7 ms·

> The Hyperloop’s thermal issues are a hard enough problem that they alone put this in the domain of materials science. That’s the same category of problems sep

> The Hyperloop’s thermal issues are a hard enough problem that they alone put this in the domain of materials science. That’s the same category of problems separating us from a space elevator. What? What are you even talking about?? That's a completely ridiculous statement. The pressure inside and outside the lunar landing module was 1,000,000,000,000,000x. 100 kPa to 10^-10 Pa. It makes zero sense to apply this kind of ridiculous relative measurement to pressures because it has no relation to how pressure works. The structural challenge in building a tube that works to .001 atmospheres is only 25% harder than building a tube that works to .2 atmospheres (same as an airplane). And it isn't any more dangerous. The only challenge that occurs is sealing in that atmosphere without any leaks and that part isn't hard either. 100 Pa is achievable by a $50 pump. I also have yet to see any compelling argument that there will be thermal issues. Floating mounts and expansion joints are hardly untested technology.

JumpCrisscross 9y ago
> The pressure inside and outside the lunar landing module was 1,000,000,000,000,000x. 100 kPa to 10^-10 Pa We didn't pump air into the lunar module from the Moon. We carried it from Earth pre-pressurized. Unless you're proposing we build and seal the Hyperloop tubes in space before bringing them down, the analogy isn't appropriate. Also, consider a soda can. It's stronger when pressurized from the inside. Modern nuclear attack submarines collapse around 730m [1]. Since for "every 33 feet (10.06 meters) you go down, the pressure increases by 14.5 psi," we're talking about water pressure of about 1,000 psi or about 72 atm. [1] https://en.wikipedia.org/wiki/Submarine_depth_ratings https://en.wikipedia.org/wiki/Submarine_depth_ratings [2] https://oceanservice.noaa.gov/facts/pressure.html https://oceanservice.noaa.gov/facts/pressure.html
- the8472 9y ago
  The force exerted by the atmosphere on an evacuated tube is the same as the force exerted on a standard-atmosphere filled submarine diving 10m beneath the surface. A 100kPa difference. And since they explicitly do not want a high vacuum - some residual air inside the tube is necessary for the hyperloop concept! - they don't have to deal with advanced tech like turbomolecular pumps. Simple displacement pumps will do.
  - schiffern 9y ago
    Note that downthread, they've already moved the goalpost from "these pumps are technologically impossible" to "but they'll need a lot of them!" https://news.ycombinator.com/item?id=15459820 https://news.ycombinator.com/item?id=15459820
    - JumpCrisscross 9y ago
      > Note that downthread they've already moved the goalpost from "these pumps are technologically impossible" to "but they'll need a lot of them!" I never said vacuum pumps are "technologically impossible". My original comment references the Space Power Facility [1] and categorises the vacuum problem as an engineering problem. A hard one, but engineering nonetheless. The comment you link to [2] replies to someone claiming the original white paper calls for a 22:1 atmosphere:tube pressure ratio. I pointed out that the figure they're referencing, Figure 11, discusses the capsule and not the tube. I'm skeptical about the economics of de-pressurising the tube, but that's an engineering problem and I've always held it as such. The materials problem is the thermal expansion of the top of the tube relative to the bottom. [1] https://en.m.wikipedia.org/wiki/Space_Power_Facility https://en.m.wikipedia.org/wiki/Space_Power_Facility [2] https://news.ycombinator.com/item?id=15459820 https://news.ycombinator.com/item?id=15459820
      - kbenson 9y ago
        > The materials problem is the thermal expansion of the top of the tube relative to the bottom. What exactly is the problem here? Is it because the top of the tube is exposed to direct sunlight? I thought it was supposed to be covered in solar panels anyway. Is it still a materials science problem if the tubes are shaded, because providing a structure that shades something with expansion from direct sunlight exposure seems quite a bit easier than to do so while trying to keep vacuum to a particular level.
      - schiffern 9y ago
        >The comment you link to [2] replies to someone claiming the original white paper calls for a 22:1 atmosphere:tube pressure ratio. I pointed out that the figure they're referencing, Figure 11, discusses the capsule and not the tube. I happen to be that someone. :) You were talking about state-of-the-art axial compressors (the GEnx-2B67), so I assumed you were talking about the axial compressor on the front of the pod. Mea culpa. But then you drew an analogy to the pressures in the SPS ("To get a sense of the engineering differences between 6:1 and 1,000:1..."), as if the Hyperloop people were trying to make a 1000:1 axial compressor. As I pointed out in my reply,[1] rotary vane compressors can easily maintain those pressures. >The materials problem is the thermal expansion of the top of the tube relative to the bottom. If that were really a problem, no pipelines of any kind could be built. Again thermal expansion joints are the solution, since with the abandonment of air-ski levitation the pod walls no longer have a requirement to be ultra-smooth. Tiny leakage on these joints is fine, since it will be made up for by the pumps located along the track. [1] https://news.ycombinator.com/item?id=15460061 https://news.ycombinator.com/item?id=15460061
- sbierwagen 9y ago
  >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.
- hwillis 9y ago
  > We didn't pump air into the lunar module from the Moon. We carried it from Earth pre-pressurized. Unless you're proposing we build and seal the Hyperloop tubes in space before bringing them down, the analogy isn't appropriate. It absolutely is appropriate, since the module was emptied and filled each time the astronauts left. > Modern nuclear attack submarines collapse around 730m. Since for "every 33 feet (10.06 meters) you go down, the pressure increases by 14.5 psi," we're talking about water pressure of about 1,000 psi or about 72 atm. Yeah- so building a hyperloop is structurally as complicated as building a tube that sits 33 feet under water. Hardly sounds complicated when put like that.
- wbl 9y ago
  An evacuated tube on earth needs to sustain 1 atm.

