Depends how much temperature variation you want to allow - according to this page, 6 feet deep gives you around a 20degF swing from winter to summer in Virginia, and you need to get down to 30 feet for temperature variation to be negligible. Digging down might help solve thermal expansion, but brings other challenges with maintenance, right-of-ways, and especially cost.
As Sam mentioned, there are a huge number of engineering challenges that are often minimized by the cheerleaders. Elon Musk’s initial back-of-the-envelope cost estimates were absurdly optimistic - and even more absurd was his proposed ticket price of $20 per person per trip between LA and San Francisco. I think the only way this ever gets built is if some government or billionaire wants to build one as a money sink, as it seems exceptionally unlikely to ever be an economic mode of transportation.
You’re discussing multiple small leaks from every direction. copperwindow was discussing a single, substantial rupture. In that case, you could end up with something like this:
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Yeah... square cube law dude.
Humans weigh a hell of a lot more than a ping pong ball, especially in relation to our surface area. You’ll have at maximum about 1 atmosphere of pressure, and that only for a few microseconds as the air expands into the vacuum, not the essentially infinite volume of air of the room’s atmosphere relative to the “cannon” and you might recall that humans weigh substantially more than a bloody ping pong ball and have significantly greater mass per unit surface area.
Not a good example. Things that work on a tiny scale don’t immediately scale up to a scale an order of magnitude higher. Especially when dealing with air related things.
The hyperloop with a rupture is basically a very big shock tube. You have a supply of pressurized air on one side and a tube with vacuum on the other. If you have a rupture, a wall of pressurized air will travel down the tube faster than the Speed of sound while the pod is travelling in the other direction with a couple of hundred mph.
The pod will basically be crushed together with everyone in it.
That sounds impressive, until you realize the speed of sound is only 0.343 km / s, 300 mph is only 0.134112 km / s (for a total of 0.477 km / s), and vehicles like lunar modules travel at 10 km/s during atmospheric re-entry. That’s not just one order, that’s two orders of magnitude greater, and no, there is no magical, steel-crushing force that obliterates everything in its path.
Air is not a solid like concrete. Air is also not a liquid, like water, with surface tension and an inability to be compressed. Air is a gas, and colliding with a gas is not nearly as catastrophic as you seem to imagine. It doesn’t have all that much mass, relatively speaking, and even the speed of sound is not an impressive figure. Heck, have a friend take a belt and snap you on the leg with it. That’s something traveling at the speed of sound hitting your bare flesh.
One way is someone puts a truck bomb or gets access to an exposed part of the hyperloop pipe and wraps it with cutting charges. Cutting the heavy steel pipe all the way around or blowing it into fragments would possibly kill trainload(s) of people in that tube who are in transit.
But it’s relative. Terrorists could also drive that truck bomb into a crowd somewhere, and with near future automated trucks, it need not be a suicide mission. Or park it next to a Federal building.
The threat we are talking about now is somehow shoots an elevated section of the pipe with an armor piercing rifle, or just an accident happens and a seam lets go partially somewhere. So it’s a moderately slow leak into the tube.
Obviously the instant pressure gets detected at a sensor in the tube, every train in it is going to begin emergency braking. The actual hyperloop prototypes are mag lev, not air hockey style like Musk’s original plan. So I guess it’s just a matter of timing.
If the trains are stopped from emergency braking and this wall of air comes down and hits the trains, it would do…what? What do people think will happen? These trains won’t have any windows and will be streamlined against the small amount of air in the tunnel. And unless it’s a complete tunnel rupture, the air is going to be at low pressure.
First of all, the lunar modules don’t return to Earth. Second, the Apollo command module which did return has a special ablative heat shield designed to burn away and carry the tremendous heat built up from hitting the atmosphere at that speed. Third, re-entry starts at 400,000 ft, where air pressure is only about 5 * 10^-8 psi, and maximum G loading from atmospheric drag occurs around 200,000 ft, where air pressure is still only .003 psi. So re-entry tells us nothing about what would happen if a 700 mph wind were to hit a capsule at 14.7 PSI.
