner than paper, would be made of carbon-nanotube composite fibers arranged in long strands, cross-braced to evenly redistribute the load if a strand were cut. Space debris that would sever a small round cable would pass through the broader ribbon, creating small holes and a manageable reduction in cable strength, letting it survive impacts from small debris and meteoroids, which would be fairly common [see image, "Cable Close-Up"].

Choosing a ribbon rather than a circular cable also greatly simplifies the design of the tread system for moving the elevator car along the cable. The climbers would pull themselves up the cable using pairs of motorized treads that clamp ~Is there significant danger clamping will damage the brittle CNT where previous repairs were made = a lump?~ the cable between them. The broad, flat treads would sandwich the ribbon, exerting significant forces against each other to grip the cable securely. The treads are based on conventional treads, the drive system is built with fairly standard dc electric motors, and the control systems are no more complex than what you'd find in a typical auto today. A round cable, on the other hand, would require a far more complex arrangement of wheeled gripping systems.

Because of the thinness of the ribbon, it would be surprisingly light: the entire 100 000-km length would have a mass of just 800 tons ~ average 8 kilograms per kilometer = 8 grams per meter~ not counting the counterweight's 600 tons ~the mass of all the thread laying climbers was about 1380 tons, including the 780 tons of micro ribbon and binder that they layed?~. But this is still obviously substantial, and it leads us to the other big problem in building the elevator: how would we get all that cable and counterweight mass up into space in the first place?

Currently, the largest rockets available can place only a 5-ton payload into the 36 000-km geostationary orbit where construction would have to begin. Remember that to keep the elevator fixed above one spot on Earth's surface, its center of gravity must always remain at the 36 000-km mark.

Launching and assembling hundreds of 5-ton payloads would be impractical, so my colleagues and I devised an alternative plan. An initial "deployment spacecraft" and two smaller spools of ribbon massing 20 tons each would be launched separately into low-Earth orbit using expendable rockets. The deployment spacecraft and spools would be assembled together using techniques pioneered for the Mir space station and the International Space Station. The deployment spacecraft would then follow a spiral course out to geostationary orbit using a slow, but fuel-efficient, trajectory.

Upon arrival, the spacecraft would begin paying out the two spools side by side toward Earth. Meanwhile, the deployment spacecraft would fire its engine again, raising it above geostationary orbit. The spacecraft's motions would be synchronized with the unreeling cable so that the spacecraft would act as the counterweight to the rest of the cable: this would keep the center of gravity of the entire elevator structure in geostationary orbit [see illustration, "View From the Top"]. When the two halves of the ribbon reached Earth's surface, a special elevator car would be attached that would ascend the elevator, stitching the two side-by-side halves of the ribbon together. ~Will electrostatic charge be sufficient to keep 100,000 kilometers of twin ribbon a meter apart from tangling with each other?/Has Brad lost it?~This initial system would have a 20-cm-wide ribbon and could support 1-ton climbers.

Other specialized climbers would then be sent up this initial ribbon, adding more small ribbons to the existing one. When one reached the far end of the elevator cable, the climber's mass would be added to the counterweight, keeping the elevator in balance so that its center of gravity would stay in geostationary orbit. After 280 such climbers ~averaging 5 tons each~ a meter-wide ribbon that could support 20-ton climbers would be complete.

The climbers, like most of the elevator system, would use off-the-shelf components wherever possible. One of the reasons the climbers would be so simple and have so much room for payload is that they would not carry power-generating equipment. Power would be delivered to climbers by lasers beaming 840-nm light from Earth onto an array of photovoltaic cells; at this wavelength, photovoltaic cells can generate electricity at an efficiency of 80 percent [see illustration, "Going Up"]. The lasers required are not yet available, but components are being tested, and free-electron or solid-state lasers at the power levels we need (hundreds of kilowatts ~won't kilowatts of this beam heat the climber and black CNT?/Perhaps CNT is not black at 840-nm, infrared?~) are expected to be available in a few years.

