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Speculating about the Hyperloop (bentilly.blogspot.com)
41 points by btilly 1825 days ago | hide | past | web | 54 comments | favorite



> People go into vehicles that I'll call cars, even though they aren't really cars. These cars can be fired by a railgun to match speeds with the tube

Most railguns I've read about produce extreme g-loads. Most people would come out as super-gross wonder-jelly if fired out of one. I think what you want is some kind of electromagnetic accelerator, but not necessarily a railgun in particular.

> On its own that space would heat up due to the friction on the gas, but you can put a heat sink (eg a block of ice) in the car and keep it comfortable inside.

The cabin of a vehicle is quite small, and the ride time is quite short, so a large block of ice probably would be feasible. But you are talking about having a car leave every second, so for each loop you would have to have a plant capable of freezing something like 2 gallons of water per second, as well as a means of refilling the ice container in each car. That's going to add quite a bit to the expense and energy use of the system you describe. Refrigeration systems become quite a bit more efficient as they become larger, so probably this is manageable.

Depending on how quickly you can turn around each car, each loop might require quite a few additional cars. (But still a fraction of the total number)


One point that just occurred to me.

If you put this tube inside of another tube, and then pumped most of the air out of the second tube, the vacuum in the inner tube would improve and the whole system would become more efficient. I believe it is linear. For instance if the outer tube reduced atmosphere to 1/3 air pressure, then the energy losses due to pushing air around drop to 1/3 of what they would have been. However local compromise of the outer tube does not significantly compromise the vacuum in the inner tube, which avoids major problems that normally exist in maintaining an evacuated tube.

A lot depends on the flaps that let air out but (mostly) not in. When a plunger goes by, you need it to be open for a very brief period to let air out. I'd be curious whether it is better to have a lot of little flaps that can open/shut fast because they are little, or to connect the flaps to a computer system that can open the flap just in time and close it again promptly.

The smart system in theory can be more efficient, but the straight mechanical system would be massively easier to design and build.

I'm also curious what breakthrough Elon had over the summer that dramatically reduced costs. The things that I'm thinking of involve the design of the plunger. Possibly a spike creating chaotic flow at the right place. Or a plunger design that more efficiently pushes air out. Or some clever design of the behind of the plunger so that the air that is left is not following at high speed. Or even the idea I put above about having 2 shells, with partial evacuation in sections of the outer one.


Hey Ben,

That's pretty clever, so you're using the 'plungers' (carriages?) as the means of partial evacuation and propulsion at the same time...

No true vacuum, just enough to reduce drag to something manageable.

One thing that still bothers me about the whole concept (besides it being a complete guess as to what Musk has in mind of course) is the stopping properties, emergency shutdown.

If you would have to suddenly decelerate in the system I proposed you'd be building up pressure in front of the car which would slow the car down until some maximum pressure i s reached at which point you can breach the walls to relieve it and keep it at that maximum. This provides a nice cushioning effect for emergencies using very little besides the components already in the system.

If you were to even partially evacuate the tube then that would no longer work and you'd need active braking at all times.

Accelerating into the at a speed slightly higher than the loop speed backing into a slug that has just passed might be a way to mitigate this to some extent, then you still have to worry about stopping the train but the braking energy could be dispersed over a much longer distance due to closer packing.

I missed the end of the interview linked by someone else in the other thread where Elon Musk says it has no 'right of way' issues so I think the whole thing is moot but it was a fun exercise anyway.


I've seen his right of way comment. But if you've got something ground based, that is 1200 km long, how can you avoid right of way issues? My guess is that he means to build it in the ocean. There aren't a lot of people living there. If so, it could look like anything you want and there would be no "right of way issues". (Update, it just hit me! If you build this in the ocean, with neutral buoyancy under water, then you don't have to worry about earthquakes or weather! Built on land and able to handle earthquakes is a lot harder.)

But the stopping properties deserve more thought. California has regular earthquakes. What happens if you've got large hunks of metal going 1200 km/h over a section of loop that just was hit by one, what happens?

My suspicion is that you build regular emergency exit tubes. (Many of which might also be good for regular tube service.) The cars are let out at the nearest one. The machinery is at risk, the people are mostly OK.

You then try to stop the plungers with regenerative braking, dumping air in, etc. Or you can build a fast emergency stop, such as having exit holes that you can drop plungers into at high speed, which carries them far enough away from the tube that they won't destroy it when they impact.

