PDA

View Full Version : To build a "space elevator" would it be best to build one the moon first?



WaxRubiks
2006-Mar-30, 08:33 PM
If you built it from the moons surface it could then take material into space to build the Earth one which could then be lowered into position from geostationary orbit around the Earth?

antoniseb
2006-Mar-30, 08:54 PM
Hmmm. You'd need to extend the ribbons from a spacecraft in Lunosynchronous orbit (27.3 days per orbit). I wonder where that is in relation to the Earth-moon L3 point. In any case, I'm guessing the ribbons would have to be pretty long.

antoniseb
2006-Mar-30, 08:54 PM
I'm moving this thread to Space Exploration. This is not an ATM.

Nicolas
2006-Mar-30, 09:01 PM
I think that the actual construction of the elevator is the least of the problems. Let's start with tackling material issues and parts production.

antoniseb
2006-Mar-30, 09:10 PM
You could build one on Itokawa (the asteroid that Hayabusa visited) cheaply enough, and with materials available today. On that asteroid you could build a space elevator with a few miles of steel cable.

The Supreme Canuck
2006-Mar-30, 09:39 PM
Hmmm. You'd need to extend the ribbons from a spacecraft in Lunosynchronous orbit (27.3 days per orbit). I wonder where that is in relation to the Earth-moon L3 point. In any case, I'm guessing the ribbons would have to be pretty long.

The best way to pull it off would be to set your asteroid at either the L1 point or the L2 point. Otherwise the whole thing is unstable.

Nicolas
2006-Mar-30, 11:24 PM
You could build one on Itokawa (the asteroid that Hayabusa visited) cheaply enough, and with materials available today. On that asteroid you could build a space elevator with a few miles of steel cable.

Which would be extremely good for a "first ever space elevator" title, and quite pointless for everything else :).

Nicolas
2006-Mar-30, 11:25 PM
The best way to pull it off would be to set your asteroid at either the L1 point or the L2 point. Otherwise the whole thing is unstable.

Depends on your definition of unstable. I thought the Lagrange points were unstable as well, as gravity does not push you towards them but pulls equally on all sides.

The Supreme Canuck
2006-Mar-30, 11:41 PM
Sure, but sticking it at either of these two L points would be far more stable than trying for a stationary orbit.

Nicolas
2006-Mar-30, 11:48 PM
I don't know. How stable are Lagrange situated satellites versus geostationary satellites?

The Supreme Canuck
2006-Mar-31, 12:07 AM
Not sure. But you need to remember that all lunar orbits are very unstable. Even those that would be okay around Earth will degrade quickly arond the Moon. The L1 and L2 points are very stable by comparison.

antoniseb
2006-Mar-31, 12:17 AM
The L4 and L5 points are where the the spacecraft would have to be that is dropping the cable to the moon.

Nicolas
2006-Mar-31, 12:20 AM
Would there be a real use in a lunar space elevator other than the construction of an earth space elevator (and construction does not seem the most important issue to tackle to me!)?

I mean, it took a SaturnV just to get the Apollo lander on the moon, if we'd have to take that approach to send things into space (rocket to the moon, elevator to space), it doesn't seem like it would be really efficient as is envisioned with space elevators.

antoniseb
2006-Mar-31, 12:34 AM
Would there be a real use in a lunar space elevator

No, not at all. The moon is a perfect place for launch by rail, and a terrible place for a space elevator. Good places for space elevators? Mars, Earth, Saturn, Uranus, Neptune. Really bad places for space elevators? Venus, Mercury, the Moon, Jupiter.

The reasons against Venus, Mercury, and the Moon are that they don't spin fast enough, and the cables would have to be very long. In the cases of the Moon and Mercury, rail guns will work fine, because they have no atmosphere. In the case of Jupiter, I think that even nanotube based ribbons will not be strong enough to hold up against the gravity.

