Yes, seriously. I'm currently trying to create a setting for a hard SF adventure game, and I need a bit of help. It might be a long shot, but I thought I'd ask around, anyway. I'm just going to copy and paste what I posted on rec.arts.sf.science:
Hello ladies and gents. I was wondering if you could help me figure out some issues surrounding something that I'd like to put into a SF computer adventure game that I'm working on.
Consider your run-of-the mill orbital skyhook. It extends up into space perpendicular to the surface of the planet that it orbits. Simple enough. Let's suppose that this skyhook is in orbit around Saturn. Now, if we were to lower the orbit of the skyhook so that its lower tip extends into the atmosphere of Saturn, it's going to experience quite a bit of atmospheric drag. Eventually, its orbit will decay and it will fall into the atmosphere, with the net result that we've just thrown a very large amount of money into a gas giant. Not good. The best solution is to keep the skyhook from ever entering the atmosphere.
As it turns out, though, that's exactly what we want to do in order to get at all the very useful He3 that you find in the atmosphere. So the other option is to use some sort of propulsion system to counteract the effects of the drag. Fortunately for us, Saturn has a rather powerful magnetosphere, which we can use to our advantage. Since the skyhook is nothing but a tether, it should be possible to use it as its own electrodynamic tether propulsion system: you pump electricity into it, and it raises itself out of the atmosphere. This should allow it to offset the atmospheric drag and the loss of orbital velocity caused by raising large amounts of He3 and other gases into orbit. And where does the power for the electrodynamic propulsion come from? We could burn some of that He3 in a fusion reactor, but we're looking to sell that elsewhere. Instead, why not put a number of other tethers in a higher orbit (not extending into the atmosphere) to generate electricity by passing through the magnetosphere and then beam it to our skyhook?
That's the plan, anyway. Here are the problems that I have:
First, I'm not sure that the electrodynamic propulsion system would provide thrust in the proper direction to counteract atmospheric drag and loss of momentum due to the constant pumping of gases up the skyhook. As I understand it, to raise an orbit, you want to thrust in the direction of your orbit; the tether propulsion system - again, as I understand it - will thrust in the direction that the tether is oriented. In this case, that would be perpendicular to the direction of travel. Is this correct? And, if it is, would this be sufficient to prevent orbital degradation or do I need to thrust in the direction of travel?
Second, I'm not sure just how much drag and momentum loss we're talking about, here. Would there be too much of either (or both) for the propulsion system, assuming it works, to counteract? What would the power requirements be? Would the electrodynamic power generation tethers be up to the task?
Third, if everything else can be made to work, would the fact that the drag on the skyhook is applied to one end only mean that the thing would tip over? Would it just lean and then become stable at an angle? If it would be a problem, is there anything that can be done to counteract it?
Fourth, just what is a reasonable orbital period and speed for such a setup? Ideally, the orbital period would match the rotation of Saturn at the equator (about 614 minutes), but I'm not sure that such an orbit is possible. If the orbital period is longer than the period of Saturn's rotation, would that mean that the atmosphere is blowing in the direction of the skyhook's travel? If so, how would this affect its orbit?
And if you can think of any other problems with the setup, I'd appreciate a heads-up.
Thanks a lot, guys.
Hello ladies and gents. I was wondering if you could help me figure out some issues surrounding something that I'd like to put into a SF computer adventure game that I'm working on.
Consider your run-of-the mill orbital skyhook. It extends up into space perpendicular to the surface of the planet that it orbits. Simple enough. Let's suppose that this skyhook is in orbit around Saturn. Now, if we were to lower the orbit of the skyhook so that its lower tip extends into the atmosphere of Saturn, it's going to experience quite a bit of atmospheric drag. Eventually, its orbit will decay and it will fall into the atmosphere, with the net result that we've just thrown a very large amount of money into a gas giant. Not good. The best solution is to keep the skyhook from ever entering the atmosphere.
As it turns out, though, that's exactly what we want to do in order to get at all the very useful He3 that you find in the atmosphere. So the other option is to use some sort of propulsion system to counteract the effects of the drag. Fortunately for us, Saturn has a rather powerful magnetosphere, which we can use to our advantage. Since the skyhook is nothing but a tether, it should be possible to use it as its own electrodynamic tether propulsion system: you pump electricity into it, and it raises itself out of the atmosphere. This should allow it to offset the atmospheric drag and the loss of orbital velocity caused by raising large amounts of He3 and other gases into orbit. And where does the power for the electrodynamic propulsion come from? We could burn some of that He3 in a fusion reactor, but we're looking to sell that elsewhere. Instead, why not put a number of other tethers in a higher orbit (not extending into the atmosphere) to generate electricity by passing through the magnetosphere and then beam it to our skyhook?
That's the plan, anyway. Here are the problems that I have:
First, I'm not sure that the electrodynamic propulsion system would provide thrust in the proper direction to counteract atmospheric drag and loss of momentum due to the constant pumping of gases up the skyhook. As I understand it, to raise an orbit, you want to thrust in the direction of your orbit; the tether propulsion system - again, as I understand it - will thrust in the direction that the tether is oriented. In this case, that would be perpendicular to the direction of travel. Is this correct? And, if it is, would this be sufficient to prevent orbital degradation or do I need to thrust in the direction of travel?
Second, I'm not sure just how much drag and momentum loss we're talking about, here. Would there be too much of either (or both) for the propulsion system, assuming it works, to counteract? What would the power requirements be? Would the electrodynamic power generation tethers be up to the task?
Third, if everything else can be made to work, would the fact that the drag on the skyhook is applied to one end only mean that the thing would tip over? Would it just lean and then become stable at an angle? If it would be a problem, is there anything that can be done to counteract it?
Fourth, just what is a reasonable orbital period and speed for such a setup? Ideally, the orbital period would match the rotation of Saturn at the equator (about 614 minutes), but I'm not sure that such an orbit is possible. If the orbital period is longer than the period of Saturn's rotation, would that mean that the atmosphere is blowing in the direction of the skyhook's travel? If so, how would this affect its orbit?
And if you can think of any other problems with the setup, I'd appreciate a heads-up.
Thanks a lot, guys.
