Space Elevator, Ho!

BWhite · 2005-08-22 08:43:37

Shaun, you cannot go up and down at the same time on the same elevator thread.

The "cost" to come down will start with the payments due on the construction cost. Assume a $10 billion elevator. At 10% amortized over ten years the monthy mortgage payment would be $132 million. If stuff is coming down, nothing can go up so the lost opportunity cost incurred bringing stuff down will be prohibitive.

It would be far more efficient to lift heat shield materials and dump your down cargo into the atmosphere instead of closing the elevator to traffic going up. Elevators need to be one way going up with the re-useable bits (laser power receivers and climbing parts for example) recovered via atmospheric re-entry and the pressurized capsules used to construct space structures.

Think one way, up only.

Shaun, I believe you are correct about the need to be at or very very close to the equator. A geo-synch elevator is dicey enough without the additional loads that would come from absorbing the Earth's rotation along a taut tether.

I think that shock loading a space elevator cable would be a bad idea.

reddragon · 2005-08-22 09:03:17

It would probably make the most sense to build an elevator with two cables: one for traffic going up and one for traffic going down. This will be initially more expensive, but will save money on atmospheric reentry and will more easily facilitate the transportation of people. Building the second cable will also be easier once the first has been built since the first can be used to bring material up.

Another possibility is to have a single, moving loop of cable with stationary cars on it. This puts all power requirements at the bottom or top, makes it easy for cargoes to be carried up and down at once, and makes monitoring and repair of the cable easier.

When we get to the point where Earth is importing things from space (PGMs, He3, etc.) and when ordinary people regularly travel to orbit or the moon for vacations or even for work, down as well as up transportation will be absolutely crucial.

Dragoneye · 2005-08-22 09:07:51

space elevator would be cool, but we honestly need to work with nano technology alot more and start getting nano machines out there helping out with building processes and what not... just immagine making diamond encased reactor cores out of a pile of coal, ropes that are highly flexible, lite, and strong.

heck you can use nano machines in bodies too. clean up any problems with the heart clogging, bad cells, fix more specific problems in the body and what not.. lots you can do.

John Creighton · 2005-08-22 09:08:20

Another possibility is to have a single, moving loop of cable with stationary cars on it. This puts all power requirements at the bottom or top, makes it easy for cargoes to be carried up and down at once, and makes monitoring and repair of the cable easier.

From what you describe I picture a gondola. A gondola would be too expensive at first and would massively increase the weight of the space elevator perhaps making it too expensive and unsafe. The first elevators will have very few climbers. Later elevators could possible have more continues flows if the risk of them falling down is not too much of a safety concern.

GCNRevenger · 2005-08-22 09:35:25

Although a moving "belt" sounds interesting, I would be worried about the mechanism being too difficult to deal with. How are you going to wind/unwind cable if you need to make an adjustment?

Two seperate, identical cables attached to a single station makes the most sense, one for up and one for down.

There is no financial reason (yet) for any private firm to invest the $10-20+ billion needed to pull this off. NASA either has to pitch in directly, or else provide a destination (Moon) worth going to.

Right now its hard to make CNTs in gram quantities, we have to come up with a better way first.

BWhite · 2005-08-22 10:49:12

I can foresee many, many up cables and many fewer down cables to provide delicate cargo and VIPs a gentle trip down to Earth. Otherwise, its more efficient to just drop PGM cargos with rudimentary heat shields.

Gee-forces are irrelevant.

John Creighton · 2005-08-22 10:51:49

I can foresee many, many up cables and many fewer down cables to provide delicate cargo and VIPs a gentle trip down to Earth. Otherwise, its more efficient to just drop PGM cargos with rudimentary heat shields.
Gee-forces are irrelevant.

Landing Genesis style. lol

BWhite · 2005-08-22 14:05:27

I can foresee many, many up cables and many fewer down cables to provide delicate cargo and VIPs a gentle trip down to Earth. Otherwise, its more efficient to just drop PGM cargos with rudimentary heat shields.
Gee-forces are irrelevant.
Landing Genesis style. lol

Hey, if the payload is a box of lunar rocks filled with PGM, smacking into the desert is not really such a bad option.

