4 ms·

> This is obvious from the kind of nitpicking that goes on (no references! 4.37 m/s^2 is the maximum motion sickness possible because an accredited expert propo

by akjj 13y ago

> This is obvious from the kind of nitpicking that goes on (no references! 4.37 m/s^2 is the maximum motion sickness possible because an accredited expert proposed it; I've never seen 4.9 m/s^2, but it's surely a barf comet!) This isn't nitpicking. Dealing with the acceleration around curves is going to be a problem for any transit system and at higher speeds it becomes a bigger problem. Unlike bathrooms, this is going to be intrinsic to any system that carries human beings. As the author points out that the hyperloop proposal has significantly higher acceleration than any other high-speed system that he knows about. So, how did the hyperloop designers decide that 4.9 m/s^2 would be safe? Did they find some study which tested this level of acceleration on average people? Did they conduct their own study which overturned conventional wisdom? Did they look at the research which led to the 4.37 m/s^2 figure and determine that it was fatally flawed? This is a critical issue and if the designers don't explain themselves, then it's fair to assume they made it up. The hyperloop proposal contains innovative elements (air suspension in evacuated tubes) and elements which are essentially common with many other transit systems (viaducts, human passengers). The designers deserve credit for the former, but they can't sweep the details of the latter under the rug. The point of references is not stylistic and not to name-check previous work. The point is that when you're doing something, which has had lots of previous research, you either use that research to justify your claims or you explain why yours will be different. > The really interesting, constructive questions: does this low-pressure air cushion thing really work? what are some factors that we need to explore to determine its true efficiency? Those are interesting questions and if Musk had proposed just the air cushion technology, then that's all we would be talking about. People would build test pieces to validate the technology and then go looking for applications. Instead, Musk choose to also include a specific proposal for a replacement for the LA-SF high-speed rail line and to estimate the costs at a tenth as much. That's what got the most media attention and so that's the aspect the OP chose to criticize. BTW, you seem to be implying that the OP is a civil engineer or a transportation engineer with an axe to grind, but he's just an amateur and if you look at his other posts you'll see that he has a lot of concerns for reducing the costs of infrastructure projects.

zeteo 13y ago
I agree that it's important to talk about motion sickness. I was pointing out that the way the article deals with it (remarking that the proposed system may involve 12% higher lateral accelerations than HSR, and instantly dismissing it as a "barf ride") is inadequate. Here are a few questions that I don't know the answer to, but seem a lot more interesting: - Do we have studies that estimate the effect of lateral acceleration on motion sickness? What do they indicate (e.g. 0.1% of passengers get it at 4 m/s^2, 2% at 5 m/s^2 etc.)? - To what extent is motion sickness in HSR comparable to the proposed system? Are the differences (no outside visibility, different vibrations model, possibly different seating) significant and if so how? - To what extent do potential passengers care about motion sickness? Has anyone compared cost and convenience trade-offs for systems with different expectations of motion sickness e.g. ferry vs. train? How comfortable are passengers with self-medication in such contingencies? Etc. And motion sickness is only one of hundreds of possible serious weaknesses in this project. But this is inherent in anything that hasn't been tried before; anyone can think of hundreds of ways in which it can go wrong. Does this mean we should never work on anything that isn't fully proven?
- akjj 13y ago
  > (remarking that the proposed system may involve 12% higher lateral accelerations than HSR, and instantly dismissing it as a "barf ride") You're comparing apples to oranges here. If I understand the OP correctly, the 4.37 m/s^2 (which is a claimed maximum, not actually in practice, but let's ignore that) is in the plane of the ground. Canting will convert that acceleration into a mixture of horizontal and vertical acceleration, as experienced by the passenger. The hyperloop track has 11.1 m/s^2 of acceleration in the plane of the ground based on their path assumptions (= (480km/h = 133 m/s)^2 / 1600 m), which the OP claims can get converted to 5 m/s^2 of vertical acceleration. This 7 times greater than "track specifications" of 0.5-0.67 m/s^2 and 5 times greater than the 0.93 m/s^2 of vertical acceleration coming from the Transrapid's 4.37 m/s^2 in the plane of the ground. To step back for a minute, the issues with curve radius for high-speed transit have existed since the beginnings of high-speed rail 50 years ago and maybe longer. It's one of the major issues preventing Amtrak from going faster in the Northeast corridor, for example. If there was some easy way (blocking windows, lowering vibrations, changing expectations, self-medication) to deal with it, our first assumption should be that someone's already tried it. If Elon Musk had some new, breakthrough ideas in this regard, then our assumption should be that he would somehow mention it, rather than expecting the Internet to figure it out from the numbers he uses. In the absence of evidence that any of the thousands of people who have designed high-speed transit allow for such sharp turns, or that the hyperloop designers have any way to mitigate the effects of high acceleration, I think the most likely possibility is that they (some of whom work for SpaceX, remember) used acceleration limits coming from the space program. Since they seem never bothered to compare to look at actual high-speed rail systems, they never noticed how different their assumptions where.