This has to be carefully worked through. But the first and most critical piece is the ability to quickly and reliably dump passengers from most of the system. And that shouldn't be that hard to do.


I read somewhere (can't find a quick reference) of a coating that resembles the structure of shark-skin, this reduces drag enormously by creating local turbulence which reduces the shear forces required to separate the layers of fluid adhering to a surface.

A coating like that on the inside of the tube and the outside of the vehicle would be an interesting component but the fluid dynamics of such a set-up are way beyond my abilities to calculate.

This whole thing is all about energy budgets, you'd have to take the amount of fuel it takes a passenger car or rail car to move from one point to another, then take some reasonable savings estimate and then work backwards from there to see how much energy you could expend on friction, drag and so on.

I have a hard time accepting that the friction in a tube while drafting would be that much higher than friction losses to a body of air that is mostly standing still and from playing around with a car and a friend of mine who is a trucker (don't try this at home kids) I've seen how low fuel consumption can go if you are properly in the slip-stream of another vehicle.

There has to be some way to make the numbers work on that. But unless 'no right of way issues' means underground (or ocean bound, which means it would only work between coastal cities and would have a pretty terrible failure mode) the whole thing is off the table anyway.

I can't stop thinking about it though :)


You could brake (at some rate) by keeping the flaps ahead of you shut. The interesting issue is to make damn sure all braking is in sync enough.


A random quick note.

The first public discussion of the Hyperloop was in http://pandodaily.com/2012/07/12/pandomonthly-presents-a-fir... starting about 43 minutes in. A revealing bit of the conversation a minute later went like this:

Elon: I have a name for it, the Hyperloop.

Interviewer: Is that like a Jetson's tunnel?

Elon: Something like that, yeah.

Interviewer: You just get in, it whisks you?

Elon: Um, yeah. Yeah.

He later on said, You're guessing in the right direction.

This suggests to me VERY STRONGLY that his technology proposal MUST be a tube of some sort. He also said that it could store enough energy to run 24x7 without using batteries. That implies that he's got a lot of material going very, very fast.

Interestingly this is the time that he made his 30 minutes downtown to downtown comment, and his solar panel comment, but said nothing indicating no right of way issues. Later on he said that he came up with a way to make it much cheaper, and to get rid of right of way issues, but hasn't been repeating the other comments that I know of.

This suggests to me that in July he might not have had right of way figured out, and suggests to me that getting rid of the right of way issues by putting it in the ocean might not be a bad guess as to the insight he had over the summer that dramatically reduces costs.

My proposal may not be on the right track. But I think it is safe to conclude from what Elon has said that it is a tube. It has a lot of mass moving. And I don't think it infeasible that he would be thinking about putting this in the ocean to solve the rights issue.


Your idea seems reasonable. It made me think of when the 4 minute mile was broken. Once it was, people began doing it on a regular basis.

Elon promotes this idea and people believe it to be possible, then suddenly it becomes possible. It makes me wonder how many breakthrough ideas are out there, that are undiscovered.

Clearly Elon is a genius. How much more of a genius would he be if he knew this principle of human nature, and used it to find the solution to a problem that he hasn't figured out. He would use the collective intelligence of the world to solve the problem.

He probably has a reasonable idea that this can be done, but what if he were to use this power he has to essentially scale his genius by harnessing the imaginations of people like you around the world?


Tubes in the ocean, whether sunken or floating, seems like a nightmare when you consider ships. Ships snagging fiber cables with their anchors is one thing, but something with people in it? Great care would probably have to be taken with this sort of thing.


I'm greatly enjoying this entire debate, but I'd just like to point out one thing: Elon said he doesn't know for sure how to do it yet or if it's even feasible (although he thinks it is). This means

a) It's possible that not all of the statements he ever made about the Hyperloop will be true for the final design (if any). I'd be especially cautious about the comment about right of way issues. This might as well just mean that he found an unused railroad track or something. And

b) Nobody knows if it can be done. Keep that in mind when debating the ideas. As far as I'm concerned, all ideas are constructive at this point.

I'm not a physicist and it's very likely that this solution is completely impossible, but let me ask anyway. If you just had a 1200 mile long train and you somehow got the mass going in a circle at 1200 mp/h, how much energy would you need to keep that momentum going? Could you cover that with solar panels covering the 1200 miles of track?