John Creighton · 2005-08-22 14:29:20

I can foresee many, many up cables and many fewer down cables to provide delicate cargo and VIPs a gentle trip down to Earth. Otherwise, its more efficient to just drop PGM cargos with rudimentary heat shields.
Gee-forces are irrelevant.
Landing Genesis style. lol
Hey, if the payload is a box of lunar rocks filled with PGM, smacking into the desert is not really such a bad option.

I’m curious how well it would work for rock and platinum group materials. Wasn’t genesis a high percentage aerogell. So it must have been really light. I’m also wondering if it would land softer if it spun like a discus.

TwinBeam · 2005-08-22 15:51:05

Edwards had a discussion group on the elevator for a while, to get ideas and discuss possible threats and so forth.

It'd be possible to have cargos move both ways, exchanging pods when they meet - but there's not much point and it makes things more complex and less safe and each climber would carry less mass, etc. It's simpler to just use the first elevator to send up the materials and build a second elevator ribbon. You can get a couple climbers going up at a time - I forget the optimal number but it was something like one climber every other day.

The "loop" idea doesn't work because the cable has to be tapered - much fatter/wider at the top. We tried figuring out some other way to transfer power along the ribbon to the climber, but I think the laser power approach is the best anyone's come up with. Some potentially useful ideas would work in atmosphere, but don't pay off much overall.

One idea that might make sense is terminating at a huge dirigible, to get above the majority of weather. In clear weather, it could lower elevator cars to pick up cargo/passengers. It could even "orbit" slowly, providing service around the globe. This mainly makes sense if the laser is moved to orbit, so it follows the elevator - the laser and power systems seem rather massive to mount on a dirigible, though not impossible. An Earth based laser makes more sense than a space based laser, as gravity falls off at about the same rate (square of distance) as the laser beam's energy density falls due to spreading.

Passenger service is a bit problematic, though not impossible. The tracked climber design is limited in speed - a trip up to GEO will take about a week if I recall correctly. Given the slow passage through the van Allen belts, some shielding would be required, adding mass and/or much higher power requirements. Alternatives like carrying up a rocket run into mass problems, but I don't recall anyone doing a study to see if there was a break-even point where it made sense.

Off-equator isn't a major problem, but the further from the equator, the more lateral tension you have to put on it. Beyond some latitude (45 degrees?) the natural "hang" would take the ribbon through the Earth unless a LOT more tension is applied.

Shaun Barrett · 2005-08-22 17:42:12

Bill:-

Shaun, you cannot go up and down at the same time on the same elevator thread.

I remember Arthur C. Clarke suggesting that this is precisely what we should be doing, in order for the potential energy of the descending car to be converted to electrical energy as it 'falls' deeper into Earth's gravity well. This electrical energy is then used to power the upward-bound car.
-- However, I'm willing to concede that practical transfer of that electrical power from the descending to the ascending car may not be possible, which is where ground-based laser power comes in.
-- Is there any reason why a sufficiently broad cable could not be able to accommodate cars moving up and down simultaneously, which is how I'd always visualized the system working? :?:

TwinBeam:-

Off-equator isn't a major problem, but the further from the equator, the more lateral tension you have to put on it.

I find this hard to visualize and harder to believe, though I'm prepared to be persuaded. Do you have any links to articles or papers (especially any diagrammatic representations) which support this idea?

reddragon · 2005-08-22 18:44:55

-- Is there any reason why a sufficiently broad cable could not be able to accommodate cars moving up and down simultaneously, which is how I'd always visualized the system working?

Interesting; I'd never thought of using a broad cable like that, but I don't see any real reason for it. Is there any advantage over two cables with it? Even for the power transfer idea, you would need to have a power line running the length of the cable. With two cables you could still have the power line running the length of each and connect them at the bottom to transfer power. But if you're running a power line up the elevator, you might as well just build a power plant at the bottom.

The tracked climber design is limited in speed - a trip up to GEO will take about a week if I recall correctly.

I'm not sure if this would be practical, but could you have the cars stop in LEO, and unload passengers and cargo at a space station attached to the cable there. The cable would still continue to GEO and beyond, to put the center of the orbit in GEO. But only the lower part of it is used.

GCNRevenger · 2005-08-22 18:56:28

A wide cable... a novel idea. Harder to make a wide cable using a narrower "seed" cable however, since it would be weaker at the edges then the middle.

Power transfer is problematic because you can't transfer power long distances with metal wires. Even a dedicated carbon nanotube power line might not be able to transmit power 36,000km.

