I realize I'm probably an idiot, but I do try to learn from my idiocy. Thanks for helping me learn. But I'm having trouble understanding how your reply takes everything into account, such as the fact that the hyperloop moves pods, not liquid. Pipelines for liquid are certainly a solved problem, but a pipeline of pods is new.
Btw, in railroads (that other long distance transportation method) the expansion is dealt with by putting a small gap between two adjacent rails. That's obviously not possible with the hyper loop as it is currently envisioned but I think that the answer is in there: don't allow the differences to accumulate over long stretches.
If that's the case, then why does the plan assume each station on the endpoints is going to absorb most of the thermal expansion? Or am I misreading? They specifically say "the stations are the ideal points to absorb thermal expansion," and the stations are on the end, so it sounds like they're aiming for a long sleeveless steel tube.
Also, I was basing my criticism on Hyperloop Alpha. Obviously, their design can change in the future. But, as presented, what gives you the impression that sleeves could be incorporated into their design without causing problems for the pods?
I can visualize sleeves which expand into each other, and if they overlap into the direction of travel, so that the edges are pointing away from the pods rather than toward the pods, maybe that wouldn't cause any problems.
The Hyperloop travel journey will feel very smooth since the capsule will be
guided directly on the inner surface of the tube via the use of air bearings and
suspension; this also prevents the need for costly tracks. The capsule will bank
off the walls and include a control system for smooth returns to nominal
capsule location from banking as well. Some specific sections of the tube will
incorporate the stationary motor element (stator) which will locally guide and
accelerate (or decelerate) the capsule. More details are available for the
propulsion system in section 4.3. Between linear motor stations, the capsule
will glide with little drag via air bearings.
Sleeves would cause an edge to sort of "protrude outwards" into the path of the pod, wouldn't they? So it'd basically be a small speedbump. Again, the edge is pointed in the direction of travel, so that would basically be like the second half of a speedbump, basically a small "drop". Except since the pod is moving at 760mph, even a small drop can result in quite a lot of force, and can cause wear on the pod itself, couldn't it? And it might feel a little jarring to the passenger.
If the interior needs to remain smooth, what sort of design could fulfill that requirement? (Maybe that's a big "if", but if the pod is going to bank off the walls, then it sounds like that's the case.)
EDIT: Also, as someone else pointed out, pipelines that need to be straight might be a new concept. Many pipelines seem to use "U" bends, which can't be incorporated into hyperloop for obvious reasons: http://demonstrations.wolfram.com/PipelineExpansionLoop/
You could simply slide one tube into the next and leave the 'gap' between the tubes sit inside the sliding portion. That way you could have ridiculously close clearance to the tube except for that tiny bit (a few cm) every hundred meters or so. Just anchor one end of each tube and let the other slide freely.
FWIW, Musk makes no mention of expansion joints in the tube, only at the ends. He's also stated elsewhere (sorry, no source) that you'll probably need to run a polishing robot through the tube to get it microscopically smooth. That doesn't sound like a surfacing requirement that would tolerate gaps.
If you care to, check out the conversation that sillysaurus3 and I had down-thread. I would love to have someone experienced in MechE glance at my napkin math!
TL;DR it looks like simply pushing all the slack to the end stations is actually feasible, and it's easier to deal with there since the cars aren't moving very fast.
It might be, but if you did want to use expansion joints to alleviate some or all of the problem you most likely could.
There are other alternatives too, such as allowing the tubing to move sideways a bit in a long curve. That would change the overall shape of the trajectory but the air cushion they talk about could take care of that.
If they do end up using expansion joints they could actually use the fact that some air might leak in to purposefully increase local air density to use that as an impromptu air bearing.
Anyway, there is plenty of challenging engineering in hyper loop, but the least interesting part is how to make a long metal tube, either in one piece or out of segments.
And of course even more challenging than the engineering will be the economics and the paper tigers.
Right-of-way issues, liability. America loves its paperwork and setting up a new very large chunk of unproven infrastructure is going to be a total headache from a paper point of view.
Hyperloop has all the drawbacks of high speed rail and has unproven tech on top of that. It'll be a legal feeding frenzy.
Each pod weighs 7,700 lb and is traveling at 760 mph. Are you sure a few-centimeter speedbump isn't a big deal? I don't know how to calculate whether or not it would be, but I'm interested in knowing why you feel it won't cause problems.
Remember, the pod comes into direct contact with the wall while banking, so it's going to take the full force of any few-centimeter bump.
> Are you sure a few-centimeter speedbump isn't a big deal?
Depends on lots of things, such as the angle of incidence of the pod relative to the direction of the wall of the tube.