Keeping mass in orbit about a point requires absolutely no energy. That is because energy is force times distance (with times being a dot product), and the force you apply is always at right angles to the motion.

It is a good thing that this is true, else the Earth could not manage to go around the Sun indefinitely!

Therefore the energy to keep a system like this going is entirely what is needed to replace various incidental losses like friction. With maglev technology there some electromagnetic drag, but the bulk of losses are air friction.

There is no way to keep a train like you describe going against air friction at 1200 mph. If you eliminated air friction, it would be very doable.


What if the entire loop was in motion? Picture a maglev train 1200km long. There is no "front" to the train, it's a loop, so air is not being compressed as it whizzes along at 1200kmph. Most of the space inside the train could be void, which could be filled with passenger cars as needed.

Would that eliminate enough of the air resistance to be feasible? Is a 1200km long contiguous structure feasible?


I am fairly sure it would not help. If you're rubbing past air at that speed, you'll have pretty serious friction issues.


I hate to be overly negative, but this is just a bunch of extra complexity layered on top of an unworkable idea. The plungers aren't relevant. If you could run this with self-propelled plungers that push cars, you could run this with self-propelled cars.

The "flaps" are just trying to play with semantics. Either they work and this is just an overly-complex evacuated tube or they don't work and this is no different than Mattheij's idea. If you land in the middle where they "kind of" work, then you likely get the drawbacks of an evacuated tube (not enough oxygen to keep a human alive) with most of the drawbacks of Mattheij's idea (too much friction to be feasible). Air becomes too thin to support human life before air resistance becomes negligible.

This proposal also has the same right of way issues that any tunnel system would have, issues that Musk specifically said Hyperloop would avoid.


I'd appreciate it if you put the same thought into the proposal that you did into throwing out criticisms that it already addresses.

If you run this with cars only, the mass of cars that you'd need to accelerate/decelerate is much higher. Also the heat issues that the cars need to deal with is also much higher. Separating plungers and cars means you accelerate/decelerate much less mass, and have far fewer heat issues to deal with. The plungers themselves will get too hot for humans, but that's OK because humans are insulated from them. (And I already said how to deal with the heat in the cars.)

If the flaps work as proposed, the air pressure directly in front of the plunger will significantly exceed the air pressure in the rest of the tunnel. The cars sit in a bubble of air maintained by that shockwave. I have not tried to do fluid dynamics calculations to estimate friction, but it is going to be many times less than with Jacques' idea. I can believe it could be low enough to make this system work.

I suspect that Elon plans to avoid right of way issues by building it in the ocean. If he doesn't do that, then it is very difficult to see how he can do anything like a “ground based Concorde” (his words) and still avoid right of way issues.


> I'd appreciate it if you put the same thought into the proposal that you did into throwing out criticisms that it already addresses.

I'm pretty sure you did not address my criticisms in your proposal.

> If you run this with cars only, the mass of cars that you'd need to accelerate/decelerate is much higher.

If you run this with cars only, you probably wouldn't make them weigh 70 tons, and then your acceleration costs drop dramatically.

> Also the heat issues that the cars need to deal with is also much higher. Separating plungers and cars means you accelerate/decelerate much less mass, and have far fewer heat issues to deal with. The plungers themselves will get too hot for humans, but that's OK because humans are insulated from them.

You only have heat issues because you're trying to push a big column of air for no reason. What is the value of this column of air? The air itself is just creating friction. The passengers can't breath it, because it's too hot for them to survive, and it's not enough to sustain life in the event of a failure, because most of this hypothetical tunnel is a vacuum.

> And I already said how to deal with the heat in the cars.

You proposed adding enough cold thermal mass to offset the heat they'd absorb (literally a block of ice). You're just adding a bunch of extra mass that you have to accelerate, and a bunch of mass you need to re-cool after every trip.

> If the flaps work as proposed, the air pressure directly in front of the plunger will significantly exceed the air pressure in the rest of the tunnel. The cars sit in a bubble of air maintained by that shockwave. I have not tried to do fluid dynamics calculations to estimate friction, but it is going to be many times less than with Jacques' idea. I can believe it could be low enough to make this system work.

