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."
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!
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.
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.
Seriously, until we have a solid info, or even a new guess, can we lay off Hyperloop?
And how will this affect the elephants?
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.
It's never that bad in practice.
There are much more exciting problems with losing an engine revolving around loss of control.
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?
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.
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.
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.
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".
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  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.
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.
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 are not just wrong in your analysis, but almost fractally wrong. http://rationalwiki.org/wiki/Fractal_wrongness
Then you're absolutely screwed! ;)
>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.
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)
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.
Though that number is always going down, because modern cars are safer than previous gens.
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.
And by the way - the title doesn't end in a question mark. ;)
Well, the acoustic idea wouldn't solve drag at all. The craft would still have to move through air.
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.
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.