> The steel expands in summer due to heat. Where does it go? Roads can expand using those interlocking teeth things, but hyperloop has to remain a closed system.

Feel free to disagree with whether or not the approach outlined is feasible, but this is specifically covered in the whitepaper. See page 5, section "Earthquakes and expansion joints" as well as page 28, section 4.2.3 "Pylons and tunnels".

To clarify, tlb's comment was in direct response to the Hyperloop Alpha whitepaper: https://news.ycombinator.com/item?id=6201728#up_6202327

He doesn't properly address thermal expansion of the entire system. A long straight steel tube from SF to LA will expand a total of 300 meters over a 40C temperature change. Where does it go?

And my response is the same: See those sections.

Sure, let's go over it together. Tlb is many things, but "rarely wrong" is probably near the top of the list. He certainly read those sections before making that comment, so there must be something here which made him as an engineering expert say that it doesn't properly address thermal expansion. I'll try to run an emulator of tlb's mind, but mine is far less sharp.

The tube will be supported by pillars which constrain the tube in the vertical direction but allow longitudinal slip for thermal expansion as well as dampened lateral slip to reduce the risk posed by earthquakes. In addition, the pillar to tube connection nominal position will be adjustable vertically and laterally to ensure proper alignment despite possible ground settling. These minimally constrained pillars to tube joints will also allow a smoother ride. Specially designed slip joints at stations will be able to take any tube length variance due to thermal expansion. This is an ideal location for the thermal expansion joints as the speed is much lower nearby the stations. It thus allows the tube to be smooth and welded along the high speed gliding middle section.

This seems like the key sentence:

Specially designed slip joints at stations will be able to take any tube length variance due to thermal expansion.

There are two stations, one in SF and one in LA: http://i.imgur.com/3TavjCY.png

(For simplicity, let's assume it's just the main route.)

That would mean each station needs to absorb 150 meters of thermal expansion. (Would anyone mind double-checking that tlb's calculation is correct?)

So it looks like the tube is resting on these things: http://i.imgur.com/srQjmps.png

and the plan is for the tube to expand due to heat, and the expansion will be absorbed at the two stations at the end.

Is that really feasible, especially given that different points along the route could have temperature differences? I drove along Highway 1 and saw how incredibly different the weather can get, and while that's south of where the route is planned, it's easy to imagine that part of the tube could be roasting in direct sunlight while the other is cooled by rain. Something like 90 degrees F at one part vs 70F at another part. That's a difference of 32.22C to 21.11C, or a total temperature difference of 11.11C.

The route from SF to LA is about 350 miles. If a tube of that length expands 300 meters, then we can divide 300 meters by 350 miles to get the total expansion over a single mile. So a variation of 40C would give an expansion of 0.85 meters within a single mile.

Since the variation in that scenario would be ~11C, multiplying the expansion by 11C/40C equals an expansion of 0.235 meters.

So I suppose the question is: Is it feasible for those things the tube is resting on to absorb a quarter meter of expansion without that expansion being pushed all the way up to the station? It's the temperature differential along the route that seems to matter, not necessarily the difference from summer to winter.

Judging by this screenshot: http://i.imgur.com/wgUJqWr.png

... it seems like an expansion of 0.25 meters might be a big deal.

But I don't know what I'm talking about. Is all of that about right so far?

I'd also be curious whether the thermal expansion results in the tube becoming bigger in the circular direction, like a bigger "O", or if it expands parallel to the tube, like becoming a longer tube. Or even both.

It seems like the plan addresses this with the first part of that paragraph:

The tube will be supported by pillars which constrain the tube in the vertical direction but allow longitudinal slip for thermal expansion as well as dampened lateral slip to reduce the risk posed by earthquakes. In addition, the pillar to tube connection nominal position will be adjustable vertically and laterally to ensure proper alignment despite possible ground settling.

But again, where precisely does 0.25m of steel go? I'm having trouble visualizing how the pillars could "allow for 0.25m of longitudinal slip over 1 mile."

I realize that a temperature variation of about 10C over 1 mile is a little extreme, but we should examine how the system handles extreme situations.