If the flaps work as proposed, then you have all the problems of an evacuated tunnel and most of the problems of Mattheij's idea, as I said. You have an inhospitable environment, need to deal with the pressure differential (throughout most of the tunnel at least), tons of energy lost to friction (or else it wouldn't be too hot for humans), etc. Even if your idea was technically possible, it seems to have no advantages over an evacuated tunnel except that you can say that it's not an evacuated tunnel.

> I suspect that Elon plans to avoid right of way issues by building it in the ocean. If he doesn't do that, then it is very difficult to see how he can do anything like a “ground based Concorde” (his words) and still avoid right of way issues.

If you build far enough underwater to avoid right of way issues (aka, no boats can get past), you're looking at worse engineering issues than an evacuated tunnel anyway. At just 10m below the surface, you're already at 2 atmospheres of pressure.

I'm also not sure that "underwater" counts as "ground based".


Here is an exercise for you.

Go to http://pandodaily.com/2012/07/12/pandomonthly-presents-a-fir.... Jump to 43 minutes in. Listen for two minutes. Come back and explain how to explain that interchange WITHOUT having a tunnel. If you listen for slightly longer when he talks about storing energy in the system so that it can run 24x7, it seems pretty obvious that whatever the system is, it has a lot of weight moving very fast.

We also know, because Elon Musk has said so, that, It is not an evacuated tunnel.

If it is going to be reasonably efficient, it can't have a lot of air in it.

So those are the design constraints for what Elon is thinking of. What can you propose?

Now why would you not want an evacuated tunnel? Well the common complaint about an evacuated tunnel is how hard it is to maintain the vacuum, and what to do if containment is breached. My proposal gives a way to maintain the vacuum (the plungers). Minor breaches of containment are not that big of a problem (the air that gets in is blown out again).


> Come back and explain how to explain that interchange WITHOUT having a tunnel. If you listen for slightly longer when he talks about storing energy in the system so that it can run 24x7, it seems pretty obvious that whatever the system is, it has a lot of weight moving very fast.

Explain that exchange without it being a giant pneumatic tube like the Jetsons. If we take it at face value, he basically agreed that it's a pneumatic tube, but I'm pretty sure that's not really what he's thinking of, and you seem to agree, given that your proposal is absolutely not a "Jetsons tube".

I agree that storing energy without batteries likely means a lot of moving mass. That doesn't mean it's a big weight flying through a tunnel. It could also be a flywheel or something else. Or it could be that the "moving mass" is an incorrect guess and it's something chemical, but not in a traditional "battery" (e.g. a breakdown of water into component gases).

> We also know, because Elon Musk has said so, that, It is not an evacuated tunnel.

We also know that Elon Musk tends to propose (and build) practical solutions. It seems unlikely that he would be proposing what amounts to an evacuated tunnel with unnecessary complexity tacked on. If he's stating that it's not an evacuated tunnel, then it's presumably not just a semantic game.

> If it is going to be reasonably efficient, it can't have a lot of air in it.

So he's designed pointless complexity into the system just for the sake of saying it's not an evacuated tunnel. "It's not evacuated. It just doesn't have a lot of air in it."

It's be a lot simpler if he just proposed a traditional partially evacuated tunnel if that's what he wants.

> So those are the design constraints for what Elon is thinking of. What can you propose?

My lack of a proposal does not make yours feasible or sensible.

> Now why would you not want an evacuated tunnel? Well the common complaint about an evacuated tunnel is how hard it is to maintain the vacuum, and what to do if containment is breached. My proposal gives a way to maintain the vacuum (the plungers). Minor breaches of containment are not that big of a problem (the air that gets in is blown out again).

Your tunnel has all the same problems as an evacuated tunnel and more. A standard evacuated tunnel also has a way to maintain the vacuum. It's called a pump. This doesn't stop a breach from being problematic. You're ignoring the fact that pushing all that air out of the way again is hard, whether it's being done with a pump or a piston. If there's a breach, your "plungers" will run into huge pockets of air that have to be evacuated. This will slow them down, potentially causing collisions depending on how large the breach is. It will actually slow them much more than an evacuated tunnel, because they form a plug in the tunnel, meaning they need to displace most of the air instead of some of it.

