Are space elevators hype?

[Sky Captain]

Carbon is very light and cable would be shaped like long flat tape so the falling cable would be slowed quickly by atmosphere. At ground level it probably would fall like a long sheet of cardboard or something like that.

[Sea Skimmer]

The general design assumption seems to be if it got moving fast enough to be really dangerous it'd get hot enough to breakup anyway. Carbon is heat resistant, but we don't need the material to vaporize, just break up enough from unusual wind loadings and thermal softening ect... that it looses all its velocity. The only serious risk would be the lowest pieces crushing an exposed worker, and given proper shelter arrangements and an exclusion zone that is an entirely manageable risk.

The elevator car itself though could fall back with significant force since its bound to be a much denser piece of material, but again if it was going to fall from very high and fast it'd largely burn up too. We'd be talking about the hazard of a single airliner falling out of the sky in pieces worst case. A few people might die, but really, manageable risk. If the cars were unmanned you could even fit a self destruct charge if you really wanted to break it up and encourage burnup. I suspect the risk of malfunction in the charge would be judged the greater overall risk though! The lasers we need to defend against space debris could also perhaps target the car, and by burning holes in it encourage overall breakup the same way Columbia was destroyed by a pinhole.

This raises a random question for me, concerning car-elevator interface. Does anyone know how exactly the elevator car is supposed to grip the cable in any of these notional designs? Carbon nanotube materials are not really known for high durability, and yet in this application the options for jacketing them into composite materials are very limited because of the parasitic weight.

[Patroklos]

I suppose you could build in some sort of track. So you might need a certain portion of the cable to be something more durable. That adds weight of course, but you can install that track after the carbon nanotube part of the elevator is already built, adding counterweight to help support the more massive track as you lay it. The weight issue is getting the cable up in the first place, once its in place the cable itself can be expanded to support any weight of infrastructure along it you require. Once you are out of the atmosphere and planet's gravity you might not use a mechanical track (is in something to clamp onto and climb using friction), but rather just use it as a guide for some sort of exhaust based pushing mechanism.

[Terralthra]

Um, Patroklos, I'm just checking in here. You realize that Luna orbits Terra with a mean orbital radius of 385,000 km, yeah?

[Patroklos]

Yeah? So? Did someone mention Luna?

[Terralthra]

Yeah? So? Did someone mention Luna?

No, but you seem to think that Terra's gravity ends at the edge of the atmosphere or low-earth orbit or geostationary orbit. None of these are the case, and no existing space elevator design is intended to take loads "out of the planet's gravity." I was asking you about Luna because that seemed to me to be a rather obvious indicator of just how far Terra's gravity well extended.

[Simon_Jester]

A space elevator will take loads up to an altitude at which the force of gravity is less than 10% of what it is at the surface, accelerate them to a stable orbit by default, and lift them the vast majority of the way out of the Earth's potential energy well.

Saying "out of the planet's gravity" may be strictly inaccurate but one could do a lot worse for purposes of brevity.

[Elheru Aran]

Would not a logical safety measure for a break in the cable, should any cars be upon it, for the cars to simply detach and hit a rocket booster of some sort? If they're high enough, it could just push them into orbit with ease; low enough, it might bring them down to a rather hard but safer than free-fall landing...

[Terralthra]

A space elevator will take loads up to an altitude at which the force of gravity is less than 10% of what it is at the surface, accelerate them to a stable orbit by default, and lift them the vast majority of the way out of the Earth's potential energy well.

Saying "out of the planet's gravity" may be strictly inaccurate but one could do a lot worse for purposes of brevity.

One could do way better, in terms of both accuracy and brevity. For example, "into orbit."

[Patroklos]

You knew what I meant, you are just being a smartass.

What I was getting at is that the difficulty in climbing the tether is significantly different depending on where you are on that tether. The energy needed and mechanism for doing so can be markedly different depending on what point you are at and what direction you are going. A bulky and heavy climbing or whatever system you are using is probably only a significant problem at a small part of the lower end of tether.

Also, has anyone yet mentioned the value of using the counterweight (or lower depending) location for launching spacecraft?

[LaCroix]

Just one (probably stupid) question...

Why not use Luna as counterweight? It would mean you have the ability of a space elevator and a connection to a moon base (which is almost more feasible than a space elevator, imho). The cable could be made at the moon and lowered towards earth with relative ease because of only having 1/6th the gravity to lift it out of Luna orbit. It will be longer and heavier, but at the same time, allow transfer from both ends. You could even make it like a space gondola, with two or more cargo pods going in either direction to counter mass. And a mid point station where you switch the 5 x a full pod extra weights to balance for Earth vs Luna gravity difference from one line to the other.

It makes the initial investment (a permanent moon base) a bit more expensive (but not necessarily more expensive than a counterweight/cable production station), and by ancoring it into the moon rock, get a sufficiently stable base counterweight that will hold about any size of cable, not fly out into space (at more than the little bit that luna already does). Also, Luna is tidally locked, so another problem of a counterweight avoided.

Luna is moving away, but you'll meed a lot more cable to lower it to earth, anyway, as it will be slack during lowering it into Earth's gravity well, and needs to be tightened, anyway. You just need to let it slip out that little bit per year that is needed.

(This idea is due to my perceived notion that the cable production is the least complicated part of the process.)