You are obviously not used to dealing with thermal expansion in other designs, but this has been done time and again before so from an engineering point of view this is mostly a solved problem and not a 'big deal'.

For instance, you could have periodic sleeves that can slide over one another so you don't end up with the expansion on the final endpoints accumulated, the trajectory could be adjusted to allow for translation of some end-to-end expansion into a sideways expansion (much like what you see in long runs of heating tubing where they throw in the occasional U or loop to make sure the tubing stays put on the longer stretches) and so on.

There are a lot of really hard technical challenges involved in the hyper loop concept, I'm not sure why you'd focus on the thermal expansion as the major one, it's trivial in comparison to some of the others.

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. We already have plenty of long pipelines, this one is just a big bigger in circumference.

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.

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/

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.

>allowing the tubing to move sideways a bit in a long curve

I hadn't considered this strategy! Using the curves as a geometric asset instead of a liability makes a lot of sense.

What do you mean by the 'paper tigers'?

> What do you mean by 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.

http://jacquesmattheij.com/you+can+not+do+that

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.

Best of luck to you. Keep making awesome things.

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.

What else do you want? A blueprint? A cad design?

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.

Let me try to explain what is going wrong here:

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.

Yer mother's a landlubber and yer father smells faintly of laundry detergent. (Am I doing https://news.ycombinator.com/item?id=8640415 right?)

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.)

For what it's worth, I found the exchange to be highly enlightening.

Classic case of personality clash! Don't be bummed out.

>Is that really feasible, especially given that different points along the route could have temperature differences?

Temperature differences would tend to cancel out, no? If one section is warmer than average and one section is colder, the overall change would be smaller.

>the tube becoming bigger in the circular direction, like a bigger "O"

Yep, by about 2 mm. No problem there.

>But again, where precisely does 0.25m of steel go?

It slides all the way to the station. There trains are moving slowly, so they could add rails to the "expansion section" between the tube and the airlock. The stationary rails slide along the inside of the tube (no seal needed). Wherever the tube ends, the pod is already on the rails!

Am I making any sense here?

Can't reply to sillysaurus3 directly (https://news.ycombinator.com/item?id=8902595), so I'm replying here:

You make a great point! I didn't quite grok it before.

>Can that expansion really be pushed 175 miles north and 175 miles south simultaneously?

>Surely at some point the weight of the tube overcomes the steel's desire to expand, so the result would be the tube either scrunching up like an accordion or going in the lateral direction.

Agreed. I have a feeling some sort of active control (basically just motorized rollers) will be needed every couple of miles to prevent "sun kinks". I haven't done the math on that though. It might be sufficient that a large hollow tube shape is much more resistant to buckling than a rail shape.

"Sun kinks": http://www.climatecentral.org/news/climate-change-warp-railr...

The trick is to click on "link" next to the comment and then write a reply. I try not to reveal that or to use it during a heated debate, since the reason the "reply" link is hidden is to discourage flamewars, but it's obviously not intended to hamper tech talk. And I wish HN had a private messaging system so that I could've mentioned that in private.

For a sense of scale, 60 mph travels one mile in 60 seconds, or 0.016 miles per second. Hyperloop goes 760 mph, which is 0.211 miles per second. (Jeez, that's fast.) So it only takes about 10 seconds for the car to traverse 2 miles, or in other words to traverse whatever effect such a motorized roller would have on the path of the vehicle. Like, if the roller causes the track to move "outwards," for example toward the ocean, by a little bit, then the pod is going to move "a little bit toward the ocean and then back" in 10 seconds.

That seems like it might be reasonable. But I suppose the question is whether a 7,700 lb pod travelling at 760 mph can tolerate, say, a 2-meter deviation in its trajectory in 10 seconds. Meaning, one meter toward the ocean, and then one meter back towards its original course. Can the pod bank against the walls by a displacement of 0.2m per second? If you hold out your arms to the length of a meter, 0.2m is pretty small, but it's not too small. And the pod would be going sideways by that amount per second.

1-meter deviation due to thermal expansion is extreme. 0.25m per mile might be a more realistic deviation. So that would be a 1-meter deviation total: 0.5m toward the ocean, then 0.5m back, which would still mean the pod needs to scrape against the inner wall by 0.1m per second.

