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Comment by jessriedel | Hacker News Reader

I can't figure out how the OP proposal differs from Charles Alexander's. Low frequency (long wavelength) waves travel around the tube at the speed of sound. By traveling at the same speed, and correctly choosing a point along the wavefront, one can take advantage of significantly reduced apparent drag. It's implicit in Alexander's suggestion that either (a) the total length of the loop is an integer multiple of the wavelength of the waves, so they are "resonant" in that sense, or (b) the envelope of the wave is much less than the length of the loop. (If neither (a) or (b) held, the wave could not be formed in the first place.)

So what distinction is OP making? That it's resonant, and so definitely (a) and not (b)? I think it would require a calculation of how fast the wave would disperse in the tube, which OP has not done.

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> I think it would require a calculation of how fast the wave would disperse in the tube, which OP has not done.

True. Two points though:

a) The rate of dispersion would be enormously dependent on the "stiffness" of the tunnel wall. If the wall is flexible at all a lot of energy would escape in its distortion. How much is a very complex calculation.

b) Ideal propagation for a ultra low frequency sound wave is probably very good. Why do I think so? I've heard that "shock waves" (actually their low frequency components) from various blasts often travel several times around the earth and are registered at least on their second lap. This knowledge has been incorporated in my gut and used to guide this "design".

I think main difference is Charles thought it with his brain, whereas I felt it with my gut.

No, jokes aside, the reason I wrote this up is that I wasn't entirely convinced of the exact physics laid out in Charles' post (I think e.g. Daniele Foresti et al [1] liked from my post may have more interesting model). Also, Charles seemed to assume that the wave would be there only to reduce drag, not to propel the capsule/car. That doesn't sound like Musk to me.

But most importantly I think I bring to the table a careful poetic analysis of the Hyperloop. My work in this case is not in physics (where I actually do have my training), but in literature.

1. http://www.pnas.org/content/early/2013/07/10/1301860110

I don't think there is any distinction. He's just reblogging. He did link the original at least.

Not so, I felt it with my own gut!

Another difference is in Charles' model you would have to travel at the speed of sound. Due to the shape not being a perfect line airflow will have to be supersonic somewhere over the surface of the craft. That's generally not good for energy efficient travel. My "design" allows for the capsule traveling at subsonic speeds.

I suspect that design by gut is not necessarily a reliable approach to this.

Yea? Why do you think so? Did you look it up in a book? You should have felt it, with your gut! [1]

1. http://www.youtube.com/watch?v=OBRKPoAPXEQ

and my gut tells me that's how our nervous system works... :)

it's slightly less wrong, as far as i can see - http://www.acooke.org/cute/HyperloopW0.html

I read your post, but I don't understand why the first link can't work. Is there a problem with a wave propagating along the tube with essentially infinite envelope? Here's a nice animation:

http://www.acoustics.org/press/151st/WaveVectorDemosmall.gif

In other words: decompose the standing wave into its forward and backward components and just keep the forward one. This is not the same thing as the air having a net velocity, so you don't bleed all your energy into friction with the walls. (You'll still lose some energy as the sound wave is naturally damped by friction due to the residual motion of the air molecules, but this might not be a large effect.)

you may be right; perhaps i have only remembered solutions with boundary conditions that would not allow such a solution. i will try to think things through again and perhaps do the maths. thanks.

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