Space elevator. Dream, hoax or reality?

Dont mistake me for a pessimist, i just like to try and see all corners of the problem. Playing devil's advocate helps bring them out. I would love to see this happen! Our debate is not a question of possibility, just one of probability.

Mark Vitrone

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Attach a frictionless pully to any point on the elevator. Run a rope from the ground to this pully. Attach mass A to the rope at the ground station and mass B to the rope at the pully station.

ALL of the energy required to raise mass A is supplied by lowering mass B if the two masses are equal.

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Question 1: How to we get mass B up there in the first place. The Work required to get B up there should be added to the equation and not considered as a separate Work. Then, if you are to get B up there, why not getting A, in the first place?


Answer 1: The pump DOES have to be primed, so either some mass has to be lifted without a counter-mass, or some mass already up there has to be used. But once you have some counter mass the process becomes "energetically" balanced. To oversimplify things, think in terms of mass A being raw materials going up to a factory and mass B being finished products comming down to the consumers.
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Question 2: The rope has weight. Any idea how much a 100,000 meter rope can weigh?


Answer 2: Keep in mind that this is a thought experiment (the frictionless pully should be a dead giveaway). Its purpose is to show how beanstalks and related devices can move things around the Solar system with much less ENERGY EXPENSE than rockets.

So I stipulate a massles rope. (In reality we might use something like a linear motor/generator with battery or flywheel storage rather than a rope.)

Construction and maintenance expenses are another question, but then all systems will have these. I haven't seen a detailed analysis of these costs for the STS vs a beanstalk program. But I'll be somewhat surprised if space elevators are anywhere nears as expensive to build and operate as rocket based systems with the same annual capacity.
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Question 3: You need a force to start B moving and get it half way, at which point, it will move A by gravity force. Any idea how to get that force? What about deflection stresses cause by the force and initial jolting (impulse)?

Answer 3: Otis makes some electric motors that could be modified for severe service.

Discounting friction (and other energy losses), it is necessary to supply power to get mass A halfway up and mass B halfway down. Then it is necessary to extract power for the rest of the trip to have them both stop at the intended locations.

Energy is power X time, and the total energy input needed to reach the halfway point is the same as the total energy recovered during the second half of the trip.
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This is not a question of pessimism or optimism. Proper engineering and physics -- thank God --- do not rely on such principle but on sound application of theories and demonstrations. I do not have answers but only questions. If I had answers, I wouldn't have started this tread.


That's OK, because I do have answers. At least some of them. (I'm not saying I came up with them, I'm saying I know what they are.)

We won't know all of the answers until we build and operate some of these things, of course. You really should check out that document I keep refering to. Some of their answers are surprising. And some of the problems they have identified (and that haven't been mentioned here) are surprising, too.
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Are we talking about the same thing?

Your original question was (approximately) "Space elevators make space travel ENERGETICALLY cheaper, right?" Emphasis added.

I've been talking about this (the comparative energy needed to move things around with an elevator vs other methods), and responding to additional questions that seemed to follow from it. Massless ropes, frictionless pullies and other ideal devices are of course restricted to thought experiments. Their use in a thought experiment can often make the underlying physical principles easier to see and talk about.

No one should contemplate their use in an actual machine design. For real world design you have to account for all of the nasty stuff - things like mass (inertia & weight), friction, vibration, wear & tear, radiation effects, accident and sabotage, magnetic effects, electrical effects, heat flow and material property changes as a function of all of the above (at least), etc. And as if these things weren't enough, you also have to factor in economic and political constraints.

But, to see how all of these things are being treated in a REAL WORLD design proposal, you have to look at something like that 80 page report I have mentioned, not at a thought experiment intended to show some of the basic physical aspects of a system in a simplified way.

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All new things face opposition and sometimes that opposition turns out to be valid. Until carbon nanotubes came along there were several problems with (then real world) beanstalk designs that kept them from being considered realistic.

Those have gone away now, but there may be new problems that will eventually assume similar proportions, and the beanstalk will go back onto the "not yet" pile along with magnetic levitation, nuclear (fission) rockets, fusion (cold AND hot), matter-antimatter power systems, etc. For example, we still haven't figured out how to make individual nanotubes long enough to ensure that a composite nanotube cable a hundred million meters long can be built from them.

Current designs are based on the assumption that we will in fact solve this problem. Progress in this area is rapid and several candidate solutions are being evaluated as we speak. But there have been similar optimistic predictions about hot fusion and look where we are with that. Bummer.

Regards,
LB

just one (side)question:
how exactly would we join nanotubes to make huge wire for optics, or any apllications like that if that would they be stable?

The intuitive mind

Even with massless rope and frictionless pulleys we has not explained how this would save energy all this could be done with the rocket and a device to store the energy lost in the return trip. And then there angular momentum that has not been accounted for.

Aside from the mechanical challenge of building the skyhook I am not sure that it is going to save as much energy as we want. The basic premise assumes that the mass flow going down is going to be roughly equal to the mass flow going up. But it seems likely that for a long time most of the mass flow will be going up. It won't be until you have a moon base and some kind of manufacturing plant that you will be able to process materials to bring back and balance out the load.