4 ms·

Some colleagues of mine also did a short feasibility assessment of the original white paper, and came to some similar conclusions. The propulsion system also ma

Some colleagues of mine also did a short feasibility assessment of the original white paper, and came to some similar conclusions. The propulsion system also made some assumptions about compressor performance that seemed too optimistic compared to existing turbomachinery. That said, I have not followed closely enough to know if the hyperloop startups out there are following the original conception or if it has significantly evolved or not. I hear that they've ditched the compressor altogether, but that's got to have a pretty big impact on expected speeds.

JumpCrisscross 9y ago
> Some colleagues of mine also did a short feasibility assessment of the original white paper, and came to some similar conclusions Likewise. It sounds like above-surface (on Earth) is probably unfeasible given (a) security and (b) thermal concerns. For security, we just contemplated debris from the track (or a bullet from an errant rifle) puncturing the tube. Hyperloop One is testing an 3.3m diameter and 500m long track [1]. That's 4,276 cubic meters [a]. "At sea level and at 15 °C air has a density of approximately 1.225 kg/m3" [2]. The air in the Hyperloop One test track thus weighs about 5,200 kg. If we use the Hyperloop's original design spec [3], a puncture means air on one side at 1 atm expanding into the space on the other at 1/1,000 atm. This simplifies to a wall of air moving at just below the speed of sound. Since the speed of sound is about 330 m/s [4], the end of the tunnel will could hit with a pulse with about 140 megajoules of energy [b]. That's the energy in about 30 kg of TNT [5][c]. > I hear that they've ditched the compressor altogether, but that's got to have a pretty big impact on expected speeds It currently sounds like a vactrain [1] with magnetic levitation [2]. [1] https://arstechnica.com/cars/2017/03/hyperloop-one-shows-photos-of-its-test-track-being-built-in-nevada/ https://arstechnica.com/cars/2017/03/hyperloop-one-shows-pho... [a] pi * (3.3 / 2) ^ 2 * 500 [2] https://en.wikipedia.org/wiki/Density_of_air https://en.wikipedia.org/wiki/Density_of_air [3] http://www.spacex.com/sites/spacex/files/hyperloop_alpha-20130812.pdf http://www.spacex.com/sites/spacex/files/hyperloop_alpha-201... [4] https://en.wikipedia.org/wiki/Speed_of_sound https://en.wikipedia.org/wiki/Speed_of_sound [b] (1/2) * (5200 / 2) * 330 ^ 2 [5] https://en.wikipedia.org/wiki/TNT_equivalent https://en.wikipedia.org/wiki/TNT_equivalent [c] (140 * 10^6 / 4.184 * 10^9) * 10^3 [1] https://en.wikipedia.org/wiki/Vactrain https://en.wikipedia.org/wiki/Vactrain [2] https://en.wikipedia.org/wiki/Maglev https://en.wikipedia.org/wiki/Maglev
- dnautics 9y ago
  "a wall of air moving at the speed of sound" What? Particles moving in through a hole against vacuum will usually be travelling at a distribution of speeds dependent on ambient temperature, usually a boltzmann distribution. It will not be a wall, but rather a gradient. It's also not so much a maglev as an inductrack, which has only really become possible recently since it's come off-patent.
  - JumpCrisscross 9y ago
    > What? Particles moving in through a hole against vacuum will usually be travelling at a distribution of speeds dependent on ambient temperature, since T = v_rms A proper answer merits CFD. Barring that, sure, one can adjust for the velocity distribution [1] and the fact that a vena contracta [2] will reduce initial flow rates. I used 330 m/s, the speed at 0 °C (which is generous since these tracks will likely be operating in hotter conditions), which one will observe is around the molecular velocity of oxygen or nitrogen. [1] https://chem.libretexts.org/@api/deki/files/68415/BolzDist-MW.gif?revision=1 https://chem.libretexts.org/@api/deki/files/68415/BolzDist-M... [2] https://en.wikipedia.org/wiki/Vena_contracta https://en.wikipedia.org/wiki/Vena_contracta
- schiffern 9y ago
  sigh another "deadly wall of air" theory ala Thunderf00t? Hasn't this been debunked already? "A little bit of physics is a dangerous thing." * A bullet or piece of debris would likely leave a hole much smaller than the diameter of the tube. A breach 1/10th the diameter of the tube will admit 1/100ths as much the air. * Even if there was a whole-tube breach, the "wall of air" will rapidly slow down and smear out into a gradual pressure rise due to friction with the tube walls. Pipes are not lossless! Within 5 km friction will have the air moving at highway speeds. So if you're so close that you can be killed by the air blast, you're so close that the pod can't brake before hitting the whole-tube breach (bad). In other words, "deadly pressure waves" don't increase your odds of dying beyond that of a regular "derailment" event. * In the event of a breach (whole-tube or otherwise), sensors in the track will signal all the pods to stop and the tunnel to undergo emergency re-pressurization. So any "wave" won't get far.
matt4077 9y ago
Weren't there published papers by NASA scientists proving, without a doubt, that it's impossible to construct a booster rocket with the structural stability to return to earth? I'm willing to give this guy the benefit of the doubt considering his track record. It's also instructive to read the comments when the original Hyperloop idea was published. Even the people thinking Musk would fail were in the minority, because everyone was certain he had no plans of even trying.
- JumpCrisscross 9y ago
  > Weren't there published papers by NASA scientists proving, without a doubt, that it's impossible to construct a booster rocket with the structural stability to return to earth? No. Going back to the 1960s, re-usable two-stage configurations were being drafted [1]. [1] http://papers.sae.org/640297/ http://papers.sae.org/640297/
  - yahna 9y ago
    But I thought this all came to our god king in a dream.
    - hwillis 9y ago
      oh stop. People are excited about reusable rockets, a mainstay of science fiction since the beginning. Why do you have to be a wet blanket? "Elon Musk didn't think of reusing rockets, and he didn't personally build them, and he doesn't have a degree in rocketry!" You're fighting a Quixotic battle against a strawman.
      - yahna 9y ago
        I think reusable rockets are cool, and I think musk is impressive. I'm just not a member of the cult of Elon. I think he follows a path of aim impossibly high, be happy with getting pretty far. People instead take it as "he's literally going to do everything".

