Comment by JumpCrisscross | Hacker News Reader

Comment by JumpCrisscross | Hacker News Reader

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> 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-pho...

[a] pi * (3.3 / 2) ^ 2 * 500

[2] https://en.wikipedia.org/wiki/Density_of_air

[3] http://www.spacex.com/sites/spacex/files/hyperloop_alpha-201...

[4] https://en.wikipedia.org/wiki/Speed_of_sound

[b] (1/2) * (5200 / 2) * 330 ^ 2

[5] https://en.wikipedia.org/wiki/TNT_equivalent

[c] (140 * 10^6 / 4.184 * 10^9) * 10^3

[1] https://en.wikipedia.org/wiki/Vactrain

[2] https://en.wikipedia.org/wiki/Maglev

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.

> ala Thunderf00t

Just watched one of his videos on this--interesting and thank you for the pointer. Agree with you on his overstating the deadliness of pressure pulses. 30 kg of TNT is a lot of energy, certainly enough to knock your infrastructure out of commission for a couple days. The "everyone dies if the tube is punctured" argument is hyperbolic, though.

The materials science problem is the thermal expansion. And not the longitudinal one that Thunderf00t mentions. It's the transverse expansion. If these are above ground, the top will heat up relative to the bottom. That's a nasty problem to solve while maintaining the structural integrity to keep a giant vacuum with speeding capsules in place.

> It's the transverse expansion. If these are above ground, the top will heat up relative to the bottom. That's a nasty problem to solve while maintaining the structural integrity to keep a giant vacuum with speeding capsules in place.

Right... you can probably solve that with a bucket of white paint. Worst case you cover it with an aluminum shield- aluminum does not absorb infrared radiation and will reflect 99.9% of ambient heat. Since the proposal included covering large sections of the tube in solar panels that isn't even a significant change.

> you can probably solve that with a bucket of white paint. Worst case you cover it with an aluminum shield- aluminum does not absorb infrared radiation and will reflect 99.9% of ambient heat

It's, unfortunately, harder than this. It's a similar problem to the ones we dealt with regarding rockets, standing fueled, on a pad. Both methods you propose were tried. The solution is to (a) paint it and (b) launch before the gradient becomes too big.

The stresses on the Hyperloop tube, when a capsule is rushing through it while it's containing a near vacuum, are comparable to those on a rocket nearing max Q [1]. The difference is with a rocket we take great care to maintain symmetry. With the Hyperloop, that isn't an option. That persistent asymmetry is what makes it a difficult materials problem, particularly if we're using any known metals (even wonderful light and thermally-conductive aluminium).

[1] https://en.wikipedia.org/wiki/Max_Q

Dude, what. That's bullshit, and the size difference will be on the order of dozens or low hundreds of microns. Not only that, but the distortion will be spread evenly across the tube because it's a tube. You're just asserting that putting the tube in shadow will somehow not block heat from the sun.

I'm also not sure you understand what I'm saying about an aluminum shield? Aluminum has an emissivity coefficient of .04. Thermal conductivity has nothing to do with it since it isn't touching the tube. It's purpose is just to not re-radiate infrared onto the tube.

I think what we have here is someone applying a known problem and solution space for one industry (rocketry) to another (civil engineering). A large vertical tube that needs to move quickly and under great stress may not allow for the same solutions that apply to a large horizontal tube that is relatively static.

Fixing a large enough solar shield above a rocket hundreds of feet in the air which has to get out of the way quickly before the rocket launches has very different requirements than fixing a shading structure above a vertical static structure a few tens of feet in the air. I'm not sure how this problem was solved in rocketry is necessarily indicative of how hard it is to solve in other circumstances.

> I'm not sure how this problem was solved in rocketry is necessarily indicative of how hard it is to solve in other circumstances

Very fair. The advantage a rocket has is you choose when it's rolled out. You don't have to design for the worst weather because you can always hide.

You can't do that for a static structure. The Hyperloop is an attempt to marry the challenges of rocketry to the standards of civil engineering. The advantage is you don't have to think about aerodynamics, which is good, because air is the worst. (You also get civil-engineering budgets.) The bad is you can't hide from the edge cases.

