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Comment: Re:It's almost like the Concord verses the 747 aga (Score 1) 143

by Rei (#49157171) Attached to: Hyperloop Testing Starts Next Year

What sort of claim is that? Since when do oxygen masks need 20kPa to function? And secondly, if there's "problematic loading on the capsules" from too much pressure on the pressure-compromised capsule, then your pressure is also way too high inside. Which means that you've repressurized the tube way too much. So the solution is: Don't do that!

Comment: Re:It's almost like the Concord verses the 747 aga (Score 1) 143

by Rei (#49157159) Attached to: Hyperloop Testing Starts Next Year

Branching at full speed is probably not possible with the Hyperloop as designed; the skis are curved to match the diameter of the tube, with a ~1mm clearance with the tube surface, so there is no passive tube design that could accommodate a "switch". In order to continue from Section A to either Section B or Section C, you'd have to make an intermediate length of tube several hundred meters long that could be physically moved at one end from B to C, with sub-millimeter precision

Wait, meaning that while it's technically possible, but it'd be really tricky to accomplish? Gee, I wish I had written something like "Branching would be really tricky, but there's no physical barriers" at the top of my post ;)

The reason is threefold: drag continues to increase at higher speeds regardless of the speed of sound

Drag is reduced in the first place by using hydrogen even at a given pressure. And you can use 1/4th the pressure and still maintain lift because you're moving four times as fast. And given how few reboosts are needed from LA to SF in the base case, a few more per unit distance hardly seems limiting.

If you consider that the steel Hyperloop pipe draped across 30m-spaced pylons will approximate a vertical sine wave, then at 700mph the allowable sag is only about 5cm

Irrelevant because earthquakes impose far more deflection that you have to be able to counter (and that the proposal calls for countering) than a craft moving past.

Mechanical braking from 1500mph in the event of an emergency is also a non-starter

What, you're picturing drum brakes or something? You're moving at high speeds in a giant steel tube. Magnetic braking couldn't possibly be easier.

a 700mph capsule will incur about 2g's of aerobraking deceleration

Where are you getting this from? Even if the tube was instantly full pressure (which it wouldn't be), a streamlined shape will not experience 2Gs at 700mph, any more than a passenger jet losing full engine power does. And anyway, 10g horizontal is not fatal even if that was the case. The average untrained individual, properly restrained, can tolerate 10g for a minute without even loss of cognitive function.

Comment: Re:It's almost like the Concord verses the 747 aga (Score 1) 143

by Rei (#49157121) Attached to: Hyperloop Testing Starts Next Year

Not only that, but if your craft is travelling four times as fast, you're sweeping through four times as much gas per unit time to compress under the skis.

Hydrogen has all sorts of advantages. And the very low pressures prevent most of the negatives. The only one that I don't know about and would require testing would be what sort of reaction would one see as a craft moves past, with any residual oxygen. If I had to guess, I'd guess that you will get some combustion, but the craft moves past so fast and the mixture will decompress so fast, I would think the rate would be quite limited.

Comment: Re:It's almost like the Concord verses the 747 aga (Score 1) 143

by Rei (#49157115) Attached to: Hyperloop Testing Starts Next Year

First off, if servicing that requires full de/repressurization is some sort of frequent event, then the whole concept is doomed for reasons entirely unrelated to anything in this discussion. Secondly, 1/5 ton of hydrogen at industrial rates is about $200. Whoop-di-doodle-doo. And the advantage is being able to travel at mach freaking 4, not about the reduction of drag at a given speed (which is, FYI, true also).

Comment: Re:It's almost like the Concord verses the 747 aga (Score 1) 143

by Rei (#49157107) Attached to: Hyperloop Testing Starts Next Year

As someone else already mentioned, it uses low pressure air because the "trains" are ground-effect aircraft, not maglev. They need air.

Secondly, the pumping budget to overcome leaks is so small, both in terms of capital and ongoing costs, that you could increase them by an order of magnitude and not have any sort of practical effect on the budget. Whatever factor you increase over the baseline increases the factor you can replace air by. You don't need 100%.

Comment: Re:It's almost like the Concord verses the 747 aga (Score 1) 143

by Rei (#49157097) Attached to: Hyperloop Testing Starts Next Year

You think keeping hundreds of miles of tubing is really going to be cheap? Go look at a highway budget sometime.

Because Hyperloop is a highway? The closest analogy is a pipeline. Except that the environmental hurdles for building an oil pipeline raise the cost dramatically. Yet Musk's budget in Hyperloop Alpha is well higher than that of an equivalent diameter per mile.

Then consider ... Then consider ... Then consider ... Then consider...

And when you're done with that, then consider that every last thing you mention here was analyzed in detail in the Hyperloop Alpha proposal, which you apparently never read.

Comment: Re:Better definition of planet (Score 0) 135

by Rei (#49155899) Attached to: One Astronomer's Quest To Reinstate Pluto As a Planet

And it is a planet. It's a dwarf planet. Dwarf means small. Planet means planet. Dwarf planet means small planet. So what's the problem?

How many exoplanets pass the current IAU definition of 'planet'? I bet a bunch don't.

Of the ones identified thusfar? I'd wager "the vast majority if not literally 100%". We can't see little stuff. Everything we see is big, which means strong orbit-clearing power. And usually also close to its star, which also helps clear the orbit.

Comment: Re:And still (Score 4, Informative) 135

by Rei (#49155829) Attached to: One Astronomer's Quest To Reinstate Pluto As a Planet

Exactly. There are many categories of planets, including but not limited to:

  * Terrestrial planets
  * Gas giants
  * Ice giants
  * Hot jupiters
  * Superearths

And so forth. Why does the concept of another category, dwarfs, enrage people?

Really, the only categorization issue that I'm adamant about is that Pluto-Charon is called a binary. The Pluto-Charon barycentre is not inside Pluto, therefore Charon is not rotating around Pluto, the two are corotating around a common point of space between them. That's a binary.

Comment: Re:Hmmm (Score 1) 143

by Rei (#49153883) Attached to: Hyperloop Testing Starts Next Year

Technically, yes, with the caveat that you'd need regular floating reboost platforms with significant power generation scattered all throughout the Pacific, and of course maintaining the track perfectly straight while floating (one presumes at a fixed depth under the water) provides its own engineering challenges. But room-temperature rarified hydrogen instead of rarified air would allow one to make the journey at about Mach 4. Faster if it's hot hydrogen.

Comment: Re:This isn't new (Score 1) 143

by Rei (#49153867) Attached to: Hyperloop Testing Starts Next Year

Are you under the misconception that hyperloop is a pneumatic tube system?

Hyperloop is a magnetically-accelerated a ground-effect aircraft operating in the sort of extremely rarified air normally only found at high altitudes. The tube's purpose is to provide such a rarified atmosphere near the ground. It's not a pneumatic train. It's not a vactrain. It's not maglev. It's a ground-effect aircraft.

Comment: Re:It's almost like the Concord verses the 747 aga (Score 2) 143

by Rei (#49153847) Attached to: Hyperloop Testing Starts Next Year

Branching would be really tricky, but there's no physical barriers. Note that even Musk's proposal isn't as far as you can take the concept. If you fill the tube with very low pressure water vapor instead of very low pressure air (via more pumping to overwhelm leaks, plus water vapor injection), your top speed jumps 40%. Fill it with hydrogen and it jumps 300% (normally hydrogen is a real pain to work with due to flammability, embrittlement, etc, but the densities in question are so low that such issues are mostly avoided). So we're talking the potential for hyperloop "speedways" for long distance runs that could blow airplanes out of the water.

The low numbers of passengers per capsule is really key to making the concept economical. Compare, say, monorail track with a full sized rail bridge. The former is vastly cheaper per unit distance because the peak loadings are so much lower, because the mass of the monorail trains are so much lower. A computer-controlled high launch rate of small, high speed capsules means you're spreading the loading out greatly, which means greatly reduced loading and thus materials costs.

Still, while Musk has been thinking of Hyperloop stations in the "airport" concept, he really needs to get out of that mindset. His proposed plan had them on the outskirts of cities. Airports are only on the outskirts of cities because they *must* be. You greatly reduce your utility by doing that, by making people catch connecting trains. Hyperloop can extend just fine into towns; with his two proposed endpoints in particular there are excellent rail routes into town that are quite straight that it could be built over.

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