4 ms·

>Unless you're proposing we build and seal the Hyperloop tubes in space before bringing them down, the analogy isn't appropriate. I am not sure what your objec

>Unless you're proposing we build and seal the Hyperloop tubes in space before bringing them down, the analogy isn't appropriate. I am not sure what your objection is here. Are you saying vacuum chambers can't be built on Earth? There are quite a few examples of large vacuum chambers, like the NASA Space Power Facility (23,400 m^3) or the Large Hadron Collider (9,000 m^3) https://scap.hq.nasa.gov/docs/Glenn_SpacePower.pdf https://scap.hq.nasa.gov/docs/Glenn_SpacePower.pdf https://lhc-machine-outreach.web.cern.ch/lhc-machine-outreach/components/vacuum.htm https://lhc-machine-outreach.web.cern.ch/lhc-machine-outreac... Both are pumped down to very high vacuum, not required for the Musk Hyperloop, (low pressure electric jet in a tube) but probably needed for the Hyperloop One. (maglev vactrain) And, obviously, both chambers were built terrestrially, and evacuated using conventional vacuum pumps.

JumpCrisscross 9y ago
> I am not sure what your objection is here. Are you saying vacuum chambers can't be built on Earth? No, I specifically linked to the Space Power Facility in my original comment! The "materials science" problem I call out is in the tube's lateral thermal expansion. The best solution I've come up with for that is to take a regenerative rocket engine [1] and make it a tube. Pumping fluid in spirals along kilometers of a vacuum tube isn't easy, but it isn't as hard as trying to invent a material that won't deform when the top gets hotter than the bottom, or the east side gets hotter than the west. [1] https://en.wikipedia.org/wiki/Regenerative_cooling_(rocket) https://en.wikipedia.org/wiki/Regenerative_cooling_(rocket)
- hwillis 9y ago
  How about wrapping the tube in insulating foam and using peltier modules to regulate temperature. Or even using double walled tubes. It would not be challenging to regulate the temperature, but nobody talks about it because it doesn't even seem necessary.
  - JumpCrisscross 9y ago
    > How about wrapping the tube in insulating foam At that point why not just bury it? It swaps the thermal problem for the water problem, but we know how to build tunnels.
    - schiffern 9y ago
      >At that point why not just bury it? Because foam is cheaper? A nice K.I.S.S. solution. Having seemingly solved this thermal expansion problem "in the same category of problems separating us from a space elevator," I'll be expecting The Fountains of Paradise to come true soon. ;)
      - JumpCrisscross 9y ago
        > We seem to have solved this thermal expansion problem I guess I'll have to telegraph my buddies in Hawthorne and at NASA :). Joking aside, no, foam doesn't solve the problem. You'll still have flexing. This is one of the limits on how long you can have a launch tank on the pad. Imperceptibly small flexing, but of the kind that weakens every metal we know.
        schiffern 9y ago
        >Joking aside, no, foam doesn't solve the problem. You'll still have flexing. Of course the flexing isn't zero, but by adjusting the thickness of the foam you can drive the diurnal/solar heating thermal gradient arbitrarily low without compromising strength by making the steel walls thinner. That was your material science objection,[1] right? >but of the kind that weakens every metal we know. That's true, but misleading. It's true that every metal gets weaker, but in steel and titanium this weakening levels off (all other metals continue to get weaker until failure). Once these two metals are at the fatigue limit they stop weakening and have unlimited flexing cycles.[2] So you can set that as your final material strength and size the tube thickness accordingly. So repeated flexing is fine, and flexing can be made arbitrarily small with insulation. Foam solves the problem. [1] https://news.ycombinator.com/item?id=15460617 https://news.ycombinator.com/item?id=15460617 [2] https://en.wikipedia.org/wiki/Fatigue_limit https://en.wikipedia.org/wiki/Fatigue_limit
