Instead of a space elevator a well

brad50

Instead of a space elevator I would like the thoughts on a well. Create a structure from Geo orbit to a altitude where a the atmosphere is thick enough to harvest . Pump the gases to a altitude where it is most practical to processing the gases for shipping. I would like to think of spinning the structure with arms out stretched at different altitudes and lengths for mutable reasons. Fo gravity, momentum launch platforms. Could a arm be long enough spin at a speed at a altitude to mach orbital speed to permit docking or would the only practical lengths and speeds be so high docking at Geo orbit be the most practical and travel down the structure to work plate forms. Would we still have to wait for carbon nanotube fiber as strong as we need for a elevator or reaching not so far in to the gravity well save enough stress could we start earlier?

Ryan_m_b

I'm confused as to what you mean, where are you pumping the atmosphere to? That sounds hugely energy intensive. I'm also not sure how you would take into account the orbital instabilities of such a structure as it's mass is spread out along it's length it wont stay in geosynchronous orbit I don't think.

As for carbon nanotubes we can produce them in vast quantities and do so all the time. The problem is that we can only make them in tiny fragments as opposed to continuous tubes tens of thousands of km long.

brad50

Sorry for not being clear I have been told some day we will have the tech to make a elevator from orbit to the earth. My thought is before we have the capacity to come all the way down we could harvest gas from the upper atmosphere for breathing and fuel. My hope is the oxygen would make it worth doing. I thinking something small compared to the elevator still must be anchored in geo

Ryan_m_b

A space elevator would by necessity be an object in geostationary orbit with a tether trailing to a counter weight in higher orbit and a down to the surface of the Earth. The tether would be a flexible "rope" essentially, not a solid tower. Harvesting gas from the atmosphere for breathing and fuel would not be necessary, it would be far easier to harvest those things on the surface (much thicker atmosphere with lot's of industry already there) and send them up the tether.

Remember the distance from surface to geostationary orbit is over 30,000km, the atmosphere trails off to pretty much nothing 100+km up. It's not a case of building something "small compared to the elevator", to make an elevator trailing a tether to the upper atmosphere would require a tether just a few hundred km shorter. In addition the tether, as I understand it, must be anchored to the Earth for the space elevator concept to work. If it just hangs it will have severe problems.

brad50

Thanks I did think if we could reach enough atmosphere earth would be just a few miles away. Wondered if the stress of those few miles where significant. Did not realize the earth anchor point was such a asset

eachus

Nanotubes don't have to be all that long to make a good space elevator cable. The best nanotubes for making a cable would be on the order of one meter long. Shorter is better if you have knobs on either end of a single walled nanotube, tens of meters long if you can put external bumps on multiwalled tubes that couple to the inner layers.

In any case, the materials problem you are trying to solve is this: How much of the total load at any point will be supported by the nanotubes, and how much by the epoxy (or whatever else) you use to bind the nanotubes into a cable?

The problem today is either finding a way to grow long nanotubes with attachment points for the epoxy, or for epoxy to bind to nanotubes without destroying their strength. I hope the solution is found in time to make a cable and carry it to the ISS. (If you really want to retire the space station, using it as the end weight for a space elevator is a very good use. Better though, is to plan the feeding of cables out of the ISS so that it ends up either around one Earth radius out, or in geosynch orbit.)

Ryan_m_b

Even metres long is optimistic at the moment! Especially in terms of error correction, and I'm not even sure if there is an epoxy as strong as CNTs.

eachus

Sigh! Designing a cable that "works" if the epoxy takes 1% of the load is not that hard. Imagine two nanotubes overlapping for hundreds of nanometers, and the epoxy binding all along that length. If you can do that, even centimeter long nanotubes are more than sufficient, and 3 cm lengths are moving from the lab to production.

The remaining (huge) problem is either making nanotubes which have structural shapes, such as knobs on the ends, or finding a material which binds to the nanotubes, and can stretch as much as the nanotubes under load. The current state of the art is an epoxy which holds up to about 0.01% of the load the nanotubes can take, then the nanotubes outstretch the epoxy and the cable falls apart.

You hit a similar problem if you depend on nanotube to nanotube (atomic) bonding. The nanotubes "creep" under load, and the cable stretches in an inelastic manner. Doesn't break until you stretch it one nanotube thin. But if you keep it under load, you will get there.

To sum up, manufacturing (single walled) nanotubes with knobs on the end, and under a meter long would do the trick. As would centimeter to meters long multiwalled nanotubes where the outermost wall has gaps. Otherwise you need to find a much better binding material. Binding strength is not the big issue--staying attached to the nanotubes is.