by akjj 13y ago

zeteo 13y ago

I agree that it's important to talk about motion sickness. I was pointing out that the way the article deals with it (remarking that the proposed system may involve 12% higher lateral accelerations than HSR, and instantly dismissing it as a "barf ride") is inadequate. Here are a few questions that I don't know the answer to, but seem a lot more interesting: - Do we have studies that estimate the effect of lateral acceleration on motion sickness? What do they indicate (e.g. 0.1% of passengers get it at 4 m/s^2, 2% at 5 m/s^2 etc.)? - To what extent is motion sickness in HSR comparable to the proposed system? Are the differences (no outside visibility, different vibrations model, possibly different seating) significant and if so how? - To what extent do potential passengers care about motion sickness? Has anyone compared cost and convenience trade-offs for systems with different expectations of motion sickness e.g. ferry vs. train? How comfortable are passengers with self-medication in such contingencies? Etc. And motion sickness is only one of hundreds of possible serious weaknesses in this project. But this is inherent in anything that hasn't been tried before; anyone can think of hundreds of ways in which it can go wrong. Does this mean we should never work on anything that isn't fully proven?

akjj 13y ago
> (remarking that the proposed system may involve 12% higher lateral accelerations than HSR, and instantly dismissing it as a "barf ride") You're comparing apples to oranges here. If I understand the OP correctly, the 4.37 m/s^2 (which is a claimed maximum, not actually in practice, but let's ignore that) is in the plane of the ground. Canting will convert that acceleration into a mixture of horizontal and vertical acceleration, as experienced by the passenger. The hyperloop track has 11.1 m/s^2 of acceleration in the plane of the ground based on their path assumptions (= (480km/h = 133 m/s)^2 / 1600 m), which the OP claims can get converted to 5 m/s^2 of vertical acceleration. This 7 times greater than "track specifications" of 0.5-0.67 m/s^2 and 5 times greater than the 0.93 m/s^2 of vertical acceleration coming from the Transrapid's 4.37 m/s^2 in the plane of the ground. To step back for a minute, the issues with curve radius for high-speed transit have existed since the beginnings of high-speed rail 50 years ago and maybe longer. It's one of the major issues preventing Amtrak from going faster in the Northeast corridor, for example. If there was some easy way (blocking windows, lowering vibrations, changing expectations, self-medication) to deal with it, our first assumption should be that someone's already tried it. If Elon Musk had some new, breakthrough ideas in this regard, then our assumption should be that he would somehow mention it, rather than expecting the Internet to figure it out from the numbers he uses. In the absence of evidence that any of the thousands of people who have designed high-speed transit allow for such sharp turns, or that the hyperloop designers have any way to mitigate the effects of high acceleration, I think the most likely possibility is that they (some of whom work for SpaceX, remember) used acceleration limits coming from the space program. Since they seem never bothered to compare to look at actual high-speed rail systems, they never noticed how different their assumptions where.