[Animats]

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.

[ericd]

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?

[Animats]

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