Soul., the idea of a nuclear powered engine is being developed, but on a different scale than you are using. First you must understand that a nuclear engine in a space travel evironment doesn’t worklike a sub or carrier. Basically the nuclear engine just creates electricity that turns a electric motor which turns the screws on the ship. In space a mechanical engine doesn’t work because in a vacuum there is nothing for the mechanical engine to push off. Now the electric engines in development are being designed for use on very small interstellar and interplanetary explorers. The way a electric engine works in space is amazingly complicated, and there are a variety of competeing ideas on how to make it work. One method basically creates a charge and sends electrons in an Arcjet from a plate into a collector at the back of the module. This creates a thrust because of the momentum created by the impact of the electroms. Knowing how small the electrons are you can imagine how small these forces are. No matter what type of engine wins the “race” it will be used on a scale where the thrust is of the magnitude .01-.1 Newtons at a specific impulse of 1000 seconds. My professor was working on a 50W Helium pulsed Arcjet for installation on orbital satilites for orbital manuvering. The specifics of this type of engine baffle me to this day, its a shame he class was at 9 am or I may have learned how it worked. All that being said not even the most ambitious proffessor has thought a nuclear/electric engine could be used to reach escape velocities and even alter orbit altitudes. The magnitudes of thrust are just way to small compared to the necessary forces.
As for the space elevator, the idea is so absurd I won’t get into the physics of it. First the wieght of such a cable would be so great the nothing could even lift it to get it to the launch platform let alone get it of the ground. Second, what would hold it up? Do you think that space is some magic place that holds things in it? Anything in space is constantly falling, and the entire length of the cord would fall very quickly because of the segments that are close to the earth.
The facts expressed here belong to everybody, the opinions to me. The distinction is
yours to draw…
Omniscient, Actually, the physics do work out in a beanstalk design. It has been shown that modern carbon fibers have the tensile strength to resist the pull on them, and buckytubes are even better. As for what holds the beanstalk up? Centripital force does. The stalk is long enough that the tangental force on the space end is great enough to pull the entire stalk taut. Think of it as a yoyo that you are spinning over your head. (Just hope the stalk don’t break, cause if it does…well thats a lot of stuff going on a long trip.)
>>while contemplating the navel of the universe, I wondered, is it an innie or outie?<<
Oh, and most proponents of the beanstalk, feel that you wouldn’t lift the entire structure at once, you would lift your end mass and a couple of cables, then start spinning the rest of the stalk, working downwards rather than upwards. (Move cables up into orbit normally at first, connect them to the stalk and then spin them downwards.)
>>while contemplating the navel of the universe, I wondered, is it an innie or outie?<<
Omniscient writes, “As for the space elevator, the idea is so absurd I won’t get into the physics of it.”
Well, it may be counterintuitive, and building one from the ground to orbit may be impossibile, but the concept itself is definately not absurd. The idea has been looked at quite seriously (by no less than NASA). There’s nothing wrong with “the physics of it” in the general case, just in certain specific cases.
> Second, what would hold it up?
Depends on the specific form the system would take. In some, the tether’s center of mass would be in geosynchronous orbit. There are many other possible designs, including tension tethers between two orbiting objects at different heights. It turns out that when you use a tether to constrain the movement of such objects, orbital dynamics cause the tether to be in tension.
The really big ones are beyond both our current materials technology and our ability to lift that much mass. But they’re not so far beyond that we should ridicule the idea.
It won’t work. First, you would need the anchor to be in Goestationary orbit. Thats 250-300 miles for a satilite, depending on how much greater this mass is it could double the needed height. Then you’ll be putting that anchor in a goestationary orbit at a certain speed (height) that allows the correct centripital force to keep that mass in orbit. Now you add a mass of a few cables, to maintain geostationary orbit, you’ll need to increase speed (ergo centripital force) and to maintain geostationary orbit then you’ll need to increase alttude. You see the coming problem? Everytime you add mass of new cables you need to add altitude and speed. Given this the total length of the cable gets longer and longer, a Catch 22. Now unfoutunately I can’t recall the equation that would dictate the increase in height with respect to the increase in mass for geostationary orbit, but i suspect that it is great enough to make this mathmatically impossible if not just practically impossible. Also recall that as mass increases the launch cost increases, and as needed altitude increases the availible mass potential decreases so the pieces of cable will need to get infinately short not only because they need to get thicker, but because they need to reach an ever increasing height. I don’t see how this could ever become cheaper than just launching to space even if it were possible, and any cost saving technology created to facilitate this would also bring the cost of launches down and negate the supposed cost benefit of the elevator.
