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Hyperloop lets you travel on a resonant acoustic wave? (conscienceofanentrepreneur.blogspot.com)
142 points by bjornsing on July 28, 2013 | hide | past | web | favorite | 83 comments

"Elon Musks final invention will be a time machine. It will only work once before exploding. Before entering it, Elon will turn around and tell us he has done his duty and given us many gifts, and now we must use those gifts to advance mankind. He hopes great things for us.

We beg for him to stay and aid us further. He smiles sadly, He looks at us with his tired but happy eyes, shakes his head, and steps into the machine. He returns to an early 21st century earth...and begins his work once again. As it has always been, as it always shall be."

Since he would be rather old when going back in time, wouldn't he rather pass his knowledge to his younger self and then iterate the process to obtain the ultimate laws of the universe.

Reminds me of Silverberg's amazing short story "absolutely inflexible".

Did you just compare Elon Musk to Jesus?

Yes. Everyone knows that Jesus had a time machine and appeared in the early 21st century.

That was beautiful.

"Godspeed, Mr Elon. Godspeed."

As a surfer, let me volunteer that it's a bit like a highway onramp: you want to match speed exactly when the front arrives. Tiny adjustments in speed, pitch, yaw, roll, and center of gravity make the difference between making it, falling off the back, and getting eaten.

Mistiming your drop leads to bad outcomes that are directly proportional to the kinetic energy of the wave and are inversely proportional to the depth of the water. This is huge kinetic energy with a distance to the wall of, probably, microns. I want to see how this thing works, but oh, Nelly, what a wipe-out!

I'm somewhat new to surfing but am thinking this is slightly different than what you are imagining. I also know next to nothing about acoustics and wave forms.

Wouldn't the wave be the height of the tube, and the vehicle be near that same height? That way you can't "miss" the wave. Unlike surfing their is no room below the wave for the vehicle do dive and throw the surfer off, and unlike surfing , a soundwave (to my knowledge) doesn't break, so there is no tumble. If you enter at the point where you're off the back of the wave, you're positioned to be picked up by the next wave, which because your vehicle height is near equal to the height of the wave, your vehicle automatically gets pushed along.

That's how I'm visualizing it, but I could be wrong.

(experienced body boarder/body surfer, novice actual surfer ;-)

I think the point is that, regardless of which wave you decide to catch, whether it be the first or second one, at some point to need to sync-up. If you miss the first one, then you still have to speed up or you're going to miss the second one too.

There's definitely an optimal timing that takes the minimum amount of energy to accelerate and synchronize with the "sweet spot" of the wave; beginners need to paddle early and paddle hard, whereas an expert has a better feel for it and will exploit his or her experience with the timing to expend less effort catching a wave.

When you surf, paddling up to speed and the timing will make or break catching a particular wave - which is very important is the "good waves" are rarer. With an acoustic loop, it's may be more predictable, but you still probably want to get it right the first time, while you're still in the main accelerator section of the tube.

The difference between sound and water waves is that sound waves are longitudinal, ie the local displacement of the medium is parallel to the wave travel direction, while in water it is perpendicular. The wave thus does not really have a 'height', it would just fill the entire tube, and there wouldn't really be a way to miss it.

Not to be funny, but it seems like every other day has another "hyperloop uses acoustic waves" article. It's a good guess, but it doesn't even contain new speculation (another article went into detailed theorising about how cars would accelerate and decelerate).

Seriously, until we have a solid info, or even a new guess, can we lay off Hyperloop?

I wouldn't mind the article if it contained some kind of real analysis as to feasibility from a physics and engineering perspective. That type of analysis would be interesting independent of hyperloop.

Worst of all this article is just a rip-off (a paraphrasing) of another article. He is kind enough to point to the original article, but he isn't adding anything new, just putting the old wine in new bottles.

No, it's not. The pointed-to article says that if you travel with the density peak of a sound wave, you get reduced drag, but you'll need some other system (like a railgun / gauss gun) to actually provide propulsion. This article says "I bet it's more than that, I bet the sound waves are actually providing the driving force like in this cool YouTube video of acoustic levitation."

Like with apple iPhone / iPad speculation, it garners pageviews. If this were forbes.com you would call it blogspam.