Ryan_m_b

I don't get this, you are claiming that a significant part of a space elevator tether can be much weaker than the required strength and that's not a problem?

eachus

Sure. You need a cable with a given strength. More to the point, you need one with a given strength per kilogram/kilometer. You don't need every part of the cable to have that strength, you need the cable as a whole to have it.

Go look at a suspension bridge. The suspension cables are probably painted, and there is a good chance that the cables were wound of steel wire around a hemp core. The hemp core, if done right reduces the amount the cable stretches due to heat.

Here we have a different issue. Even if you could create 60 thousand kilometer long nanotubes, making a space elevator out of them would mean that one (micrometeorite) flaw in each of several strands could bring the elevator down. So you need to cross-link between nanotubes. As long as you are going to do that, you might as well use the cross-linking material to link shorter strands together as well. The epoxy, or whatever you use for cross-linking doesn't have to be as strong as the nanotubes. As long as the epoxy creates n bonds between two nanotubes, the epoxy can be 1/n times as strong as the nanotubes. If it is possible for all of those bonds to be simultaneously load-bearing.

Or you can use something that is a mechanical only link. Imagine that you can make cables of 10 meter long nanotubes woven together to avoid knotting. If the connectors on either end of the cable total 1 centimeter long, they can weigh 100 times as much as the cable per unit length without contributing significantly to the overall weight.

I think that best solution will be a cable with (bumpy) nanotubes in an epoxy matrix. Others think that (relatively) long stretches of parallel nanotubes with occasional bridges will be the best solution. In reality though, the first to meet engineering and manufacturing goals will likely be the solution used in the first space elevator.

Oh, one other note. It doesn't make sense to send humans to Mars unless they take along a space elevator. Using (your choice) one of the Martian moons as a counterweight, the weight is probably less than any other (manned) landing system for Mars. There are some tricks to a lunar space elevator, but one would probably be good practice for Mars.

Ryan_m_b

Right, so if I have this correctly you're saying that if the epoxy strength is 1/10th of the CNTs one could just use 10 times as many links. Do you know of any real life examples of this?
Mars would actually be a bad choice for an elevator, Phobos's orbit crosses the equator four times a day. Your elevator would be hit by it at some point, using one of the moons is out as well unless you bring along enough energy to drastically change the orbits (unlikely and if you did have it you could just use that energy to land on the surface directly.)

Alternatively I wonder if Phobos could be used as a skyhook.

eachus

I just realized that I need a disclaimer here. Yes, a space elevator would be neat, and I think that one or more will be built from Earth to space, although it will be installed from orbit.

The real interest in stronger nanotube cables is for space tethers. See http://spacetethers.com/ for a lot of the ongoing work.

Also note that combining space tethers and an elevator cable results in a much better and cheaper system than a space elevator alone. Such an elevator would hoist cargoes from Earth to around 200 kilometers (125 miles). From there tethers would shoot the cargo into an orbit that would meet the elevator near geosynchronous orbit about 12 hours later. No wear and tear on the main elevator cable, and you get to geosynch much faster.

eachus

Go to the nearest harbor, and look at all the running and standing rigging on the boats and ships in harbor. There are some (very high tech) cases where the standing rigging will be replaced by a single stainless steel wire on racing sailboats. Other than that, everything you see will depend on friction between strands of hemp or steel. With hemp, of course, the strands are in the tens of centimeter range, and friction between adjacent strands is the only thing that keeps a several hundred meter (or foot) long cable in one piece. Since hemp does this well, the overall strength of the cable per weight/length is about the same as the individual strands of hemp. Steel cables do the same trick but over longer lengths. (Oh, and if you race sailboats you will know that you need to inspect those single wire systems very carefully for cracking. If one lets go under stress, you are lucky if the mast doesn't go, and there is a significant risk of fatalities.)

The problem with nanotubes is that they make nylon look rough. This is why I was talking about engineering the shape of the nanotubes. With multi-wall nanotubes, you can get that thousands of connections between an outer and inner tube. Now chemically bonding to the outer tube won't damage the strength of the inner tube. Doing it in a lab is easy. Creating a 60,000 kilometer cable is not.That's why I gave you your choice. ;-) If your space ship weighs enough, just lower it from Phobos. If necessary you can start it swinging like a pendulum. (Reversing that process is pretty tricky though.) As for an elevator attached to Diemos, there are a number of ways of making sure that the cable always misses Phobos. You can move the attachment point on Mars, vibrate the cable, or just have thrusters at the right altitude to swing it out of the way. I think a vibrating cable is the most elegant solution, but any of them would work.