4 ms·

The pressure tube doesn't need to be at near-vacuum. By the drag equation, air resistance is proportional to pressure; merely cutting pressure by 90% would redu

The pressure tube doesn't need to be at near-vacuum. By the drag equation, air resistance is proportional to pressure; merely cutting pressure by 90% would reduce air resistance by the same amount, and that's trivial with perfectly ordinary pumps. Serious vacuum pumps are not required. This is not to say that keeping it airtight won't still be a major engineering problem, it just isn't as near-impossible as near-vacuum would be. To cope with expansion, you'd need to use sliding plates--which, yes, will make it harder to keep the air out. That's going to be an interesting challenge.

wlesieutre 9y ago
Doesn't the Hyperloop concept rely on not having a perfect vacuum? It's supposed to suck in air on the front and blow it out the bottom as an air cushion. Without the air cushion you just have "maglev in a vacuum" instead. I'm sure that's also a fast mode of travel, but like you said, the vacuum part of it takes a lot of money.
- Robotbeat 9y ago
  Does the vacuum part take a lot of money? I would think that if anyone knows, it'd be someone like Elon Musk who has built a spacecraft company from the ground up. For many of Elon Musk's projects, I get the idea that Elon is mostly just bringing broad, multi-domain knowledge to bear on industries that have been siloed for a very long time. So everyone scoffs at a long vacuum tube and using turbomachinery, etc, but for someone with a physics background with extensive knowledge of the spacecraft environment, turbopump rocket engines, and all the subsystems and ground testing systems that enable all this, it really isn't far-fetched at all. Most physicists (of the experimental kind, i.e. those who have to have hands-on knowledge fabricating things in addition to theoretical background) that I've talked to understand his ideas and think they're fairly reasonable. People seem to base most of their criticisms on the fact that it's different than what we already do without a fundamental, first-principles understanding of the system.
  - wlesieutre 9y ago
    A real vacuum (or very close to it) would be expensive, yes. That's what Musk designed the hyperloop to avoid; it works with a "low pressure" tube instead. His original hyperloop proposal paper outlines this in the intro: http://www.spacex.com/sites/spacex/files/hyperloop_alpha.pdf http://www.spacex.com/sites/spacex/files/hyperloop_alpha.pdf Another extreme is the approach, advocated by Rand and ET3, of drawing a hard or near hard vacuum in the tube and then using an electromagnetic suspension. The problem with this approach is that it is incredibly hard to maintain a near vacuum in a room, let alone 700 miles (round trip) of large tube with dozens of station gateways and thousands of pods entering and exiting every day. All it takes is one leaky seal or a small crack somewhere in the hundreds of miles of tube and the whole system stops working. However, a low pressure (vs. almost no pressure) system set to a level where standard commercial pumps could easily overcome an air leak and the transport pods could handle variable air density would be inherently robust. Unfortunately, this means that there is a non-trivial amount of air in the tube and leads us straight into another problem. The "another problem" being that if you just do low pressure, it means your pod has air in front if it and has to pushing that air around (or if that air has nowhere else to go in the forward tubing, trying to compress it, because not enough of the air can squeeze around the pods at the edge of the tube). That's what the hyperloop is getting around by sucking air in the front and blowing it out the bottom. Working as an air cushion is something of a bonus, and if everything works out right it also lets you avoid the expense of maglev.

wlesieutre 9y ago

Doesn't the Hyperloop concept rely on not having a perfect vacuum? It's supposed to suck in air on the front and blow it out the bottom as an air cushion. Without the air cushion you just have "maglev in a vacuum" instead. I'm sure that's also a fast mode of travel, but like you said, the vacuum part of it takes a lot of money.

Robotbeat 9y ago
Does the vacuum part take a lot of money? I would think that if anyone knows, it'd be someone like Elon Musk who has built a spacecraft company from the ground up. For many of Elon Musk's projects, I get the idea that Elon is mostly just bringing broad, multi-domain knowledge to bear on industries that have been siloed for a very long time. So everyone scoffs at a long vacuum tube and using turbomachinery, etc, but for someone with a physics background with extensive knowledge of the spacecraft environment, turbopump rocket engines, and all the subsystems and ground testing systems that enable all this, it really isn't far-fetched at all. Most physicists (of the experimental kind, i.e. those who have to have hands-on knowledge fabricating things in addition to theoretical background) that I've talked to understand his ideas and think they're fairly reasonable. People seem to base most of their criticisms on the fact that it's different than what we already do without a fundamental, first-principles understanding of the system.
- wlesieutre 9y ago
  A real vacuum (or very close to it) would be expensive, yes. That's what Musk designed the hyperloop to avoid; it works with a "low pressure" tube instead. His original hyperloop proposal paper outlines this in the intro: http://www.spacex.com/sites/spacex/files/hyperloop_alpha.pdf http://www.spacex.com/sites/spacex/files/hyperloop_alpha.pdf Another extreme is the approach, advocated by Rand and ET3, of drawing a hard or near hard vacuum in the tube and then using an electromagnetic suspension. The problem with this approach is that it is incredibly hard to maintain a near vacuum in a room, let alone 700 miles (round trip) of large tube with dozens of station gateways and thousands of pods entering and exiting every day. All it takes is one leaky seal or a small crack somewhere in the hundreds of miles of tube and the whole system stops working. However, a low pressure (vs. almost no pressure) system set to a level where standard commercial pumps could easily overcome an air leak and the transport pods could handle variable air density would be inherently robust. Unfortunately, this means that there is a non-trivial amount of air in the tube and leads us straight into another problem. The "another problem" being that if you just do low pressure, it means your pod has air in front if it and has to pushing that air around (or if that air has nowhere else to go in the forward tubing, trying to compress it, because not enough of the air can squeeze around the pods at the edge of the tube). That's what the hyperloop is getting around by sucking air in the front and blowing it out the bottom. Working as an air cushion is something of a bonus, and if everything works out right it also lets you avoid the expense of maglev.