> I don't know how to calculate whether or not it would be, but I'm interested in knowing why you feel it won't cause problems.
I don't know how to calculate rockets landing on barges either, but SpaceX seems to be able to pull it off. There is nothing in the realm of engineering that says you can't make a bunch of tubes sliding over each other relative to the direction of travel for some vehicle to travel within that tube as long as you respect a couple of basics. One simply solution would be something like this:
seg2----- --seg1
----- ----------
0000000
Where the '0' are the elements of the pod in contact with the wall (I would assume they're using rollers there, anything solid would wear out the wall in no time at all).
That way there is no speedbump at all... It's not as if you're going to do right angle turns at the joint of two segments.
Anyway, I really think you're chasing a ghost here, the problems with hyperloop are not basic engineering.
That solution is interesting, but does it account for the fact that the tube curves all throughout its route? Some parts need to be curvy, some need to be straight. Are you sure that expansion can be coaxed into a curve shape rather than linear? How?
Also, curves still result in a speedbump with that solution. The pod will be traveling directly toward the gap just below "seg2" if it's banking against the wall. You could make the edge a sharp edge, like a sword's edge, but that doesn't help because due to expansion that edge will still protrude out into the path of the pod.
I know it seems like it shouldn't be a hard problem, but it's looking like one. Remember, 300 meters of expansion over 350 miles is 0.85m of expansion per mile. Hold out your arms to the length of a meter and picture how much steel that is. That's what your proposed joints need to deal with: 0.05cm of expansion per meter. So if you have your joints every 20 meters, that's still a 1cm gap, which means a speedbump around curves. Wouldn't that make curves very unpleasant for the passenger and the pod? And you can't have them every meter without increasing the engineering complexity and weakening the structural integrity, right? Yet whatever solution they come up with would need to be placed less than every 20 meters, otherwise they'll get a 1cm gap.
I have a feeling that no matter how many times I'm going to answer your 'but this is impossible' with showing you how in fact it is possible you're not going to concede that in fact it probably is possible nor are you going to stop so in the interest of conserving bandwidth I'll bow out of the conversation here with this final bit.
The solution for the above is that you need to take into account the radius of curvature and you're clearly not doing that. The inside of that tube needs to be relatively smooth in the direction of travel, not perpendicular to it. You could also interleave to two segments if you wanted it to be even smoother.
So no, this is not a hard problem in spite of your continued insistence that it is because you can't solve it.
If you don't have time, fine. If you don't want to talk, fine. But I have some strong words which, out of respect for HN's guidelines, I won't say. Being dismissive of a newbie's concerns while killing their interest in the field isn't great, especially after they've put hard work into giving you a clear-cut example of the technical reasons why something looks hard to them.
And I still don't understand how your solution wouldn't leak air into the hyperloop unless the steel sleeves were pressing very firmly into each other. Like the inner sleeve forcefully pressing into the outer sleeve.
I have no idea why you refuse to admit this is at the very least a nuanced problem.
> If you don't have time, fine. If you don't want to talk, fine.
I'm not on your payroll and you're stuck in a groove.
> But I have some strong words which, out of respect for HN's guidelines, I won't say.
Feel free, I'm not made of sugar. But you're arguing from a position that makes no sense to me, you're apparently entirely ignorant of the basics of mechanical engineering and you're going to tell the likes of Elon Musk and his band of merry engineers what they can or can not do. They just came within a hair of planting a friggin' rocket on a barge from space. Do you really believe they couldn't get a bunch of tubes to slide inside each other without overshooting their air intake budget? That's mostly a factor of 'how big are the pumps' and 'how many such sliding spots do we need' (a few hundred would do just fine) and how much room there is for additional seals (plenty).
> And I still don't understand how your solution wouldn't leak air into the hyperloop unless the steel sleeves were pressing very firmly into each other. Like the inner sleeve forcefully pressing into the outer sleeve.
No, you don't need that you can use regular seals for that, just like we do on high pressure air connections, fun fact, they work better when the difference is higher, go figure.
> I have no idea why you refuse to admit this is at the very least a nuanced problem.
Because it is a nuanced problem for you, but not for anybody that has the wherewithal to be hired on the engineering parts of a project like this. Just like my little brother would find assembly language a 'nuanced problem' and I find it relatively easy. It all depends on whether or not you're doing this (mechanical engineering) on a daily basis or whether you want to be back-seat driving nay sayer. Once you've decided on the latter any solution will be inadequate.
I don't know if you're having a bad day or have gotten into a strange mindset, but I just never expected this level of "personally insulting" out of Jacques Mattheij, especially against someone who freely admitted at the very top of the thread that I'm an idiot who is arguing from a position of ignorance. So I'll go try to find someone else to geek out with an interested newbie. Like, say, someone who finds engineering enjoyable.