I wonder what the limits on the banking are...

Ahh, thanks for the tip, and the wonderful discussion!

I agree, that would not work. I was suggesting that motorized rollers move the track longitudinally instead of sideways. Essentially they would take up the slack and actively push it to the end stations, counteracting the friction from the passive rollers. You could use thermometers and position sensors to model the stress on the tube.

Whoops, I see. Sorry for the confusion!

That's an interesting idea, but are you sure that thermal expansion imparts enough force against 175 miles of steel in order to push it all the way to the station? 175 miles of steel would be incredibly heavy. It seems like the heating/cooling might even cause the steel to weaken enough such that, rather than rolling the tube towards the station, it would start to scrunch like an accordion. But I wish a structural physicist would step in with some calculations to prove whether or not 175 miles of steel on rollers could be pushed by thermal expansion without compromising hyperloop's structural stability.

Maybe we could ask /r/askscience?

EDIT: After re-reading your comment, I think you're saying the rollers would actually provide the force necessary to move 175 miles of steel, rather than letting the expansion passively push it. But those rollers would have to grip onto something in order to do that. They wouldn't be rollers so much as tank treads.

Hmm... Do you think that might work? Could 175 electric motors working in unison provide enough force to tank treads that bite into the concrete pillar below, and into the steel tube above, in order to successfully shift 175 miles of steel? And the system has to have temperature sensors every mile, and those sensors must not ever report incorrect readings or else the whole tube breaks. And those motors must never break down or run out of power...

I'm sounding pessimistic, but honestly I have no idea whether that would work.

EDIT 2: Ah, I think I've figured out why it can't work. It's not a straight tube. It bends all along the route. http://i.imgur.com/3TavjCY.png

So motorized rollers would actually need to bend the tube in order to successfully transmit the thermal expansion. In other words, when a piece of steel tubing is initially created, it's bent with a certain curvature. But if rollers are to transmit the expansion all the way to the station, the exact location of where the steel is bent needs to be dynamically defined at runtime, rather than statically assigned at compile time. Did I phrase that very clearly? It would help to be in-person, or maybe I could draw a picture if needed.

It doesn't seem like rollers could dynamically bend the steel all along the 175 mile track in order to carry the expansion.

>are you sure that thermal expansion imparts enough force against 175 miles of steel in order to push it all the way to the station?

Nope. That's why you may need to push it! :)

Assume the rollers have the Crr of a passenger train wheel (0.0020). If each section weighs 193 tonnes (23 mm thick by 2.23m diameter by 100' by 8 g/cm³), then each roller will provide ~400 N of friction. Over 175 miles (worst case) that works out to 3.56 meganewtons of resistance to motion.

For a thin-walled cylinder, buckling stress is modeled using the large deflection approximation[1] as:

σ_x = 0.238 Et/r

Where σ_x is the bucling pressure, E is the material's Young's modulus, t is the wall thickness, and r is the radius. Plugging in 190 GPa, 20 mm, and 1.125 m yields…

58 meganewtons, or about 16 times as strong as the predicted frictional force

I guess they may not need active control after all!

But what about curves? Won't it result in a lot of sideways force? Needs more calculations…

[1] The weaker of the two. http://www.dtic.mil/dtic/tr/fulltext/u2/a801283.pdf

Sweet calculations! Thanks! Check out my above comment again, since I was apparently writing out an "EDIT 2" while you were doing these awesome calculations.

Basically, what do you think about the fact that it's not a long, straight tube? The rollers would need to dynamically bend the steel tube in order to carry the expansion all the way to the station, wouldn't they? Is that possible?

Even if there aren't any rollers, the tube still needs to re-bend itself somehow. But how? That would mean steel curves near the endpoints would move a whopping 150 meters in total. So it seems like part of the tube manufactured as a curve in summer would need to become a straight length of pipe by winter.

Heh, I love that we thought of two different but related problems simultaneously!

First yours, since it's much cooler. The minimum curve radius is 3.67 km, or a length difference of 18 mm between the left and right side over the length of one pylon.

