The original Hyperloop paper quotes its design pressure as “about 1/6 the pressure of the atmosphere on Mars” [1]. Martian atmospheric pressure is “about 0.6% of Earth's mean sea level pressure” [2]. So 0.1% of Earth’s surface pressure, or a 1,000:1 pressure change.
To put that in perspective, the Boeing 787’s GEnx-2B67, the most powerful GEnx engine variant, generates a 43:1 pressure ratio [3]. To get a sense of the engineering differences between 6:1 and 1,000:1, look at NASA’s Space Power Facility [4].
The Hyperloop’s thermal issues are a hard enough problem that they alone put this in the domain of materials science. That’s the same category of problems separating us from a space elevator.
The Hyperloop always seemed like a transportation system not for this planet. The thermal issues, too, become trivial to solve on Mars: bury the tube. Mars does not appear to be too seismically active [5] and has no existing property rights to take into account [citation needed]. (The lack of water also makes steel more viable.)
> The Hyperloop’s thermal issues are a hard enough problem that they alone put this in the domain of materials science. That’s the same category of problems separating us from a space elevator.
What? What are you even talking about?? That's a completely ridiculous statement. The pressure inside and outside the lunar landing module was 1,000,000,000,000,000x. 100 kPa to 10^-10 Pa. It makes zero sense to apply this kind of ridiculous relative measurement to pressures because it has no relation to how pressure works. The structural challenge in building a tube that works to .001 atmospheres is only 25% harder than building a tube that works to .2 atmospheres (same as an airplane). And it isn't any more dangerous.
The only challenge that occurs is sealing in that atmosphere without any leaks and that part isn't hard either. 100 Pa is achievable by a $50 pump.
I also have yet to see any compelling argument that there will be thermal issues. Floating mounts and expansion joints are hardly untested technology.
> The pressure inside and outside the lunar landing module was 1,000,000,000,000,000x. 100 kPa to 10^-10 Pa
We didn't pump air into the lunar module from the Moon. We carried it from Earth pre-pressurized. Unless you're proposing we build and seal the Hyperloop tubes in space before bringing them down, the analogy isn't appropriate.
Also, consider a soda can. It's stronger when pressurized from the inside. Modern nuclear attack submarines collapse around 730m [1]. Since for "every 33 feet (10.06 meters) you go down, the pressure increases by 14.5 psi," we're talking about water pressure of about 1,000 psi or about 72 atm.
Both are pumped down to very high vacuum, not required for the Musk Hyperloop, (low pressure electric jet in a tube) but probably needed for the Hyperloop One. (maglev vactrain) And, obviously, both chambers were built terrestrially, and evacuated using conventional vacuum pumps.
> I am not sure what your objection is here. Are you saying vacuum chambers can't be built on Earth?
No, I specifically linked to the Space Power Facility in my original comment! The "materials science" problem I call out is in the tube's lateral thermal expansion.
The best solution I've come up with for that is to take a regenerative rocket engine [1] and make it a tube. Pumping fluid in spirals along kilometers of a vacuum tube isn't easy, but it isn't as hard as trying to invent a material that won't deform when the top gets hotter than the bottom, or the east side gets hotter than the west.
>The "materials science" problem I call out is in the tube's lateral thermal expansion.
I admire your method of argumentation.
The comment I replied to said nothing at all about thermal expansion. Your original comment, which I wasn't replying to, was mostly strange references to pressure ratios, and a single line about thermal problems, with no numbers cited. In that comment you did link to the Space Power Facility... as an argument against Hyperloop One! You did something similar downthread: https://news.ycombinator.com/item?id=15459777 But by asserting that you addressed the problem in your original comment, you make the person you're arguing with look like a bully, without having to actually address their points. Very efficient.
>The best solution I've come up with for that is to take a regenerative rocket engine [1] and make it a tube. Pumping fluid in spirals along kilometers of a vacuum tube isn't easy, but it isn't as hard as trying to invent a material that won't deform when the top gets hotter than the bottom, or the east side gets hotter than the west.
Using the phrase "regenerative cooling" in this context is another headscratcher. Regenerative cooling in rocketry is running propellant through channels in the nozzle, then either dumping it overboard, using it to power a gas generator, or burning it in the rocket. It's a great way to get rid of megawatts of heat.
