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I'm greatly enjoying this entire debate, but I'd just like to point out one thing: Elon said he doesn't know for sure how to do it yet or if it's even feasible (although he thinks it is). This means

a) It's possible that not all of the statements he ever made about the Hyperloop will be true for the final design (if any). I'd be especially cautious about the comment about right of way issues. This might as well just mean that he found an unused railroad track or something. And

b) Nobody knows if it can be done. Keep that in mind when debating the ideas. As far as I'm concerned, all ideas are constructive at this point.

I'm not a physicist and it's very likely that this solution is completely impossible, but let me ask anyway. If you just had a 1200 mile long train and you somehow got the mass going in a circle at 1200 mp/h, how much energy would you need to keep that momentum going? Could you cover that with solar panels covering the 1200 miles of track?

Keeping mass in orbit about a point requires absolutely no energy. That is because energy is force times distance (with times being a dot product), and the force you apply is always at right angles to the motion.

It is a good thing that this is true, else the Earth could not manage to go around the Sun indefinitely!

Therefore the energy to keep a system like this going is entirely what is needed to replace various incidental losses like friction. With maglev technology there some electromagnetic drag, but the bulk of losses are air friction.

There is no way to keep a train like you describe going against air friction at 1200 mph. If you eliminated air friction, it would be very doable.

What if the entire loop was in motion? Picture a maglev train 1200km long. There is no "front" to the train, it's a loop, so air is not being compressed as it whizzes along at 1200kmph. Most of the space inside the train could be void, which could be filled with passenger cars as needed.

Would that eliminate enough of the air resistance to be feasible? Is a 1200km long contiguous structure feasible?

I am fairly sure it would not help. If you're rubbing past air at that speed, you'll have pretty serious friction issues.

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