ONCE AN ELEVATOR IS DEPLOYED, keeping it operating would be the next big challenge. Serious threats to an elevator would come from:

The weather—lightning, wind, hurricanes, tornadoes, and jet streams
Airplanes, meteors, space debris, and satellites
Erosion from atomic oxygen in the upper atmosphere
Radiation damage
Induced oscillations in the cable
Induced electrical currents
Terrorists
Some of these challenges would be met merely by locating the elevator's Earth anchor in the eastern equatorial Pacific, west of the Galápagos Islands, where the weather is unusually calm and the threats from hurricanes, tornadoes, lightning, jet streams, and wind are greatly reduced. This location is also about 650 km from any current air routes or sea lanes, significantly reducing the chance of an accidental collision and making the site easier to secure against terrorists. An anchor in the Pacific obviously implies a floating platform, but such structures are already commercially available, thanks to the offshore oil industry [see illustration, "Elevator Ahoy"]. ~more next post~

Hi NYdoc: Using your numbers (5 megohms for 100,000 kilometers/ perhaps correctly) I got 50 ohms per kilometer. Watts = I squared R = 10,000 times 50 = 500 kilowatts per kilometer = 500 watts per meter if 100 amps is flowing which is more than enough to melt the ice and evaporate the water. I'm guessing the one ton ribbon is 200 milimeters wide and averages 0.001 millimeters thick = 0.2 square milimeters, cross sectional area. There are 1000 cubic millimeters in one cubic centimeter and one cubic centimeter has a mass of 2 grams if the density of CNT with binders and coating is 2. One meter of ribbon is 200 cubic milimeters = 0.2 cubic centimeters = 0.4 grams. One kilometer of  ribbon = 0.4 kilograms. 100,000 kilometers = 40 metric tons, which is double, but extra material is likely necesary in Earth's upper atmosphere to prevent damage by atomic oxygen. More material is also needed near GEO altitude = the ribbon is tapered = less than 0.001 milimeters thick in the atmosphere.
If the resistivity is 0.4 milliohms per centimeter cube = 400 micro ohms per centimeter cube, that is 225 times the resistivity of copper.
As you suggested the ribbon resistance is heavily dependent on the polymer (or other binder used). I'm hoping a high temperature binder is found as I suspect the CNT can tolerate 1000 degrees c in  the absence of oxygen and it may have to in some senarios.   Neil

I posted the following at   www.space.com   Please embellish, correct or comment:  Beaming power by lasers or microwave is low efficiency small scale, likely best medium scale, with rapidly escalating problems at very large scale. At   www.liftport.com   you can find details on a single wavelength solar panel in infrared that is 80% efficient and the laser to produce that wave length. The plan is to beam the laser energy to the photovoltaic panel mounted on the climber/lifter going up the space elevator which we may build in 12 or 15 years. The climber needs to be powered by the laser beam all the way to Geo sychronous orbit = 36,000 kilometers. Average efficiency is perhaps 1% with soon to be available technology which is acceptable for the space elevator, but not for moon energy needs or to replace fossil fuel use on Earth. Microwave energy beaming may be 10% overall efficiency, which makes it marginal for energy trasmission in most applications. Near term we can use klysitrons which require several amps at 100,000 volts. Ten of them in a series string operating at a million volts dc from a large number of solar panels in series. There is perhaps 5 million watts of energy in the beam (12 million watts from the solar cells) so several of these systems are needed to provide the energy needs desired by a few hundred moon colonists. 100 mylar or other mirrors in low moon orbit can reflect the beam to rectenna sites all over the moon, with typical rectennas receiving a beam 90 plus percent of the time. Some of the reflected beam will miss the rectenna, so 3 megawatts falling on the rectenna is optimistic and 2 megawatts dc output from the rectenna is optimistic. Some applications can use the dc, but 400 hertz 120 volts may be the standard. 100 inverters in series can produce 50,000 volts ac at perhaps 80% average efficiency = 400,000 watts if the output current is ten amps. Locations near each rectenna can be connected to a local grid with a pair of wires carring both the high voltage dc and the high voltage ac.   Neil