None of these things would be useful for a hyperloop tube? You don't want to run kerosene/liquid oxygen/hydrazine/whatever through a cooling jacket then dump it on the ground, you don't need a gas generator for anything, and there's no way to feed the heated propellant to the actual hyperloop car. And if you could, you wouldn't want to, since if you combusted it in the car it would just dump the exhaust in the tube, killing the vacuum.
Presumably active cooling of a hyperloop tube would use a closed refrigerant cycle, which has little to do with regenerative cooling, besides the idea of cooling channels.
Talking about regenerative cooling in this context isn't wrong, exactly, it just betrays a rather shallow understanding of the problem at hand, as seen in your first comment.
> Regenerative cooling in rocketry is running propellant through channels in the nozzle, then either dumping it overboard, using it to power a gas generator, or burning it in the rocket
"Regenerative cooling" is a rocket term. It came to mind because I used to be an aerospace engineer. There's no requirement in the definition of regenerative cooling for the coolant to be dumped.
> Presumably active cooling of a hyperloop tube would use a closed refrigerant cycle
I don't think one can just presume that. You've already got lots of pumps for pumping air. Given (a) the seal on the tube will be periodically broken (for entry, exit and maintenance) and (b) a safety factor, you'll have more pump capacity than you'll need. Filtering and then compressing atmosphere, running it through a heat exchanger, and then letting it expand through the cooling channels before dumping it doesn't seem obviously worse than having kilometers of refrigerant running around.
> which has little to do with regenerative cooling, besides the idea of cooling channels
See above. Also, I assumed if you did this you'd use it to boost SpaceX's nozzles' economies of scale.
>"Regenerative cooling" is a rocket term. It came to mind because I used to be an aerospace engineer. There's no requirement in the definition of regenerative cooling for the coolant to be dumped.
?
Regenerative cooling in rocketry uses propellant. Propellant is always dumped overboard, because it's propellant.
Please show me a rocket that uses closed loop cooling of the rocket nozzle.
>Filtering and then compressing atmosphere, running it through a heat exchanger, and then letting it expand through the cooling channels before dumping it doesn't seem obviously worse than having kilometers of refrigerant running around.
My turn to be pedantic about definitions: this sure sounds like closed loop cooling to me! The working fluid is air, you draw it from a big reservoir, (the atmosphere) cool something with it, then return it to the reservoir. (Something you can't do with open-loop regenerative cooling of a rocket nozzle, since the coolant gets burned at the end of the cycle)
However, maintaining the air dryers and replacing the filters sounds like it wouldn't be any cheaper than conventional phase-change cooling, and having to build custom vacuum/compressor pumps for the hyperloop project is going to be more expensive than buying COTS vacuum pumps.
> Please show me a rocket that uses closed loop cooling of the rocket nozzle
The most famous one is the under-development SABRE [1], which includes a closed-loop helium cycle. For cryogenic rockets, closed-loop cooling of the nozzle has been explored [2] to avoid hydrogen embrittlement and oxidation of the nozzle channels, as well as to simplify plumbing.
In any case, we've devolved into arguing semantics.
> My turn to be pedantic about definitions: this sure sounds like closed loop cooling to me!
I was figuring on dumping the air once done versus worrying about a reservoir. That said, I haven't done any math on the benefits of saving the return piping (and reservoir cost and maintenance) versus using something traditional.
> maintaining the air dryers and replacing the filters sounds like it wouldn't be any cheaper than conventional phase-change cooling, and having to build custom vacuum/compressor pumps for the hyperloop project is going to be more expensive than buying COTS vacuum pumps
Fair enough. As you observe, it's a problem I haven't seen a suitable solution to (apart from burying, which trades the thermal problem for, in my view, the better water management problem and the scarier land-use problem.)
>The most famous one is the under-development SABRE [1], which includes a closed-loop helium cycle.
From the linked article:
>>The 'hot' helium from the air precooler is recycled by cooling it in a heat exchanger with the liquid hydrogen fuel.
It's open-loop regenerative cooling, with helium as an intermediate coolant. The heat still ends up in the propellant, which gets dumped overboard. Not closed-loop.