Note that for Saturn, Uranus, and Neptune, the planet-side anchor would be in the clouds. The high end would potentially have trouble with the rings.

Van Rijn
2006-Mar-31, 11:45 AM
Here's a previous thread on this subject:

http://www.bautforum.com/showthread.php?t=13863

and my post with some links:

http://www.bautforum.com/showpost.php?p=304433&postcount=18

For a fixed lunar skyhook, you would have the cable go through either L1 or L2 (nearside or farside) with the ballast somewhat beyond. That would put tension on the cable, since past the Lagrange point things would tend to move away from the moon.

Oh, hey! I just googled and found a Universe Today article on the subject:

http://www.universetoday.com/am/publish/lunar_space_elevator.html

Van Rijn
2006-Mar-31, 12:02 PM
No, not at all. The moon is a perfect place for launch by rail, and a terrible place for a space elevator.


Have to disagree with you on the lunar space elevator. Becase of the moon's lower mass, we could build a space elevator with existing materials - we don't need carbon nanotubes to get a decent taper. Of course, rail launch is good too.



Good places for space elevators? Mars, Earth, Saturn, Uranus, Neptune. Really bad places for space elevators? Venus, Mercury, the Moon, Jupiter.

The reasons against Venus, Mercury, and the Moon are that they don't spin fast enough, and the cables would have to be very long. In the cases of the Moon and Mercury, rail guns will work fine, because they have no atmosphere. In the case of Jupiter, I think that even nanotube based ribbons will not be strong enough to hold up against the gravity.

Note that for Saturn, Uranus, and Neptune, the planet-side anchor would be in the clouds. The high end would potentially have trouble with the rings.

Actually, the earth is about the limit for carbon nanotubes in a synchronous skyhook. The taper factor (much like a rocket's mass ratio) for a tensile space elevator would be much worse for all the gas giant's, and for the same reason - the planets are much more massive and have a much higher escape velocity. However, there is a type of space elevator that could get around that issue, called the space fountain (http://en.wikipedia.org/wiki/Space_fountain).

(edit to add: I notice that Wiki disagrees with me on building a tensile space elevator for the smaller gas giants, assuming advanced materials, though they do note the taper would be much greater. The thing is that even for earth, it is pushing things assuming near theoretical strength material and even a little parasitic mass, so . . . )

A space fountain is an active structure, using the momentum of constantly moving "pellets" to support itself. It is essentially a mass driver loop. It isn't something we would build today, but then we aren't likely to need one soon.

Also, there are non-synchronous skyhook schemes, such as the rotovator (http://en.wikipedia.org/wiki/Hypersonic_skyhook#Rotovators) that could work for Venus and Mercury as well as Earth, Mars, etc. There actually are quite a number of "momentum bank" and other momentum transfer schemes that might turn out to be more practical than the synchronous skyhook.

ryanmercer
2006-Mar-31, 01:11 PM
Someone should ask liftport ( www.liftport.com/ ) since they are the ones behind constructing one here on Earth and are currently developing all needed technologies.

Nicolas
2006-Mar-31, 05:22 PM
I do fear that they'll need to keep that "currently developing all needed technologies" line for quite some years on their website...

GOURDHEAD
2006-Apr-01, 03:26 PM
http://service.spiegel.de/cache/international/spiegel/0,1518,408953,00.html describes conditions that the design of any ocean anchored space elevator will have to consider.

Ara Pacis
2006-Apr-01, 10:38 PM
I've been a fan of a rotovator concept for the moon and earth. It doesn't require as much of the materials and might avoid some weather problems and also some political problems. On earth we might be able to catch the rotovator with airborne craft. These could be stationary aerostats at very high altitudes (like JP Aerospace's Dark Sky Stations) or aerodynes either at a ballistic apogee or at a velocity synch in motion. Later innovations could use railguns on the surface to launch dead weight cargo for interception.