Thanks Frank and Cthayer: I suspect climbers will continue to go weekly (or oftener) to the far end to inspect the ribbon for damage and make repairs and upgrades. Some climbers may carry rocket engines to assist with trancient management should an emergincy develop. The far end of the ribbon travels faster than escape velosity, so I would think it will be used frequently to send pay loads throughout the inner solar system at low cost.
Someone explained that the ribbon would move Earth minutely to compensate for excessive traffic up to GEO orbit. I think this requires that the average tension on the ribbon/tether be positive. Allowing large portions of the tether/ribbon to be slack may cause the entire tether to fall toward Earth.
I mentioned the possibility of out running your own stretch transient (compression trancient if breaking beyond GEO orbit) as I hoped someone could analyze that condition. I agree it is possible that could cause a doubling of the stress. I believe all kinds of transients travel slowly on slack portions of the ribbion just as a vibrating string goes to a lower pich as we reduce string tension on a musical instrument.  Neil

If the anchor ship is unable to move temporarilly, there is a fair chance the ribbon can be moved sufficiently by reeling in or reeling out ribbon. There is also a fair chance the ribbon can be moved sufficiently, by reversing the direction of a climber, or elevator for an hour or less, so there are back up possibilities which are quite predictable.   Neil

I posted this answer on  www.bautforum.com  so please comment if errors are likely.
The space elevator described at   www.liftport.com  seems to be much the same as the Dr. Edwards  elevator. The CNT = carbon nano tubes needs to be hundreds (would you believe 24 or 34?) times stronger than Kevlar for the starter ribbon to have a mass of 20 tons and the finished ribbon 200 tons. The climbers = lifters or elevator car will be only 5% to 10% of the total ribbon mass, so the total movement is rather small. Perhaps more important, nothing happens a few miles up the ribbon when a climber starts up the ribbon. The ribbon stretches locally due to the weight which is increased by the climber accellerating up the ribbon. The stretch transient travels up the ribbon at perhaps 500 kilometers per hour, so the climber could outrun it's own stretch transcent.
If my 500 kilometer per hour estimate is average, it is 200 hours (The FAQ suggest 7 hours as the natural resonance/could this possibly be correct?)before the far end starts to fall toward Earth. By then the climber is likely past GEO altitude, and is now pushing (if applying the breaks) the ribbon away from Earth which partially cancels the pulling it was doing the first 36,000 kilometers. If the climbers stop at or before GEO altitude or leave the ribbon for destinations elsewhere in the solar system instead of parking at the far end as a counter-weight, there are acumulating problems keeping the ribbon from falling toward Earth. Over periods of weeks, it will require careful management of the transcients traveling on the ribbon. Over compensating can mean the ribbon is tryng too hard to move away from Earth which may break the ribbon due to excessive tension. This is an unlikely problem, unless telemetering gives bad data and/or humans make a  bad judgement call or micro meteorites damages the ribbon just before a strong transcient arrives. Please  embelish, refute and/or comment as I may have this wrong.   Neil

Hi Bob: Perhaps 100 of those small nukes need to detonate close to Earth's surface in the first seconds, (even using conventional explosives to lift orion the first meter) to produce one g to oppose the one g of Earth, to get the the Orion accelerating upward. Can the pusher plate survive? If Orion is built in orbit, much less than one g can be produced per one minute interval.
 Assuming Orion can lift off the ground, The rest of the fallout will be the 900 nukes and the pusher plate errosion and the new isotopes made from Earth's atmosphere which I think is much less harmful than vaporized launch pad. Also Orion can be rotated (soon after lift off) so perhaps 1/4 of the reaction mass leaves Earth's atmosphere.
 Humans have built huge projects successfully such as Orion without proto types or much testing of subsystems, but more often the results have been unsatisfactory.
 We would need to risk the CNT for a 100 ton capacity SE on a single launch instead of the CNT needed to support a one ton climber = lifter. The CNT may be half the total cost of the first SE = space elevator.   Neil

My guess is 1 or 1000 space elevators will change the weather very little. enviromental changes will be small to negligible. As far as I know, only a few ions will be discharge by the the space elevator with neglible impact.
EMP damages unprotected electronics, but neither EMP nor the ozone layer will be changed significantly.   Neil

