And my response is the same: See those sections.
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.
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.
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/
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.
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
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.
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.
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.
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.
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.
Best of luck to you. Keep making awesome things.
What else do you want? A blueprint? A cad design?
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.)
Classic case of personality clash! Don't be bummed out.
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.
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.
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'?
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.
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 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.
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...
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...
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.
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.
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
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.
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.
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.