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Comment by a11r | Hacker News Reader

I am assuming that you are talking about the thermal expansion of the tubes (ducts ?). A long time ago, the rails making up train tracks used to have gaps every few meters to accommodate thermal expansion (same problem). Modern rails in high speed train tracks are welded into one continuous bar that can be over a kilometer long. The trick is to stretch the rail just enough and then pin it down - kind of like a slightly stretched rubber band. That stretch can accommodate thermal expansion.

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That's a good direction to pursue, but CWR (continuous welded rail) segments are only a few kilometers long at most. That, plus the fact that the cross-section of a rail is much less than the cross-section of a hyperloop tube, makes the tensile stress at the ends of the rail small enough that they can be managed by anchoring the ends of the rail to the rail bed.

The tensile stress at the ends of a hyperloop track would be orders of magnitude higher if you tried this approach. I haven't done the math, but I'd suspect you'd need an anchoring system comparable to what you see on large suspension bridge cables, if not more. Not only that, but I suspect you'd start to reach the limits of what steel can structurally sustain. With this solution, you've got a O(100km) long tube that is not only under vacuum but now also under enormous tension. Before this can be considered a viable solution someone needs to do the math on that, and AFAIK no one has.

So I still want my put options.

Hmmyeah, but the result is so-so, it experiences a few troubles in the warmest days of the summer and trains (especially high-speed trains who both need perfect rails and put more strains on them) have to be speed-limited or in a few cases even cancelled.

Isn't space super-cold? I mean, if you pull air out of a tube, the stretched air absorbs calories (–or cools down, which is the principle of a fridge with other chemicals and a liquid/gas transition). So can this effect keep the metal cold?

Space isn't cold. It has very low temperature. These are not the same thing.

Because space has extremely low density thermal radiation doesn't really work very well... it's very hard for a vehicle in a vacuum to dump heat...there is no medium to receive the energy!

> Space isn't cold. It has very low temperature. These are not the same thing.

That's news to me. What do you think "cold" means if not a synonym for "low temperature"?

> Because space has extremely low density thermal radiation doesn't really work very well... it's very hard for a vehicle in a vacuum to dump heat...there is no medium to receive the energy!

No, that's completely wrong. It is very easy to radiate heat into space as long as your radiator is not pointed at the sun. What you can't do is convect heat away, which makes it hard to transfer heat from the hot part of a spacecraft to the cold part. But getting rid of the heat once you've transferred it to a radiator is really easy. In fact, some parts of a spacecraft need heaters to keep them warm if they don't get sun exposure.

Not a good analogy. One of the main Hyperloop points it is to have near vaccum inside the tubes, which will reduce air resistance and allow faster speeds, otherwise it's just a train inside a tube. This means they must be totally sealed, so no gaps allowed.

It won't be a complete vacuum though? I mean it doesn't have to be. Even removing 70% 80% of the air would greatly reduce drag. Plus don't they also need safety systems in case of issues so the tube segments can be locked and re-pressurized without damaging the train?

Drag reduction is, I think as important as decreasing the shock wave generated by breaking the local speed of sound. Partial vacuum does both. Not sure how to optimize for cost and reliability of construction vs max sped

So... you put in a flexible air tight seal in the tube. It will have to be modular anyway. Doesn't seem that challenging or cost prohibitive. A quick google turns up NASA projects that were looking into flexible airlock seals.

> near vacuum inside the tubes

This isn't true at all. The tube will still have air inside of it, you just need higher pressure behind the car than the air in front.

The cars themselves work a lot in the same way a cylindrical air hockey table would work.

The current hyperloop company has deviated substantially from Musk's design - now it's just narrow train in a semi-evacuated tunnel.

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