Naked Science Forum
General Science => General Science => Topic started by: lightspeed301 on 03/01/2012 18:07:45
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For those who have not heard of this, the concept is simple. You assemble a small space station in geosynchronous orbit. Then build a carbon nano tube cord down to the ground. You simply lift one payload up, and perhaps for balance sent another payload down. Sort of like the counter weights in a building elevator.
OK. I have been reading about these things for decades [Arthur C Clark, the science fiction author, proposed them many decades ago]. Not one time have I even heard one single discussion concerning the conservation of angular momentum.
Specifically, payload velocity on earth at the equator is 1,000 mph. The GS space station has an orbital velocity of nearly 7,000 mph. http://www.centennialofflight.gov/essay/Dictionary/GEO_ORBIT/DI146.htm So. The elevator going up needs to accelerate an additional 6,000 mph and the one going down needs to shed the same amount.
Accordingly, it seems to me you need to retain some sort of propulsion device on each elevator. The system still seems theoretically more efficient then a boost to orbit. For instance, the lift system itself would be mechanical, and relatively slow. No need for giant rockets to get out of the atmosphere.
However, a satellite for low earth orbit would need to be released from the elevator and accelerated at right angles to the elevator to at least 17,000 mph.
Any thoughts on space elevators?
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Only problem is the cable, a little beyond current materials. You do need a fairly big hunk of rock or whatever in an orbit a bit past GEO, to provide the tension in the cable to keep it taut. You do not really want to have anything in orbit below the tether, as then you would have to track everything and move the cable constantly to avoid them, and eventually you will get hit by something big enough to snap the cable. Not good for whatever is in the way of the resulting tsunami or wherever the cable hits land along the equator.
Read 2063 for some good info about the space elevators, and a good story as well.
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The construction of a space elevator will be a monumental task. It won't be easy to get that much material up into space, and as mentioned, failure of the cable could be devastating.
A cable from the Moon to the Earth-Moon Lagrangian 1 or 2 point would be longer, but simpler to construct, and might be an alternate means for getting lots of material into space, but would first require a permanent lunar colony. The moon also has less issues with wind resistance, so a surface mounted orbital accelerator would also be effective without the need for a space elevator, at least to move material upward.
It is unclear if cabon nanotubes will have adequate strength/density for the task, and whether we will be able to build and weave long-chain nanotube cables. Spider webs are interesting because they not only contain chain polymers, but also have a type of built-in spring/flex elements that would reduce material fatigue. Overall a space elevator will have to have some long-chain super-strong, super-light elements with a similar type of fatigue arresting element. Certainly one wouldn't want to have to replace the entire space elevator every couple of years due to material fatigue, or weathering.
For the required density of the cable, it will likely consist entirely of the first 9 elements on the periodic table. Carbon making 4 bonds, and also making delocalized ring half-bonds, is an excellent choice for a primary element in the construction. Silicon, immediately below Carbon on the Periodic table is likely too heavy. This means that the list of possible construction materials is much more limited than one might think.
Certainly, as with any elevator, it requires power to go up. It is unlikely though, that one would use a cable like an elevator to tow the car up because length/weight/density/tensile strength issues of construction so you couldn't really construct it like a ski chair-lift with cars going down adding power those going up. And, obviously, one of the issues is WEIGHT, so sending empty cars down the cable might be problematic just for the added weight. One might be able to use surplus cars, old satellites, and etc, for raw materials for construction in space. But, one would have to control the collection of too much space-junk.
One of the proposals is to power cars using lasers or mirrors and photo-electric panels. Graphite type rings as in carbon nanotubes are also good conductors, but electric power always requires two conductors, which might not be practical in the lift, so current designs are for self-contained "climbers".
As far as speeds. A geostationary orbit will be a higher energy orbit than the low-earth orbits. Likely, to get to LEO, one could take the satellite up the cable to a higher level than the desired orbit then drop it off, and let it fall and accelerate to the perfect orbit. Potential collisions could be an issue as mentioned.
Many things that are currently in a LEO orbit could be done at a higher orbit, for example a space station would be good at a geostationary orbit. The current one was just constructed in LEO because of the greater expense to access it with current rocket technology. Weather satellites, however, are currently being designed to do rapid orbits around the planet in LEO, and polar orbits may require LEO.
Anyway, I don't foresee a space elevator being completed in the next half century... perhaps sometime... or perhaps on the moon and elsewhere. But, there are a few reasons why an Earth space elevator might not be too practical.
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cliff - Re dropping a satellite from GS to LEO. That sounds about right. And certainly it is an engineering problem that might have a better solution. For instance launch from a very large lighter then air vehicle at very high altitude.
But my original question still stands. Assuming the elevator exists, how do you solve the problem of conservation of angular momentum. I see no other option but to provide the payload with right angle thrust to the elevator cable.
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To get the satellite into geostationary orbit, you could take it just beyond geostationary orbit for release. That will cause it to be going faster than the corresponding orbital altitude. Release it, and it should move to a higher orbit and slow down... then over days or weeks it will move to the desired location, at which time you move it to a lower orbit with minimal thrust.
For lower orbits...
you are right that you would need to add angular velocity to the satellite, and thus you would likely have to accelerate it somewhat from the cable.
If your goal is an altitude of 600 km
So you raise the satellite to 30,000 km
Then drop it.
You wouldn't need to impart a significant angle to its trajectory to force it into a tangent at 600 km. Or, you would likely target a highly elliptical orbit, of say 160 km altitude at the closest approach, then slowly modify the orbit into a circular orbit.
It would require some energy to push the satellite into a 600 km circular orbit, but a lot less than would be required to launch it from the surface of the planet.
I agree that a lot could be done with lighter than air launch vehicles. However, it would be a huge endeavor to design a blimp capable of holding the weight of the Space Shuttle (about 4 million lbs takeoff weight), and carrying it to somewhere around 40 km altitude, which is still quite a bit short of the 160+ km altitude for "space" and LEO. However, if the platform was large and sturdy enough, one might be able to gain some additional speed with some kind of a slingshot launch, then use the wind to re-accelerate the platform.
Another option that has been tested with some success is to use a conventional aircraft as a launch vehicle somewhere between 20 and 40 km altitude, and either at subsonic or supersonic speeds. The advantage is that the initial boost up to say Mach 4 & 40 km altitude, could be done with only carrying the fuel, and not the oxidizer. And, one would get to an altitude where wind resistance would be minimal.
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I'm not very sanguine about an Earth based Space Elevator in the near future.
First the materials to make it don't exist in a practical form at the moment, carbon nanotube rope isn't up to the job. Graphene is another contender, but again, not there right now.
Beyond that, there's the radiation issue; the Van Allen Belts are above the equator; and for somebody riding the elevator they would get a nasty dose. It wouldn't kill them, and it wouldn't .... quite... give them radiation sickness, but it's still a nasty hit, and far above statutory limits.
You'd think you could shield against it, but adequate shielding is too heavy until you get to VERY big elevators.
The other issue is performance, Space Elevators are very slow to get to orbit, and the time between launches is pretty long because the cable is inevitably quite fragile low down near the Earth.
All in all, not very promising.
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But my original question still stands. Assuming the elevator exists, how do you solve the problem of conservation of angular momentum. I see no other option but to provide the payload with right angle thrust to the elevator cable.
You don't need to, as the car climbs the cable tilts slightly due to coriolis effect and that gives the angular momentum, the Earth essentially tows the payload up to speed without doing anything. Its actually free energy you're stealing it from Earth's rotation (!) but the Earth has an enormous amount so it's not significant.
The effect is more significant above GEO, you could get very large amounts of energy by throwing stuff off the top. There's plans to throw stuff to Jupiter like that if they ever build one; free energy and no propellant!!!!!