JumpCrisscross 9y ago

> Some colleagues of mine also did a short feasibility assessment of the original white paper, and came to some similar conclusions Likewise. It sounds like above-surface (on Earth) is probably unfeasible given (a) security and (b) thermal concerns. For security, we just contemplated debris from the track (or a bullet from an errant rifle) puncturing the tube. Hyperloop One is testing an 3.3m diameter and 500m long track [1]. That's 4,276 cubic meters [a]. "At sea level and at 15 °C air has a density of approximately 1.225 kg/m3" [2]. The air in the Hyperloop One test track thus weighs about 5,200 kg. If we use the Hyperloop's original design spec [3], a puncture means air on one side at 1 atm expanding into the space on the other at 1/1,000 atm. This simplifies to a wall of air moving at just below the speed of sound. Since the speed of sound is about 330 m/s [4], the end of the tunnel will could hit with a pulse with about 140 megajoules of energy [b]. That's the energy in about 30 kg of TNT [5][c]. > I hear that they've ditched the compressor altogether, but that's got to have a pretty big impact on expected speeds It currently sounds like a vactrain [1] with magnetic levitation [2]. [1] https://arstechnica.com/cars/2017/03/hyperloop-one-shows-photos-of-its-test-track-being-built-in-nevada/ https://arstechnica.com/cars/2017/03/hyperloop-one-shows-pho... [a] pi * (3.3 / 2) ^ 2 * 500 [2] https://en.wikipedia.org/wiki/Density_of_air https://en.wikipedia.org/wiki/Density_of_air [3] http://www.spacex.com/sites/spacex/files/hyperloop_alpha-20130812.pdf http://www.spacex.com/sites/spacex/files/hyperloop_alpha-201... [4] https://en.wikipedia.org/wiki/Speed_of_sound https://en.wikipedia.org/wiki/Speed_of_sound [b] (1/2) * (5200 / 2) * 330 ^ 2 [5] https://en.wikipedia.org/wiki/TNT_equivalent https://en.wikipedia.org/wiki/TNT_equivalent [c] (140 * 10^6 / 4.184 * 10^9) * 10^3 [1] https://en.wikipedia.org/wiki/Vactrain https://en.wikipedia.org/wiki/Vactrain [2] https://en.wikipedia.org/wiki/Maglev https://en.wikipedia.org/wiki/Maglev