- sbierwagen 9y ago
  >The "materials science" problem I call out is in the tube's lateral thermal expansion. I admire your method of argumentation. The comment I replied to said nothing at all about thermal expansion. Your original comment, which I wasn't replying to, was mostly strange references to pressure ratios, and a single line about thermal problems, with no numbers cited. In that comment you did link to the Space Power Facility... as an argument against Hyperloop One! You did something similar downthread: https://news.ycombinator.com/item?id=15459777 https://news.ycombinator.com/item?id=15459777 But by asserting that you addressed the problem in your original comment, you make the person you're arguing with look like a bully, without having to actually address their points. Very efficient. >The best solution I've come up with for that is to take a regenerative rocket engine [1] and make it a tube. Pumping fluid in spirals along kilometers of a vacuum tube isn't easy, but it isn't as hard as trying to invent a material that won't deform when the top gets hotter than the bottom, or the east side gets hotter than the west. Using the phrase "regenerative cooling" in this context is another headscratcher. Regenerative cooling in rocketry is running propellant through channels in the nozzle, then either dumping it overboard, using it to power a gas generator, or burning it in the rocket. It's a great way to get rid of megawatts of heat. None of these things would be useful for a hyperloop tube? You don't want to run kerosene/liquid oxygen/hydrazine/whatever through a cooling jacket then dump it on the ground, you don't need a gas generator for anything, and there's no way to feed the heated propellant to the actual hyperloop car. And if you could, you wouldn't want to, since if you combusted it in the car it would just dump the exhaust in the tube, killing the vacuum. Presumably active cooling of a hyperloop tube would use a closed refrigerant cycle, which has little to do with regenerative cooling, besides the idea of cooling channels. Talking about regenerative cooling in this context isn't wrong, exactly, it just betrays a rather shallow understanding of the problem at hand, as seen in your first comment.
  - JumpCrisscross 9y ago
    > Regenerative cooling in rocketry is running propellant through channels in the nozzle, then either dumping it overboard, using it to power a gas generator, or burning it in the rocket "Regenerative cooling" is a rocket term. It came to mind because I used to be an aerospace engineer. There's no requirement in the definition of regenerative cooling for the coolant to be dumped. > Presumably active cooling of a hyperloop tube would use a closed refrigerant cycle I don't think one can just presume that. You've already got lots of pumps for pumping air. Given (a) the seal on the tube will be periodically broken (for entry, exit and maintenance) and (b) a safety factor, you'll have more pump capacity than you'll need. Filtering and then compressing atmosphere, running it through a heat exchanger, and then letting it expand through the cooling channels before dumping it doesn't seem obviously worse than having kilometers of refrigerant running around. > which has little to do with regenerative cooling, besides the idea of cooling channels See above. Also, I assumed if you did this you'd use it to boost SpaceX's nozzles' economies of scale.

