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Continued Success for Space Elevator Tests
Jacki O writes "According to their Web site the Space Elevator company Lifport recently managed to get their platform and climbing robot to the mile-high mark over the Arizona desert." From the announcement: "A revolutionary way to send cargo into space, the LiftPort Space Elevator will consist of a carbon nanotube composite ribbon eventually stretching some 62,000 miles from earth to space. The LiftPort Space Elevator will be anchored to an offshore sea platform near the equator in the Pacific Ocean, and to a small man-made counterweight in space. Mechanical lifters are expected to move up and down the ribbon, carrying such items as people, satellites and solar power systems into space."
The robot only made it around 1500 feet. The cable was a mile long.
The article said that the platform (held up by baloons) at the end of the teather was a mile up. The climbing device reached 1500 feet, 500 feet further than previous attempts, but still quite a bit short of a mile.
I'm out of my mind right now, but feel free to leave a message.....
For those who have not experienced this particular pleasure: the obligatory Wikipedia reference.
Take a string, tie a rock to it and swing it around your head. Then you'll get the picture.
http://en.wikipedia.org/wiki/Space_elevator
The most common proposal is a tether, usually in the form of a cable or ribbon, that spans from the surface to a point beyond geosynchronous orbit. As the planet rotates, the inertia at the end of the tether counteracts gravity and keeps the tether taut. Vehicles can then climb the tether and escape the planet's gravity without the use of rockets. Such a structure could eventually permit delivery of great quantities of cargo and people to orbit, and at costs only a fraction of those associated with current means.
There was an article in Analog (WAAAAY back when) on the math behind space elevator cables, and they indicated that unless a material such as carbon fibers (nanotubes and the like weren't even on the horizon then) were developed to commercial viability then the required strength to weight ratio would make the cable waaay too wide at its halfway point.
Less is more.
The centripetal force is what holds it down, not what holds it up. From an inertial frame of reference, there is no force that holds it up; that's simply a function of its own inertia. If you wish to use the Earth as your reference frame (as you are doing) you must invent a force, called a centrifugal force, to account for the fact that a spinning object is not an inertial reference frame.
I have seen the future, and it is inconvenient.
No. 62 miles is the completely arbitrary definition of "space", but a space elevator that ended at that altitude would simply fall back down. By necessity, the center of mass (radially from the surface of the Earth) must be at or near geosynchronous orbit, so it naturally remains centered over its ground anchor. Geosynchronous orbit is at 22,241 miles above sea level. So, by gradually tapering the cable and extending it past GEO, the center of mass ends up there. Alternatively, you can have a large mass like a captured asteroid or something as an anchor just on the far side of GEO, although you should also have some counterweights you can move around on the cable to keep the center of mass in the right place as a load moves up from the surface. Additionally, keeping the center of mass just a little bit further out that necessary ensures that the space elevator will have just enough tension to keep it taut, giving the climbers an easier job.
You must still be somewhere on the slopes of the Mogollon Rim and not in Flagstaff, yet. The city is actually at 7,011 feet (or 1 1/3 miles) above sea level. Climb Humphrey's peak just north of the city, and you'll be at 12,633 feet.
But who knows, maybe they do mean 62,000 miles? I thought the elevator's main purpose was to get things in and out of just the atmosphere, as to avoid all the problems with expensive and dangerous rocket launches and dangerous re-entries.
We don't use rocket to get above the atmosphere. Planes can pretty much do that. Balloons can (and regularly do) do that. That's the easy part.
We use rockets to get velocity, because you need a ridiculous velocity in order to actually orbit the Earth at a low height.
You do not, however, need a ridiculous velocity in order to orbit at a very, very high height. At geosynchronous orbit, you need no velocity, because you've already got the speed from the Earth's rotation.
So yes, they do mean 62,000 miles (100,000 km). And the benefits you get from a cable like that are insane. Costs/pound to launch things into space become negligible. Transit to the Moon becomes cheap and fast, because the end of the cable is actually moving faster than orbital velocity.
In fact, if you climbed all the way to the end of the cable, and let go with good timing, you'd end up past Jupiter (and on a direct trajectory, too, no mucking about in Lagrange points).
Yes, it's moderately insane. Yes, it's ridiculously difficult. But it would also end up being one of the biggest changes in human industry that has ever occurred. Space solar power plants beaming down power becomes feasible. Large-scale structures built in space become easy.
Plus, once we get the technology, we can build them on other planets as well. The Moon. Mars. It basically eliminates almost all of the serious difficulties of space flight.
Actually, the counter weight is at 62,000 miles. That can be launched by conventional rocket to 32,000 and the tether let off in both directions from there. As was pointed out elsewhere, the tether is the hard part. These guys have a mile long tether so I guess your comment is legitimate.
All the climber (elvevator car) needs to do is go up to 100 miles to do what the space shuttle does and only 62 miles to do what Spaceship-One did. So in the case of the climber part it is 1 down and 99 to go.
I have to agree with the GP it feels to me like the ribbon is the more difficult part
Of course it does. For one thing, you can understand how a climber can climb. So if you see one climbing, you say "hey, that's easy, I could've built that." But designing something that reliable and that optimized is very, very difficult.
I'd imagine if you were a material scientist working with carbon nanotubes, you might feel that the ribbon is easier. Especially because we really don't have to get all the way to 50-100 GPa. We can just taper it more, which will cost a lot more and have other problems, but we can design around those then.
You can, for instance, build a space elevator on the Moon out of, say, Kevlar. You'd still need a super-ultra reliable climber, though.
If it takes 6 months to get something into space it may end up being cheaper to use conventional methods especially if only one or two climbers can operate at a time.
First, it might be cheaper in terms of getting-to-orbit, but that's not the whole story. You don't need to design for stresses with a space elevator, for one. And second, keep in mind that once you climb to the end of the space elevator, you're moving very, very fast. You can let go of the cable and you'll have enough momentum to get to Jupiter.
Second, if it's too slow, there's an easy solution. Drop more elevators. The expensive part of the elevator is launching the first mass into space. Once it's up there, if you need to increase your capacity, it's a very small marginal cost.
There is absolutely no way that a space elevator wouldn't completely revolutionize space travel.
Yes, the earth will slow down, but only by femtoseconds/day (millionths of a nanosecond - or for the numerically challenged, approx 0.000 000 000 000 001 second) for every few million tonnes we get up there.....
As the earth is slowing is spin buy a much larger amount due to tidal drag, it's a non-issue...
Um, you can go to infinity and not escape the Earth's gravity well.
m l). If you just want to go to the moon you're going to want to cast off from the cable at a significantly lower altitude, otherwise you're going to make a BIG crater.
The critical factor is how fast you're going in relation to how hard gravity is pulling on you. When you're in geosynchronous orbit you're moving fast enough to stay forever at the same height. If you're HIGHER than geosynch, but still moving at the same speed (1 rotation / 24 hours) you're going to drift AWAY from Earth if you let go. If your cable is long enough you can go a LONG way away. A 62,000 mile cable is more than enough to go to Jupiter (http://www.isr.us/Downloads/niac_pdf/chapter7.ht
Other facts about geostationary orbits:
Other geostationary orbits are not a problem... there are however many other obits that COULD intersect with the cable though.
Have you thought for yourself today?
You are right in saying that this is not enough energy to maintain an orbit. According to spaceelevator.com You would come off the cable at 3.1 km/sec and would need a booster to bring you up to enough speed to maintain Low Earth Orbit at 7.7km/sec.
But you saved on the lower stage and you don't have to worry about atmosphere anymore, so it would be a good Shuttle replacement. Even now when a sattelite is released from the shuttle, a booster is required if you want to get it to a higher orbit. On the other hand if the elevator is ever built low Earth orbit will probably not be used that much anymore. It may in fact be dangerous to have things that may hit the cable and most things would be brought to geosynchronous orbit.
IAARRS (I am a retired rocket scientist, and have participated in a NASA
Space Elevator workshop, and been on a science panel with one of the Liftport
guys - I guess that makes me a relative expert)
A tower going up from the ground meeting a cable coming down from orbit is
more efficent than a cable going all the way to the ground, if, and this is
important, the strength of the cable is substantially less than the depth
of the earth's gravity well.
Here's why: As you build a longer cable or a taller column of constant area
under gravity, the stress gets higher. In a column the maximum stress is at
the bottom, and in a cable it is at the top. Eventually you exceed the
strength of the material.
The Earth's gravity well is equal to one gee times the radius of the planet
= 6,378 km. A space elevator is centered at GEO, which is 97% of the way out
of the Earth's gravity well, so we need to span 6,167 km at one gee.
The strongest readily available carbon fiber that is not made of nanotubes
is about 1 million psi in strength. It has a density of 0.067 lb/in^3, so
if you had a cable 15 million inches long under one gee, it would be at the
limit of it's strength. 15 megainches = 381 km, which is a factor of 15
below what we need.
You can build towers or cables longer than the strength limit if you make
them progressively wider to keep the stress below the limit of the material.
Each 15 inches of length in the cable above adds one millionth to the stress,
therefore the area has to increase by one millionth. Over a 381 km length,
the area of the cable increases by a factor of e (2.718...). This length,
found by dividing strength by the density of the material, is called the
scale length. If you have 16.2 scale length to cover (6167/381), your
cable area increases by e^16.2 = ~10 million.
A graphite/epoxy composite is needed for a tower. Bare fibers are okay in
tension, but you need to stiffen them for a compression structure. Typically
using the same fibers, the composite will be 30% as strong in compression as
the bare fibers are in tension. Now assume you build a tower up and a cable
down with the same area ratios from bottom to top. The tower's scale height
is 114 km, so the combined scale heights for the tower + cable = 495 km.
Now you need 6167/495 = 12.5 scale heights. e^12.5 = ~250,000, which is
a factor of 40 improvement.