A space station at any orbit lower then GEO is a bad idea for one very simple reason: the station would not be moving at orbital velocity. As such, part of its weigh would be "hanging" from the counterweight, and place enormous strain on the cable. You would also not be in zero-gravity.

BWhite · 2005-08-22 19:57:25

-- Is there any reason why a sufficiently broad cable could not be able to accommodate cars moving up and down simultaneously, which is how I'd always visualized the system working?

Of course maybe you can do it, but elevator company A sending two cars UP on both sides of that wide will very likely make more money than company B sending one car UP and one car DOWN.

Shaun Barrett · 2005-08-23 05:34:01

Bill:-

Of course maybe you can do it, but elevator company A sending two cars UP on both sides of that wide will very likely make more money than company B sending one car UP and one car DOWN.

I understand your point. It all depends on the system and the costs.
In the case of passenger cars, it might be different. I envisage a multi-tonne multi-storey car with bedrooms and communal areas, including an observation deck - after all, it's a one week trip each way, and riding one of these cars to GEO will be a spectacular experience, enjoyed perhaps only rarely in a human lifetime.
-- Imagine passing through gravity equivalent to 0.5g, then Martian gravity of 0.38g, then Lunar gravity of 0.16g, on your way to zero-g at the GEO station! What a buzz! And don't forget the incredible view of Earth receding beneath your feet, as seen from the observation deck!!

Returning these "Pullman cars" of the space-age to Earth in free-fall, using a heat-shield and parachutes, will not only be impracticable but undesirable, especially for older passengers.
-- By all means, employ reusable containers with disposable heat-shields for cargo, if you wish, or if it turns out to be better from a financial viewpoint, but for people it won't be as attractive, I think.

As for power transmission, a separate carbon nanotube power line may well be up to the job in transferring potential energy from the descending car to the ascending car. Who knows what electrical-conduction properties we may be able to induce in these amazing tubes? Perhaps we won't need a ground-based laser at all. We'll have to wait and see.
-- In any event, I think Space Elevators must surely make more sense than the "Roman Candle" routine we have to go through these days! Just a few thoughts.

C M Edwards · 2005-08-23 07:19:03

I wonder how this stuff would work for balloon applications.

Hmm... Sheets of kevlar are already heat sealed onto sheets of PET for balloon gores. I see no reason this material couldn't be used instead of kevlar. This stuff could produce balloon fabric of the same strength at a fraction of the weight and thickness.

Cool! 8)

GCNRevenger · 2005-08-23 08:11:51

CNTs have conductivity theoretically tens of thousands of times better then plain old copper wire, but practical conductivity is probobly only a few hundred or thousand times. While this is really good, it isn't good enough to transmit power from the ground all the way to GEO most likly.

Heat-bonding pure CNT sheets might not work well either, if you can't get the matrix (like PET in balloons) to entangle with the CNT mesh.

Complete, total, "no kidding" reuseability of the elevator cars on a seperate "down" track will probobly trump the cost of a dedicated, independant reentry vehicle for cargo in the long run.

reddragon · 2005-08-23 09:22:13

The "loop" idea doesn't work because the cable has to be tapered - much fatter/wider at the top.

I see the reason for tapering, but I would think it would create problems for the climber. Any climber that grips around the cable will need to constantly be readjusting the size of its grip. And CNT doesn't give much traction, I think. Would a non-tapered cable be possible, or would the weight simply be too much?

Shaun Barrett · 2005-08-23 21:13:20

Reddragon:-

Would a non-tapered cable be possible, or would the weight simply be too much?

I don't remember Brad Edwards' "HighLift Systems" Company discussing tapering of the ribbon as part of their plan to put up a SE in the next 10-15 years. I'm not sure tapering is actually necessary with CNTs(?). :?:

Just in case you didn't catch this news item earlier in the year:-
http://www.azonano.com/news_old.asp?newsID=808
-- It was an announcement by "Liftport Group" that they were to commence commercial nanotube production in June 2005. (Presumably, production is well under way by now.)
From the article:-

LiftPort Group, the space elevator companies, has announced plans for a carbon nanotube manufacturing plant, the company's first formal facility for production of the material on a commercial scale. Called LiftPort Nanotech, the new facility will also serve as the regional headquarters for the company, and represents the fruition of the company's three years of research and development efforts into carbon nanotubes, including partnering work with a variety of leading research institutions in the business and academic communities.