Assuming that all sections start as straight tubes, how much sideways force would that apply on the pylon? For that we need to calculate the "flexural stiffness"[1], which for a hollow cylinder is given as:

EI = E pi/4 (D⁴ - d⁴)

Where E is Young's modulus, and D and d are the outer and inner diameter.

Therefore the bending moment[2] M is:

M = EIκ = E pi/4 (D⁴ - d⁴)κ

Where κ is the curvature, equal to 1/r.

Finally, muddling through the equations on the bending moment article (and treating the tube as a 200' beam with a load in the center) yields:

f = M / (100')²/(200') [not sure about this one]

A whopping 300 tonnes•f of lateral force on the pylons! Probably doable, but it means if your tubes are constructed straight your pylons are going to be buttressed at a ~60° angle. Also, note that this only applies to the slip distance (~150m, or three pylons), since the tube sections well within the curve will never move enough to be in a non-curved section of track, so they can be built curved from the beginning.

Also you can vary the curvature smoothly[3], which can substantially reduce the forces involved. For example, if you eased into the curve over 30 pylons and it only moved by 3, the sideways force would be reduced by 10x (since the curvature is always "about right"). Further calculations here are left as an exercise to the interested reader. :D

As for my own objection… Even on the sharpest turns successive pylons will only vary from straight line by 0.15°, so the lateral forces caused by expansion and contraction shouldn't exceed 9 tonnes force. Piddly by comparison.

[1] http://www.explorelifeonearth.org/cursos/CB-18%20simple%20st...

[2] https://en.wikipedia.org/wiki/Bending_moment#Example

[3] i.e. the curve doesn't start suddenly, but gradually; this is how actual tracks/roads are laid out. https://en.wikipedia.org/wiki/Euler_spiral

TL;DR as long as you ease into the curves (which is good for comfort anyway), it looks totally doable even without motorized rollers.

Do you have an email address we could continue this at? I'm going to need a little while to get back into a happy mindset after a rather unfavorable exchange upthread. I'm really looking forward to picking your brain though. Thanks for chatting with me.

Sent. Thanks for the convo!

forgive my amateur and completely unscientific postulation here, but is it possible that this conversation is moot seeing as normal train tracks do not have to extend at normal train stations, despite the rails being steel and continuous?

Continuous steel rails do have problems with large temperature swings. See the link I posted about "sun kinks." They cause about 50 derailments per year. http://www.climatecentral.org/news/climate-change-warp-railr...

Rail lines have more curves which can take up that slack, and the positional tolerances are much more forgiving. Because of its high speed operation the hyperloop is both incredibly straight and incredibly sensitive to kinking.

That makes total sense, but I don't think it would work. The expansion doesn't really cancel out. Imagine heating 1 mile of hyperloop by 11C, but only 1 mile. The result is 0.25m more steel that has to go somewhere.

Can that expansion really be pushed 175 miles north and 175 miles south simultaneously?

I'm not going to say that's impossible, because I'd imagine someone having similar objections to, say, the invention of A/C current. "Can pushing electrons back and forth really propagate thousands of miles?"

But if you visualize the implications of that, it's intuitively difficult to believe the expansion can be transmitted to the endpoints across the entire length of a solid steel tube 175 miles away. That would move an incredible amount of mass. Surely at some point the weight of the tube overcomes the steel's desire to expand, so the result would be the tube either scrunching up like an accordion or going in the lateral direction.

(Jacques said sleeves could be the solution so that thermal expansion can't accumulate, but it seems like sleeves would introduce problems for the pod, possibly severe ones. When a pod travels over a sleeve, it would feel like a jolt, if I'm understanding his idea of "sleeves" correctly.)

I'm so happy someone is actually engaging with the comment and tossing ideas around with me though.

There was a 1970s proposal for an underground tunneled very-high-speed transport quite similar to the hyperloop. Partially-evacuated tubes, relatively small vehicles. Cost and other considerations torpedoed it.

Look up R.M. Salter at RAND:

"The Very High Speed Transit System" (August, 1972)

"Trans-Planetary Subway Systems -- A Burgeoning Capability" (February, 1978)

http://www.rand.org/pubs/papers/P4874.html http://www.rand.org/content/dam/rand/pubs/papers/2008/P4874....

http://www.rand.org/pubs/papers/P6092.html http://www.rand.org/content/dam/rand/pubs/papers/2009/P6092....