[Patroklos]

The moon is not in a geosynchronous orbit, so there is no way to anchor it on the Earth end. Or am I missing something,

[Eternal_Freedom]

Indeed, the Moon is, amusingly, too far away. Plus, the studies I've seen say you only really need a counterweight if you dont want the cable extending far beyond geostationary orbit. One version I saw had the cable pushed out to 144,000 km altitude and that alone was the counterweight.

To expand on the "too far away" bit. You want it to be in geostationary orbit so it stays over the same point on Earth's equator. If you anchored the cable to both Earth and Lunar, it would get pulled taut and snapped by the rotation of Earth. Unless you're using cartoonishly-strong cables, in which case they get wrapped around Earth.

[LaCroix]

Oh, my bad - I accidentally lumped tidally locked and gestationary into the same category. My fault.

[Elheru Aran]

You *could* theoretically build a massive track spanning the entire diameter of the Earth for the terrestrial end of the cable to ride upon...

More likely, perhaps you could have the cable extend from Luna and dangle in "midair", so to speak?

Far more probably, there would simply be two skyhooks, one on Earth side and one above Luna, and travel between the two would simply be a shorter distance than Earth to Luna. Or you could do that super-long cable and just shoot rockets straight up it.

[Terralthra]

Yeah. What we essentially would want is the center of weight (not mass) of the cable to be in geostationary orbit (~35 Mm), which means you either need a few times the length of cable past that, or a honking counterweight somewhat past that.

[Sea Skimmer]

Indeed, the Moon is, amusingly, too far away. Plus, the studies I've seen say you only really need a counterweight if you dont want the cable extending far beyond geostationary orbit. One version I saw had the cable pushed out to 144,000 km altitude and that alone was the counterweight.

Thing is that's a lot of extra expensive nanotube cable to make, a whole fucking lot. So much so that the engineering problems of a counter weighted cable may not be a big deal. We aren't even remotely close to making even single strands of this stuff long enough to be usable in a space elevator but methods are tending to be very slow and expensive, so its hard to know, and that's why scienceisn't super decided on counterweight or not. Really short fibers ones we can churn out now, which we could not a decade ago, but they are only useful for stuff like car bodies or airplane parts with a large percentage of binder.

[Eternal_Freedom]

Using a captured asteroid as a counterweight would also double as a nifty mining operation as well, so it probably is the most likely option.

[Patroklos]

You *could* theoretically build a massive track spanning the entire diameter of the Earth for the terrestrial end of the cable to ride upon...

Would that provide an anchor though? That would have to be one hell of a track and one hell of a car it's attached to to keep things straight. Not only that but since the moon isn't just not geosynchronous but it also doesn't rotate around Earth's equator so ther would be lots of lateral stress that would need to be compensated for as well as slack pulled in and pulled out as the distance from said track and the Luna anchor varies.

More likely, perhaps you could have the cable extend from Luna and dangle in "midair", so to speak?

Far more probably, there would simply be two skyhooks, one on Earth side and one above Luna, and travel between the two would simply be a shorter distance than Earth to Luna. Or you could do that super-long cable and just shoot rockets straight up it.

pretty much. With a cable 55k km long you can simply release objects with escape velocities to get to the the L1 and L2 Lagrange points as well as Luna. You could do the same thing with a Luna based elevator to get back to Earth. Essentially climbing the elevator is the only energy you need to get between the two bodies plus maybe some maneuvering thrusters to ensure a smooth docking.

As far as powering that climb the two methods I've seen discussed the most is using the conductive properties of nanotubs to transfer power to the climber or use ground based lasers to push it. Both these options benifit from not requiring power generation on the climber itself thus saving mass.

[Simon_Jester]

You *could* theoretically build a massive track spanning the entire diameter of the Earth for the terrestrial end of the cable to ride upon...

More likely, perhaps you could have the cable extend from Luna and dangle in "midair", so to speak?

The Earthside end of the cable would be moving at supersonic speeds.

[Elheru Aran]

You *could* theoretically build a massive track spanning the entire diameter of the Earth for the terrestrial end of the cable to ride upon...

The Earthside end of the cable would be moving at supersonic speeds.

Which is kind of pretty damn cool in a sense, but highly impractical, I'll grant

[Patroklos]

Could that be harnessed into power generation? Obviously its a pie in the sky idea from start to finish but if you were going to do that you are talking about a lot of "free" kinetic energy.

[Elheru Aran]

If it's being pulled around by nothing more than a massive cable extending into the sky... well, sure, yeah, that's pretty much 'free', I guess. You'd have to work out how to do it without unduly slowing the cable-end along its track, or you're going to tear stuff up. But perhaps if it rode in some kind of tunnel along a magnetic rail or some such, that could create a fairly tremendous vortex behind it, which could drive some sort of wind turbine. Or turn the tunnel into some sort of electric generator using the movement of the carriage through it? This thing would probably go around the Earth a few times each day... turn it into free transportation? The route would be extremely predictable so you could have intercontinental transport hubs sprouting off it.

[Sea Skimmer]

Put supersonic wind turbines on the dangling cable! Then use steering microwave antennas to transmit the power to a series of fixed ground stations 20-30 miles apart around the earth. That would be far cheaper then a track.

The sonic booms from this is are going to be able to level buildings though if we build the cable lowenough. Which would be awesome.

[madd0ct0r]

Won't work. Energy being pulled out of the system via wind resistance must come from somewhere - PE of cable lowering ie the cable getting dragged lower.