4 ms·

Yes, I'm aware of this post. Unfortunately, its author failed to discuss the solution from the Hyperloop paper. The author incorrectly assumed the tube is attac

by ash 10y ago

Yes, I'm aware of this post. Unfortunately, its author failed to discuss the solution from the Hyperloop paper. The author incorrectly assumed the tube is attached rigidly to the pillars and then proceeded to criticise this wrong assumption.

lisper 10y ago
You need to go back and re-read this paper more carefully. The author does discuss exactly the proposal in the paper. He even quotes it: "But the real problem isn’t with the slip joints at the ends; it’s with the pillar supports along the way. Musk thinks the dampers, “tucked away inside each pylon,” need to accommodate only “the small length changes between pylons.” Wrong." In other words, the original hyperloop white paper assumes that the movement at each pylon is small, and that the only large-scale movement is at the ends. But this is wrong. There is large-scale movement throughout the system. The thermal expansion motion reaches a maximum at the ends, but it doesn't magically go away as soon as you leave the station. If the tube at the station is moving 100m then the tube near the station is moving 100 meters minus epsilon.
- ash 10y ago
  This quote is from the first part of the paper for which Musk "apologize[s] in advance for my loose use of language and imperfect analogies". It's just an intro. The later parts of Hyperloop paper are more precise. I've already quoted "longitudinal slip for thermal expansion" from section 4.2.3. And, unfortunately, leancrew.com article author missed the proposed solution for the problem they spent many words talking about. > If the tube at the station is moving 100m then the tube near the station is moving 100 meters minus epsilon. Yes, of course. I agree that the problem of 100m thermal expansion is real and needs to be solved. And I'd like some arguments about why won't Hyperloop "longitudinal slip" solution work (where tube could freely move past the pillars).
  - lisper 10y ago
    Would you please post a copy of the text that you consider to be "the proposed solution for the problem" that you think leancrew missed? Because I just re-read all of 4.2.3 and I don't see it.
    - ash 10y ago
      I've already quoted it in my very first comment in this thread. You could argue that writing "pillars which constrain the tube in the vertical direction but allow longitudinal slip for thermal expansion" and "slip joints at stations…" is not detailed enough. I've concluded this "longitudinal slip" means some kind of linear bearing on top of each pillar, where straight tube could slowly move without constraints in the direction of the cumulative thermal expansion, while being constrained vertically. v v ------------- tube ------------- ^ ^ | | | | After thermal expansion: v v ------------- tube ------------- ^ ^ | | | | (Note how "tube" sign written on the tube has moved past two pillars.) Leancrew author missed the "longitudinal slip" part, because most of the article proves that thermal expansion accumulates. Of course it does! Thus the "longitudinal slip" solution. Yes, Hyperloop paper could be more detailed and more clear. Indeed, it confused leancrew author. Do you anyone knowledgable (in mechanical engineering?) who could comment on "longitudinal slip" solution? How feasible is it?
      - lisper 10y ago
        > You could argue that writing ... is not detailed enough. Indeed. > How feasible is it? Not even remotely feasible. You would need to have bearings at each column. Friction slip joints would introduce way too much stress. Remember: the tube is not only moving hundreds of meters at the ends, it's moving >>1m along (very nearly) its entire length.
        ash 10y ago
        > Not even remotely feasible. You would need to have bearings at each column. Friction slip joints would introduce way too much stress. That's the kind of discussion I'd love to have! Of course, you need linear bearings [1] at each column. But what kind of bearings? Ball or roller bearings? [2] Plain/solid bearings? [3] Something else? What are the forces the tube would produce while expanding? What would be the linear expansion speed? What bearings would be able to sustain those speeds and forces? Would the tube and the pillars be able to sustain them? Do bearings with necessary parameters even exist? How expensive would they be? So on and so on. Unfortunately, I haven't seen such discussion yet. I could try doing the calculations myself (starting at [4], for example). But I'm afraid I would make some silly newbie mistake in the process. > Remember: the tube is not only moving hundreds of meters at the ends, it's moving >>1m along (very nearly) its entire length. Yes, of course the tube is moving by hundreds of meters along most of its length. The questions are: how fast? with what load against the pillar? [1]: https://en.wikipedia.org/wiki/Linear-motion_bearing https://en.wikipedia.org/wiki/Linear-motion_bearing [2]: https://en.wikipedia.org/wiki/Ball_bearing https://en.wikipedia.org/wiki/Ball_bearing [3]: https://en.wikipedia.org/wiki/Plain_bearing https://en.wikipedia.org/wiki/Plain_bearing [4]: http://www.pbclinear.com/Blog/Calculating-PV-for-a-Plain-Bearing-in-3-Steps http://www.pbclinear.com/Blog/Calculating-PV-for-a-Plain-Bea...

by ash 10y ago

lisper 10y ago

You need to go back and re-read this paper more carefully. The author does discuss exactly the proposal in the paper. He even quotes it: "But the real problem isn’t with the slip joints at the ends; it’s with the pillar supports along the way. Musk thinks the dampers, “tucked away inside each pylon,” need to accommodate only “the small length changes between pylons.” Wrong." In other words, the original hyperloop white paper assumes that the movement at each pylon is small, and that the only large-scale movement is at the ends. But this is wrong. There is large-scale movement throughout the system. The thermal expansion motion reaches a maximum at the ends, but it doesn't magically go away as soon as you leave the station. If the tube at the station is moving 100m then the tube near the station is moving 100 meters minus epsilon.

ash 10y ago
This quote is from the first part of the paper for which Musk "apologize[s] in advance for my loose use of language and imperfect analogies". It's just an intro. The later parts of Hyperloop paper are more precise. I've already quoted "longitudinal slip for thermal expansion" from section 4.2.3. And, unfortunately, leancrew.com article author missed the proposed solution for the problem they spent many words talking about. > If the tube at the station is moving 100m then the tube near the station is moving 100 meters minus epsilon. Yes, of course. I agree that the problem of 100m thermal expansion is real and needs to be solved. And I'd like some arguments about why won't Hyperloop "longitudinal slip" solution work (where tube could freely move past the pillars).
- lisper 10y ago
  Would you please post a copy of the text that you consider to be "the proposed solution for the problem" that you think leancrew missed? Because I just re-read all of 4.2.3 and I don't see it.
  - ash 10y ago
    I've already quoted it in my very first comment in this thread. You could argue that writing "pillars which constrain the tube in the vertical direction but allow longitudinal slip for thermal expansion" and "slip joints at stations…" is not detailed enough. I've concluded this "longitudinal slip" means some kind of linear bearing on top of each pillar, where straight tube could slowly move without constraints in the direction of the cumulative thermal expansion, while being constrained vertically. v v ------------- tube ------------- ^ ^ | | | | After thermal expansion: v v ------------- tube ------------- ^ ^ | | | | (Note how "tube" sign written on the tube has moved past two pillars.) Leancrew author missed the "longitudinal slip" part, because most of the article proves that thermal expansion accumulates. Of course it does! Thus the "longitudinal slip" solution. Yes, Hyperloop paper could be more detailed and more clear. Indeed, it confused leancrew author. Do you anyone knowledgable (in mechanical engineering?) who could comment on "longitudinal slip" solution? How feasible is it?
    - lisper 10y ago
      > You could argue that writing ... is not detailed enough. Indeed. > How feasible is it? Not even remotely feasible. You would need to have bearings at each column. Friction slip joints would introduce way too much stress. Remember: the tube is not only moving hundreds of meters at the ends, it's moving >>1m along (very nearly) its entire length.
      - ash 10y ago
        > Not even remotely feasible. You would need to have bearings at each column. Friction slip joints would introduce way too much stress. That's the kind of discussion I'd love to have! Of course, you need linear bearings [1] at each column. But what kind of bearings? Ball or roller bearings? [2] Plain/solid bearings? [3] Something else? What are the forces the tube would produce while expanding? What would be the linear expansion speed? What bearings would be able to sustain those speeds and forces? Would the tube and the pillars be able to sustain them? Do bearings with necessary parameters even exist? How expensive would they be? So on and so on. Unfortunately, I haven't seen such discussion yet. I could try doing the calculations myself (starting at [4], for example). But I'm afraid I would make some silly newbie mistake in the process. > Remember: the tube is not only moving hundreds of meters at the ends, it's moving >>1m along (very nearly) its entire length. Yes, of course the tube is moving by hundreds of meters along most of its length. The questions are: how fast? with what load against the pillar? [1]: https://en.wikipedia.org/wiki/Linear-motion_bearing https://en.wikipedia.org/wiki/Linear-motion_bearing [2]: https://en.wikipedia.org/wiki/Ball_bearing https://en.wikipedia.org/wiki/Ball_bearing [3]: https://en.wikipedia.org/wiki/Plain_bearing https://en.wikipedia.org/wiki/Plain_bearing [4]: http://www.pbclinear.com/Blog/Calculating-PV-for-a-Plain-Bearing-in-3-Steps http://www.pbclinear.com/Blog/Calculating-PV-for-a-Plain-Bea...