You have no idea what you are talking about. Think about the destruction caused by 150 mph hurricane winds. Now consider that the energy in the wind goes up with the square of the speed.
And yes, the speed of sound is an impressive figure. Capt. Brian Udell and his navigator ejected from an F-15 at 800 mph. The navigator was killed instantly, and Udell had both his legs snapped in half, his arm dislocated, his helmet torn off and all the blood vessels in his face broken by the force of the windstream. This despite being strapped to a chair that wad supposed to protect him and wearing serious protective gear. I believe he is the only pilot to survive an ejection at that speed.
Ever see what happens to an aircraft if it loses streamlining at those kinds of speeds? It’s usually converted into confetti with a few heavy pieces. The Reno Air Races have had several fatalities where a failed control surface caused the aircraft to basically disintegrate, killing the pilots instantly.
I think someone in this thread really needs to do some math, because there are two widely divergent opinions and that’s unnecessary when there is actual math that can be done on the subject.
I am amenable to either view, but it needs to be supported by something beyond assertion.
So now what? Conceding that there are scenarios where there might be a low pressure leak that forces the trains to stop, you now have people sitting in an enclosed steel tube in the California sun, potentially hundreds of kilometers from the nearest exit. Are you going to cut them out? Remember, Musk’s design calls for a one piece solid welded steel tube. There are other reasons that can’t work, but assuming it did, what’s the emergency procedure? are they expected to limp along at 50k for hours while baking in an enclosed tube?
Aside from metal expansion, the big thing to worry about is the vacuum itself. Have you see what happens to a tanker car when it loses just a few pounds of pressure due to cooling? It can crush flat. Tanker cars have roughly the same wall thickness as the Hyperloop spec. And they have special valves that allow pressure to equalize if the liquid or gas payload cools down and therefore lowers air pressure in the tank.
I believe the Mythbusters only had to drop a tanker car to about 4 psi (after disabling the safety valves) before it suddenly crushed like a pop can.
But here’s the thing: Let’s say these problems are potentially fixable. The thing that makes me highly suspicious of the various hyperloop companies is that they seem to be focusing their engineering efforts on things like making little maglev cars go faster, or hosting design competitions for futuristic terminal buildings, instead of trying to figure out how to handle 1300 ft of expansion between night and day, or buckling loads from differential heating, or expansion joints that can hold a vacuum and not require constant maintenance, or how to draw a vacuum in reasonable time from hundreds of kilometers of tubing, or…
Real engineering companies tackle the hard and potentially show-stopping problems first, so they can validate the feasiblility of the project before investing more money. Companies that focus on flashy public events and headline grabbing stunts are generally trying to attract money from governments or private investors, and the actual engineering can come later - or never. I have seen this kind of rent-seeking startup with a bogus product too many times to count. This has the smell of the same kind of thing.
He just converted the speed to km/s, I guess because he somehow thinks that this makes it look slower or less threatening or something.
Let’s put it in another perspective: A .44 magnum bullet travels about 450 m/s, (or if you like, a measly .45 km/s) and weighs 16 grams. A cubic meter of air at sea level weighs about 1.3 kg.
Air may seem insubstantial, but it has inertia like any other mass. The faster you hit it, the more energy you must expend either moving it out of the way, or compressing it to slow you down. At the speeds we are talking about, air isn’t quite a brick wall, but it’s enough to turn a lightweight capsule into a jumbled mess in very short order.
By the way, it’s a myth that surface tension is what makes water hurt when you dive into it from a height. What makes water hurt (or kill you) if you fall into it from a height is simple inertia. The water can’t move out of the way fast enough, so the energy get dissipated by your body instead. And once you jump from a certain height (250 ft or more) you might as well jump onto concrete.
Ok that part is addressable. Obviously the train would have onboard batteries so in the near term, it’s not going to overheat because there life support would be running off the batteries… There would have to be doors that can be closed, with safety interlocks, so you can separately pressurize-depressurize sections of this thing. So the solution would be to close the doors before and after the stopped train and then open some pressurization vents to fill the tunnel in that section.
