Hyperloop has sex appeal, but it seems like Maglev or just plain old high speed rail makes more sense from an economics perspective. I can't imagine it's cheap to keep a tube evacuated. Either way, it seems good to try and build these sort of high-tech industries in the US.
The speed is supposed to be even higher than maglev, so it can compete with planes over longer distances.
But yes, any kind of high speed public transport would be great.
Hyperloop tubes were never meant to be fully evacuated, they operate at pressure differentials with air bearings (imagine a cylindrical air hockey table).
It may not be totally evacuated but the pressure difference between it and the outside world is still supposed to be very large isn't it? And if you want a hyperloop that actually goes anywhere that's going to represent quite a large volume of low pressure space that you need to create and maintain.
Surely this has to be quite energy intensive and generally difficult to maintain, especially since you have to get people in and out of it all the time. I'm not saying these problems are physically insurmountable but they sure sound expensive.
Another thing, is that anyone that punches a hole big enough in the tube basically transforms half of the hyperloop into a bomb. Energy = pressure * volume. That's a lot of energy smashing through a hyperloop capsule. Basically, imagine if 3% of the tube was filled with pipe bombs, but in reverse.
As long as pod moves, there's a constant pressure build-up of air in front of it, to the point that the design includes a compressor to pump air from the front of the pods because passively preventing the buildup by letting it blow through the pod would be insufficient.
So a leak would be a piffling little annoyance in comparison - there'd not be enough volume of air inside the pods to cause a problem compared to the air the compressor on the pod will be actively pumping from the front of the pod to get it out of the way.
That compressor is absent from every single design we've seen right now, The duct for air to go from front to back would also use up a significant amount of cabin space, and the electric-powered multi-stage high compression ratio high-speed compressor would have massive power usage, weight, and cost. Power usage that would have to be palliated via batteries, as the capsule supposedly has no contact with the tube.
Because of this, no full-scale design yet has incorporated a compressor.
The tube would need egress points, interfaces to stations. I think the difficulty of building one tube between stations is like building a 100km long airplane fuselage. But any proposal is little more than "'just' build a tube!"
The difference between what an aircraft fuselage and what the hyperloop tube needs to do actually seem to be quite massive (caution, back of napkin understanding from somebody who is casually interested but by no means an engineer).
The hyperloop is apparently to be pressurised at around 1/1000th of the pressure of the atmosphere (around 0.015psi). An airplane cabin is apparently pressurised to around 11-12 psi and at an altitude of 36,000 feet apparently the atmospheric pressure is about 3.3 psi so actually the plane fuselage only has to manage a pressure difference of about 3.6 times. So it's like building a 100km long airplane fuselage that has to deal with pressure differences that are something like 280 times greater (please excuse/correct me if this is not the relevant comparison) - although you probably claw back a lot of the difference from a generalised perspective of engineering difficulty from not having to put the thing in the air to begin with.
Edit: I suppose it's more like building 100km of space craft cabin, which probably underscores my understanding of the difficulty a bit better.
The only thing I can find which specifically mentions a pressure says 1 millibar, which is quite low.[0] The pressurization/depressurization cycle will also fatigue the pods, which will reduce their life-expectancy.
I recall at the time it was first proposed, Elon Musk said 0.1kPa which agrees with another comment stating 1 millibar. At the time, criticism comparing it to the old vactrain idea was dismissed as "but this tube is not fully evacuated". But it's about the same value that was proposed by vactrain, and is 99.9% evacuated.
It doesn't have to be that good a vacuum. It's probably really no more expensive than conventional high speed rail.
The hyperloop proposal is operating at approximately 1 millibar of pressure. That sounds CLOSE to a vacuum..and it is, roughly 0.01 atm, but with vacuums getting down to around there is easy. Getting down to, say, 0.00001 atm is much much much harder.
But you aren't just doing it a chemistry lab. You're doing it in an enormous tube and the tube has openings to allow pods in and out, so it's a lot more complicated.