I think a lunar rotovator could be a proof-of-concept for an earth system. It might be economical in the interim between now and a thriving space economy in the future by allowing for safer non-ballistic landings on Luna by cargo and passengers. It also would save weight by removing the need for landing and launching engines and fuel at a time in space exploitation where weight is still at a premium.

Perhaps in the future a system of rotovators could be used to send things back and forth between Earth, Luna and into trajectories to other planets. I haven't calculated masses and velocities, so I'm not sure how likely this is. However, I think it would at least reduce the amount of fuel and the size of engines needed.

Van Rijn
2006-Apr-02, 12:52 AM
Perhaps in the future a system of rotovators could be used to send things back and forth between Earth, Luna and into trajectories to other planets. I haven't calculated masses and velocities, so I'm not sure how likely this is. However, I think it would at least reduce the amount of fuel and the size of engines needed.

Hans Moravec wrote about orbital transfer tethers long ago (it was one of the first articles I read on this subject). Here is one page on them:

http://members.aol.com/Nathan2go/xferteth.htm

The nice thing about them is that they act like momentum banks for back and forth travel. You can also add momentum slowly with ion drives or other efficient techniques, then make a quick withdrawal. Or you can add and subtract momentum by two way travel between tethers. They would be especially handy for cargo - you could put packages in interorbital or interplanetary "virtual pipelines" using this technique quite efficiently.

eburacum45
2006-Apr-03, 06:40 PM
The nice thing about them is that they act like momentum banks for back and forth travel.
And the nasty thing about them is that they need very good timing. If a cargo package misses a rotovator it will continue moving with the same momentum as it had previously- in most cases, if not all, the cargo package would be in an unstable orbit and be lost.

With extremely good automation such a system might be feasible, perhaps some sort of very sophisticated autopilot.
Although I think that such extremely good automation will be available eventually, the threat of cargo pods in unstable orbits re-entering and falling to Earth every time a mistake is made might not be a selling point.

Van Rijn
2006-Apr-04, 04:48 AM
And the nasty thing about them is that they need very good timing. If a cargo package misses a rotovator it will continue moving with the same momentum as it had previously- in most cases, if not all, the cargo package would be in an unstable orbit and be lost.


If you're talking about a rotovator for earth to orbit transport, the craft would be well under orbital velocity, perhaps only mach 6 or 7. It isn't going to go into an odd orbit - or at most, it will quickly intersect earth. There really isn't much point in using a rotovator if you are going to use conventional rockets for much more than half of the velocity change to get to orbit.

A rotovator or some similar concepts might mesh up well with suborbital followup craft to SpaceShipOne.

For interorbit transfers that go wrong I would expect emergency rockets and procedures would be mandatory. Actually, you pretty much need fine control and "final wave off" capability if there is an emergency.

The nice thing is that you can afford to waste a bit of mass on safety, since there are ways of getting the energy back.

Jerry
2006-Apr-04, 01:56 PM
It makes more sense to start with something much more simple - like a suspension bridge connecting New York with Las Angelos. Come on guys! no matter what technology you are using, an enormous mass would have to be lifted into space, and suspended with enough strength to survive the bowing tension of hurricane force winds.

Even a cable between asteroids or legrange points is not feasible - the relative velocities are always changing. Finally, any material other than heavy metals and perhaps brittle carbonites, is going to be subject to rapid degradation from the solar UV, ionized wind, (think strong acid) and cosmic rays.

antoniseb
2006-Apr-04, 05:10 PM
It makes more sense to start with something much more simple - like a suspension bridge connecting New York with Las Angelos.

I believe that the space elevator is possible, but I am grateful to Jerry for making a very nice image of just how hard a job this is. Some people write things that just assume we can build this thing tomorrow, when the reality is that getting it done in the next 94 years will be a challenge, and very expensive.

The fact is that this cable (ribbon) is going to have to be so long that it could be wrapped around the Earth's equator multiple times. What do you suppose that ribbon will weigh?

