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Comment by Inthenameofmine | Hacker News Reader

The tube would not be evacuated. The train sucks the air in front of it and pushes it around and especially below, creating an air cushion just like an air hockey, further decreasing energy requirement for movement. That way you also get rid of the air evacuation cost.

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The tube would be evacuated on earth. Which is why they need a tube in the first place:

Mars

According to Musk, Hyperloop would be useful on Mars as no tubes would be needed. This is because Mars' atmosphere is about 1% the density of the Earth's.[10][45][46] For the hyperloop concept to work on Earth, low-pressure tubes are required to reduce air resistance. However, if they were to be built on Mars, the lower air resistance would allow a hyperloop to be created with no tube, only a track.[47] https://en.wikipedia.org/wiki/Hyperloop

The important point is 0.01 ATM is a long way from high vacuum. Pumps can get down to that pressure very easily and individual Pumps can cover wide sections of track.

> a hyperloop to be created with no tube, only a track

Don't we call that a “train?”

yep

The original design did not have a 'track' along the full route. Cars would more or less hop between different sections. On Mars that represents clear reliability issues, but the ability to avoid building load barring bridges over gorges could be a major cost savings.

Sure, but I think it's fair to call a train that is powered with electromagnetic pulses from stationary coils something different than a train that is powered by an onboard engine.

You just described a Maglev

Yes, although I think maglev trains use continuous linear motors while if I understood the original proposal correctly, the hyperloop would be more like a series of railguns evenly spaced along the route.

"What is Hyperloop?

"Hyperloop is a conceptual transport system in which passengers are loaded into pods and fired through vacuum tubes at more than 600mph (1,000km/h)....

"Pumping the air out of the tubes reduces resistance, allowing high speeds to be achieved, potentially using less energy than a train."

The journalist is wrong/simplifying

See wikipedia [0]: "pressurized capsules ride on an air cushion driven by linear induction motors and air compressors"

[0] https://en.wikipedia.org/wiki/Hyperloop

You cut off that quote:

> incorporating reduced-pressure tubes in which pressurized capsules ride on an air cushion driven by linear induction motors and air compressors.

So the tubes are low pressure. Perhaps not vacuum, but in reality so called vacuum tube delivery systems aren't actually a vacuum either, that's why a more accurate name for them is Pneumatic Tube Systems.

I'm not sure what it is exactly you're trying to argue, above you made it sound like the system isn't low pressure at all.

Same source: "The Hyperloop resembles a vactrain system but operates at approximately one millibar (100 Pa) of pressure." That would be a "medium vacuum" according to Wikipedia.

Both BBC quotes are in full agreement with the document at http://spacex.com/sites/spacex/files/hyperloop_alpha.pdf . Page 12, "100 Pascals", that's roughly 99.9% of the air removed.

The distinction between "very low pressure" and "vacuum" that is implied by the "hyperloop is not a vactrain" camp is a fantasy.

It is pretty relevant at the speeds being discussed.

For the design of the vehicle, yes, for the design of the infrastructure, not so much. Those 100 Pa are actually much lower than what previous vactrain proposals suggested. The only difference is that hyperloop is trying to do something useful with the unavoidable trace atmosphere instead of just overcoming it.

The MIT prototype, and some of the other proposals, are really a maglev system. The original Hyperloop proposal called for aerodynamic support, with very tight tolerances, but that turns out to be iffy.

A maglev system needs a much more expensive track and/or a lot of energy from the vehicle to power the lift. This is the big problem with maglev - track cost. Maglev works fine; the Transrapid system in Shanghai has been running for 12 years now without serious problems. Top speed 300 MPH.

The Hyperloop is only interesting if it's far cheaper than a maglev. The Transrapid maglev system has far more capacity, can corner better, and can climb steeper grades.

Ah that explains all the maglev. I could have sworn that the original version rode on air and used its electric turbine for propulsion.

Anything good you've read that explains why they've abandoned that route?

The Berkeley team proposed aerodynamic suspension. There's an analysis with ANSYS which indicates that the design can be tweaked so that at no point does the air go supersonic.[1] But nobody seems to have published a stability analysis for the vehicle in the tube.

The "flying height" in the original proposal was to be about 1mm, so this thing has to be really well behaved aerodynamically. And the tube has to be really smooth. (Expansion joints, emergency escape hatches, pressure doors, and switches would be problems.) The whole point of the Hyperloop is that it's cheap because the tube is simple and dumb.

When you look at the MIT design [2] it's really a maglev monorail that operates in an evacuated tube. (Or outside of one, as they've demonstrated by sending down a rail). It doesn't have any of the fan propulsion or air cushion lift or tight tolerances from the original Hyperloop proposal.

[1] http://www.ansys-blog.com/20130925hyperloop/ [2] https://www.youtube.com/watch?v=yiyPQU48Ef0

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