Your plungers also have to form a tight seal in order for them to be effective. I'm not even sure that's possible. 70 tons of mass flying through a tunnel with a tiny gap all around. What happens when we hit that breach and this mass wobbles a bit? How big does the gap have to be to ensure safety? How big before the plunger isn't actually expelling gas anymore? And you admit that the friction the plungers encounter will make them hot enough to be deadly to humans. This is all wasted energy.

P.S. You also said this thing is going to be underwater. How on earth do you imagine than an underwater breech would be minor? And how is this air being expelled underwater anyway? Are you planning on building an exterior tunnel big enough to hold an air gap plus the transit tunnel (plus the return tunnel) all underwater? For a tenth of the cost of a bullet train?


Why are you inventing constraints that don't exist?

Elon's pauses make it clear to me, and presumably to you, that he was checking what she said versus what he was thinking, and finding that, to a layman, the two would seem similar. Even if underlying technologies are different.

His wording suggested strongly to me that his solution was a tube. Did it not suggest that to you?

No matter how many times you say "pointless complexity", that doesn't mean that there really is pointless complexity. (And we're disagreed on the complexity.)

I'm quite aware of how hard it is to push air out of the way. A quick comparison of the mass of the plunger and the mass of the air says that a bubble of air will not actually slow the plunger down much. Being in air constantly would, but a bubble of fixed size wouldn't. (You'd have to think about what effects it might have on passengers inside.)

There is absolutely no need for a tight seal, as long as the flaps open and shut at the right time. You can stand a long ways away from a moving truck and still feel the wind pushing away. And the closer that it gets to Mach 1, the farther away you can feel that gust.

The friction losses are hard for me to estimate. The upper limit on the temperature of the plungers is caused by the heating of air that is suddenly compressed to 1 atmosphere, and the friction from air moving past at hundreds of miles per hour. However if only a little bit of air is involved, the heating might happen at reasonable rates. Without parameters that I don't have and a fluid dynamics simulation that I am not prepared to do, I can't tell. That said, I can tell you that the rate of heating per volume (and therefore the ease of dealing with it) goes down as you make the plungers larger, up as you speed up, and down as you reduce air pressure. So if you have little enough air and large enough objects moving through it, the heat generation gets more and more reasonable.

And about your PS, I'm trying to guess at how Elon gets around right of ways. Underwater is not part of my basic idea at all.


What constraints do you imagine I'm inventing? That it's practical?

Musk's pauses indicate to me that he was internally asking himself whether his proposal is somehow like the Jetsons tubes. That doesn't necessarily mean he was looking for literal parallels. And if he were, I don't see why "pneumatic tube that sucks a single person up" would turn into just "tube" to a layman, either. He's also mentioned the Concorde and a railgun. How do those fit into your vision? Will a layperson see those in your car/tunnel system?

I'm not just saying "pointless complexity". I'm telling you why it's pointless. Your system is certainly more complex than an evacuated tube, and without a compelling reason to embrace such complexity, then it is indeed pointless.

Let's step back a bit, though. You're saying that your system is easier to implement than an evacuated tube. Let's assume that's true (though I don't think it is). Is it also easier to implement than a partially evacuated tube? Those are expected to travel at or past Mach I, and don't need the car to move nearly 100% of the air in the tunnel. How is your proposal superior? You talk about using the plungers to evacuate air, but the energy cost to remove the air must still be paid. Your 70 ton slugs might carry more momentum, but a car in a partially evacuated tube wouldn't need to displace all the air it encounters, and so wouldn't need as much momentum. You're constantly paying the cost to push around a bunch of air. That cost is largely avoided with a more traditional design.

I'm not certain about your idea that you don't need a tight seal. Yes, at Mach I a gust would blow pretty hard. But in a near-vacuum you'll not be blowing much air. Remember that you're going to have to blow with >1 atmosphere of pressure to push anything out and past the flaps. Also remember that this "gust" is the result of resistance caused by the giant plug you're pushing down the tube, resistance that another design might just avoid or at least minimize. And any air not evacuated by the "gust" will contribute to further drag as the plunger pushes past it.


It will be wonderful until the first time a plunger smashes a car with a family to smithereens because the railgun timing was slightly off due to a voltage irregularity.


There are clearly a lot of details that you have to get perfectly right. However getting them right is an engineering challenge, not a physics challenge, and engineering for safety is something we as a society tend to do pretty well. (As opposed to engineering for cost containment, which we as a society are often horrible at.)

