Space Elevator Could Cost Less Than You Thought

WolfWithoutAClause writes: "We've had Space Elevator stories before on Slashdot, mainly saying how impractical they are for the foreseeable future. Now however, there's an 8M pdf paper on NASA Institute of Advanced Concepts [NIAC] website that says it may now be possible with existing materials and can be done for about $40 billion. That's less than the entire launch market for a single year. If he's right, the first elevator may be complete in 10 years time, with the second and third following 2-3 years afterwards."

Comments

9/11

[Anonymous Coward]

Not to bring up any bad memories, but if history has taught us anything - this will be a target. How could you keep something this long and lanky safe from planes?

Re:9/11

[WolfWithoutAClause]

Surface to air missiles and/or aircraft.

Re:9/11

[xah]

Who marked this post as flamebait? It makes a legitimate point.

Among others, this "space tether" would be vulnerable to the following terrorist attacks: missle, bullet, bomb, acid, human piloted aircraft, remote controlled aircraft, ground vehicle, laser, and fire.

These are just a few of the feasible methods to cut such a cable. If a terrorist wanted to place an infiltrator inside the space elevator, more attack options would be available to them.

This space elevator idea doesn't sound feasible when the security problems it would engender are considered.

Re:9/11

[Suidae]

Don't forget one of the more obvious attacks, building a fuel-ladden craft that is to be raised on the elevator and which just happens to detonate while being raised. Depending on how tough the climbers are cable are, and how well protected the cable is from other attacks, it might be a practical attack channel.

It's a tether

[Dr. Tom]

This version of the Space Elevator doesn't go all the way to the ground. That's why it can be built with existing materials. You still need a (hydrogen fueled) rocket to get to the dock at the lower end of the tether, which is about 250 km up. However the dock is moving significantly slower than orbital velocity, which increases payload and allows cheaper (more reliable & maintainable) rockets.

Re:It's a tether

[bofh31337]

A full 35,000km long space elevator would not be practical. Something of that great length would span many g-forces and you would need a large counter-weight above geo stationary to have zero velocity at ground level. Having that kind of taper from bottom to top would require a huge mass The big difference with a 250km tether is the center of attracting (and mass for that matter :)) can be in many more places. I'm thinking the best idea isn't so much a space elevator but a space slingshot using a pair or more of gravity stabilized fully rotating cables. It's an interesting idea that's been thrown around for many years.

Re:It's a tether

[WolfWithoutAClause]

> A full 35,000km long space elevator would not be practical.

That's what I thought, but read the paper. He claims it's possible; and describes how, how much and how long. The carbon nanotubes are strong enough now; or atleast that's the claim.

Re:It's a tether

[JohnPM]

Not sure you were reading the same document as I was. The cable certainly does go all the way to the ground. Also, the cable described cannot really be built with existing materials because it relies on carbon nanotubes. While this material does exist, I don't believe it has ever been used as a construction material outside the laboratory.

Re:It's a tether

[JabberWokky]

While this material does exist, I don't believe it has ever been used as a construction material outside the laboratory.

NASA may have its current problems, but it has a beautiful history of advancing materials science and using labratory materials in real world situations with incredible results. I do not doubt that if they set out to do this, and choose to use carbon nanotubes, that not only will it be built, and carbon nanotubes become a practical building material (in whatever level of expense they wind up settling at), but also that the public as a whole will forget that it was NASA that spearheaded the practical use of the material, and will continue to perpetuate the myth that NASA spent our tax dollar developing zero G pens while the Soviets used pencils.

--
Evan

Yes, it does go all the way to the ground

[iktos]

It's about a 96000 km, fixed at the bottom end, with a counterweight at the far end.

It's 50 mm wide and with a cross section of 2 mm^2 (which makes it good for lifting 20 tons, payload 12, every 97 hours). But upgradeable, of course. Cable mass 572 tons, counterweight 621.

Many parts of the building are pretty well thought out, like first sending down a thin cable and build the rest by having climbers adding more, and then using the used climbers as the counterweight. (Also, the climbers increase in mass as the cable grows stronger, from a total of 619 kg to 20 tons. Beam powered from the ground.)

The initial cable would mass 19.8 tons, with fuel the deployer would mass 190 tons, but that's still a reasonable number of Shuttle missions.

