7 ms·

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

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

vidarh 12y ago
And my response is the same: See those sections.
- sillysaurus3 12y ago
  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 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 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 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.
  - jacquesm 12y ago
    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.
    - sillysaurus3 12y ago
      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 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/ http://demonstrations.wolfram.com/PipelineExpansionLoop/
deleted 12y ago
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vidarh 12y ago

And my response is the same: See those sections.

sillysaurus3 12y ago
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 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 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 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.
- jacquesm 12y ago
  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.
  - sillysaurus3 12y ago
    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 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/ http://demonstrations.wolfram.com/PipelineExpansionLoop/

deleted 12y ago

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