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I read somewhere (can't find a quick reference) of a coating that resembles the structure of shark-skin, this reduces drag enormously by creating local turbulence which reduces the shear forces required to separate the layers of fluid adhering to a surface.

A coating like that on the inside of the tube and the outside of the vehicle would be an interesting component but the fluid dynamics of such a set-up are way beyond my abilities to calculate.

This whole thing is all about energy budgets, you'd have to take the amount of fuel it takes a passenger car or rail car to move from one point to another, then take some reasonable savings estimate and then work backwards from there to see how much energy you could expend on friction, drag and so on.

I have a hard time accepting that the friction in a tube while drafting would be that much higher than friction losses to a body of air that is mostly standing still and from playing around with a car and a friend of mine who is a trucker (don't try this at home kids) I've seen how low fuel consumption can go if you are properly in the slip-stream of another vehicle.

There has to be some way to make the numbers work on that. But unless 'no right of way issues' means underground (or ocean bound, which means it would only work between coastal cities and would have a pretty terrible failure mode) the whole thing is off the table anyway.

I can't stop thinking about it though :)




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