The facts expressed here belong to everybody, the opinions to me. The distinction is
yours to draw…
Yeah, two objects would be in tension, but what two objects and what orbits that could possibly facilitate any benefit in the beanstalk?
I’d also like you guys to explain the beanstalk a bit. Do you mean one end is thicker and the stalk get exponentially smaller? If so where would you put the thick end and where would you put the thin end? If the thick end is at the top because it needs to support he mass of the rest of the cable, ok, but the mass of the operative payload is heavier near the surface of the earth.
One final contradiction, when you send up a payload the mass of the system will increase and the station would need to reach a higher altitude. So everytime you deployed a payload you would need to expend more thrust to increase and then decrease the altitude (assuming you sent that payload away from the elevator).
SoulFrost writes, “OK…let’s say we refitted our already modified shuttle to be nuclear-powered (say, with more efficient engines than our modern submarines have).”
Well, there are a few different ways to propel spacecraft. Some, like solar sails, only exist in theory and involve light pushing against a large surface. But among the ones we can actually build right now, they all involve shoving reaction mass out the back of our spacecraft at high speed to increase it’s velocity in the opposite direction, according to conservation of momentum. You can make this more effecient by ejecting the reaction mass at very high speeds.
A “nuclear powered” spacecraft still has to have some sort of reaction mass, and that’s really the issue in the end. Just having a reactor on board won’t really buy you anything on that front - reactors don’t create mass, and they don’t speed mass up. (Well, there are some designs for using this energy to speed up mass, but they’re all just theorical and likely to remain that way for quite a long time).
The reason a submarine can use a reactor for power is that is has the luxury of having water to push against, which you can do with a screw. Our spacecraft doesn’t have anything like that to shove against!
Some unmanned spacecraft, especially those that need to operate far from the sun, do use what’s called a Radioisotope Thermoelectric Generator (RTG) to generate electricity. This is definately not a nuclear reactor, but it does use the heat from the decay of some isotope of plutonium to generate electricity to power the craft’s systems. The Voyager probes, for instance, have such a system.
Omniscient write, “The entire concept is foolish.”
No, it’s not foolish, Omniscient. It’s been studied quite extensively, in fact. You can probably find NASA’s technical papers on the subject if you’re really interested in learning about it instead of dismissing it.
A tether in tention between two orbiting masses is useful because you can reel stuff up and down it, and it conserves energy. Say you have a large space station in a high orbit. In normal use, a lot of what you lift to the station (say, people, science experiments, things that turn into garbage after use), gets taken back down. Now, with normal chemical propulsion, you have to expend energy to lift all this stuff up, and then expend yet more energy (albeit less) to get it back down again. With a tether, you get back the energy used to lift the stuff be sending other things back down the pipe, so to speak. It’s a major net win, and if it wasn’t, nobody would be bothering to study it. Rotating tethers have other, but still useful, applications.
> You see the coming problem? Everytime you add mass of new cables you need to add altitude and speed. Given this the total length of the cable gets longer and longer, a Catch 22.
What I see, is that a little knowledge is a dangerous thing :-).
If you’re really interested in this topic, here’s a NASA page that has a bunch of papers on the subject, mostly written for a layperson audience, so they’re easy to digest.
At the very top of the page: “Tethers are now being seriously considered for use with the Space Shuttle and the Space Station for raising or lowering payloads for various scientific and engineering purposes” … so, in short, don’t dismiss something just because you don’t understand how it could work
At one point, I also had a page with links to more technical papers on the topic, but I can’t seem to find it now :-|. I’ll post again if I manage to hunt it down.
I seem to remember an article in Discover magazine a few years ago that discussed the possibility of “beanstalk” elevators.
If memory serves (and it often doesn’t), not only was the elevator from earth to orbit considered, but also one from orbit to various other paths… usually something near the moon. The conclusion was that it was feasible, as long as certain probable technological advances were made.
I’m afraid you aren’t so “omniscient” if you think that orbital periods are dependent on the mass of the orbiting body, or that geosynchronous orbits are anywhere near 250 miles.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams
