Can SpaceX Successfully Revolutionize Transportation with the Hyperloop?

[eprparadox]

SpaceX is hosting an open competition for pod designs to test on the hyperloop 1 mile test-track they're going to build. http://www.spacex.com/hyperloop

My question is this: to even begin THINK about designing a pod the way Elon envisions it, what sort of physics do I need to know? Any textbooks that would help me gain the skillset necessary from a physics perspective? What resources can I go through to help me learn more?

I have no plans to compete in this competition, but I would like to learn so hopefully in the future, I may be able to contribute meaningfully to this.

Thanks!

 

[rumborak]

I would think two things will be important:

- Fluid dynamics. While the tube will be held at a mere 1 millibar, that's air that needs to get out of the way. Also, air is used for levitation. Getting all that into a compact design will be challenging
- Control. At that speed you will likely have to actively control the pod to keep it perfectly aligned. Any misalignment could be catastrophic.

 

[mfb]

- Finite-element calculations for the mechanical engineering
- some electronics and power management

I would not rely on active controls I think - the timescale is very short. Passive mechanisms are more failproof.
Hard drives are a great example - the head is just a few nanometers above the surface which moves at up to tens of meters per second. That gives a typical timescale of less than a nanosecond - no way to actively control that. The head hovers above the surface based on a tiny layer of air.

It is interesting that Elon Musk does not want to develop that with his companies. He has a good track record of making good ideas successful, so if he thinks this is a good idea, why wouldn't he try to make a product out of it?

 

[rumborak]

Another thing will be, what's actually supposed to be the means of propulsion? The tube is at 1 millibar, I.e. 100 Pa. If you wanted to use the air for propulsion, that would constitute the maximum pressure differential on the vehicle, and with a 3m diameter, you get a measly 700N of force. That's not going to accelerate you much, is it?

EDIT: Ah, linear induction motors. So, that will be another challenge, alongside the batteries needed to power those.

 

[mheslep]

I expect one of the more difficult problems will be maintaining alignment and internal smoothness of the tube over time and over changing environmental conditions. With the typical temperature variance linear expansion of a steel tube over that 300 mile proposed route is something like a hundred meters, which has to be allowed continuously over all pylon supports due to the near vacuum, and accommodated at the terminals. Most of the other aspects of hyperloop have some similar precedent in other venues: air lift, linear motor drive, air compressors, aerodynamic analysis. But there's nothing very similar at all to this 300 mile gap-less tube.

 

[mheslep]

why wouldn't he try to make a product out of it?

The man already has three market changing companies. Tesla still runs at a loss, against $2 gasoline, and is building a single battery factory to double the planet's battery making capacity, though the company has no history of making cells. SpaceX is profitable but the last launch failed explosively, and the first of a kind controlled descents on land have all failed (so far). It's not as if the Musk companies made t-shirts. Now he's to take on first of kind, supersonic ground transport? I think his choice of brainstorming the idea and letting go of it was remarkably sound.

 

[A.T.]

But there's nothing very similar at all to this 300 mile gap-less tube.

https://en.wikipedia.org/wiki/Nullarbor_Plain#Railway_line
"The railway line has the longest straight section of railway in the world (478 km, 297 mi)"

So you would need an airtight equivalent of this:
https://en.wikipedia.org/wiki/Breather_switch

 

[mheslep]

...
So you would need an airtight equivalent of this:
https://en.wikipedia.org/wiki/Breather_switch

I've been pondering the expansion problem since the HL proposal was released. I've not yet seen a workable solution, aside from allowing axial travel and the ~100M expansion (~500 km steel, 20degC) at the terminals. That breather switch used for rail has sliding sections, which can't seal. Gas/fluid pipe typically goes with U sections and the like AFAICT, also not an option for HL.

 

[A.T.]

hat breather switch used for rail has sliding sections, which can't seal.

The sealing doesn't have to be at the sliding interface. It could be outside, encompassing the entire section, where the pipes overlap.

U sections and the like AFAICT, also not an option for HL.

Well, some high-g loops could make the journey more interesting. ;-)

 

[mfb]

A 1/5000 relative expansion makes curves impractical: to limit displacement to 1 meter, the curvature radius can be at most 5 kilometers, which limits speed to 150m/s for 1/2 g horizontal acceleration. Would work, but that is assuming the track is a full circle - an impractical track design for transportation.

Sliding sections plus flexible outside sealing (can be U-shaped) should work.

@mheslep: in other words, he has an excellent track record of marketing unconventional things.
There were rockets that landed again, just the combination with an actual orbital rocket launch is still open (could happen later today for the US / in the night to Tuesday for Europe).

 

[mheslep]

The sealing doesn't have to be at the sliding interface. It could be outside, encompassing the entire section, where the pipes overlap.

That then entails an axial compressible, airtight encompassing section strong enough to hold the vacuum and withstand environmentals. Unless the encompassing section is to also be structurally strong, all the joining sections would have to meet outside the pylon support.

Inside, a smooth surface must be maintained to +/- a few mm, somehow mounted atop the sliding/telescoping sections.

I'm curious about existing, somewhat similar expansion problems. Pipelines, bridges, use non-applicable solutions. Do large pressurized aircraft simply expand nose to tail? Subs? The ISS?

 

[mheslep]

@mheslep: in other words, he has an excellent track record of marketing unconventional things.

Agreed. And if I read the track record of those unconventionals correctly, they required a great deal of direct attention by the man himself to escape disaster, and the man himself doesn't scale. It's to his credit that he recognizes limits, because many don't.

Kevin Doyle: No, 'cause there are safeguards!

Eddie Morra: Against aggressive overexpansion? There aren't because there are no safeguards in human nature. We're wired to overreach. Look at history, all the countries that have ever ruled the world - Portugal, with its big, massive navy... All they've got now are salt cods and cheap condoms.

[crowd laughs]

Eddie Morra: And Brits? Now they're just sitting in their dank little island, fussing over their suits. No one's stopping and thinking, 'Hey, we're doing pretty well. We got France, we got Poland, we got a big Swiss bank account... You know what? Let's not invade Russia in the winter, let's go home, let's pop a beer and let's live off the interest.'

http://www.imdb.com/title/tt1219289/quotes