Similar / more: http://en.academic.ru/dic.nsf/enwiki/6107059

How about hedging your statement to "not CURRENTLY feasible using TODAY'S materials and structural concepts (trusses, etc)". Space flight and industrial production of fertilizer were both considered impossibilities when first proposed.

Sure, I'll reduce it to "Unless people make an advancement in structural materials similar to the invention of steel itself, which is to say 'fundamentally world-changing,' hyperloop is nothing more than an inspirational pipe dream, similar to world peace." (EDIT: This comment may actually get called out in the future once Hyperloop is fully deployed, as an example of some idiot's words. So it's probably more than a little dumb of me to have said this. In my defense, it's based on Hyperloop Alpha, as it's presented in the whitepaper. Their design could change radically before deployment.)

If you're planning on building a closed loop out of steel, and there's nowhere for the steel to go when it expands in summer, the plan may need some rethinking.

The best I can think of is some kind of inner lining, so that it has a segmented hull, like a caterpillar: https://www.google.com/search?q=caterpillar&tbm=isch

But it would need to be very strong lining in order to prevent any leaks between the segments. And any kind of a leak would need to be detectable and would shut down the hyperloop until it's resolved.

Between the wildly optimistic cost projections and the fundamental engineering difficulties, I'm much more inclined to call it a pipe dream than a perhaps-future-possibility.

See my comment above - the expansion is specifically addressed. If you're going to criticize it, at least read the full whitepaper first.

> But it would need to be very strong lining in order to prevent any leaks between the segments.

The entire point of not requiring vacuum is to avoid having nearly as strict tolerances for leaks.

It's a "reduced-pressure tube." A leak is still a big deal, isn't it?

Regardless, the core structural engineering is the focus of the criticism, not whether it might leak. Also, since Space X's whole point is to compete on cost savings, it was a little strange to hear cost projections that were so optimistic.

The entire purpose of the design is explicitly to keep the tolerance for leaks at a level where, to quote the whitepaper "standard commercial pumps could easily overcome an air leak and the transport pods could handle variable air density"

Section 4.5.4 otherwise covers this, and does point out that for large leaks, braking would be required, but smaller leaks are intended to be dealt with as part of regular maintenance, with no indication that it'd need to affect operations.

> Regardless, the core structural engineering is the focus of the criticism, not whether it might leak.

That may be so, but most of the criticism I've seen of it appears to just ignore large sections of the whitepaper.

EDIT: For example, with respect to handling expansion/contraction, consider that the world has gas and oil pipelines that has to deal with far worse conditions without leaks.

Gas and oil pipelines typically use expansion loops to handle thermal expansion/contraction by flexing the pipe: http://demonstrations.wolfram.com/PipelineExpansionLoop/

Because the hyperloop cars are discrete (as opposed to the continuous flow in a pipeline), I'd expect them to cause a lot of stress on the pipeline walls if they had to run around the bends in an expansion loop.

That's why Elon's plan is to let the endpoints move instead.

Humans have been engineering expansion joints for various types of continuous structures for well over a hundred years, including sealed pipelines.

The new design requirements and constraints posed by the Hyper Loop concept just increase the complexity of the problem. Certain engineers love this kind of challenge.

of course a profound technical concept will provoke questions regarding its technical feasibility.

so what is your point? that it's impossible? that it shouldn't be attempted? that it's not a problem worth working through?

"naysaying" doesn't mean "saying no". "naysaying" is the inability to see a little further into the future based on the knowledge we have today.

> "The steel expands in summer due to heat. Where does it go?"

a cliche example but- i'd love to be the anachronistic fly on the wall listening to all the "where will the hot metal go???" people talking to the Wright brothers.

Just let it bow up and down when it expands, and call it a rollercoaster. ;)

"Imagination is more important than knowledge." - Albert Einstein

Despite down votes, I still strongly believe the idea outweighs the blame for not thoroughly answering all technical questions.

And as others have already pointed out that the technical problem isn't as big as it sounds.

Also Elon Musk has announced to build a 5 mile test hyperloop track in Texas yesterday.