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Comment by Robotbeat | Hacker News Reader

Indeed, Hyperloop Transportation Technologies (the crowdsourced one) said they'd use Inductrack technology for their version of Hyperloop: https://en.wikipedia.org/wiki/Inductrack

This uses permanent magnets arranged in a Halbach Array for levitation.

This method of levitating a pod is incredibly efficient and gets more efficient at higher speeds (in a vacuum). You can actually exceed typical rail efficiency (rolling resistance of 1:1000, i.e. an effective coefficient of friction of 0.001 or a lift-to-drag ratio of 1000) at 300m/s since levitation power remains basically constant above a certain speed (as opposed to increasing linearly in the case of friction or rolling-resistance). And because you're in a vacuum, you're far exceeding what could be possible in a non-hyperloop/vactrain design: https://www.google.com/patents/US20100064929

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Yes but maintaining a vacuum over long distances is a huge challenge.

Any public information on what Musk thinks he can get the leakage rates to?

I assume the metric here would be something like cubic-yards-per-mile-per-day or something like that.

Or acres per fortnight, perhaps?

I'll say, mine always gets caught going around corners or on chairs...

It's a 'soft' vacuum, so doesn't have anywhere near the difficulty of a 'hard' vacuum. IIRC the detailed specs of what they intend to do aren't out, so it's all just speculation.

This might be a case where no one has actually tried it with a maker mentality. Musk was saying the other day that you need 1 atmosphere of pressure capability to create a vacuum, but 5-6 to hold back the watertable related to his tunneling network. Seems like from a first principles perspective making a vacuum should be much easier than an underwater tunnel. I'm guessing that you could create a partial vacuum in such a tunnel fairly easily. But I haven't 'done it' and that's the rub.

I don't think the problem is the pressure - holding up one atmosphere is not big deal - I think it's the porosity. It just takes one hole, anywhere, and pscchht you start losing your vaccum. You have to continuously run pumps to get rid of the air that finds its way in, and their efficiency is inversely proportional to the pressure difference.

This is also true of below-water-table tunnels and pipelines of various sort, which are often at much higher pressure and with much greater environmental risk.

It is quite a bit easier to hold back a liquid than a gas. Different minimum pore size and you get hydrophobic chemistry and surface tension on your side.

Especially at such big pipe diameters as required.

You can use cheap materials like modified concrete to patch a hole in an underwater tunnel, not so in a vacuum tube - that needs something much better and more expensive.

Not true. You can use concrete to hold back a vacuum and cheap materials like a thin polymer coating, too. Remember, Hyperloop does not require an ultra hard vacuum.

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