Have you seen better discussion than the link provided by the OP? http://charlesalexander2013.wordpress.com/2013/07/18/hyperlo...

It's easy to get excited, but I remember the hype surrounding the Segway.

The hyperloop is feeling like the segway at the moment. I remember the hype before the reality of the segway and everyone being disappointed when they found out it was just a slightly altered scooter.

The hype before the Segway was so immense, that even if they announced something exactly like the Hyperloop people would have felt disappointed. People actually considered anti-gravity belts to be a possibility at the time.

Segway was a business with patents, investors, and stuff. This is a gift to the humanity although if somebody has to implement it, it will be Elon Musk, I'm sure.

For someone who has taken nothing more than undergraduate physics as a requirement for my degree, can someone explain the benefits/risks of using an acoustic wave system over others? Has anyone previously looked at this sort of system for travel? If the wave were to be disrupted and these capsules come to a screeching halt, how fast of a deceleration are we talking about? Never more have I wanted to crack open my physics texts again.

And how will this affect the elephants?

This depends entirely on the drag coefficient of the train car. If the engines on a supersonic jet cut off, it's essentially a rock traveling at the speed of sound through still air. I don't believe the acceleration is cataclysmic for the occupants (although probably jolting). Remember also that normal commercial gets travel at like 80% of the speed of sound, and they can easily handle a loss of engine power. (Although maybe typical engine cut-outs are more gradual than a suddenly ruptured hyperloop tunnel?)

So the train car need only have aerodynamics similar to an air plane in order to avoid bad stopping forces. An aerodynamic design makes sense anyways since you want to avoid drag.

If the passenger cabin were smaller than the capsule, the cabin's movement could be softened by hydraulic dampers. This could also prevent casualties (e.g. through brain trauma, as often happens in train crashes) in case of an instant halt.

Assuming a constant "engine" output power, which doesn't sound all that unreasonable, the worst case deceleration would be numerically equal, but opposite direction, to the worst case acceleration from a standing start at takeoff.

It's never that bad in practice.

There are much more exciting problems with losing an engine revolving around loss of control.

You point to an important omission in my scientific article! My gut just assumed the reason would be 'drag', specifically reducing it.

I guess the question is: would it require less energy to push capsules/cars through the tube with an acoustic wave than it would to force them through the tunnel e.g. with some sort of propulsion?

> the tunnel would be opened up to the outside atmosphere. That would immediately ruin the resonance and all capsules should come to a screeching halt.

Correct me if I'm wrong, but my intuition is that this would also lead to quite a bad explosion. I mean, the amount of energy I imagine it would take to initiate a standing wave between LA and NY should be massive, (and certainly not a small amount to maintain it either, though far less than the initial amount if it truly stores the energy well). But if it is _storing_ this energy, then any rupture should release it quite... quickly. No?

A standing wave is not nothing.. it is a huge amount of pressure oscillating back and forth in an underground cavity. A controlled hole in the cavity wall could release it properly, but a crack in the side could effectively lead to rupture, couldn't it?

Another thing, if the air pressure inside the tunnel is indeed quite high, the train cars would also need to be pressurized. This leads to some other potential dangers.. not to mention the time to pressurize would be added to the total trip time.

It doesn't need to be a standing wave, it doesn't need to be from LA - NY (it only needs to be the length of the train), and it doesn't need to be in an enclosed tube.

A Hyperloop sounds to me like a marketed trademarked copyrighted patented implementation of a traditional Launch Loop aka Space Fountain. A wiki link:



You could buy a kids toy circa 2005 implementing a model of the design using what looks like a shoelace and a blacklight. My kids thought it was pretty cool.

So, take a magnetically controllable loop, like a wagon wheel tire. It'll collapse under its own weight it stood up. Turns out if you spin a rope ridiculously fast it inevitably turns into a circle and can support well over its own weight. Basically you build a linear induction motor to get dragged along and remove the clamps at the appropriate time. Think of how you can ride upside down in a roller coaster as long as you exceed critical speed (too slow and you fall out, whoops)

Compared to a space elevator its well within modern current day tech to build one. I donno if its economically within modern day ability.