You're not being insulted, you merely are trying to argue about things that you admit that you have no clue about and you persist in doing so regardless of the effort made to show you that you are wrong. So either stop arguing (I do know a thing or two about mechanical engineering) or start researching what I gave you above as the bits and pieces of the solution.
To have an enjoyable conversation with an interested colleague, rather than someone whose tone is "shut up and listen to your elders." Besides, having a crotchety back-and-forth makes for boring reading, and we should be respectful of the community.
Thanks for the pieces you've provided so far. I'll bring up my objections elsewhere. Hopefully anyone who's been having my same concerns will be able to do the same.
You argue that something is impossible but at the same time you outline your ignorance. The normal position for being ignorant is to study the subject, not to argue that something is impossible.
It's like saying a moonshot never happened because you couldn't do it yourself. There are lots of things that each of us individually can not do, but collectively we can do amazingly complex things. The specialization required takes time and effort. If you're unwilling to put in the time and/or make the effort then you should not take up positions that tell those that did make that effort what they should not be able to do according to you.
If mechanical engineering interests you then maybe you should go and study the subject rather than to take up strong positions like these and to be rigid in your inability to shift on those positions.
> I'll bring up my objections elsewhere. Hopefully anyone who's been having my same concerns will be able to do the same.
Basically that says that you want to argue, not learn and that you'll only take what agrees with your viewpoint.
But no, actually, you smashing huge holes into my logic was exactly what I was hoping for, because that's how ignorance is transcended. I dislike both the peanut gallery and peanuts, so I think I came across wrong. Starting the conversation by vaguely disparaging your merry band of engineers probably wasn't great, but my mind's malleability is play-dough, definitely not steel.
Finding someone who agrees with my viewpoint is the most useless thing, both to me and to anyone that might be reading the conversation. Someone with an ability to point out why I'm wrong and to actually debate with me is both rare and valuable.
Please try to understand that some people find the basics interesting, so even though you're saying you don't, it's important to kindle that spark. (I still don't see how this is basic, since the entire SpaceX engineering team somehow missed your segmentation approach, or considered it and chose not to pursue it for some reason, but you're probably right. And I have other qualms about the approach, but I don't really trust re-opening this debate without some kind of mutual understanding that we'll focus on the ideas in the debate, not on the people involved.)
Btw, in railroads (that other long distance transportation method) the expansion is dealt with by putting a small gap between two adjacent rails. That's obviously not possible with the hyper loop as it is currently envisioned but I think that the answer is in there: don't allow the differences to accumulate over long stretches.
If that's the case, then why does the plan assume each station on the endpoints is going to absorb most of the thermal expansion? Or am I misreading? They specifically say "the stations are the ideal points to absorb thermal expansion," and the stations are on the end, so it sounds like they're aiming for a long sleeveless steel tube.
Also, I was basing my criticism on Hyperloop Alpha. Obviously, their design can change in the future. But, as presented, what gives you the impression that sleeves could be incorporated into their design without causing problems for the pods?
I can visualize sleeves which expand into each other, and if they overlap into the direction of travel, so that the edges are pointing away from the pods rather than toward the pods, maybe that wouldn't cause any problems.
Still, the interior margin from pod to wall is very small: http://i.imgur.com/YsLDXBe.png
Also, consider this paragraph:
The Hyperloop travel journey will feel very smooth since the capsule will be guided directly on the inner surface of the tube via the use of air bearings and suspension; this also prevents the need for costly tracks. The capsule will bank off the walls and include a control system for smooth returns to nominal capsule location from banking as well. Some specific sections of the tube will incorporate the stationary motor element (stator) which will locally guide and accelerate (or decelerate) the capsule. More details are available for the propulsion system in section 4.3. Between linear motor stations, the capsule will glide with little drag via air bearings.
Sleeves would cause an edge to sort of "protrude outwards" into the path of the pod, wouldn't they? So it'd basically be a small speedbump. Again, the edge is pointed in the direction of travel, so that would basically be like the second half of a speedbump, basically a small "drop". Except since the pod is moving at 760mph, even a small drop can result in quite a lot of force, and can cause wear on the pod itself, couldn't it? And it might feel a little jarring to the passenger.
If the interior needs to remain smooth, what sort of design could fulfill that requirement? (Maybe that's a big "if", but if the pod is going to bank off the walls, then it sounds like that's the case.)
EDIT: Also, as someone else pointed out, pipelines that need to be straight might be a new concept. Many pipelines seem to use "U" bends, which can't be incorporated into hyperloop for obvious reasons: http://demonstrations.wolfram.com/PipelineExpansionLoop/