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Comment by macinjosh | Hacker News Reader

For all of you whose panties get wet at the sound of Elon Musk's name Hyperloop is essentially just Aérotrain in a tube:

https://en.wikipedia.org/wiki/A%C3%A9rotrain

This guy isn't really a genius, he's just rich.

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Not really. There are similarities in the car's design, just as there are similarities among airplanes.

Aérotrain is a magnetic levitation train, Hyperloop does not use magnetic levitation. Read the PDF.

Aérotrain is not magnetic, it uses air resistance too. Read the link, ahem.

Aérotrain uses air, like the hyperloop.

430 km/h (Aerotrain) vs 1200 km/h (Hyperloop) is a big deal. That is possible with the tube design. Aerotrain operated on the "free air" and that was the reason to lose the competition with TGV.

man, the air cushion behavior at 1200 km/h (speed of sound at normal pressure, and 1.2 Mach at the proposed decreased pressure in the tube) would be a subject of a lot of PhD works. Or in other words - it will be hell of an engineering (stability of the cushion and parasitic oscillations come to mind - that is with respect to compressor created cushion, and in addition to the compressor created air cushion the cushion's airfoil will be riding its own shock wave between tube surface and the airfoil in front of the cushion's high pressure volume) to make supersonic ground effect from concept into product.

I just double-checked the pdf. It says:

"The capsules travel at 760 mph (1,220 kph, Mach 0.91 at 68 ºF or 20 ºC)"

1200 km/h is still subsonic

Mach value is lower for lower pressure, about 250 m/s for hyperloop proposal

Wolfram Alpha disagrees. It says:

Mach(20 C, 1 atm): 343.051 m/s

Mach(20 C, 0.1 atm): 343.182 m/s

Not much higher, but definitely not lower.

http://www.wolframalpha.com/input/?i=speed+of+sound+in+dry+a...

Unfortunately, it refuses to show Mach(20 C, 0.001 atm), since the lowest supported pressure is 0.098 atm.

There's still a chance that Wolfram Alpha is incorrect; I am unable to prove or disprove that, because I don't know aerodynamics.

The problem is that atmospheric Mach numbers don't follow from first principles, and the Wolfram Alpha values emanate from an empirically derived approximation. An analytical relationship between the speed of sound and air pressure arising from first principles would require a solution to the Navier-Stokes equations, for which solution a million dollar prize lies unclaimed:

http://en.wikipedia.org/wiki/Navier%E2%80%93Stokes_equations

Quote: "The Navier–Stokes equations are also of great interest in a purely mathematical sense. Somewhat surprisingly, given their wide range of practical uses, mathematicians have not yet proven that, in three dimensions, solutions always exist (existence), or that if they do exist, then they do not contain any singularity (smoothness). These are called the Navier–Stokes existence and smoothness problems. The Clay Mathematics Institute has called this one of the seven most important open problems in mathematics and has offered a US$1,000,000 prize for a solution or a counter-example."

> There's still a chance that Wolfram Alpha is incorrect ...

At the moment there is no "correct". It's all based on field observations, numerical modeling, and estimation.

You are right, my bad with the low pressure speed of sound . Anyway the high transonic flow isn't a piece of cake as well.

I'll go out on a limb here and wager that SpaceX engineers know the Mach value for various permutations of temperature and air density. But I might be wrong...

Just the simple fact that the Aerotrain was on a track and not in a tube makes all the difference in the world. And I don't get this "look someone proposed something similar before" thing (everything from vacuum tubes to maglev to this thing). These previous inventors failed to get their ideas realised. Now a visionary has his own tweak on things and should have a good shot. Rejoice!

>This guy isn't really a genius, he's just rich.

And a self-promoter. This is how you maintain you image of a super-hero.

The Hyperloop is a cool idea, and I hope its the future. But anyone who believes this can be built, today, for $6BB is out of their mind.

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