If you want to grapple with this problem live, rent (or borrow) a thermal camera and make a model. Aluminum or tin foil would probably work for something on the window. I've only done this upright, to simulate storing an unfuelled vehicle outdoors in "ready-to-launch" mode, but you'll run into similar problems with a horizonatal configuration. At first, the shade works. Then thermals develop. You can foam it, and that looks like it works for a few days. Then someone instruments the inside and, lo and behold, hot spots. Turns out foam doesn't really help with heat that recurs in the same place, day after day. (Our solution: slowly rotate it.) You could completely isolate the tube, which is what NASA does in its vehicle assembly building [1], but at that point you might as well (a) bury it or (b) have a fleet of Concordes flying on loop, because either will be cheaper.

I'm not saying it's impossible. But it's much harder than the pressure problem, which is itself hard to get economical. If you want the tube above ground, I don't think it works with existing materials. My criticism of the Hyperloop One project is they didn't bother solving these issues with models. (Note: this is how Elon did it with the Falcon 1.) Instead, they decided to build a maglev track.

[1] https://en.wikipedia.org/wiki/Vehicle_Assembly_Building

So, to make sure I understand the problem correctly, you expect thermals to develop under shaded portions that still cause heat, and that to affect the structure's top and bottom heat differential to a degree that it would still cause problems? Is this different than oil pipelines because of the low pressure and lack of a heat transferring medium to even the temperature of the tube? I'm trying to figure out how this would affect a proposed hyperloop system, when it seems sufficiently solved for other above ground pipeline systems.

I can see relative size, inside medium, building material differences, shading structures and acceptable tolerances all affecting the outcome one way or the other, but I'm not sure to what degree each one would affect the outcome, so I'm not sure if it's actually as hard as you make it sound or whether a solution is known and achievable.

> Is this different than oil pipelines because of the low pressure and lack of a heat transferring medium to even the temperature of the tube?

Most hydrocarbon pipelines run HTHP: high temperature, high pressure. This keeps their contents viscous. That, in turn, means heat emanates relatively uniformly from inside the pipe. For pipelines subjected to asymmetric expansion (e.g. when starting up or shutting down), they "walk".

"Walking behaviour occurs as the pipeline is heated, and expands asymmetrically, until the point when pipeline expansion is fully mobilised. Expansion is ‘fully mobilised’ when a virtual anchor forms near the centre of the pipeline. The virtual anchor is then stationary, while pipe to each side expands away from the anchor as the temperature continues to rise. Once expansion is fully mobilised, walking ceases for that cycle." [1] As long as it walks laterally, pipeline owners tend to be fine with it

The solution to walking is typically laissez faire (taking care to ensure the deformation occurs laterally, i.e. side to side, versus sticking a butt up into the water.) Needless to say, this isn't an option for the Hyperloop.

Granted, the contents of these pipes operate at 130º to 170º C. They're also narrower, resist a smaller pressure differential, face fewer such asymmetric events and don't face the stress of capsules periodically whizzing past inside them. Our tube won't displace by meters. Its displacement, moreover, won't be problematic on day one. But over time it will critically weaken known materials. Big, dynamically mechanically stressed, close to vacuum and above ground is hard.

[1] https://www.researchgate.net/profile/David_Bruton/publicatio...

yeah Thunderf00t's hyperloop video is dumb, moreover, yes a lot of pressure can cause a tiny metal bearing to punch through a glass tube but the net force distributed over a mass scales with the size of the object squared, but the mass scales with the size of the object cubed. So the effects of pressure on a tube 2 meters wide is going to be far less dramatic than one 2 cm wide.

> If these are above ground, the top will heat up relative to the bottom.

The heat will quickly conduct to the other side. It's one of the main reasons for choosing steel. Add white paint to that (or solar panels) and you'll have reduced heat absorption to a minimum.

> The heat will quickly conduct to the other side

I take it you haven't had the pleasure of watching carefully machined cylinders buckle in the Arizona sun :). White-painted aluminum and shaded, mind you.

Those were structures on the order of meters. These problems become nasty with scale.

"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.

> 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-M...

[2] https://en.wikipedia.org/wiki/Vena_contracta

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