Hyperloop - Hackdot

Re:Hyperloop by VladRussian2 on Tuesday August 13, 2013 @02:58PM (#6207554)

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.

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Re:Hyperloop by krasin on Tuesday August 13, 2013 @09:58PM (#6209553)

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

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Re:Hyperloop by VladRussian2 on Tuesday August 13, 2013 @10:03PM (#6209579)

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

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Re:Hyperloop by krasin on Tuesday August 13, 2013 @10:08PM (#6209596)

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.

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Re:Hyperloop by VladRussian2 on Tuesday August 13, 2013 @11:37PM (#6209806)

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.

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Re:Hyperloop by DonPedro on Wednesday August 14, 2013 @03:54PM (#6213814)

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...

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Re:Hyperloop by krasin on Tuesday August 13, 2013 @10:16PM (#6209619)

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

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Re:Hyperloop by lutusp on Tuesday August 13, 2013 @10:42PM (#6209677)

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.

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