Three major problems, if that dude splits at the wrong time its an unholy mess as it augers in, and there's something of a dynamic stability puzzle to stabilize movement and temperature, and finally you need a plan of sorts if the clamp or whatever you call it sticks and hauls the first stage back for another go of it.

One optional problem is to build a loop that stores energy by spinning a bit fast, faster than a vehicle re-entering for a second loop can handle nose first. That makes the "stuck clamp" hollywood disaster even worse. Of course a smart engineer would never do that... would they?

There's a fixation on SSTO, of course. However, aside from economic issues, you can make a real loop thats as worthless as you want, and any delta-v it gives for free to a launch vehicle is a net win. So make a "mere" 10 mile diameter one that only replaces the first stage of a launcher, thats cool. How about reducing the mass fraction for a "SSTO" from 95% to maybe 50%? That would still be cool, even if its not "SSTO".

If you can't mentally handle this rotating stuff, think of it like a really long aircraft carrier catapult... so long it can loop back on itself no problemo. And think of cablecars, although it'll probably be magnetic not literally clamping onto a cable, although that would be a charming steampunk aesthetic.

Except that Musk has said it's the 5th mode of Earth Transport after Trains, Cars, Airplanes and Boats.

Excellent point! Throwing in space travel would be obvious feature creep.

There's been a lot of speculation on the hyperloop without accounting the 'air-hockey table' comment from Elon. I personally thought it meant using the ground effect.

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.

> 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:


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.

I can't wait for Elon Musk to silence all the low self-esteem full-of-envy idiots talking shit about him and mentioning "vaporware" without even understanding how vaporware works! I bet $100 he'll have a prototype to showcase given the engineering power at his disposal.

I'll take that $100 bet.

And I'll hold your money. Bitcoin, anyone?

How might one get on and off this thing at these speeds? Perhaps a 2nd loop near entry/exit points?

Just like a normal train would - you route off the main track. As far as actually stopping it, it could just be a "rail gun in reverse" which would allow it to regain a huge portion of its energy upon deceleration.

Whatever it ends up being, this is exciting!


Just to start with, I suspect that you may be confusing light with sound.

You are not just wrong in your analysis, but almost fractally wrong. http://rationalwiki.org/wiki/Fractal_wrongness

> if you travel with a speed close to speed of light and you immediately stop...

Then you're absolutely screwed! ;)

If the solution involves a tunnel or tube from SF to LA then I fail to see how it can be as cheap as has been claimed. I suggest it's an accelerator that launches capsules ballistically, "skips" them to maintain velocity, and recaptures a good deal of energy on arrival.

Do you ever think that Elon has no plan, but he's just put this idea of a plan out there and let us crowd source a solution. That would be neat way to solve an 'impossible' problem.

6,437km/h (4,000mph) to zero is going to be one hell of a stop.

I don't think it is as fast as you say. It is some where between 650-750km/hr. The image that this blog shows explains how that might look and work http://www.techcrump.com/travelling-the-world-in-6-hours-elo... but the fact that it is anti-crash is totally unimaginable if the traffic increases it is likely that the crashes would be reported, lets see what they put together i eagerly waiting for it to launch.Just a little too exited

Where do you get that figure from? The speed of sound is around 1000km/h,right? Still one hell of a stop but it might not be that fast, since basically its the air drag doing the braking.

Yeah, where did you get those numbers? The distance from SF to LA is about 380 miles, and the speed of sound is about 760 mph. Whether or not the principles of sound movement are involved in Musk's plan, the speed is roughly equivalent if we're talking traveling back and forth in half an hour. Still a jolt, but you're talking about six times that.

I see now what I was reading wasn't HyperLoop it was a similar concept only using a vacuum tunnel.


>Safety is no small concern when you're talking about speeds in excess of 4,000 mph (6,437 km/h). After all, we've all seen the wreckage that can be caused in a 60 mph (96 km/h) car crash. The kinds of tube tracks we're talking about here would have to stretch thousands of miles in order to reach their optimum level of benefit – that's thousands of miles of safety risks. What happens when an earthquake strikes and cracks the pressure seal or destroys the tube completely? A vehicle traveling 4,000 mph is going to eat up some serious distance in an emergency stop situation.