NEOWatcher
2006-Apr-04, 06:04 PM
Maybe that should be an episode of "Extreme Engineering".
I think they did a show about a bridge across Bering, and a tunnel from US to Euro.

Nicolas
2006-Apr-04, 07:13 PM
US-Euro tunnel? On wheels over the ocean floor or what? I mean, plate tectonics... :)

NEOWatcher
2006-Apr-04, 08:00 PM
US-Euro tunnel? On wheels over the ocean floor or what? I mean, plate tectonics... :)

Magnetic Levitation (http://dsc.discovery.com/convergence/engineering/transatlantictunnel/interactive/interactive.html)(not the tunnel, but the trains) http://www.cosgan.de/images/smilie/figuren/g025.gif

Nicolas
2006-Apr-05, 07:19 AM
How do those anchors solve the plate tectonics problem? I know they have some slack, but they must be replaced from time to time I assume (couldn't find it on the site).

Maybe we should split this one off to general science?

WaxRubiks
2006-Apr-05, 08:10 AM
don't the continents seperate about an inch or two a year? Probably easy enough to engineer for.

ryanmercer
2006-Apr-05, 11:28 AM
US-Euro tunnel? On wheels over the ocean floor or what? I mean, plate tectonics... :)

They wanted to lay tubes down on the ocean floor... bad idea...

However... I'd do it tunelling below the seafloor... their was talk of constructing a nuclear subselene tunneler (and iirc the government has a patent on both designs) that melts/bores glass-like tunnels underground, however no one thinks they have ever actually made and used it... I imagine they did though during the cold war when they were making those tunnels throughout the midwest and deserts so that they could drive 2 ICBMS side by side on trucks to keep the nuclear arsenal moving. In the 80' Los Alamos National Laboratory and from Texas A&M University (under contract to NASA) for use of one on the moon *nods*. That is mainly how the america-euro tunnel would have to be constructed.

NEOWatcher
2006-Apr-05, 12:22 PM
How do those anchors solve the plate tectonics problem? I know they have some slack, but they must be replaced from time to time I assume (couldn't find it on the site).

Maybe we should split this one off to general science?

Done:
Tunnel on over to this thread. (http://www.bautforum.com/showthread.php?t=40164)

Van Rijn
2006-Apr-06, 09:55 AM
I believe that the space elevator is possible, but I am grateful to Jerry for making a very nice image of just how hard a job this is. Some people write things that just assume we can build this thing tomorrow, when the reality is that getting it done in the next 94 years will be a challenge, and very expensive.


Another one of the nice things about the rotovator and similar non-synchronous designs is that you can meet it halfway. That is: It can handle half the velocity change required to reach orbital velocity and rockets can handle the other half. This dramatically reduces the mass ratio of the rocket and dramatically reduces the strength to weight ratio requirement of a decent tether design. That brings it within the range of existing materials. Such a tether would also be much shorter than a synchronous tether.

My expectation is that there will be an evolutionary development of space tethers for limited in-space velocity transfer and for use as electrodynamic tethers for orbital velocity change. Synchronous tethers would come later.




The fact is that this cable (ribbon) is going to have to be so long that it could be wrapped around the Earth's equator multiple times. What do you suppose that ribbon will weigh?

Here:

http://www.isr.us/Downloads/niac_pdf/chapter5.html

They calculate an initial cable mass of 19,800 kg. Keep in mind that advanced carbon nanotube material has a truly amazing strength to weight ratio. A typical plan is to put up a "starter cable" and use that to add additional cables and capacity.

Economical mass production of the extremely high strength material will be one of the key issues for such a design. I am not a big proponent of the synchronous Earth skyhook, at least for the short term. However, I do suspect that non-synchronous skyhooks will play a big factor in opening space and quite possibly sometime within the next two decades.

Van Rijn
2006-Apr-06, 10:13 AM
It makes more sense to start with something much more simple - like a suspension bridge connecting New York with Las Angelos.


