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Comment by seanflyon | Hacker News Reader

The design only calls for one station at each end. Many people have said that the best way to deal with thermal expansion is to let the ends (at the 2 stations) move ~500 feet at each end. The alternative is expansion joint every so often along the tube.

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If you do that, though, then all of your support structures the whole way through need to allow the tube to move inside of them. The tube is supposed to be supported every 100 feet or so (if I remember correctly) so that means that the last pylon has to be at least 500 feet from the station and that the tube has to be able to slide inside the support.

I suspect that you'd see a lot of wear on the tube that's sliding over the pylon supports as it might go through at least one if not several heating and cooling cycles daily. I could see two cycles if you've got side heating after dawn, midday shade under the solar panels, and then late afternoon heating after the solar panels stop casting a shadow over the tube. You might get another cycle if you have two parallel tubes with two parallel lines of solar panels above them.

I think that there are a lot of options for reducing friction wear. My first thought would be to put wheels between the tube an its supports (attached to the supports, not the tube).

You're increasing the cost a lot there because the wheels need to be on bearings because you've going to have to hold the tube with some amount of preload. You can't just let it rest on the wheels, the wheels have to be pressed onto the tube at all times because the vehicle is moving very fast and any kind of minute deviation up or down will result in substantial forces between the tube and the vehicle which have to be transmitted through the wheels and into the foundation and eventually into the ground.

So now you have wheels, bearings, preload springs, hinges, travel arms, etc. Instead of a big piece of plate that is welded to the tube and bolted to the foundation you're talking about a big apparatus with moving parts and precision bearings. The plate and welding might cost $500 per. The other might cost $20k apiece. A person can buy a lot of rebar and concrete and forms and whatnot for $20k.

I don't imagine any sort of suspension would be needed. The system is always loaded (with only a slight increase when the vehicle is over a given support because the tube is most of the weight). It needs bearings, but they don't have to move quickly or be particularly low friction. Even if the bearings failed the tube would just slide; you would want to replace them before too much wear, but it would not be urgent.

If that is too complicated, just put in replaceable slide plates. More regular maintenance, but fewer moving parts.

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