Either way, pretty much any accident will result in everyone's death.

Car accidents at 50 MPH result in 80% death.

760 MPH is much much much more, considering the amount of energy needed to be dispersed grows quadratically (E = 0.5 * m * v^2)

"Car accidents at 50 MPH result in 80% death."

Actual stats from Great Britain in 2008 on motorways, presumably always speed limit above 50 MPH, "6% of the total killed, 3% of total seriously injured, 5% of total with slight injuries." at least per wikipedia.

So there's about a 9 in 10 chance if you crash on a GB motorway, at least in 2008, you'll walk away without a scratch, rather than your 1 in 5 stat.

Also all minor aircraft accidents involve flying faster than 50 MPH although death from anything but fire is relatively rare. True though, that controlled flight into terrain at cruise speed doesn't usually leave many large pieces on the mountainside.

Given traffic in the UK, I don't think it's a safe assumption all those accidents were at > 50mph.

You are confusing the speed limit with the speed at which the crash happens. Most people apply breaks before the crash, and do not crash at full speed.


Though that number is always going down, because modern cars are safer than previous gens.

My gut tells me that must be a couple of times the speed of sound...

Wolfram|Alpha gives 343.2 m/s (1236 km/h) at 20 °C and 1 atm.


Of course, in a closed system that isn't directly touching humans, it doesn't necessarily have to use those conditions. Still, I don't think you're going to get anywhere near 6400km/h.

If the deceleration is slow then you don't feel too much.

Like most titles that end with a question mark - "no". Why would you use some kind of acoustic wave system over tried and true maglev?

It probably would still use maglev to eliminate the friction between surfaces. The acoustic waves would be a means of propulsion.


And by the way - the title doesn't end in a question mark. ;)

It does on HN : P

Well, the acoustic idea wouldn't solve drag at all. The craft would still have to move through air.

I refer you to Charles Alexander's idea, which goes through a bit of the math for why traveling with the peak of an intense, low-frequency sound wave would give freakishly low drag.


Ah, I see. That's an interesting idea. Sounds impractical - if you're going to do all that engineering work, might as well just get rid of the air in the first place - but cool concept.

Do you know how difficult it is to build and maintain an evacuated tube of that size? Not all loopy tubes are created equal.

As opposed to building this thing, filling it (perfectly) with ultra-high-power resonant soundwaves of the precise characteristics, and flinging in shuttles and hoping nothing goes wrong? I don't know, the evacuated maglev is sort of sounding like the safe, easy option?

The reason evacuated tubes are expensive and difficult is because of the tube, not because of the evacuation. Once you have a great big tube, reducing the air inside isn't going to be that big a deal. You don't even need to get rid of all the air, just maintaining 90% vacuum would be fine, you could whiz down there at 1000kph as easy as kiss my hand.

Anyway, I'm not a physicist, I am just wondering if people have a full understanding of the forces involved flinging stuff around at 1000kph+ in an enclosed space at full air density. It is some serious shit. People are comparing it to aeroplanes, but at sea level it would be 3x the shockwave a plane has to deal with. High speed trains in Europe and Japan already match or exceed the air-displacement issues faced by airlines - and that's one third the proposed speed of this device. I don't know, man. Sure, in ideal circumstances you might be able to engineer some low drag trick. Still sounds like unicycling along the edge of a cliff to me. Near mach 1 in a fully pressurised tunnel? Dear christ.

You're right! Well, I didn't put it there... That I can remember at least.

> it's not an evacuated tunnel

Really? I thought it was already established that the Hyperloop is a vactrain (http://en.wikipedia.org/wiki/Vactrain). Using small cars (say, six passengers) accomplishes the "leaves when you arrive" and putting solar panels on top of the tunnel accomplishes the storing energy part. I wonder what the alternative would be and what its advantages over a vactrain would be.

He's said explicitly that it's not, although I can't find a primary source for this quote.

Solar panels don't store energy. They only convert it.

Thats clearly nothing to think of :P

I have a flight to LA tomorrow. Any chance this will be ready by tonight?

You're not flying to LA tomorrow. They haven't built the plane yet.

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