It went on to say:-

LiftPort Nanotech will make and sell carbon nanotubes to glass, plastic and metal companies, which will in turn synthesize them into other stronger, lighter materials (also known as composites) for use in their applications. Already being used by industries such as automotive and aerospace manufacturing, carbon nanotube composites are lighter than fiberglass and have the potential to be up to 100 times stronger than steel.

I'm optimistic that we're on our way!

SpaceNut · 2005-08-24 05:17:59

Reddragon:-
Would a non-tapered cable be possible, or would the weight simply be too much?
I don't remember Brad Edwards' "HighLift Systems" Company discussing tapering of the ribbon as part of their plan to put up a SE in the next 10-15 years. I'm not sure tapering is actually necessary with CNTs(?). :?:

Tapering was always the plan if it were a ridgid pole design, wider at the base getting smaller the higher up as a load barring weight reduction.

How does one anchor a cable to and orbital platform, without pulling the platform from orbit?

reddragon · 2005-08-24 09:26:13

How does one anchor a cable to and orbital platform, without pulling the platform from orbit?

By having another, equally heavy cable going the other way. Thus the center of mass is in geosynchronous orbit. The cable need not be rigid, and the end is not supported by being on the ground. If it is too be tapered at all, the wide end is at the top.

I'm optimistic that we're on our way! Smile

So am I. I'm kind of surprised NASA hasn't been putting much thought, that I know of, into a space elevator. If a private company builds it, NASA and other parts of the government (such as the military) will be paying them every time they want to launch something, while if a government owns it it can reap in the tax dollars from private use.

SpaceNut · 2005-08-24 10:03:11

How does one anchor a cable to and orbital platform, without pulling the platform from orbit?
By having another, equally heavy cable going the other way. Thus the center of mass is in geosynchronous orbit. The cable need not be rigid, and the end is not supported by being on the ground.

So then we are using centripicle force of Earth spining by length to geosynchronous orbit with mass multiplier of attachment in orbit to keep cable up but then as each payload is put onto the climber, the apposing force on the cable by the mass of the payload will pull it back to Earth. The geosynchronous mass then must be more than the mass of the cables and the payload to which we wish to bring to orbit in order to keep the cables out stretched.

TwinBeam · 2005-08-24 11:42:53

I'm kind of surprised NASA hasn't been putting much thought, that I know of, into a space elevator. If a private company builds it, NASA and other parts of the government (such as the military) will be paying them every time they want to launch something, while if a government owns it it can reap in the tax dollars from private use.

Uh, I'd prefer they DON'T catch on. Government, including NASA, can't seem to accomplish anything without it costing about 10x what it'd cost if done by a corporation or other non-government entity. So they'd keep costs high and skim off those customers who are willing to pay the higher price - doubly discouraging private investment in an affordable elevator. Sort of like the shuttle did...

But, that said, NASA is paying some attention to the elevator - google it.

Also - I said earlier that a loop isn't possible. There are some ways to make it work "in theory" - e.g. many loop segments, each slightly thicker than the segment below - but I suspect the technical problems make this impractical, especially early on.

BTW - people might want to check out http://liftport.com/ - they have a good discussion forum.

reddragon · 2005-08-24 16:54:23

Uh, I'd prefer they DON'T catch on. Government, including NASA, can't seem to accomplish anything without it costing about 10x what it'd cost if done by a corporation or other non-government entity. So they'd keep costs high and skim off those customers who are willing to pay the higher price - doubly discouraging private investment in an affordable elevator. Sort of like the shuttle did...

True. Of course a private space elevator isn't guaranteed to solve these problems, although it's probably better than the government. I was more expressing surprise that they hadn't been doing it, than commenting on whether they should or shouldn't.

Shaun Barrett · 2005-08-24 19:58:41

Hi SpaceNut!
-- Forgive me if I'm wrong but I think I detect some confusion in your mind about the basic mechanics of Space Elevators (SEs). :?:
In the days before CNTs, people tried to design SEs as best they could using the materials available at the time. The stumbling block, of course, was always the lack of a material with a tensile strength great enough to support even just its own weight over a length of tens of thousands of kilometres, never mind that plus the weight of multi-tonne 'climbers'! For this reason, it was found necessary to taper the cable - making it thicker and stronger the higher up you went, in order to support the weight of cable below. Working in our favour was the gradually lessening pull of gravity the higher you went, but nevertheless any cable they could devise at the time became exponentially wider as you approached Geosynchronous Earth Orbit (GEO).