If I saw Elon Musk standing up there with cost projections, I'd be optimistic that they were in the right ballpark. If I saw virtually anyone else there, for a project of this type, I'd be assuming that they missed order of magnitude cost issues.


Our current forms of public transportation are certainly not without risk. A runaway DC metro train killed 9 people a few years back. Maintenance workers are regularly killed on the tracks. Extrapolating from a sample of admittedly just one system, I doubt that this would be any less safe than the high speed rail they are proposing.


While that would be horrible, it's still probably a lot less likely than that same family being involved in a fatal car accident.


You might want to read this: http://www.daniel-irimia.com/2012/04/19/daryl-oster-vs-henri.... It's been done as a research project in Romania. Not sure if it can be patented...


Transporting mining material, freight, etc. seems like an excellent proving ground. Much less chance of a disaster killing someone and killing the project. Often there's a large amount that has to go from one place to another place, not people who have many different places to go. I guess the only problem is that freight often doesn't have to move fast. If it takes your mining output an extra 8 hours to get to processing it doesn't matter. If it takes a person an extra 8 hours, they'll pay less or use other forms of transport, or require stopping stations...


How about this:

http://www.youtube.com/watch?v=0DfDOlUXEBo

It's chinese concept of a train that doesn't stop - you have the main body in constant motion (going around in a loop), and the passenger cars just attach to it and detach from it via magnetic forces. No need to stop the big train, you can catch on at any time, and this likely could work with - properly outfitted - cars.


I feel quite certain that Elon Musk will be remembered as our generation's greatest entrepreneur.

Even if this idea doesn't succeed, the scale of his ambition is breathtaking.


What are the advantages of using air instead of liquid? Is it just the breathablity aspect?

I posted a few comments in the past on my guess. A liquid filled loop with a (super)hydrophobic inner surface. Energy stored as kinetic, solar accelerated.

I have been looking, and haven't been able to find much info on reduced energy loss by using hydrophobic surfaces. It's a new enough tech that all the good info is behind academic paywalls.


Would pressure differences throughout the tube caused by differences in altitude be something to worry about? I'm not sure how, if at all, that would effect the hydrodynamics.

Also, I would be concerned that with that much fluid moving through it that fast, a possible failure mode could be "temporarily turns into a horizontal launch loop". (which would be awesome ;)


* Failure would look Awesome

Always a winning bullet point.


Any hypothesis that does not include electric drivetrains (ala Tesla) powered by solar (ala Solarcity) is going to be wrong. Also, the 'loop' part strongly implies that it will store excess energy. I think the 'immune to weather + no need for right-of-way' issue is the most intriguing here.


"No need for right of way" is also enough to know that none of these tube proposals are correct. You can't build a tube from SF to LA without running into right of way concerns.


Yeah. That is my assumption too. But all these proposals just seem to gloss over it.

edit: Jacques admitted it was an oversight.

Still, it's interesting reading these ideas even if they might not be what Elon is thinking.


One of the central supports against patents is that engineers given a problem will all come up with roughly the same solution. So given that Elon has dropped a number of hints the question then becomes: can a bunch of engineers come up with his proposed solution before he reveals what it is.

That's a very interesting puzzle to me and I think this may be a way in which the patent office could do their reviews: ask a community of smart cookies to come up with what is in the (secret to that point) patent application, the one that gets it right gets a part of the filing fee and the patent is denied, if nobody can crack it in some set time (say 4 weeks) then the patent is granted.


There are not right of way issues if your tube is floating under water in the ocean. This also protects you from earthquake risks.

He has said that his system is like a “ground based Concorde”. His specs give you a speed of over 1200 km/h. If you're going 1200 km/h on the ground and aren't in a tube, you've got a problem.


Okay, but if you're floating under water, you've added a ton of new concerns. You're looking at a lot more than 1 atmosphere of pressure if you're under water, and a containment failure goes from stopping the transit to drowning everyone. Also you now don't have the solar panels Musk mentioned (they don't do so well underwater), and the oceans are not magically immune from the effects of earthquakes. Tsunamis can most definitely cause plenty of damage.

LA is also 17 miles from the coast, so if you're getting from downtown SF to downtown LA in 30 minutes, you've got less than 15 minutes to travel nearly 400 miles underwater in order to leave enough time to even attempt to get to LA under the deadline. You're now travelling at Mach II, almost twice as fast as Musk proposed.


