Space Elevator within 15 years?

Great Debates
81

The thickening you are addressing is the handle the longitudinal forces along the cable, not the transverse forces which are going to be far greater.

Picture this: you are swinging a long 4x4 around. Let go of it and notice how it rotates as if flies away from you. What kept it from rotating that way before? Your arms provided a major transverse force to keep the end of it from rotating the way it wanted to. Scale up a million times. What are the transverse forces now? “Ohmygosh” sums it up nicely. It has to be extremely thick at the bottom to handle this sideway force. Thickening in the middle is a low order term.

I ignore the wind when I refer to instability. Gravity (Earth and Moon) alone will break it or crash it within an hour. The postings about its center of gravity are too naive to even try to respond to. It isn’t a point mass, it cannot be treated as a point mass. Spheres can be, this is obviously not a sphere.

Again, what was the situation 1 hour before final hookup? You’re telling me this mountain-sized end on a string is going to come swinging down thru the atmosphere at top re-entry speed, not burn up, and then just “stop”. And along its whole length too?

If you can’t see the obvious flaw in this, you’ve been reading too much Science Fiction.

82

I don’t believe that is true. When you’re swinging it around, the only forces acting on the beam (your 4x4) are centripetal; they’re what keeps the thing moving in a circle. By virtue of the fact that the beam has a length, the velocity at one end of the beam and the other end must be different (ie. because the distant end has to go farther in order to make a revolution in the same amount of time). At the moment you let go, it is this differential velocity which provides the spinnng you’re referring to. Looked at in a different way: The angular momentum the beam has when you’re swinging it around has to be conserved, and goes into spinning the beam about its center of mass when you let go.

For reasons in addition to the one you mention, you’ll need to provide Monstrous Braking to your unreeling mechanism in order to keep the tether deployment down to subsonic speeds. Sure, it’ll take days (weeks?) to deploy, which re-raises the question of the thing drifting away from the base station before you get ahold of it. But you can’t let it deploy at re-entry speed. In fact, as you get near the end of deployment, you’ll want to slow it down further, which will be difficult, but not impossible.

83

You are seriously misunderstanding the physics here. Nothing kept the beam from rotating before you released it; it was always rotating as long as you were swinging it. Your arms are not providing any shear (or “transverse”) forces at all on the beam if you are swinging it at a constant velocity. The only force involved is a tensile force keeping the beam from flying away from you.

In a tether, the only forces of any significant magnitude are the tensile forces generated by the fact that the tether is spinning end-over-end once per day in sync with it’s rotation around the earth.

Who said anything about re-entry speed? You can lower the end of the tether as slowly as you like through the atmosphere. Except for the wind, it’s stable the whole way down. Without an anchor it may drift around randomly, but it’s not going to have any major want to move sideways at high speed.

Consider a sattalite in geosynchronous orbit. It travels around the earth once per day. Now start to extrude two cables from the sattalite - one towards the earth, one away from it. Tidal forces will cause the sattalite-and-tether arrangement to align itself so that one tether is always pointing at the earth, and the other is always pointing away. The arrangement is naturally stable, and since the sattalite is in a geosynchronous orbit, the lower tether is always pointing at the same spot on the earth.

If you continout extending the tether long enough in both directions, the lower tether will eventually touch the ground at the spot directly below the sattalite. Since the entire mass is in geosynchronous orbit, there is no sideways motion of the stack (other than random drift) that needs to be dealt with, and tidal forces keep the whole thing stable and aligned the whole way. Where is this supposed instability coming from?

84

And that would work on an airless body, but it presents serious difficulties when trying to work with the Earth.

How do you plan to get this tether to the ground, considering the wind forces on it? This is not trivial - wind forces on a building that’s only a few hundred feet high are enough to cause the entire building to sway back and forth. Now imagine something being hit by winds along a 100,000 foot length. And the winds in the upper atmosphere can be going hundreds of miles per hour.

Not only that, but the wind isn’t all going in the same direction. As you increase in altitude, the direction of the wind changes. So this thing is going to be tugging on the satellite unevenly - the tether going away from earth is providing forces in tension only, while the one going through the atmosphere is swinging around wildly.

These forces are going to be far greater than reaction jets on a satellite could control.

This problem isn’t solved by sending down a really small tether first and then ‘spinning’ new strands once its anchored, because even a small tether will generate huge forces. The wire struts on a biplane can take 40 mph off the top end speed, and they are only maybe 1/4" in diameter. Circular cross-section disrupt airflow and cause turbulence behind them, which creates immense amounts of drag. If you don’t believe me, next time you’re driving in a car try holding a pencil out of the window. You’ll be surprised by how much force is acting on it.