That patent also specifies a heat exchanger to the propellant tank.
>I was figuring on dumping the air once done versus worrying about a reservoir.
A poorly telegraphed joke. The atmosphere, here, is the reservoir. I have edited my comment.
>However, maintaining the air dryers and replacing the filters sounds like it wouldn't be any cheaper than conventional phase-change cooling, and having to build custom vacuum/compressor pumps for the hyperloop project is going to be more expensive than buying COTS vacuum pumps.
Just want to point out a facet of such a project that you probably haven't considered. The EPA puts strict requirements on the operators of refrigerant systems, the way they categorize operator class is based on pounds of refrigerant. Thus even though a COTS phase change refrigerant system seems like the obvious choice for Hyperloop One, because of regulatory burden it would almost certainly unfeasible. A refrigeration system big enough to cool a hyperloop would have to have every inch of refrigerant line spray tested every 3 months or an insanely expensive monitoring system.
How about wrapping the tube in insulating foam and using peltier modules to regulate temperature. Or even using double walled tubes. It would not be challenging to regulate the temperature, but nobody talks about it because it doesn't even seem necessary.
Because foam is cheaper? A nice K.I.S.S. solution.
Having seemingly solved this thermal expansion problem "in the same category of problems separating us from a space elevator," I'll be expecting The Fountains of Paradise to come true soon. ;)
> We seem to have solved this thermal expansion problem
I guess I'll have to telegraph my buddies in Hawthorne and at NASA :).
Joking aside, no, foam doesn't solve the problem. You'll still have flexing. This is one of the limits on how long you can have a launch tank on the pad. Imperceptibly small flexing, but of the kind that weakens every metal we know.
>Joking aside, no, foam doesn't solve the problem. You'll still have flexing.
Of course the flexing isn't zero, but by adjusting the thickness of the foam you can drive the diurnal/solar heating thermal gradient arbitrarily low without compromising strength by making the steel walls thinner. That was your material science objection,[1] right?
>but of the kind that weakens every metal we know.
That's true, but misleading. It's true that every metal gets weaker, but in steel and titanium this weakening levels off (all other metals continue to get weaker until failure). Once these two metals are at the fatigue limit they stop weakening and have unlimited flexing cycles.[2] So you can set that as your final material strength and size the tube thickness accordingly.
So repeated flexing is fine, and flexing can be made arbitrarily small with insulation. Foam solves the problem.
> We didn't pump air into the lunar module from the Moon. We carried it from Earth pre-pressurized. Unless you're proposing we build and seal the Hyperloop tubes in space before bringing them down, the analogy isn't appropriate.
It absolutely is appropriate, since the module was emptied and filled each time the astronauts left.
> Modern nuclear attack submarines collapse around 730m. Since for "every 33 feet (10.06 meters) you go down, the pressure increases by 14.5 psi," we're talking about water pressure of about 1,000 psi or about 72 atm.
Yeah- so building a hyperloop is structurally as complicated as building a tube that sits 33 feet under water. Hardly sounds complicated when put like that.
The force exerted by the atmosphere on an evacuated tube is the same as the force exerted on a standard-atmosphere filled submarine diving 10m beneath the surface.
A 100kPa difference.
And since they explicitly do not want a high vacuum - some residual air inside the tube is necessary for the hyperloop concept! - they don't have to deal with advanced tech like turbomolecular pumps. Simple displacement pumps will do.
> Note that downthread they've already moved the goalpost from "these pumps are technologically impossible" to "but they'll need a lot of them!"
I never said vacuum pumps are "technologically impossible". My original comment references the Space Power Facility [1] and categorises the vacuum problem as an engineering problem. A hard one, but engineering nonetheless.
The comment you link to [2] replies to someone claiming the original white paper calls for a 22:1 atmosphere:tube pressure ratio. I pointed out that the figure they're referencing, Figure 11, discusses the capsule and not the tube.
I'm skeptical about the economics of de-pressurising the tube, but that's an engineering problem and I've always held it as such. The materials problem is the thermal expansion of the top of the tube relative to the bottom.
>The comment you link to [2] replies to someone claiming the original white paper calls for a 22:1 atmosphere:tube pressure ratio. I pointed out that the figure they're referencing, Figure 11, discusses the capsule and not the tube.