If he really meant the actual downtown of Los Angeles, then there is definitely a problem getting out of there in time to make anything feasible. And all of the ground, air and underground around there is going to be an issue. Heck, http://en.wikipedia.org/wiki/Westside_Subway_Extension shows 4-8 billion just to extend a subway through part of it.

As for oceans, I think the risks are less than you think. A tsunami does great damage when it hits land, but in the open ocean you just bob a few inches. The mechanics behind that are simple - a wave coming in to shore starts to get bigger once the depth of the land is the same as the depth of the wave. Tsunamis are very deep waves.


I don't pretend to know what Musk has been hinting at. I don't see why you're clinging desperately to the "not an evacuated tunnel" constraint while dismissing other constraints you don't like, though. I assume Musk said "downtown" intentionally, since he said it twice (once for each city). Maybe his idea simply isn't feasible, given the constraints he's mentioned, but if we're going to dismiss inconvenient constraints, "Mach I without a vacuum" seems like a good place to start.

As for oceans, a Tsunami is still going to put a lot of stress on a 400-mile underwater tunnel. And at the points you get in and out of the tunnel, you're going to be at the shore anyway, where the Tsunami is its strongest. Also boats.


...but if we're going to dismiss inconvenient constraints, "Mach I without a vacuum" seems like a good place to start.

OK, let's discuss Mach 1 without a vacuum. A condition that I had not through, but which actually makes surprising sense as a design limit with the system that I am suggesting.

The Chapman–Jouguet condition says that a supersonic shock wave travels at the speed of sound in the air behind it. The ideal gas law says that this speed is almost entirely determined by temperature, not pressure. If you were to go anywhere above Mach 1, the temperature behind the shock wave gets hotter than room temperature very, very quickly. At Mach 1 exactly you should be able to maintain a thin (~0.2 micrometers is quoted by Wikipedia as the measured value) bow shock.

That shock can trap a huge pressure differential behind it, which could allow an open container to maintain normal air pressure while moving through a much lower density of air. If you are moving below that speed, the pressure differential that you maintain is much less than if you speed it up. Therefore for efficiency you'd want the vehicles to be moving at Mach 1 relative to the gas. Assuming that the gas is dragged along, this would let you actually go slightly faster than Mach 1. But if we assume that we have a lot of tunnel and occasional objects, the air would move much closer to the speed of the tunnel than the objects moving through, and therefore the top speed would be approximately Mach 1.

The exact relationship between the air pressure of the tube and the air pressure behind that flap is very complex. But with the flap design that I suggested, at pressures above 1 atmosphere there would be leakage and an opportunity to remove air. If the pressure fell below 1 atmosphere, that opportunity would disappear. Therefore the flap system that I suggested would do a good job of letting air leak out of the tunnel at about the necessary rate to maintain 1 atmosphere inside of the open containers as the speed of the whole system increases and air leaves the tunnel. (It should not be hard to design ways to bring air into the tunnel...)

This reinforces the point that the tunnel need not be evacuated, and explains why Elon said it would run at the speed that he predicted.


To mince words, Musk used the word "tunnels" (and not "tubes"), perhaps intentionally?? Tunnels are know for leaking water (and air), while tubes (or pipelines) are implied to not leak.

At any rate, the subject of Evacuated Tube Transport Technologies (ET3) tm was disclosed in US patent 5,595,543 in 1999. For the record, Coanda's system was pneumatic (and took alot of power to move the air). Search for "evacuated tube transport" on youtube for an overview. I have not noted any claims for 'hyperloop', that are not exceeded by ET3 (tm). see www.et3.com and www.et3.net


"You're looking at a lot more than 1 atmosphere of pressure if you're under water"

There are already some very long vehicular tunnels at depth (the Chunnel, Seikan Tunnel, etc.). I don't think one atmosphere more or less would be a showstopper.


The point is that if you can deal with many atmospheres of pressure, you would just do a proper evacuated tunnel and get the benefits of lower friction, etc., instead of trying to find a way to make it work with air still in the tunnel.

The Chunnel also cost about $17 billion in today's dollars and is only 31 miles long (including the above-ground parts). I don't think this is a feasible model for long-distance transit, especially when Musk claimed a cost of $6 billion for SF to LA.


The Chunnel was also built by governments with contractors, subcontractors, sub-sub contractors and so on.

