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Comment by tlb | Hacker News Reader

He doesn't properly address thermal expansion of the entire system. A long straight steel tube from SF to LA will expand a total of 300 meters over a 40C temperature change. Where does it go?

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"By building a system on pylons, _where_the_tube_is_not_rigidly_fixed_at_any_point" [...] "A telescoping tube, similar to the boxy ones used to access airplanes at airports would be needed at the end stations to address the cumulative length change of the tube"

So, the tube is lying loose on the pylons, and he assumes it will keep its shape while expanding.

I think that is highly optimistic for a tube that is a straight line, even more so for one that is curved.

If he manages to accomplish that, the end stations will move over 300 meters, in total. Hopefully, when the tube contracts again, the motion will be reversed perfectly. Otherwise, that end station in LA could slowly move north or south over the years.

>If he manages to accomplish that, the end stations will move over 300 meters, in total.

Where are you getting this idea from? That seems like an insane implementation compared to using accordian-like flexible section[s].

From what I cited: "A telescoping tube, similar to the boxy ones used to access airplanes at airports would be needed at the end stations to address the cumulative length change of the tube"

I guess that's because accordion-like flexible sections and carriages moving at 1000 km/h on a tiny air cushion don't go well together, or because such sections, being a lot leakier then welds, would let too much air in.

Right, so the tube changes length. The entire station doesn't move. Also, the trains aren't going 1000 km/h at the station.

If I read it right, the movement is absorbed by shock absorbers that need to be installed for earthquake protection anyways. You just need to make sure that the segment between the lateral movement/shock absorbers are sized such that the differential thermal expansion doesn't violate the deflection requirements.

The paper describes having a telescoping section on each end to accommodate thermal expansion, but the previous commenter is stating that it is likely this telescoping method will not be sufficient, as the variation in length of the structure will cause significant bending of the pylons themselves. Between the ends, there is no mechanism to allow for thermal expansion, which will either cause massive axial stress on the steel tube, bending of the pylons, or both. The shock absobers will not have sufficient travel to address this, (100s of meters); axial bearings on the tube may be required.

Each 30M long tube expands 15mm over 40C of temperature change. You could imagine a 15mm expansion joint at each connection, which is roughly what oil/gas pipelines do, but the paper explicitly says tubes will be welded together for straightness.

How does rail address this?

The gaps between each section of rail can be quite large.

Prior to the use of continuous welded rail there were gaps between each 39 foot section of rail.

Nowadays with welded rail, careful compliance with construction standards is required. Slow orders are issued during heat waves and frequent track inspections are conducted to look for potential or actual sun kinks.

A good summary of the issue can be found in the Heat Orders FAQ from Virginia Railway Express. [1]

[1] http://www.vre.org/feedback/frequently_asked_questions/faq_h...

I don't know how to do the math, but couldn't it be absorbed by the horizontal play at each plyon? Imagine the tube "zig-zagging" a tiny bit at each column.

To absorb the 15mm expansion over each 30M long tube, you'd need to zig-zag nearly a meter. At full speed you're passing through 10 tubes per second. It would literally shake your head off.

Does it have to be steel? Any other materials we can be considering? Some interesting new forms of carbon...?

If you want it building for a reasonable cost then it's steel or nothing.

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