I happen to be that someone. :) You were talking about state-of-the-art axial compressors (the GEnx-2B67), so I assumed you were talking about the axial compressor on the front of the pod. Mea culpa. But then you drew an analogy to the pressures in the SPS ("To get a sense of the engineering differences between 6:1 and 1,000:1..."), as if the Hyperloop people were trying to make a 1000:1 axial compressor. As I pointed out in my reply,[1] rotary vane compressors can easily maintain those pressures.
>The materials problem is the thermal expansion of the top of the tube relative to the bottom.
If that were really a problem, no pipelines of any kind could be built. Again thermal expansion joints are the solution, since with the abandonment of air-ski levitation the pod walls no longer have a requirement to be ultra-smooth. Tiny leakage on these joints is fine, since it will be made up for by the pumps located along the track.
> If that were really a problem, no pipelines of any kind could be built
The Trans-Alaska Pipeline system, which I believe is the largest at least in the United States, is 1.2m in diameter [1]. We're talking about a pipe almost 3 times wider that needs to hold itself against the atmosphere and keep capsules neatly contained.
Side note: long pipelines zig-zag to allow for thermal expansion and contraction [2]. You can't do that with the Hyperloop. (Bridges handle this with various ingenious methods, most of which will work for the Hyperloop's longitudinal expansion.)
> thermal expansion joints are the solution
Scaling pipe expansion joints where they maintain the near vacuum and deal with the structural stress of a capsule whizzing by will be difficult. By "difficult" I mean these are problems NASA (for the ISS) and Schlumberger (for pipes) have been grappling with for years and with billions of dollars in R&D.
> The materials problem is the thermal expansion of the top of the tube relative to the bottom.
What exactly is the problem here? Is it because the top of the tube is exposed to direct sunlight? I thought it was supposed to be covered in solar panels anyway. Is it still a materials science problem if the tubes are shaded, because providing a structure that shades something with expansion from direct sunlight exposure seems quite a bit easier than to do so while trying to keep vacuum to a particular level.
The worst thing about the Hyperloop concept is how it takes different smart people with slightly different assumptions about what is being described, how it should work, and how it does work in the fields they are familiar with or experts in and the other fields they are only conversant in, and turns those people into rabid defenders of their own calculations that somehow lose the ability to reassess where mistakes might have been made. I'm not sure how or why this happens, because as far as I know none of this is theoretical science, it's just a matter of applying the principles accurately (which seems to be the sticking point). For example, here's a whole playlist of people arguing over the physics of it, which does devolve into name-calling at points.[1] There appears to be good science involved in the assessments though, but could be said for the most part about both parties. The interesting parts are where people step outside their areas of expertise and their unfamiliarity with the application of the principles causes mistakes.
> Wait, who said that they need to compress the air all the way to Earth pressure?
No one. The Hyperloop alpha paper explicitly contradicts this assumption on page 18, where the flow diagram gives the compressor input at 99 Pa and the output at 2.1 kPa (a 21:1 pressure ratio).
> The Hyperloop alpha paper explicitly contradicts this assumption on page 18, where the flow diagram gives the compressor input at 99 Pa and the output at 2.1 kPa (a 21:1 pressure ratio)
One standard atmosphere (atm) is defined at over 100,000 Pa [1]. The 21:1 pressure ratio in Figure 11 on page 18 is for the "passenger plus vehicle capsule" [2]. I'm talking about the tube.
Going from 99 to 2.1 isn't hard. Going from 100,000 to 100 and then keeping it there is.
But that's no challenge at all. You're just describing a usual vacuum pump. And just a single-stage roughing pump (Harbor Freight, anyone?) can get down to less than 1 Pascal, so 100Pa is a piece of cake. That's only needed for slight leaks in the tube, not for the pod itself.
Keep in mind that the absolute pressure difference is far less for Hyperloop than for a typical natural gas pipeline, and yet the latter can keep leaks to an absolute minimum in spite of thermal expansion, etc.
Scaling is hard. The largest vacuum chamber we've built is a fraction of the size of the proposed Hyperloop. It was a very, very hard problem [1]. It's expensive, sucks up loads and loads of power and needs lots of thermal management, structural reinforcement and vacuum-off maintenance.
