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Comment by RoboTeddy | Hacker News Reader

On viability: what's the state of the tunnel thermal expansion issue?

(Last I heard, it seemed like there might not be easy workarounds)

I am not an engineer outside of IT, but what if you submerged the hyperloop in the pacific between SF and LA? Would thermal expansion no longer be an issue because you're using the ocean as a thermal sink? And you can provide enough positive air pressure to keep water out of your loop.

Yeah, but one of the big bullet points is that the tube should be at negative air pressure in order to reduce air resistance.

Right, but what if its not feasible on land due to thermal expansion, but is feasible underwater with slightly higher power requirements to overcome air resistance? Just a thought.

The design is intended to be robust with regard to leaks (pumps can overcome some leakage to keep interior at a near-vacuum). This would not work underwater.

Pumps can overcome a small amount of leakage - from tiny cracks between bolt threads, etc - places that an un-reactive gas can make it through a huge pressure differential. In shallow water, one has the viscosity of water to work with, which makes this much easier in liquid phase.

What I don't know is how pumps, and cracks, would react to gaseous H2O versus gaseous N2; Or if perhaps a liquid with low offgassing potential could be used as a boundary layer to seal cracks instead of water (which, while much higher viscosity than a gas, remains lower than most liquids).

One big advantage of any submersed liquid approach: Earthquake resistance.

Additional notes: My main theory before the announcement was that the concept used a fast-speed-of-sound, low-density STP hydrogen tube and maglev rather than slow-speed-of-sound air tube, or an expensive-to-seal vacuum tube. An STP hydrogen tube hovercraft levitation model still seems pretty damn attractive to me, especially with a sealing liquid buffer of some sort - it cancels out the problem of water-vacuum interface, and in water, flammable seals, as well.

I wonder if they could use carbon fiber? It has near zero thermal expansion.

I have this awesome vision of some big machine sitting at both ends where they knit fibres together, cure it then just basically extrude the thing the whole way and meet in the middle.

Yes it's expensive but with the magic of science and a bit of hand waving surely our processes have advanced to the point where this is becoming feasible?

Unfortunately I have no idea if a carbon fibre tube is suitable to operating at sub atmospheric pressures...

My understanding is that the thermal expansion is taken care of near the stations with a telescoping section. So the tube then has to be free floating, i.e. not attached to the pylons.

http://www.leancrew.com/all-this/2013/08/hyperloop/ goes into this in more detail (and is skeptical that it can work as described in the Hyperloop Alpha white paper).

That link doesn't even entertain the notion that the tube is not fixed to the pylons (even though it quotes the white paper saying exactly that).

No, it specifically addresses that. The white paper says that rather than rigidly fixing the tube to each pylon, it would be fixed to a damper that would "absorb the small length changes between pylons." The problem with the white paper's approach, as the blog post explains, is that the length changes are far from small.

Yeah, this seemed like the biggest killer, besides inertia and politics.

My friends and I were wondering aloud if you could put a slight bow in the pipe - so instead of expanding lengthwise, the pipe might raise and lower at certain places. Of course, none of us are engineers in that sense.

1. Most of the tubes would need some bowing anyway for curves, since they can't find a perfect straight path from LA to SF

2. Tests show that tubes where L >> D, thermal expansion is linear along the length. If you restrained both ends you'd get the buckling in the interior sections that you describe. That might even be a bigger problem though because it would affect the path geometry and alter the attitude control requirements of the pods. I think the wrinkling in the tube skin would also be more dangerous than just letting it stretch.

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