With SpaceX Elon has demonstrated an ability to reduce costs by a factor of 10 in large part by eliminating the use of contractors. If you look on the PandoDaily interview, just before the Hyperloop is discussed he was railing on about how inefficient the 405 highway expansion is. I wouldn't be surprised if he thinks that he could save an even bigger factor in that kind of public work than he did with SpaceX.

I'm not saying that he's right. I'm saying that his cost projections are extremely unlikely to start with existing construction projects as a base line.


I dont know about 'hyperloop' but ET3 (tm) tubes (if placed underground) require less than 1/15th as much earth removal than a train tunnel or freeway tunnel. Our patented (US 5,950,543 and others) ET3 system optimizes tube size to strike a balance between low cost and high utility (as cars do), we use a freeway network design philosophy (not like a train track) see www.et3.com and www.et3.net to learn more.


Far easier than an evacuated tunnel, though, is a tunnel built on standard-pressure hydrogen/helium. Light gasses have a much higher speed of sound, and their lower density means lower drag. A system of hydrogen-filled maglev tubes that can max out at 2500mph with freight-weight loads over long hauls would be more than a little bit valuable.


A tunnel filled with hydrogen seems to be a very bad idea.

http://en.wikipedia.org/wiki/Hindenburg_disaster

A tunnel filled with hydrogen has a large surface area for potential leaks.


I don't think I need to debunk what happened to the Hindenburg yet again (and, incidentally, what didn't happen with it, its sister-ship the Graf Zeppelin II or its ancestor the Graf Zeppelin I in 682 other flights).

Suffice it to say, the challenges of an electrically grounded hydrogen tunnel lined with heavyweight materials using modern technologies & sensors are a hell of a lot different than the challenges of an ultra-lightweight, ultra-thin flammable gas bag before the advent of plastics, in an electrical storm.

A tunnel filled with vacuum has it a hundred times worse, because you're dealing with 15PSI trying to get in rather than 0PSI pressure difference diffusing out. We already regularly use larger-molecule methane at thousands of PSI.


I'm not sure what exactly you mean when you say "debunk what happened to the Hindenburg". The thing became engulfed in flames and crashed. Not much debate on that. Or are you talking about debunking the claim that hydrogen airships are fundamentally unsafe, a claim I didn't actually make? (Incidentally, the fact that other hydrogen airships didn't suffer the same fate doesn't mean they were actually safe. Lots of homes have aluminum wiring but have managed to avoid burning down. You'd still be wise to replace any aluminum wiring you find in your home.)

Obviously a modern hydrogen tunnel would be much different than a zeppelin, and have different challenges. Still, 400 miles of 10-foot diameter tunnel would have plenty of opportunities for problems (whether it's evacuated or filled with hydrogen or something else). I'm not sure an evacuated tunnel is necessarily worse from a safety standpoint, though. It might be harder to engineer, but the failure mode seems safer. A hydrogen tunnel means that a leak will mix hydrogen with an oxidizer. If it's at 0 PSI, this could mean a leak lets oxygen into the tunnel (either due to high-pressure weather or due to hydrogen rising from the leak, leaving a low-pressure environment in the tunnel). If you wanted to fill a tunnel with hydrogen, I think it would be wise to keep at least a small positive pressure to ensure than any leaks are outward only.

I'm totally unclear why methane stored at thousands of PSI is relevant. Is there some 400-mile long, 10-foot diameter, above-ground methane tunnel I'm not aware of?


Why does everyone expect this "tube" to be filled with air? A liquid is more interesting and it also stores a lot more energy when it moves ...


If you can find a liquid with low enough friction to be feasible, I'm all ears.

The only ones that I'm aware of are thin gases or helium-II. The latter is rather impractical to use for a great many reasons. But I'd be interested if anyone has other suggestions.


Are you talking about friction between carriage and liquid or walls and (moving) liquid?

The liquid will move at the desired travel velocity, so I believe the latter will be important.


Air is a fluid, too. It's also got a lower viscosity than say, water, so it will create less resistance.

Saying it stores a lot more energy when it moves is like saying that we should make our train cars heavier so that we can recover more energy by regenerative braking.


Because the resistance of any economically feasible liquid is probably higher than that of air. The energy needed to overcome that friction doesn't drop just because the water carries more momentum.




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