The coolant water will exit the condencer at about 43 degrees f and can improve the efficiency of refrigerators, deep freeze ice makers and water coolers. At about 45 degrees, it can condence potable water from the air and cool hotels etc. At about 55 degrees f it can cool the roots of crops then go to the high pressure pumps which power tubines far below the surface at 60 degrees f.
For best operation the pressure in the condencer and turbine output should be high vacuum = 2 milibarrs?
The warm water from the surface of the ocean boils at about 75 degrees f at 20? millibarrs, so the turbine has only a tiny pressure and temperature gradient to generate electricity. To produce 200 megawatts it needs to be about 1000 times bigger = ten times taller + ten times wider + ten times longer  than a high pressure/high temperature turbine that produces 200 megawatts. Some waste heat above the 85 degrees water temperature can increase the electrical output considerably by super heating the water before it goes to the boiler. On sunny days solar energy can super heat the boiler water and/or the low presure water vapor that drives the turbine. Please embelish, comment and/or refute as I am mostly guessing how the system works.   Neil

The utility of of the electrical generation is somewhat exaggerated, so it is reasonable to suspect the water condensed on cold pipes, air conditioning the hotels etc with cold salt water and the much increased yield of food crops with cooled roots is also exaggerated. My guess is the system is barely competitive with good luck and few engineering errors in very few locations.
The weak point is bringing the cold 39 degree f water (up three miles and horizontal several miles to the condenser. The condenser can be located at the high tide mark and the turbine perhaps 5 meters higher at some risk that a tsunami will flood and damage the system. Higher means megawatts to pump the huge volume of water up hill.
Ten kilometers of 2 meter pipe has considerable friction loss if the flow through the pipe is at 100 kilometers per hour. Lower speed means the condenser will occasionally fail to condense, producing a black out or brown out. Worse, the column of 40 degree f water has (3%?) more the mass than the column of 60? degree water being pumped downward to turn water turbine powered pumps to pump the 40 degree water upward. Submersable electrical pumps are possible almost 3 miles below the surface, but the extreme pressure and salt water mean poor reliability.
A concentric pipe is likely with the 40 degree f water at the center, 2 meters in diameter, then thermal insulation, the 60 degree f water descending which will warm the 40 degree water to perhaps 41 degrees by the time it reaches the condenser. The perhaps 3 meter pipe needs thermal insulation for the perhaps 2 kilometers below the surface where the outside water cools to about 65 degrees to avoid heating the 60 degree f water.
The 60 degree f water is pumped downward at very high pressure requiring megawatts of pump power. The very high pressure means the concentric pipe is very costly to assure good reliability immersed in salt water. Actually a third concentric pipe is desirable as the deepest turbine should get the high pressure  60 degree f water first with the pumps closer to the surface getting the turbine discharge consecutively.   Neil

The duty cycle of lightning is very low, so it is unlikely it can be used to power anything. especially the space elevator which is located where lightning is rare.
Perhaps a quick reel in is possible, but more likely we will concentrated on repairing the break before the ribbon lands in the ocean.
My guess is CNT = carbon nano tubes are extremely resistant to high temperatures (in a vacuum) and to salt water corrosion, but the binders (poymers/epoxy?) may deteriate rapidly.
 Atomic oxygen = one atom per molecule may make CNT impractical in the upper atmosphere, but 99.9% of the ribbon is not in the upper atmosphere, so a good solution is likely.
My guess is minor repairs will be made to the tether 24/7, except during hopefully rare emergencies. After perhaps ten years the ribbon will be so lumpy from numerous repairs (some not very skillful) we will sell peices of it to use as rope etc.  Neil

I agree most of these problems are covered by probable solutions. Ocean water has enough salt that a metal ship has a better ground than most structures on land. We won't launch if a rare bad storm is likely. Even with precautions, repairs and a large safety factor, a broken ribbon is likely several times per decade. In most scenarios the gap takes hours to widen by kilometers, so repairs will often be practical, if repair teams stay on high alert.
My guess is the electrical conductance of the ribbon will be less than 1/10 that of copper even if we sacrifice strength to get low resistance, so the tether is not usable to send large amounts of electricity for hundreds of kilometers. My guess is we should instead sacrifice some strength to get high resistivity. This will reduce vulnerability to lightning and other possible electric current flow such as EMP = electromagnetic pulse.   Neil