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Comment by ericd | Hacker News Reader

I wouldn't dismiss the space elevator out of hand. It requires carbon nanotubes of a few meters length to achieve the required strength, and you wouldn't need to lift it pre-built - you could build it with a guideline and cable laying cars traveling up and down, adding to the cable, much like they do with suspension bridges. Long term, it seems like far and away the best approach if we can solve the materials science problem.

EDIT: I found this book to be a pretty great primer on the subject: http://www.amazon.com/Space-Elevator-Earth-Space-Transportat...

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I would certainly dismiss elevators because there is a much more practical alternative that doesn't require materials that don't exist in sufficient quantities to reach a quarter of the way to the moon.

The Lofstrom Loop (http://en.wikipedia.org/wiki/Launch_loop) could be built with materials we have today, although it requires sufficient amounts of money and land that only large countries or multibillionaires could attempt it.

I never have understood why the space elevator gets so much attention, and the launch loop is almost unknown. The elevator obviously had some high-profile proponents (like Arthur C. Clarke), but given how realistic the launch loop seems (only 8 billion dollars??), I would think it would be the thing everyone talks about.

The fact that it is not makes me think that it is less realistic or more constrained than it is made out to be. I certainly don't have the skills necessary to evaluate that, but I bet someone here does...

The launch loop must be perpetually active once established. That's a lot of continuous power needed for something we do pretty rarely--launch stuff into space.

And if the power turns off, it falls back to Earth. A heavy, high-speed belt falling 50 miles down along a length of 1,200 miles...not an easy problem to solve. Even if it drops into uninhabited territory, it's not going to be in great shape.

A space elevator is a passive system. Once established, it stays up. If you lose power you can't go up it, but it doesn't fall down. In that respect it is more like the passive infrastructure we're comfortable with, like highways, bridges, buildings, etc.

If the power is turned off the Launch Loop will not immediately fall out of the sky. As the loop slows it will gradually fall. This is also why most pictures show it operating over the ocean. We have a lot of ocean at the equator.

A space elevator is not passive. It needs active dampening systems to avoid resonances that would tear it apart. The failure modes for a space elevator are also pretty terrifying. The lower the break the less harmful it is, but higher breaks will cause huge amounts of damage. Worst case scenario, the whole cable comes down. That is 75000 km. (Assuming the traditional double-spool deployment.) The Earth's circumference is only 40000 km. It will wrap around the earth twice! 'Blue Mars' has more about what such a disaster might look like.

The loop is not more well known because it was only invented in 1981 and requires non-intuitive physics to explain. Space elevators are "simpler" (if you ignore the active control problems) and have been around since 1895, though the modern design dates to 1959.

> If the power is turned off the Launch Loop will not immediately fall out of the sky. As the loop slows it will gradually fall.

That's for a loop constructed only of superconductors and magnets. If you use active electromagnets instead of the superconductors for saving money (or if your superconductors get too hot), the failure mode is way more harmful.

Anyway, I really don't know why the launch loop is so overlooked. Specifically, I don't know why nobody's trying to build an intercontinental bridge. It may be because of our anti-nuclear culture, and that those things only make any sense when coupled with nuclear power, but that's just speculation.

The linked Wikipedia page addresses turning the loop on and off. In fact, it can't be run continuously because it would overheat.

Edit: actually, I may be mistaken. Here is one of the relevant passages:

"When at rest, the loop is at ground level. The rotor is then accelerated up to speed. As the rotor speed increases, it curves to form an arc. The sheath forces it to follow a curve steeper than the rotor's natural ballistic curve, which [clarification needed] , in turn, exerts a centrifugal force on the sheath, holding it aloft. The loop would be anchored to the ground to remain at a fixed height.

"Once raised, the structure requires continuous power to overcome the energy dissipated. Additional energy would be needed to power any vehicles that are launched."

So is this talking about raising it only the first time? And then it has to have continuous power for the remainder of it's operational life? It seems like having power off capability shouldn't be an insurmountable task.

The failure mode of a launch loop is really bad though. The constant power requirement is a pretty big problem and the solutions to loss of power aren't that promising. Compared to the elevator where there's a larger safety margin.

I liked Elon Musk's note on the space elevator; he basically said "we should look at it once we have a bridge from Los Angeles to Tokyo, because that's far easier to build".

I disagree with that argument. Great solutions require great problems, and moving things between LA and Tokyo just isn't anymore. Right now, shipping a person costs some $1k (return included) and takes less than 12 hours; a 40 ft container of cargo around $800 and 22 days.

Launching a satellite still costs millions. The closest thing to inexpensive space flight is probably Virgin Galactic's SpaceShipTwo, which is still in testing. Space remains interesting and unsolved. And as such, I'll bet you a beer we'll have a space elevator sooner than either a bridge or a tunnel directly connecting LA and Tokyo.

What would the guideline be made of?

The same stuff as the main cable, just thinner and less redundant. The main issue with the space elevator is the cable weight vs. cable strength.

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