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Lunar Space Elevator Instead?
koa writes "We have all seen articles on building a Space Elevator on the earth, how about this article about experimenting with the Moon first since the technology we have available to us is sufficient, as the Moon's gravity is 1/6th that of Earth's (the cable weight would require less exotic materials such as carbon nano-tubes). One could make a very good argument for commercialization of Space if getting materials to and from the Moon's surface was vastly cheaper and easier."
I thought we were going to use the moon to anchor this to , instead of the typical big bulky sattellite.
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Relocating to the Moon won't help the project a bit if the raw materials (whatever) has to be brought from Earth . A mining/harvesting camp on Moon would be at least a few decades away, until then this can wait on the backburner. An orbital platform harvesting asteroids for heavy metals would rock ! (literally) . Would be nearer to earth and it would put solar sails in the domain of practical rather than as Sci Fi book fodder.
Hmm... all the differential equations in Rocket Science confuse the hell out of me . I suppose the space elevator doesn't have the rocket's exponentially growing weight problem ?. (Now I know why they say "It ain't rocket science)
I'd rather vote on the space catapult to launch rockets at Mach 3 (or higher) with something (jet aeroplanes or Maglev rails on mountains) . If the initial acceleration can be supplied by ground based non-moving power equipment, the rocket could go a looong way in reducing weight.
Sadly the word space Catapult brings into mind unnecessary images of North Elbonia and
Quidquid latine dictum sit, altum videtur
orbital_radius^3 = (3,600^2 * surface_gravity * surface_radius^2 * orbital_period^2) / 2*pi^2
...the height of the elevator, therefore; for the moon it's 190000km. In other words, five times higher than one on Earth! That's nearly half-way to Earth; the gravitational disturbances from Earth's much greater mass could well make the whole thing infeasible.
Think about this - carbon nanotubes are lightweight and burn up rather nicely. The highest you're going to get to break an elevator cable is a few km above the surface. The bit above the break stays in orbit due to a combination of forces, the bit below will fall and burn if it's high enough to gain enough speed to be dangerous.
And of course if you anchor the elevator offshore (makes most sense for security anyway) then all you're going to get is a splash.
How many people can read hex if only you and dead people can read hex?
But you are absolutely right. Having no atmosphere, the Moon is the ideal place to put a railgun. Besides Heinlein, many other authors have used that concept, among them Gerard K. O'Neill, who popularized the L5 orbit concept.
It would like when they were building the first highways across America. The company would build a "seedling mile" of good highway along a stretch of crummy road near some town. After people tried that mile of good road in the middle of a stretch of washboard, it was a lot easier to get them to vote for the taxes to pave more of it.
One line blog. I hear that they're called Twitters now.
I am not a physicist or rocket scientist, but a few questions pop out:
The cable would be 58,000 km long. This is the distance from the Moon to the L1 point, which is the balance point of gravity between the Earth and Moon.
Wouldn't the cables center of mass need to be at L1 or slightly above (relative to the Moon), rather than the end of the cable? If one end of the cable were at L1 and the other end on the moon, the moon's gravity would have a greater effect than the Earth's gravity, so the cable would be pulled back down to the moon, correct? Or am I missing something?
This has nothing to do with centrifigal force, like an Earth-based elevator where the counterweight keeps the cable taut.
Sigh. This has everything to do with centrifugal force. It is exactly the same as a terrestrial space elevator. That's why you could build a cable to L4, or L5, even though gravity doesn't appear to balance there at all. It's all about solving the 3 body problem in a rotating frame.
When we talk about a space elevator for Earth, we're talking about building a cable to geosynchronous orbit. At that point, objects orbiting Earth appear to stay above the same point on the Earth's surface because they rotate with the same angular velocity as Earth.
At the L1 point, objects appear to stay at the same point with respect to the Moon because they rotate with the same angular velocity as the Moon. This doesn't happen at the naive calculation that you'd do to get "lunar synchronous orbit" (which is something like half the distance to the Earth) because the Earth is the dominant gravity player in the Earth-Moon system. That's why you have to solve the 3 body problem, whereas for Earth's GEO case, you can ignore the Moon because Earth rotates so fast and Earth is so massive. But centrifugal force does play a role.
However, the cable is an extended object. From an orbital mechanics point of view, the cable stays at L1 because it's in orbit at L1. From the lunar surface's point of view, though, the cable stays upright because it's taut under a large amount of tension, caused by centrifugal force and gravity.
And on a terrestrial cable, the counterweight doesn't keep the cable taut. You don't need a counterweight. You just need the center of mass roughly at GEO.