08 October 2015

Are Space Elevators going to be necessary?

​From a recent discussion with an interested friend of whether our potential future interplanetary civilization will necessarily rely on Space Elevator technology for traffic to and from the surface of Earth. ​

​[We have to take] into account the fact that the rocket equation applies to any rocket (i.e., any system that achieves escape by pushing against itself), whereas a space elevator (or any one of a number of other related technologies) does not, and thus is freed from the inexorable logarithmic logic of rockets, which is that the cost of lifting mass to orbit grows exponentially with mass. There's just no way around it. Chemical rockets, atomic rockets, fusion rockets, any rockets. You can improve efficiency but even with 100% efficiency you are still faced with very steep costs to lift substantial mass from a gravity well like Earth's. Only magic, i.e., anti-gravity or something of the sort, can get around this in a self-contained vessel. If you look at the curve of increasing materials strength, and the kinds of materials being researched now (fullerene and carbyne fibers, and even more exotic things), it seems to me far less speculative to infer that it's likely humans (or, by the same logic and some time or place, others), would solve these problems than that they would invent something for which no theoretical basis exists at all.

I rest my case, other than to throw in that I'm pretty sure this is the direction most people who dream of building things like Banks Orbitals are thinking. It's not so much getting up there to build stuff, because that could be done with rockets, but if you want to have traffic, i.e., ordinary people going into space and back routinely, rockets, of any kind, just aren't going to work. (The whole issue of the near impossibility of building rockets that have low environmental impact is a whole other can of worms; and if you're talking about atomic rockets of any kind, then that becomes even worse of an issue).

Of course, if you are talking about leaving the vast majority of humans to fend for themselves on Earth, while a whole new subset of humanity becomes Homo caelestis or whatever, then it won't matter. Rockets of one kind or another work just fine in interplanetary space, away from large gravity wells.

And then there's the whole issue of interstellar travel. Rockets are very poor for that, too, for a set of different reasons. No practical solution for moving significant masses to other stars yet presents itself. For example, a fusion rocket that's 99% efficient would have to carry enough fuel to power the entirety of human civilization for several years just to get a schoolbus sized payload vehicle to .1 of the speed of light, and slow it back to planetary velocities at the other end. The fuel to payload ratio has to be something like 100,000 to 1. And that's still slow enough that it couldn't be practical without some form of suspended animation.

I do believe we will eventually send long range probes to suspected habitable planets orbiting so called nearby stars (<50 l.y. or so), and even longer range, if our culture survives and becomes a thriving spacefaring civilization here in the Solar System, our descendants will figure out some way to go there. But that, I think, will require a quantum leap in technological abilities that we can't really foresee except in general at this point. Somewhere, others have probably done this, but, as I always harp on, the fact that we don't see them is prima facie evidence that it's very hard, and, at least so far in the history of the universe, doesn't happen all that often. ​

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