So how exactly do you get this tether down to the ground?

85

Yeah, wind is going to push the tether all over the place as it’s descending. I don’t think that alone is an insolvable problem - the wind forces shouldn’t be strong enough to break our unobtanium tether. It will make it nearly impossible to drop the tether precisely on one spot, so what you’ll have to do is drop the tether kinda near the anchor point, then grab it with something large and heavy and tow it to where it’s going to be permanently anchored.

86

I wasn’t thinking about the tether snapping - I was thinking about the extruding satellite’s orbit being messed with. There will be a tremendous force acting on that satellite. Especially the plan outlined in the OP, in which a small satellite that fits into the shuttle would extrude a very, very thin tether down to the Earth. In that situation, I could easily imagine that the wind forces on the tether would simply pull the entire satellite out of orbit. If you look at the amount of kinetic energy stored in a satellite that can fit in the shuttle, how does that compare to the total wind forces exerted on a tether that has to cut through 50 miles of atmosphere?

What happens if the Satellite is pulled into a lower orbit? It’s no longer geosynchronous, and now the whole shootin’ match is slowly making its way around the earth.

I must admit that I’m having a hard time picturing the dynamics of this whole thing, but I’m certain that the problem isn’t as simple as extruding cables in opposite directions until one touches down.

That’s why I said in my first message that the devil in projects like this is always in the details. The ‘big picture’ concept is simple enough, but attempting to actually implement it will take a ton of R&D and probably a lot of new engineering techniques. You know, little thinks like being able to easily spool and unspool the nanotubes in space, electric charges, wind forces, safety, emergency procedures, you name it.

87

I was starting to post a reply to Sam Stone’s latest post, and then realized that we’re pretty much in agreement on everything, so I stopped elevating the violent agreement. :slight_smile: My summary of our viewpoint: Sure, if you have the materials, then building a space elevator is possible. But working out all the engineering details to get it to work will be a, um, female dog. (I’m not clear on cursing: are general swear words not allowed outside the Pit, or is it just insults (esp. ones that use ‘bad words’) that are verboten?) And the economics are not clear, but can’t be ignored.

Personally, I’m more interested in the steps you’ll have to take to get from where we are now (100 tons to orbit per year, only a few people in space at a time) to where we would be with a space elevator or similar facility (millions of tons to orbit per year, thousands or millions of people permanently in space). I guess I should start a new thread to ask that question, though.

88

Let’s just all remember what happened the last time we tried to build a massive tower to reach the sky. In Babel, as I recall. :wink:

In all seriousness - good discussion, chaps. I’ve always wondered about the practicalities since I first saw A C Clarke’s idea as a kid.

A possibly foolish thought: what if the technology existed to build this thing quickly. Like, ‘close to the speed of light’ quickly. Would that overcome some of the problems associated with an unfinished cable? Certainly ftg’s “an hour before completion” scenario wouldn’t exist!

I know, I know. Stupid.

pan

89

Dammit Kabbes, I was thinking on the way in this morning about posting something about Babel.

90

Forget my earlier suggestion about removing the atmosphere; I have a Better Idea; temporarily increase the rate of the Earth’s rotation to such a point that the centripetal (or is it centrifugal? - I’m never sure) force exceeds that of gravity, then we can just let the tether reel out from the gound.

91

Hmmm. I never thought about extending the cable past the GEO point. Let’s just say I wouldn’t want to work out there. Even my emergency safety tether wouldn’t give me a feeling of security.
Read Robert L. Forward’s book Indistinguishable from Magic. He lists a number of ways of building a “virtual space elevator” that don’t require a single strand of unobtainium with its impossibly high tensile strength. I’m not sure I like his suggestions, but at least you wouldn’t have to worry about an enormous cable breaking and wrapping itself catastrophically around the earth.

92

Assuming you had the ability to materialize 60,000 miles of matter instantaneously, why bother building a space teather? Just materialize whatever you want directly to orbit.

93

Much as I am hesitant to defend what is quite possibly a foolish statement, I more meant that the cable would be constructed as already mentioned in earlier posts, but that the construction would not take years but seconds. There would be no time for significant stresses and loads prior to completion, because within seconds it would be complete.

It’s a matter of having these nanobots we’re talking about working unimaginably quickly rather than at current speeds.

pan

94

You generally don’t use microscopic robots to build macroscopic projects. In any event, the more exotic you make the construction process (nanobots, stopping the Earths orbit, etc) the more you prove that this project won’t be started in 15 years.

