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Comment by lisper | Hacker News Reader

> There can be absorption points along the track

Sure, but these have to hold a vacuum. No one has figured out a way to do that.

I am assured by those in a position to know that thermal expansion joints which hold a near-vacuum are an entirely solved and essentially trivial problem.

I do have some concerns about whether they're a trivial problem within this context. An engineering solution which works in small numbers for PhDs in a lab isn't necessarily scalable to mass low-cost manufacturing, deployed in the field and irregularly serviced by workers of uncertain provenance. But evidently there is a way to do this, at least.

And, more importantly, can be certified for real world use. Considering the speeds used, scrutiny will be extremely high. Even if you use it for freight at first, the track will be close to settlements and any accident could end up in a disaster. The behaviour of trains on tracks is very well understood (learning from accidents), the behaviour of a vacuum train tube is not.

This is a very fair point. There might be components that are routinely used in non-life-safety-critical applications, but this is no guarantee that they will pass certification scrutiny when they are in the critical path for thousands of lives every day.

I worked on an autonomous vehicle project here in the UK about 10 years ago (the Heathrow Pod). The certification process was absolutely intense. And this was for a system that was capped at 25mph; had we gone any faster than the regulatory burden would have become very significantly more onerous (below 25mph it wasn't necessary to test vehicles and infrastructure to destruction; above 25mph, it is).

In this domain, there's a huge difference between a cool engineering testbed, a private pilot project, and running actual service for the public. If Hyperloop is going to be doomed by anything, failure to appreciate this fact is probably at the top of the list.

I work in a lab with vacuum systems that get heated and cooled and we just have segments of flexible bellows like this:

http://www.pchemlabs.com/subcatagoryb.asp?pid=flexible-bello...

Not sure if the same thing will work at larger scales, but I'm not sure its a fundamental problem.

Wowza, 5" bellow is $450, how cheap could they make 6' one, even in bulk?

Not exactly like that, but yeah, the same sort of idea I'm sure would work.

Gas pipelines hold hundreds of bar of pressure; I don't see how one atmosphere is a problem.

Gas has no problem flowing around corners. Gas pipelines are usually not straight lines. Hyperloop will have to be a straight line, unless you want to kill passengers.

Gas pipelines use Expansion Loops [1] to pick up expansion slack. The problem is that, in order to push people through the tube at 700mph at acceptable G's, you can't put bends like that in the HyperLoop. If you try to send people down a pipeline at 700mph, you would end up with paste at the other side. Gas pipelines are surprisingly un-straight, bendy, and bumpy

[1] https://static.interestingengineering.com/images/import/2017...

Except for the people who invented the accordion and bellows.

Isn't this what a syringe does?

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