JumpCrisscross 9y ago

> I am not sure what your objection is here. Are you saying vacuum chambers can't be built on Earth? No, I specifically linked to the Space Power Facility in my original comment! The "materials science" problem I call out is in the tube's lateral thermal expansion. The best solution I've come up with for that is to take a regenerative rocket engine [1] and make it a tube. Pumping fluid in spirals along kilometers of a vacuum tube isn't easy, but it isn't as hard as trying to invent a material that won't deform when the top gets hotter than the bottom, or the east side gets hotter than the west. [1] https://en.wikipedia.org/wiki/Regenerative_cooling_(rocket) https://en.wikipedia.org/wiki/Regenerative_cooling_(rocket)

hwillis 9y ago
How about wrapping the tube in insulating foam and using peltier modules to regulate temperature. Or even using double walled tubes. It would not be challenging to regulate the temperature, but nobody talks about it because it doesn't even seem necessary.
- JumpCrisscross 9y ago
  > How about wrapping the tube in insulating foam At that point why not just bury it? It swaps the thermal problem for the water problem, but we know how to build tunnels.
  - schiffern 9y ago
    >At that point why not just bury it? Because foam is cheaper? A nice K.I.S.S. solution. Having seemingly solved this thermal expansion problem "in the same category of problems separating us from a space elevator," I'll be expecting The Fountains of Paradise to come true soon. ;)
    - JumpCrisscross 9y ago
      > We seem to have solved this thermal expansion problem I guess I'll have to telegraph my buddies in Hawthorne and at NASA :). Joking aside, no, foam doesn't solve the problem. You'll still have flexing. This is one of the limits on how long you can have a launch tank on the pad. Imperceptibly small flexing, but of the kind that weakens every metal we know.
      - schiffern 9y ago
        >Joking aside, no, foam doesn't solve the problem. You'll still have flexing. Of course the flexing isn't zero, but by adjusting the thickness of the foam you can drive the diurnal/solar heating thermal gradient arbitrarily low without compromising strength by making the steel walls thinner. That was your material science objection,[1] right? >but of the kind that weakens every metal we know. That's true, but misleading. It's true that every metal gets weaker, but in steel and titanium this weakening levels off (all other metals continue to get weaker until failure). Once these two metals are at the fatigue limit they stop weakening and have unlimited flexing cycles.[2] So you can set that as your final material strength and size the tube thickness accordingly. So repeated flexing is fine, and flexing can be made arbitrarily small with insulation. Foam solves the problem. [1] https://news.ycombinator.com/item?id=15460617 https://news.ycombinator.com/item?id=15460617 [2] https://en.wikipedia.org/wiki/Fatigue_limit https://en.wikipedia.org/wiki/Fatigue_limit
sbierwagen 9y ago
>The "materials science" problem I call out is in the tube's lateral thermal expansion. I admire your method of argumentation. The comment I replied to said nothing at all about thermal expansion. Your original comment, which I wasn't replying to, was mostly strange references to pressure ratios, and a single line about thermal problems, with no numbers cited. In that comment you did link to the Space Power Facility... as an argument against Hyperloop One! You did something similar downthread: https://news.ycombinator.com/item?id=15459777 https://news.ycombinator.com/item?id=15459777 But by asserting that you addressed the problem in your original comment, you make the person you're arguing with look like a bully, without having to actually address their points. Very efficient. >The best solution I've come up with for that is to take a regenerative rocket engine [1] and make it a tube. Pumping fluid in spirals along kilometers of a vacuum tube isn't easy, but it isn't as hard as trying to invent a material that won't deform when the top gets hotter than the bottom, or the east side gets hotter than the west. Using the phrase "regenerative cooling" in this context is another headscratcher. Regenerative cooling in rocketry is running propellant through channels in the nozzle, then either dumping it overboard, using it to power a gas generator, or burning it in the rocket. It's a great way to get rid of megawatts of heat. None of these things would be useful for a hyperloop tube? You don't want to run kerosene/liquid oxygen/hydrazine/whatever through a cooling jacket then dump it on the ground, you don't need a gas generator for anything, and there's no way to feed the heated propellant to the actual hyperloop car. And if you could, you wouldn't want to, since if you combusted it in the car it would just dump the exhaust in the tube, killing the vacuum. Presumably active cooling of a hyperloop tube would use a closed refrigerant cycle, which has little to do with regenerative cooling, besides the idea of cooling channels. Talking about regenerative cooling in this context isn't wrong, exactly, it just betrays a rather shallow understanding of the problem at hand, as seen in your first comment.
- JumpCrisscross 9y ago
  > Regenerative cooling in rocketry is running propellant through channels in the nozzle, then either dumping it overboard, using it to power a gas generator, or burning it in the rocket "Regenerative cooling" is a rocket term. It came to mind because I used to be an aerospace engineer. There's no requirement in the definition of regenerative cooling for the coolant to be dumped. > Presumably active cooling of a hyperloop tube would use a closed refrigerant cycle I don't think one can just presume that. You've already got lots of pumps for pumping air. Given (a) the seal on the tube will be periodically broken (for entry, exit and maintenance) and (b) a safety factor, you'll have more pump capacity than you'll need. Filtering and then compressing atmosphere, running it through a heat exchanger, and then letting it expand through the cooling channels before dumping it doesn't seem obviously worse than having kilometers of refrigerant running around. > which has little to do with regenerative cooling, besides the idea of cooling channels See above. Also, I assumed if you did this you'd use it to boost SpaceX's nozzles' economies of scale.