Re:Yes, it does go all the way to the ground

[Anonymous Coward]

One of the interesting things about this design is that the counter weight must be in a strange state of having superorbital speed.

Take the differential element of the wire that sits at the geostationary orbit. That element sits in geostationary orbit, and would be weightless.

The thing is, all points on the wire would have to have exactly the same orbit time, if the wire is to stay straight.

The counter-weight would ALSO have to have an orbit time of exactly one day. This means that it would be moving faster that objects would naturally at that orbital radius. How would that be done? By having the wire support tension, just like flinging the counterweight around on the end of a string under tension.

The base of the wire would have to be attached to the earth in a very strong manner to support that tension.

A nice pair of scissors would send the counter-weight into a very large orbit indeed. :-)

Re:But...

[WolfWithoutAClause]

That would probably be bad, although it might sort itself out, I don't know if anyone really knows what happens. I think that the tether would be designed to be an insulator.

the paper, not the slides

[blamanj]

That 8M download only gives you the slides - pretty pictures but no text. The actual phase I paper is here. It's a 15M download - and you can year the server creaking under the strain.

Re:the paper, not the slides

[WolfWithoutAClause]

Actually, to be correct; the 8M pdf file is the phase II paper, which was written later; and contains more information if anything; but less text.

Angular momentum

[p3d0]

When something's going up the elevator, where does it get all the angular momentum it needs to stay in orbit? Does the climber have rockets? I don't see them on the diagram.

Re:Angular momentum

[p3d0]

Ok, thanks. But doesn't that impart equal and opposite angular momentum to the cable? What happens to that momentum?

Re:Angular momentum

[p3d0]

Perhaps. It still feels like we're getting angular momentum for free here. It's like we're neglecting the biggest coriolis effect on the planet.

I think when a weight goes up the rope, stealing some of the rope's angular momentum, the rope will not swing like a pendulum as a result. Rather, it will very, very slowly wrap itself around the earth.

Re:Angular momentum

[CedgeS]

You are probably not thinking about angular momentum, but energy in circular motion.

Energy contained in circular motion is equal to:
(This is the energy associatied with angular momentum)

E = 1/2 I * (w^2)

Where I is the moment of inertia and w is the angular frequency (in this case about 7.27 x 10^-5 1/s because the period of rotation will be 24 hours). The moment of inertia will increase as the load gets further away from the Earth.

I = m * (r ^ 2)

m is mass
and r is radius from the center of the earth.

So, the energy in circular motion at each height would be:

E = 1/2 * m * (r^2) * (w^3)

To get the formula for the total energy at each height, add the potential energy from Earth's gravitational pull.

To answer your question, the increase in angular momentum of the payload is a result of the force exerted by the elevator doing work on the payload, resulting in a change in energy of circular motion.

If you are worried about what is called conservation of angular momentum, the increase in angular momentum comes from a decrease in the angular momentum of the Earth. Conservation laws are usually written like so:

initial = final

So,
L (angular momentum) initial = L final

I forgot, angular momentum = I * w

Where L is the sum of the angular momentums in the system.

So,
L(earth) + L(payload) + L(elevator) initial = L(earth) + L(payload) + L(elevator) final

Because the radius from center of mass of the elevator and the Earth don't (negligibly) change during the lifting of the payload (this would affect I) and that for the payload does, the final angular frequency of something must be slower. Since they are all tethered together going at the same angular frequency, their angular frequencies must remain the same, and the anular frequency of the Earth will decreas very slightly (negligibly actually) and days will become slightly shorter while the payload is in space. You wouln't notice it though, because this happens every time any payload is sent into space -- every satellite space ship, etc.

When you drink too much physics, alchohol just doesn't make any sense anymore.

Re:Angular momentum

[p3d0]

You are probably not thinking about angular momentum, but energy in circular motion.

Perhaps, but I really think I'm talking about angular momentum.

To answer your question, the increase in angular momentum of the payload is a result of the force exerted by the elevator doing work on the payload, resulting in a change in energy of circular motion.

How can you get angular momemtum arising from forces that act perpendicular to the direction of the desired orbit?

If you are worried about what is called conservation of angular momentum, the increase in angular momentum comes from a decrease in the angular momentum of the Earth.

The cable is not rigid, and unless I'm mistaken, it is perpendicular to the Earth's surface, so I don't see any way it can transmit a moment to the Earth.