Once that happens, pressure interlocks would allow for the doors inside the train to be opened from the inside, and there would have to be some headlamps or something in the emergency kit. Passengers could then walk to the nearest emergency airlock which you could put every few kilometers or so.
But yes, all this hardware adds cost. One thing discussed is building much smaller cargo hyperloop systems first. Since cargo systems would need far less safety mechanisms and would need not be as reliable. This has historical precedence - the first elevators were used in factories and were considered too dangerous for passengers.
Also, these heavy pressure doors? Located in unguarded stations every few kilometers? Terrorists could probably hot-wire a door to close, putting a steel door in the way of a train going several hundred mph.
Of course, eurorail has the same vulnerability. I am not even sure why this doesn’t happen more often. All a terrorist would have to do would be to drive a cement truck or something onto the tracks, ramming through any protective fences, a few seconds before a train is about to come.
Let’s take a more mundane problem with a hyperloop: capacity.
The D.C. Metro’s cars can hold 175 people each. It can run 8-car trains at rush hour, so that’s 1400 people per train. it can run trains at 5-minute intervals, so that’s 12*1400 = 16,800 people per hour.
To match that capacity, a hyperloop with 1-person pods would have to shoot them through the tunnel at intervals of roughly 1/5 of a second apart. Can the engineering handle that? I’m skeptical. And if it can’t, then it’s not a meaningful mass transit solution.
I am sure you can see quite trivially that the hyperloop has more capacity per track due to the higher speed. If you did 1400 person hyper-trains, you could launch them every 2 minutes or sooner because the higher acceleration means the tracks are cleared sooner.
With that said I don’t think the hyperloop is all that feasible. The problem with building ones is a local government has to agree, clearing the needed land and helping with the funding. At numerous levels, not just 1 government, all the governments along the route. Especially somewhere like California, where landowners can file a suit and block progress until the cases are decided in a court. Even if the landowners always eventually lose, adding 10 years to a project easily makes it infeasible.
Less expensive airline travel, or autonomous busses are just more practical. The simple reason is if I invent an autonomous bus or minivan tommorrow :
a. Nobody has to agree but the NTSB*. As long as they approve my vehicle for the road, it can start making revenue immediately without delay. (while a train is useless unless the entire track and 2 full stations are completed)
b. Hyperloop stations are in fixed locations. As ridership changes, autonomous busses can just go to whereever they are summoned.
c. You can easily change the size of the bus, from a minivan to an actual bus, or use multiple sizes.
d. You can make the vehicles actually drop people off where they are needed, it’s an end to end solution.
e. Slower speeds and armor in the vehicle itself can make fatalities uncommon.
f. Yes, it takes longer but the busses would have wifi and charging plugs and reclining seats and automated security.
*this isn’t true today but Congress is reasonably likely to pass a bill giving the Feds exclusive power to regulate autonomous vehicles and not state governments. It’s obviously within their power as it’s clearly interstate commerce.
No, I converted to km/s to make a direct comparison to another figure generally given to km/s that hit’s a “wall” of air at a considerably faster rate and doesn’t instantly crumple as the person I was responding to seemed to think it would.
So what? You literally just threw out random numbers without considering any of the implications. By your quoted figures, a small, soft piece of metal travels through air at 450 m/s (about the speed of the “wall of air” impact) and does not deform, but I’m to believe a hardened structure at several times the mass will fold like paper.
So they wont build the hyperloop out of balsawood. What, did you think it would require some infinitely light vehicle to achieve a measly 300mph (the number the above poster gave)? Japanese bullet trains move faster than that without a vacuum and are certainly not flimsy constructions. Even if we go with the proposed speed of about twice that, it’s only 4 times the kinetic energy and not enough to crumple solid steel.
Which is why I specifically explained that air can be compressed as the major difference between it and a liquid.