JumpCrisscross 9y ago

> The pressure inside and outside the lunar landing module was 1,000,000,000,000,000x. 100 kPa to 10^-10 Pa We didn't pump air into the lunar module from the Moon. We carried it from Earth pre-pressurized. Unless you're proposing we build and seal the Hyperloop tubes in space before bringing them down, the analogy isn't appropriate. Also, consider a soda can. It's stronger when pressurized from the inside. Modern nuclear attack submarines collapse around 730m [1]. Since for "every 33 feet (10.06 meters) you go down, the pressure increases by 14.5 psi," we're talking about water pressure of about 1,000 psi or about 72 atm. [1] https://en.wikipedia.org/wiki/Submarine_depth_ratings https://en.wikipedia.org/wiki/Submarine_depth_ratings [2] https://oceanservice.noaa.gov/facts/pressure.html https://oceanservice.noaa.gov/facts/pressure.html

the8472 9y ago
The force exerted by the atmosphere on an evacuated tube is the same as the force exerted on a standard-atmosphere filled submarine diving 10m beneath the surface. A 100kPa difference. And since they explicitly do not want a high vacuum - some residual air inside the tube is necessary for the hyperloop concept! - they don't have to deal with advanced tech like turbomolecular pumps. Simple displacement pumps will do.
- schiffern 9y ago
  Note that downthread, they've already moved the goalpost from "these pumps are technologically impossible" to "but they'll need a lot of them!" https://news.ycombinator.com/item?id=15459820 https://news.ycombinator.com/item?id=15459820
  - JumpCrisscross 9y ago
    > Note that downthread they've already moved the goalpost from "these pumps are technologically impossible" to "but they'll need a lot of them!" I never said vacuum pumps are "technologically impossible". My original comment references the Space Power Facility [1] and categorises the vacuum problem as an engineering problem. A hard one, but engineering nonetheless. The comment you link to [2] replies to someone claiming the original white paper calls for a 22:1 atmosphere:tube pressure ratio. I pointed out that the figure they're referencing, Figure 11, discusses the capsule and not the tube. I'm skeptical about the economics of de-pressurising the tube, but that's an engineering problem and I've always held it as such. The materials problem is the thermal expansion of the top of the tube relative to the bottom. [1] https://en.m.wikipedia.org/wiki/Space_Power_Facility https://en.m.wikipedia.org/wiki/Space_Power_Facility [2] https://news.ycombinator.com/item?id=15459820 https://news.ycombinator.com/item?id=15459820
    - kbenson 9y ago
      > The materials problem is the thermal expansion of the top of the tube relative to the bottom. What exactly is the problem here? Is it because the top of the tube is exposed to direct sunlight? I thought it was supposed to be covered in solar panels anyway. Is it still a materials science problem if the tubes are shaded, because providing a structure that shades something with expansion from direct sunlight exposure seems quite a bit easier than to do so while trying to keep vacuum to a particular level.
    - schiffern 9y ago
      >The comment you link to [2] replies to someone claiming the original white paper calls for a 22:1 atmosphere:tube pressure ratio. I pointed out that the figure they're referencing, Figure 11, discusses the capsule and not the tube. I happen to be that someone. :) You were talking about state-of-the-art axial compressors (the GEnx-2B67), so I assumed you were talking about the axial compressor on the front of the pod. Mea culpa. But then you drew an analogy to the pressures in the SPS ("To get a sense of the engineering differences between 6:1 and 1,000:1..."), as if the Hyperloop people were trying to make a 1000:1 axial compressor. As I pointed out in my reply,[1] rotary vane compressors can easily maintain those pressures. >The materials problem is the thermal expansion of the top of the tube relative to the bottom. If that were really a problem, no pipelines of any kind could be built. Again thermal expansion joints are the solution, since with the abandonment of air-ski levitation the pod walls no longer have a requirement to be ultra-smooth. Tiny leakage on these joints is fine, since it will be made up for by the pumps located along the track. [1] https://news.ycombinator.com/item?id=15460061 https://news.ycombinator.com/item?id=15460061
sbierwagen 9y ago
>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.
hwillis 9y ago
> We didn't pump air into the lunar module from the Moon. We carried it from Earth pre-pressurized. Unless you're proposing we build and seal the Hyperloop tubes in space before bringing them down, the analogy isn't appropriate. It absolutely is appropriate, since the module was emptied and filled each time the astronauts left. > Modern nuclear attack submarines collapse around 730m. Since for "every 33 feet (10.06 meters) you go down, the pressure increases by 14.5 psi," we're talking about water pressure of about 1,000 psi or about 72 atm. Yeah- so building a hyperloop is structurally as complicated as building a tube that sits 33 feet under water. Hardly sounds complicated when put like that.
wbl 9y ago
An evacuated tube on earth needs to sustain 1 atm.