At that stage, the shorter you could make the actual cable, the better chance you had of making the SE work because it resulted in a massive reduction in the necessary thickness of the cable at GEO. So, it was thought that building a self-supporting tower from the ground up would be a good idea - especially if that tower could be higher than anything ever conceived of by humanity before.
-- I recall seeing a website somewhere, in which they discussed the theoretical possibility of constructing a tower rather like the Eiffel Tower in Paris as far as the shape was concerned, but reaching an altitude of 50 kilometres!! (Some 6x higher than Mt. Everest.)

Whichever way they looked at the problem, though, the practicality of any solution they arrived at failed because of the lack of a material they dubbed 'unobtainium', from which to make the cable itself.
-- Now we have CNTs, the theoretical difficulties of building the SE have disappeared. We've obtained 'unobtainium'! Now, it's just a matter of engineering.
-- As far as I'm aware (and this is something I'm prepared to be corrected on), CNTs are sufficiently strong not to require thickening towards GEO. As I mentioned in a previous post, I don't remember anyone at Liftport Group or HighLift Systems talking about tapering the cable.

Your quote:-

So then we are using centripicle force of Earth spining by length to geosynchronous orbit with mass multiplier of attachment in orbit to keep cable up but then as each payload is put onto the climber, the apposing force on the cable by the mass of the payload will pull it back to Earth. The geosynchronous mass then must be more than the mass of the cables and the payload to which we wish to bring to orbit in order to keep the cables out stretched.

The way it works is this: GEO is at 36,000 km altitude, which is where you have to keep the centre-of-gravity (c.o.g.) of your satellite in order to maintain stability. From your satellite, you lower your CNT cable down toward your ground-station. Obviously, the act of doing this shifts your c.o.g. downward, too, which destabilizes your orbit. So, you have to either gradually shift your satellite slightly further away from Earth as you lower your cable, or keep the satellite where it is and simultaneously extend a length of cable outward. Either way, you can keep your c.o.g. precisely at GEO and maintain orbital stability.

Let's assume you're going to extend a cable both downward and upward, simultaneously, in order to keep your c.o.g. at the required height above Earth (36000 km). The length of cable you release upward will have to be greater than the length of cable you release downward, because the cable going downward is getting closer to Earth and therefore experiencing a stronger gravitational pull, while the outward-bound cable is getting farther away and experiencing less gravity. This is compensated for, to some extent, by centrifugal force, which causes more outward force on the outward-bound cable the higher it extends above GEO.
-- But the upshot is you need to release roughly twice as much cable upward as you do downward, to keep the c.o.g. at the geosynchronous orbit. This means you need about 100,000 km of cable - 36,000 km of it hanging down to the ground and 64,000 km of it hanging up and out into space!

In theory, the lower end of your cable needn't be attached to the ground-station at all. It could end just 1 metre above the ground and, as long as the c.o.g. of the whole system remains stable at an altitude of 36,000 km, the bottom of your cable will stay 1 metre above the ground!
-- But as soon as you try and attach a climber to the cable, the c.o.g. of the system is now disturbed and you're starting to pull the satellite downward out of its stable orbit.
-- That's why we have to attach the lower end of the cable to the ground and deliberately shift the c.o.g. of the system a little farther out, in order to use centrifugal force to put many tonnes of outward tension on the cable. This is easily achieved, once the lower end is secured, by simply releasing more cable outward from your satellite.
-- With your cable now taut, you can attach your climber and up you go!

Another advantage of balancing your c.o.g. by extending cable up and away from Earth at the same time as you're lowering the other end to the ground, is that you get yourself an interplanetary launching system into the bargain.
-- The outer end of your cable is whizzing around the world once every 24 hours at an altitude of 100,000 km above the ground. It's speed is roughly 28,000 km/hr. If you send an interplanetary spacecraft out to the end of the cable .. and let it go .. it will leave Earth's gravitational influence and head off into deep space, using Earth's rotation to propel it instead of rocket fuel!
-- Depending on the moment you choose to release it, you can send it off toward any planet you like.

Brilliant, isn't it?!

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