95

How are these nanobots supposed to move? Down the tether as it’s completed? If so, then if it only took 10 seconds to build a 24,000 mile long tether, the nanobots would have to be building at a rate of 8,640,000 mph. It would be impossible to get a nanobot to MOVE that fast, let along BUILD that fast.

96

Look, no offense intended, but where are you folks getting some of the outrageous assumptions about the topic? Sure, there are enormous problems with building a ‘beanstalk’. I doubt it will be built in 50 years. Possible never. But I swear I’ve seen more made-up assumptions lately posing as facts, than actual issues regarding the proposed materials and construction.

Examples:

  • ftg says the cable would be coming down at reentry speed (!), cobalt seems to confirm this. Where the heck does it say that the tether would descend at high speed in the published articles? Cite please? Everything I’ve seen implies that the cable might as well descend at walking speed if it’s convenient, and that this is under the control of the ‘extruder’ at geosync orbit.

  • kabbes contention that the cable would be built ‘in seconds’. Where did you read that one?

Finally, ftg, there are no transverse forces operating on the cable (although we’ll treat weather as a sepate & important topic). It is in a stable position from start to finish and the forces are in balance. You are currently moving at 1000 mph if you are on the surface of the earth. So, what transverse forces are operating on you? None? Yep. If you had a personal tether to the center of the earth, what forces would operate on the tether. None? Yep.

OK, one more to ftg:

97

You need to re-read kabbes posts regarding that one, noting carefuly the manner in which the idea was suggested.

98

Thankyou mangetout.

Squeegee, I hardly think that my suggestion for how a problem might possibly be overcome given sufficiently advanced technology counts as a “contention”. I’m not for a moment saying that this is how it would be done.

msmith - No. Not within 15 years. Hell, not within 50 years. But reading this thread has pretty much convinced me of that anyway.

Sam - Is it theoretically impossible for nanobots to construct at that speed? Or is it just extremely unlikely? In 100 years? 500 years? 9 million miles per hour is still just a fraction of the speed of light.

I can’t believe that I’m seriously defending this. But I don’t see how else the construction could be achieved. Leave a dangling cable there for any significant length of time and you’re screwed.

pan

99

At those kinds of speeds, the nanobots would require enormous amounts of energy. They would also absorb immense amounts of frictional heating from the atmosphere and their own motors. Think about the shuttle’s tiles glowing red-hot. Now imagine something going 100 times that speed.

I won’t say that it’s *impossible without thinking about it some more, but certainly it would require a technology that is not going to be available in the lifetimes of us or our grandchildren, for that matter. Hell, I’ll go out on a limb and say it’s impossible.

But more to the point, if we had the kind of energy and technology to make million mph nanobots that can build a beanstalk in a matter of seconds, then we’ll already be so advanced that a beanstalk will be a trivial problem anyway. So forget the nanobots.

If this thing is going to be built in our lifetimes, it would have to deal with atmospheric forces. That’s extremely difficult for a satellite in orbit to do. The math isn’t that hard, but it’s tedious. You’d have to figure out the cross-sectional area of the tether at each altitude, the speed and average density of the air at each altitude, then come up with a figure for drag forces. Then figure out how long this thing will be hanging in the atmosphere before it’s anchored, and you can figure out the reaction mass required in your satellite to compensate for the motion.

But then you have another problem - the tether going out into space will oscillate if you move the satellite around underneath it, and start whipping around. So you’d have to engineer a system that can compensate for changes in drag pretty much instantly.

The most cost-effective solution might involve extruding the tether down into the very thinnest altitude, then flying up another tether from the ground that’s already made and connecting the two. Once they are connected, you start paying out the tether in the opposite direction to put the whole thing under tension, and now you’re in business.

That would minimize the time the tether is causing satellite problems. But these are the kinds of engineering problems that would have to be analyzed and solved before we could do this. That’s one reason why the 15 year timeframe is ridiculous.

Remember a book called “Colonies in Space” by TA Heppenheimer? Or the original goal of the L5 society? I still remember when I was a kid, and hearing scientists seriously talking about how we could have 20,000 people living in floating space colonies by the year 2000. This was back around 1975 or so. Here we are in 2002, and I don’t see them. This 15 years-to-build-a-tether idea falls in the same category - something that only sounds do-able until you start looking at the fine print and coming to grips with the sheer amount of engineering work required.

100

In fact, Mangetout, since it is said that “Greed is the root of all evil,” there would be an even greater benefit: The greedy guys would hang on to the stuff and would experience a modern version of King Midas’ touch. :smiley: