The article says, "His conclusion: The space elevator could be built with existing technology—minus the super-lightweight tether necessary to make the whole thing work." In other words, it cannot be built with existing technology.
NASA uses the idea of "Technology Readiness Level (TRL)" [1] to help assess speculative technology, the (obvious) idea being that each stage of deployment of a technology relies on passing tests at previous stages.
On this scale, it's not clear to me that space elevators are at TRL1 ("basic principles observed and reported") yet. Space elevators depend critically on having a material that's strong enough to build the cable. Feasible designs for climbers, debris avoidance systems, power transmission and so on, can't make up for the lack of this critical component.
Carbon nanotubes are seductive. They are very strong, diamond like, in that circularized plane. But when you extend the tube to macro dimensions, you have problems with phonons and the 'rogue waves' that add up over such long distances (relative to atoms). These phonon effects can and do add up to break the nanotube as there is so much energy in the phonon that it overcomes the intermolecular bonds. You can help a bit with an onion layering of nested nanotubes, each a little bit bigger. But that doesn't get you much further. Doping the tubes with other atoms reduces the strength by a lot, but may increase the length. Isotopes of carbon, like C10 or C14, do help a fair bit. However, they tend to be too radioactive to be stable for a project like this. You'd have to continually replace the rope as the dopant isotopes would decay away and the nanotubes would self destruct again. In the end, we need new ideas to help with this. Perhaps some strange electron-phonon interaction? I don't know.
Yes, but so what if you need to continuously replace parts of the cable? As long as you can identify the damage and cheaply perform the repair, it doesn't seem that bad.
What I never seem to see addressed, is the following: As the elevator ascends the cable, it's "ground speed" must continually increase. What is the source of the force that provides that acceleration (parallel to the ground)?
A traditional rocket expends a great deal of energy firing its rockets with a component parallel to the surface of the earth to gain acceleration in that direction. Where is the equivalent coming from w/ the space elevator?
> As the elevator ascends the cable, it's "ground speed" must continually increase
That is backwards. The higher you are the less speed you need. This is true in all orbits. Contrast the orbital velocity of Mercury vs Neptune.
The rocket is aiming for low orbit. I.E. you go fast enough that you fall around the earth rather than into it. The only reason a rocket's ground speed increases as it's altitude increases because it's hard to add a lot of speed while still in the denser sections of the atmosphere. Altitude is needed merely to clear the drag imparted by the atmosphere.
We go fast and low because it's the easiest way to get something to stay in orbit using rockets. But mechanically as we gain altitude we go slower relative to the ground. Eventually when you go high enough you are standing still relative to the ground (geosynchronous orbit) or even go backwards relative to the earths rotation (high orbit).
A space elevator is aiming to go so high that there is no parallel acceleration necessary. The rotation of the earth provides all the acceleration needed. A space elevator that ends in geosynchronous orbit requires no parallel acceleration relative to the ground. A space elevator in high orbit could get something of a free ride out of earth's gravity well powered by the rotation of the earth.
That is not at all how it works. The elevator does not go into low-Earth orbit(LEO). It is part of the elevator system whose center of gravity is at Geostationary Orbit. So the craft itself, and the ribbon, is always over the same spot on the Equator.
The earth's rotation. Much like how your rotational velocity increases when you climb stairs. Also, the cable is not vertical, you want a large mass past geostationary orbit which pulls the cable fairly thought though there is some bend.
PS: Try and calculate just how much energy is in earth rotation it's a rather large number.
A space elevator only provides access to geostationary orbit. One can climb up to LEO altitudes along an elevator, but that doesn't mean much. You wouldn't be in orbit, but sitting stationary as if atop a tall building. So we would still need to launch rockets for any space use in LEO (GPS and imaging sats for example). And any such orbiting objects would of course have to be navigated around this obstruction. Many useful orbit types would have to be outlawed to protect the elevator.
I wonder, is easier to launch into a LEO from the ground or from a fixed point at say 300km altitude? If you climbed a space elevator you would still need 8km/s of speed laterally. So you jump off and fire your rocket. You still fall towards the ground, requiring some thrust to keep out of the atmosphere. Without the arcing trajectory of a ground launch you would have to accelerate roughly twice as quickly, requiring larger engines. Is that really any better than starting from the ground as we do today?
Or you could climb to a near-geostationary position, burn retrograde until you touch the atmosphere, then aerobrake down to LEO. That's still a heck of a lot of effort.
Any height is an advantage in achieving orbit. It's an exponential problem - you need fuel to go up, and you need more fuel to lift the fuel you need to go up, etc.
Also, air resistance is much more at the surface than higher up.
So by starting your ascent at a high altitude you need much less fuel both because you need less fuel, and because there is less air resistance (friction).
Space elevators still need to use the same energy to lift something, but they use an external power source so do not need to lift their own fuel, and they lift more slowly and greatly reduce the effect of air resistance.
So no, a space elevator won't get you to "outer space", but it creates a stepping stone which greatly reduces the cost of getting there.
I believe you have a fundamental misunderstanding of the speed of the top of a space elevator. The counterweight at the top of a space elevator would be approximately at geostationary orbit. If you were to be up there and "drop" a baseball, say, it would appear to hang there motionless, as it would also be (approximately) at geostationary orbit.
Neither. For LEO the object is released at well above the desired orbit and as it drops it acquires the needed velocity. Some fuel is still needed but not nearly so much.
You are right jumping off at LEO altitude would be silly. Which is why you would go to GEO then transfer to LEO. GEO is something like 35,000 km high. You burn your rockets to fall towards low orbit, then pick up all the velocity you need falling towards LEO. Burn again once at perigee (or aerobrake) to steer your orbital path into circular rather than elliptical.
A transfer from GEO to LEO is significantly cheaper than reaching LEO from the ground. Additionally transfer from GEO to anywhere else in the solar system is really cheap. It is hard to overstate just how much of an advantage a space elevator would be in reaching any location in the solar system including LEO.
The only disadvange is travel time. Rockets to 300km get there quickly. Elevators to 35000km get there slowly.
You wouldn't have to contend with gravity loss (energy expended by performing work against gravity) or atmospheric drag, so all else being equal it would be a net energy win. Definitely not a free ride though.
A space elevator on Earth is very hard to build and would require exotic materials which we can't yet synthesize in bulk. But you can make a decent lunar space elevator with Kevlar.
I certainly believe an underthought of this article, which is that improved capabilities with materials such as carbon nanotubes might lead to a paradigm shift in the possibilities of what can be done with materials.
My understanding of the biggest challenge is that each point in the cable must be strong enough to carry the weight of all the cable beneath it, which by the time you reach the upper atmosphere would be tremendous even using carbon nanotubes. The cable would also be perpetually struck and damaged by high velocity micrometeoroids.
[+] [-] garethrees|10 years ago|reply
NASA uses the idea of "Technology Readiness Level (TRL)" [1] to help assess speculative technology, the (obvious) idea being that each stage of deployment of a technology relies on passing tests at previous stages.
On this scale, it's not clear to me that space elevators are at TRL1 ("basic principles observed and reported") yet. Space elevators depend critically on having a material that's strong enough to build the cable. Feasible designs for climbers, debris avoidance systems, power transmission and so on, can't make up for the lack of this critical component.
[1] http://www.hq.nasa.gov/office/codeq/trl/trl.pdf
[+] [-] eternauta3k|10 years ago|reply
[1] https://en.wikipedia.org/wiki/Skyhook_%28structure%29
[+] [-] cl42|10 years ago|reply
[+] [-] fsloth|10 years ago|reply
[+] [-] Balgair|10 years ago|reply
[+] [-] icebraining|10 years ago|reply
[+] [-] w1ntermute|10 years ago|reply
https://twitter.com/elonmusk/status/559557786514632704
[+] [-] CountHackulus|10 years ago|reply
[+] [-] meddlepal|10 years ago|reply
[+] [-] matt-attack|10 years ago|reply
A traditional rocket expends a great deal of energy firing its rockets with a component parallel to the surface of the earth to gain acceleration in that direction. Where is the equivalent coming from w/ the space elevator?
[+] [-] johngalt|10 years ago|reply
That is backwards. The higher you are the less speed you need. This is true in all orbits. Contrast the orbital velocity of Mercury vs Neptune.
The rocket is aiming for low orbit. I.E. you go fast enough that you fall around the earth rather than into it. The only reason a rocket's ground speed increases as it's altitude increases because it's hard to add a lot of speed while still in the denser sections of the atmosphere. Altitude is needed merely to clear the drag imparted by the atmosphere.
We go fast and low because it's the easiest way to get something to stay in orbit using rockets. But mechanically as we gain altitude we go slower relative to the ground. Eventually when you go high enough you are standing still relative to the ground (geosynchronous orbit) or even go backwards relative to the earths rotation (high orbit).
A space elevator is aiming to go so high that there is no parallel acceleration necessary. The rotation of the earth provides all the acceleration needed. A space elevator that ends in geosynchronous orbit requires no parallel acceleration relative to the ground. A space elevator in high orbit could get something of a free ride out of earth's gravity well powered by the rotation of the earth.
[+] [-] CapitalistCartr|10 years ago|reply
[+] [-] Retric|10 years ago|reply
PS: Try and calculate just how much energy is in earth rotation it's a rather large number.
[+] [-] parenthephobia|10 years ago|reply
[+] [-] sandworm101|10 years ago|reply
I wonder, is easier to launch into a LEO from the ground or from a fixed point at say 300km altitude? If you climbed a space elevator you would still need 8km/s of speed laterally. So you jump off and fire your rocket. You still fall towards the ground, requiring some thrust to keep out of the atmosphere. Without the arcing trajectory of a ground launch you would have to accelerate roughly twice as quickly, requiring larger engines. Is that really any better than starting from the ground as we do today?
Or you could climb to a near-geostationary position, burn retrograde until you touch the atmosphere, then aerobrake down to LEO. That's still a heck of a lot of effort.
[+] [-] jasonpeacock|10 years ago|reply
Also, air resistance is much more at the surface than higher up.
So by starting your ascent at a high altitude you need much less fuel both because you need less fuel, and because there is less air resistance (friction).
Space elevators still need to use the same energy to lift something, but they use an external power source so do not need to lift their own fuel, and they lift more slowly and greatly reduce the effect of air resistance.
So no, a space elevator won't get you to "outer space", but it creates a stepping stone which greatly reduces the cost of getting there.
[+] [-] mixedmath|10 years ago|reply
[+] [-] CapitalistCartr|10 years ago|reply
[+] [-] johngalt|10 years ago|reply
A transfer from GEO to LEO is significantly cheaper than reaching LEO from the ground. Additionally transfer from GEO to anywhere else in the solar system is really cheap. It is hard to overstate just how much of an advantage a space elevator would be in reaching any location in the solar system including LEO.
The only disadvange is travel time. Rockets to 300km get there quickly. Elevators to 35000km get there slowly.
[+] [-] didgeoridoo|10 years ago|reply
[+] [-] ceejayoz|10 years ago|reply
No, it provides access well above geostationary orbit.
https://en.wikipedia.org/wiki/Space_elevator#/media/File:Spa...
Time things right and you can basically fling off the end like the tip of a whip and head anywhere in the system.
[+] [-] unknown|10 years ago|reply
[deleted]
[+] [-] Symmetry|10 years ago|reply
http://hopsblog-hop.blogspot.com/2012/09/beanstalks-elevator...
[+] [-] mixedmath|10 years ago|reply
[+] [-] joshuagross|10 years ago|reply
Does anyone know what he's referring to in this comment?
[+] [-] mattmanser|10 years ago|reply
[+] [-] jrcii|10 years ago|reply
[+] [-] Fargren|10 years ago|reply
[+] [-] icebraining|10 years ago|reply
[+] [-] cgrubb|10 years ago|reply
[+] [-] Beltiras|10 years ago|reply
[+] [-] blammail|10 years ago|reply