# Space elevator twists and alternatives

From I what I understand, the greatest difficulty in making a Space Elevator is material strength – that is, having a cable strong enough to support all the weight of the miles and miles of cable necessary.

So I understand that material research is making a lot of progress, but we’re still a long way away.

So how can we get around this, or make the problem easier to solve?

How about this: A geosynchronous satellite, with a long cable, but it doesn’t go all the way to the ground – in fact, it doesn’t even get close to the ground. It just barely dips into the highest altitude that can be reached by helium or hot-air balloons (or some other aircraft – balloons were just the first to pop into my head). Then, in what would undoubtedly be a challenging feat of ballooning, balloons (or other aircraft) could relatively cheaply bring small payloads up to the cable which could bring them into space.

Would this work? Would it make the materials problem easier?

If it’s a long structure with its long axis pointing at the Earth, only part of it will be in the right place to be in geosynchronous orbit - the bottom end and top end will be wanting to do something else (and even more so when you start moving mass up and down it)

That cable danging in the atmosphere will create a lot of drag on the satellite. It would need to be constantly refueled to stay in position.

Good point. Hopefully, the fuel could go up on the balloons – it would be an interesting math problem to see if the fuel could be brought up fast enough to make it economical.

OK, let’s say a little space station at the top maintains its position (using thrusters or something) in geosynchronous orbit – what would be happening to the dangling cable?

I had thought that geosynchronous orbit meant, essentially, that you could stand underneath the satellite and look up, and it would be there, and it would still be there hours and days later if you look up from the same spot. So I would have understood that any dangling cable would still be “up” from the same point on Earth.

Geosynchronous orbit is over 22,000 *miles *up. You’re going to need a big balloon.

Please read the OP again. The balloons aren’t going to go any higher than balloons can go.

Although when comparing how high balloons can go to 22K miles, perhaps my proposal wouldn’t even help defray the material strength requirements by a significant amount

The highest that balloons can possibly go is close to the ground. For a really high-altitude balloon, you might be able to get to 200 miles. That’s about 1% of the way to geosynchronous. Cutting 1% off of the required cable length doesn’t really make any practical difference at all.

I think you missed the part where he said there would be a cable hanging down into the atmosphere.

It does when you consider security, defense and political issues.

You are correct that there will be some amount of drag from the difference between rotational speed and the motion of the atmosphere, but that is easily dealt with (or at least as easy as anything about this can be) by locating the orbital terminus slightly beyond geostationary orbit, ensuring that the cable remains in a state of tension.
However, beyond the essential material strength required by the cable or ribbon are a multitude of other, highly challenging problems, such as how to fabricate and assemble the system (it has to be lowered from orbit, which means either shipping all materials to orbit or manufacture in situ), how to anchor it on the ground (the best is probably to used a submerged platform that is free to move), how to protect it from orbital debris (need to be able to divert either debris or the cable), how to repair it in service (damage due to impact, UV radiation, abrasion, thermal damage, et cetera), how to control the oscillations and resonances that will most certainly occur in this effective multibody system, how to protect it from attack, not to mention all of the legal and logistical issues associated with access and ownership.

There are huge unresolved problems, and constructing a “beanstalk” to orbit would be a technical challenge that would dwarf any previous engineering project in history even if we had a material with the required material strength and length in hand today.

Stranger

I didn’t miss that part. Did you miss the part where he said that not having the cable go all the way to the ground would save an appreciable amount of weight? He’s still got 21,900 miles of cable. Using balloons doesn’t save you anything of value.

In his book Indistinguishable from Magic physicist Robert Forward devotes a chapter to space elevators, and comes up with some intriguing (though, I suspect, unworkable) alternatives in the absence of ultra-high-strength materials. Worth a look, though.

http://www.amazon.com/Indistinguishable-From-Magic-Robert-Forward/dp/0671876864

You’re right about what a geosynchronous orbit is, but that’s only possible with an object that is a specific height - a dangling cable does not meet that specification - in fact, it won’t dangle straight down - it will (I think) tend to be in a faster orbit - and any counterbalancing parts (you’ll need those) that extend above geosynchronous orbital distance will tend to lag.

The effect is that your vertically-hanging rope isn’t going to stay vertical - this is not just a problem for the scenario you propose, but is also a problem for the setup of a more conventional attached space elevator - i.e. getting it into position

Actually, it would stay vertical. It’ll be trying to leave its “orbit”, but the force acting on it to cause it to leave that orbit will be pulling straight down, and hence will be balanced out by the tension in the cable.

How do you get it into that configuration? - if you start with two big spools of wire in geosynchronous orbit and pay them out in opposite directions (one toward the Earth’s centre and the other away), the bit of wire you’re lowering is going to have more angular momentum than it needs to stay below you - and will advance ahead of your orbit. The wire you pay out in the opposite direction will do the opposite.

You can’t lower the wire. An orbit is stable. You have to push your elevator stalk down and up, so let us imagine it’s made of jointed sticks or something. You push the first stick down. Thing is, it’s still moving with your orbital velocity, which is too slow for its new orbital altitude, so it starts to fall, or rather, there is a centripetal force pulling it straight down. Similarly, with the stick you push up, it’s moving too fast for its altitude, so centrifugal force pulls up on it.

Now you’re getting somewhere. You add more sticks, and continue pushing. The tension on your elevator grows, because the bottom of it is moving much too slow for its altitude, and the top is moving much too fast. The tension keeps it vertical. If the elevator snaps in the middle, the bottom part falls until it acquires enough velocity to acquire a new, stable, much more eccentric orbit, and the top rises until the same (cf. Larry Niven’s “The Integral Trees”).

What you are doing is using tidal forces to “expand” your elevator, which is essentially a tidally-locked moon with a weird skinny shape. I don’t think you’re getting something for nothing, however. I would bet the total energy you expend in the little pushes ends up being of order what it would take to raise the whole thing to orbit.

Why submerged?

You wouldn’t have to push your wires out; they’ll be pulled out on their own. And yes, if you do it too quickly, they’ll end up curved and swinging around, but if you spool them out slowly, it’s no problem. Things in orbit naturally tend to take on a configuration where their long axis is pointing at the center (this is the mechanism responsible for the Moon always showing us the same face, for instance).