dnautics 9y ago
"a wall of air moving at the speed of sound" What? Particles moving in through a hole against vacuum will usually be travelling at a distribution of speeds dependent on ambient temperature, usually a boltzmann distribution. It will not be a wall, but rather a gradient. It's also not so much a maglev as an inductrack, which has only really become possible recently since it's come off-patent.
- JumpCrisscross 9y ago
  > What? Particles moving in through a hole against vacuum will usually be travelling at a distribution of speeds dependent on ambient temperature, since T = v_rms A proper answer merits CFD. Barring that, sure, one can adjust for the velocity distribution [1] and the fact that a vena contracta [2] will reduce initial flow rates. I used 330 m/s, the speed at 0 °C (which is generous since these tracks will likely be operating in hotter conditions), which one will observe is around the molecular velocity of oxygen or nitrogen. [1] https://chem.libretexts.org/@api/deki/files/68415/BolzDist-MW.gif?revision=1 https://chem.libretexts.org/@api/deki/files/68415/BolzDist-M... [2] https://en.wikipedia.org/wiki/Vena_contracta https://en.wikipedia.org/wiki/Vena_contracta
schiffern 9y ago
sigh another "deadly wall of air" theory ala Thunderf00t? Hasn't this been debunked already? "A little bit of physics is a dangerous thing." * A bullet or piece of debris would likely leave a hole much smaller than the diameter of the tube. A breach 1/10th the diameter of the tube will admit 1/100ths as much the air. * Even if there was a whole-tube breach, the "wall of air" will rapidly slow down and smear out into a gradual pressure rise due to friction with the tube walls. Pipes are not lossless! Within 5 km friction will have the air moving at highway speeds. So if you're so close that you can be killed by the air blast, you're so close that the pod can't brake before hitting the whole-tube breach (bad). In other words, "deadly pressure waves" don't increase your odds of dying beyond that of a regular "derailment" event. * In the event of a breach (whole-tube or otherwise), sensors in the track will signal all the pods to stop and the tunnel to undergo emergency re-pressurization. So any "wave" won't get far.

matt4077 9y ago

Weren't there published papers by NASA scientists proving, without a doubt, that it's impossible to construct a booster rocket with the structural stability to return to earth? I'm willing to give this guy the benefit of the doubt considering his track record. It's also instructive to read the comments when the original Hyperloop idea was published. Even the people thinking Musk would fail were in the minority, because everyone was certain he had no plans of even trying.

JumpCrisscross 9y ago
> Weren't there published papers by NASA scientists proving, without a doubt, that it's impossible to construct a booster rocket with the structural stability to return to earth? No. Going back to the 1960s, re-usable two-stage configurations were being drafted [1]. [1] http://papers.sae.org/640297/ http://papers.sae.org/640297/
- yahna 9y ago
  But I thought this all came to our god king in a dream.
  - hwillis 9y ago
    oh stop. People are excited about reusable rockets, a mainstay of science fiction since the beginning. Why do you have to be a wet blanket? "Elon Musk didn't think of reusing rockets, and he didn't personally build them, and he doesn't have a degree in rocketry!" You're fighting a Quixotic battle against a strawman.
    - yahna 9y ago
      I think reusable rockets are cool, and I think musk is impressive. I'm just not a member of the cult of Elon. I think he follows a path of aim impossibly high, be happy with getting pretty far. People instead take it as "he's literally going to do everything".