Re:Angular momentum

[PhuCknuT]

I don't agree however that you could cancel the effect out by timing the subsequent launches. The issue of total angular momentum needs to be addressed. Subsequent launches could only cancel the angular momentum if they were somehow launched from the end of the cable with opposite angular momentum.

I think you misunderstood what I meant. The cable would lean west (very slightly) during a launch, and once the payload finished its ascent, the cable would swing back towards vertical. If left alone at this point, it would continually swing back and forth. It's not the transfer of angular momentum away from earth that I was talking about cancelling, it's the swinging of the cable. If you time your launches so that they occur while the cable is swinging back east, the eastward momentum would be canceled out by the westward momentum that the cable gains from the cargo.

As for why it would swing like a pendulum, I'll try to explain the best I can without drawing pictures. :) As long as the cable is leaning westward, the cable will be gaining eastward angular momentum from the earth. This is simply because the cable is under tension and no longer perpendicular to the orbit. Likewise, when it leans east, the cable tension will have a westward component pulling on the counterweight. As the cable swings back and forth, the angular momentum of the whole system (earth, cable, counterweight) will remain the same, but a small amount of the momentum will slowly transfer back and forth between earth and counterweight.

Re:Angular momentum

[WolfWithoutAClause]

> The cable is not rigid, and unless I'm mistaken, it is perpendicular to the Earth's surface, so I don't see any way it can transmit a moment to the Earth.

You're exactly right. It can't be perpendicular to the earths surface and transmit moment to the Earth.

In fact, as an object goes up the tether, it tends to drag the tether to the west, because the tether is accelerating the payload sideways as it goes up towards orbit at geosynchronous altitude- it needs a few klicks/sec up there relative to the ground, and its stationary at ground level, so it accelerates as it goes up the cable. The only way a cable can do that is to form a shallow v shape.

This V shape means the tether is tilted to the ground. The tension on the tether at the ground will therefore pull on the earth and slow it down; but I wouldn't exactly lose sleep over that bit.

In fact, the tether's position can be controlled by moving the payload up and down on the tether and careful timing.

It's a bit more robust than you think...

[Anonymous Coward]

First of all, if you break the cable down low, the bulk of it just "springs back" into an elliptical orbit with the same perigee.

Secondly, a lot of plans call for the cable to join a 15 km high tower, since building up from the earth is feasible for that altitude, and chopping mass off the bottom end of the cable translates directly to increased cargo capacity. 50,000 ft is higher than most planes fly.

The scary scenario is the cable breaking up high, e.g. the counterweight coming loose. The cable would fall to earth, wrapping around the equator multiple times as it does so, cracking like a whip. All of the energy spent launching it would come down in a long thin bang.

I see some major overlooked features...

[szyzyg]

Firstly - they seem to have ignored the fact that a cable this long would snap under its own weight - even made of this carbon nanotube material. Ideally you want to maintain a constanst strain on the material throughout the tower so you need to start with very thick cable at the midpoint then taper it out towards the ends - the midsection would be maybe 10 times thicker.

Now, even if they've accounted for this then the depolyment is in trouble, since they have to spool out the material from a drum, which means that you start spooling from one end or the other which means that you can't follow the ideal thickness profile without exceeding your structural limits during some point in the unroll procedure.

The design for the deployment should instead extend the upper and lower half in both directions simultaneoulsy. The problem here is desiging a mechanism which can deploy this towards the end when the strain becomes highest.

Another minor issue is how quickly you can deploy such a a large sturcture - the more patient the better, but you're dealing with 100,000km of cable - taking at at 10km/hr would take over a year to deploy, acceleration and deceleration of the deployment would induce oscialltions in the cable which would be difficult to damp...

As for the danger of a break - not only would it fall down by wrapping itself around thew world a couple of times, but the tension on the structure would be like a strethed rubber band - the stored energy would be huge - think in terms of a nuclear powered rubber band.

Re:I see some major overlooked features...

[JabberWokky]

Now, even if they've accounted for this ...

Why don't you get a physics degree and submit your own paper.

Or, here's a netball, left field idea - read this paper, see if you understand the math involved, and if not, drop back, read a bit more, figure out what we've learned about orbital mechanics, structural engineering and materials science over the past 8,000 years, and apply Clarke's second law.

--
Evan "Armchair physicist who knows where his knowledge ends and learning begins" E.