If you have carbon nanotube cable which has, say a 10 million psi strength,
your scale length is 3810 km, and your area only needs to grow by a factor
of 5 from bottom to top, so the reduction possible by using a tower is much
less helpful. Of course, we are not making 10 million psi cable in useful
quantities yet.
Daniel
I was on digg earlier, (sorry /.) and was seriously taken aback by the ignorance of the geek masses there.
I never thought someone who spends time looking at cutting edge science would have problems concieving of the validity of the space elevator. The tensile strength of the filament has been built to about 1/3 the necessary strength in less than 3 years. A method is in process to produce the filament en-mass when it gets up to strength, and NASA is backing the robot climber contest. every aspect of the project is being chipped away at relentlessly (and with notable progress). The location in the mid-pacific has been scoped out. (few storms, intl waters, far far away from anything but more pacific ocean, etc.)
There are preliminary designs for the sea platform. The counterweight as currently concieved will consist of a.) the least mass possible bunch of research oriented electronica, and b.)the first hundred or so test run ballasts.
These people are serious. check before scoffing.
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has a lesser mass than
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Aside from that, if you build the tower first, you can launch from the tower to build the rope, and start getting significant returns much sooner.
Last of all, it's easier to blow the second example free in a case of terrorist attack. It's rather hard to do much to the first. And if it does break free, it does tons less damage in the first case (the tower+rope).
Correct Horse Battery Staple: 72 bits of entropy. Enter "Correct H" into google. When it generates the phrase, that's
I never called the robotics "trivial", I called them simple in comparison to the CNT ribbon. I am a materials scientist working on Carbon Nanotubes (to also reply to your post below), and while growth isn't my concentration, I do know from the literature that the fastest growth acheived for CNTs is 10-100 microns/sec.
Now CNTs are only strong if they are continuous. In other words, if you spin a thread of them the tube to tube bonding would probably not be strong enough for the elevator. So to build the ribbon you have to grow continuous nanotubes to the length you want the ribbon. If we assume the upper limit on the nanotube growth rate I stated above, then it would take approximately half a million years to grow one mile of ribbon.
Since I'm not working directly on the ribbon I could be wrong about a few things, but the point is that there are several very tough technological obstacles to growing the ribbon. In contrast the climbers build on technology we already have, so that's why I said they are simple to build in comparison to the ribbon.
Well, I can't say how much something like that would cost to build, but it probably wouldn't provide enough speed to get something into orbit. Velocity given constant acceleration over some distance is given by:
v[f]^2 = v[i]^2 + 2ad
So, from a standing start, taking optimistic values for acceleration (say 10 G's), and the length of the ramp (say 100 km):
v^2 = 2*10g*d
v^2 = 2*10*9.81*100000
v^2 = 19620000
v = 4425 m/s
Which isn't even close to what you need for orbit, so you still need a significant rocket. Except now, you need a rocket that can handle your launch ramp, which isn't trivial.
You'd end up spending a lot of money for not much gain. You'd save some fuel, but complexity is already the expensive part and you're increasing that quite a bit.
I rarely criticize things I don't care about.
"Pretty much" only scores with horseshoes and hand grenades
The Shuttle SRBs shut off at nearly the same altitude as balloons reach. Scientific balloons are up at 40-50 kilometers. At that point, you're above 99.9% of the atmosphere. If you really wanted to, you could get almost arbitrarily high - it's just a question of how large you'd like the balloon to be. Like I said. But you don't use balloons instead of the SRBs, because the SRBs supply humongoid amounts of velocity as well.
To orbit, you have to get all the way out of the atmosphere
To orbit, you need velocity. Whether or not there's atmosphere only tells you how long you're going to orbit for as your velocity bleeds away thanks to air resistance.
Heck, the Space Station is still in the atmosphere, and it's orbiting.
There are a whole host of ways to get things to high altitude, but none of them really work clearly better than rockets because you need velocity - that is, none, save the space elevator, which accelerates very, very very gently over a very, very long cable.
Orbiting tethers, for instance, could pick up a payload off of a balloon-launched payload. That'd get you to a high altitude, but in order to pick up the velocity required, the payload would experience a supremely ridiculous amount of stress when the tether picked it up.
Or you could launch a rocket off of a balloon-supported platform. But again, the stress would be insane because the amount of velocity you need to gain in such a short time is so high.
Or we could build a really, really tall tower. But unless we get out really, really far, that tower won't do a tiny bit of good, because the (angular) velocity you need is so freaking high that, again, the stress would be nuts. Or you'd use a rocket - but the fuel savings on the rocket wouldn't be that large.
The atmosphere is essentially gone by 50 km. It's down 3 orders of magnitude. At 100 km, it's down 6 orders of magnitude. At 150 km, it's down 9 orders of magnitude. But even building a gigantic tower out to 150 km wouldn't significantly help with launching a spacecraft. You'd still need a rocket.
Actually no. At geosynchronous height you still need orbital speed.
Yah, yah, it should've said "angular" velocity there, not velocity. I do, however, commend you on saying that in one paragraph rather than 5 as the other poster did.