Air is not like water because air can be compressed. Air gives way readily, water does and cannot because water is a liquid. Surface tension adds a small amount of force but is a non factor. The physical state of water is what is more important (and its density).
That’s cute and all, but no one was saying that the hyperloop would hit air at that speed, so the heat you ramble about is entirely irrelevant. My point was that it’s insufficient crushing force to crumple the module like a tin can. Anything else is you either trying to look smart or missing the point.
Wooden framed structures filled in with drywall are not the same as a solid steel tube. There’s also a significantly larger surface area involved in hurricane force winds against a building, and wind passing down a tube dude. That’s not even remotely comparable.
And bare flesh is comparable to steel in your mind? Why do you keep making comparisons to really, really, soft materials as if they’re relevant? Yes, I’d hate to be buck naked hitting the air at that speed. So what?
Yeah, drag can definitely rip off the joints on a piece of metal sticking up from a plane’s surface, and that can chain into a cascading failure of the structure. So, instead of having ailerons, flaps or other potentially jutting structures, the engineers would be sane enough not to add silly jutting structures that are only needed for controlling aircraft and just use a bullet or other substantially lower drag creating design.
Mode of Transport X has more capacity than Mode of Transport Y if X is faster than Y and everything else is the same. But can you accelerate a 1400-person train in a vacuum tube? Doesn’t the whole idea rely on the capsules being fairly light in order to get them up to a few hundred mph really quickly?
Maybe you can take your time to accelerate that hypertrain if it’s going from LA to SF, but if it’s just going to Dodger Stadium from a nearby subway stop, or to O’Hare from downtown Chicago (these are two of Musk’s hyperloop proposals), how’s that going to work?
You can’t accelerate any faster than passenger comfort allows anyway, which limits you to a G or so. It only takes a couple minutes of this acceleration to get you to any reasonable speed you would be looking for.
For shorter jaunts, if hyperloops are used at all, lower speeds would be used.
The O’Hare to Chicago would be about 10 miles, about the shortest that such a track would have any use. You could get up to speed on that route, if you wanted to, or just top out at 200mph or so, still get there a bit faster than through traffic.
I will agree that one person pods is not a great idea, unless the pods are capable of linking together and acting as a single unit.
This is also news to me, every design I’ve seen or read about has been more like a train car. Sure, each car might operate independently, but not as pods for individuals. I can’t see individual pods being particularly more useful than a train car anyway. With a train-car, all of the “stopping time” at a station is essentially in parallel for each passenger, no need to gum up the tracks doing it sequentially. If this hyperloop is intended as a mass transit system and not a luxury of the ultra-rich, then having 20+ cars waiting to slowly align to the station, load/unload and then launch again seems inefficient. Way worse so if it’s more like 200+ (BART) or 2000+ (Tokyo) passengers at each stop.
I also don’t know why you’d need a bullet train or faster for a short run like 10 miles. Metro lines are more than sufficient for that, already in place, and probably cheaper to operate. I’d imagine a commuter would best be served by, say, boarding in one city then deboarding in another and transferring over to the local metro system. Likewise, taking the local metro to the hyperloop and then looping across country or between cities. The idea that it’d replace cars sounds like Elon being, well, Elon.
I agree that all the talk of the hyperloop going on is just silly. You don’t need high speed trains to go to Dodger Stadium.
If anybody wanted to build one, it should be transcontinental. Otherwise, what’s the point? It’s supposed to be a viable alternative (faster, cheaper, safer) to flying.
I envision it as being like the “subshuttles” of Gene Roddenberry’s pilot “Genesis II”. To wit:
An elaborate “Subshuttle” subterranean rapid transit system was constructed during the 1970s, due to the vulnerability of air transportation to attack. The Subshuttles utilized a magnetic levitation rail system. They operated inside vactrain tunnels and ran at hundreds of miles per hour. The tunnel network was comprehensive enough to cover the entire globe.
The two pilot movies never addressed what happens if the train breaks down three miles down and 2000 miles from the coast in the mid-Atlantic or mid-Pacific. I know I’d want to know before I’d ride it!