And why is this simpler?



Come on guys! no matter what technology you are using, an enormous mass would have to be lifted into space, and suspended with enough strength to survive the bowing tension of hurricane force winds.


Not that massive but very strong, under significant tension, and with a very small cross section.



Even a cable between asteroids or legrange points is not feasible - the relative velocities are always changing.


No idea what you are getting at here.



Finally, any material other than heavy metals and perhaps brittle carbonites, is going to be subject to rapid degradation from the solar UV, ionized wind, (think strong acid) and cosmic rays.

Cable proponents have not ignored wind issues, UV, oxygen degradation, and so forth. See here, for instance:

http://www.isr.us/Downloads/niac_pdf/chapter10.html

These are engineering issues, not the laws of physics.

NEOWatcher
2006-Apr-06, 12:10 PM
And why is this simpler?
It's shorter, closer to the ground, in a breathable atmosphere, easier to get by safety opponents, proven technology (just much bigger), etc.


Not that massive but very strong, under significant tension, and with a very small cross section.

Could be, time will tell.


No idea what you are getting at here.

The distances are not constant.


These are engineering issues, not the laws of physics.
They are very closely related. Why build a bridge? Probably because of gravity.

Van Rijn
2006-Apr-06, 08:56 PM
It's shorter, closer to the ground, in a breathable atmosphere, easier to get by safety opponents, proven technology (just much bigger), etc.


Assuming a basic synchronous skyhook, this superbridge would be much more massive, total cable length would be greater, it would be a more complex structure, would have to deal with a wide variety of local conditions, etc.



Could be, time will tell.


No, it is fundamental to the design.



The distances are not constant.


Still not getting it. What exactly is being suggested, and what is the relevance to earth or lunar skyhooks?



They are very closely related. Why build a bridge? Probably because of gravity.

Missing the point. There are engineering issues - there always are. But Jerry didn't present any "show stoppers." That is, nothing that would make the concept impossible.

Ara Pacis
2006-Apr-07, 06:03 AM
If you're talking about a rotovator for earth to orbit transport, the craft would be well under orbital velocity, perhaps only mach 6 or 7. It isn't going to go into an odd orbit - or at most, it will quickly intersect earth. There really isn't much point in using a rotovator if you are going to use conventional rockets for much more than half of the velocity change to get to orbit.

A rotovator or some similar concepts might mesh up well with suborbital followup craft to SpaceShipOne.

For interorbit transfers that go wrong I would expect emergency rockets and procedures would be mandatory. Actually, you pretty much need fine control and "final wave off" capability if there is an emergency.

The nice thing is that you can afford to waste a bit of mass on safety, since there are ways of getting the energy back.

Are we talking about a rotovator that "walks" across the earth so that it has a relative speed of zero as it dips to it's nearest point? If that is the case then it would not even need a mach 6 craft to catch up to it. We could simply have large balloons at the rendezvous.

Van Rijn
2006-Apr-08, 05:21 AM
Are we talking about a rotovator that "walks" across the earth so that it has a relative speed of zero as it dips to it's nearest point? If that is the case then it would not even need a mach 6 craft to catch up to it. We could simply have large balloons at the rendezvous.

There are tricks with matching the rotation of a long rotovator with the rotation of the earth to reduce the relative speed at closest approach . . . but a balloon is a bit much. Also, if the rotovator does this much work, it increases the strength to weight ratio and length requirements. It can have lower requirements than a synchronous skyhook, but the requirements are still quite impressive, and well beyond what is currently possible. A half orbital velocity design is much more attainable in the not too distant future.

publiusr
2006-Apr-12, 09:31 PM
Are we talking about a rotovator that "walks" across the earth so that it has a relative speed of zero as it dips to it's nearest point? If that is the case then it would not even need a mach 6 craft to catch up to it. We could simply have large balloons at the rendezvous.

That synchs well with JP Aerospace.