> Keep in mind that the absolute pressure difference is far less for Hyperloop than for a typical natural gas pipeline
Pipes are pressurized from the inside. See my comment from elsewhere in the thread on why vacuums are different [2].
All this said, I generally agree with you. I don't think building a giant vacuum is beyond current technological capability. I do think building one structurally sound and thermal-expansionwise stable enough to carry passengers at high speeds is.
>The largest vacuum chamber we've built is a fraction of the size of the proposed Hyperloop. It was a very, very hard problem [1].
Do we really have to say that a long extruded tube that only has to maintain low vacuum is a lot simpler than the SPS?
SPS: big thing made up of many custom one-off parts that runs at 1/380,000,000ths of an atmosphere and can blast test articles with intense simulated sunlight.
Hyperloop tunnel: big thing made up of many identical parts that runs at 1/1,000ths of an atmosphere.
>It's expensive, sucks up loads and loads of power
I think they know that, don't you? There's no line-by-line breakdown, but the paper allocates $260 million for the station+pumps and 21 MW for electricity consumption (pumping, accelerating pods, and charging pod batteries).
>needs lots of thermal management
I agree that you need it, but what specific problem do you see?
>Pipes are pressurized from the inside. See my comment from elsewhere in the thread on why vacuums are different [2].
You said a soda can is stronger when pressurized from the inside. That's true, but the obvious solution is to make it thicker than a soda can. :) The paper calls for 0.8-1.0 inch thick steel, which my math says is more than adequate.
Who's they? Hyperloop One? No--they've just punted the "hard" stuff to the end. (Kind of like the Wright Brothers designing the seats of their plane before getting it flying. Oh look! [1])
Elon Musk? Yes--I do. That's why he's waiting.
> what specific problem do you see?
The top heats up and the bottom doesn't.
> The paper calls for 0.8-1.0 inch thick steel, which my math says is more than adequate
Adding mass adds strength while increasing the time the structure takes to reach thermal equilibrium. The thermal gradient isn't itself a problem. But if you look at the forces necessary for the materials in question to tear themselves apart, and then consider their thermal coefficients of linear thermal expansion, you can derive a maximum tolerable thermal gradient given the size of each tube segment (we'll assume the problem of reticulating vacuum seals is solved).
When you solve for strength, you get too much material for the system to reach equilibrium before inclement weather either causes (a) the structure to buckle, laterally or (b) the outside of the tube to start shearing itself from the cooler inside.
When you solve for thermal stresses, you lose your strength. Microbuckling and microfracturing may not seem like a big deal, but it is when you're talking about 1 standard atmosphere bearing down from the outside with a capsule swinging about on the inside.
We need a strong material that either (a) conducts heat really well or (b) doesn't change shape when asymmetrically heated. We don't have something that meets those requirements yet that we can manufacture at scale.
>No--they've just punted the "hard" stuff to the end.
Citation needed. The link provided gives no support.
I would be interested in seeing your calculations on the thermal side. An ANSYS multiphysics simulation (the software SpaceX uses) would show the problem, no?
SpaceX has already built and used a mile-long, 11' diameter vacuum tube in Southern California, which would seem to put this matter to rest (the proof is in the pudding, after all).
> An ANSYS multiphysics simulation (the software SpaceX uses) would show the problem, no?
Yes. But SpaceX != Hyperloop One.
> SpaceX has already built and used a mile-long, 11' diameter vacuum tube in Southern California, which would seem to put this matter to rest (the proof is in the pudding, after all)
I'll go ahead and predict that the strength of that tube will have materially decreased after 1 year in the elements. I'll even posit that will occur independent of rusting, which appears to have unfortunately taken place, due to the formation of microfractures within the metal due to repeated lateral thermal flexing. That said, this was a demo track. It wasn't designed to withstand one standard atmosphere while whizzing fast, heavy capsules inside it for years on end.
>I'll go ahead and predict that the strength of that tube will have materially decreased after 1 year in the elements.
With steel, the real question is not the weakening from Year 0 -> Year 1, but from Year 4 -> Year 5. After an initial period of weakening the strength of steel levels off.[1]
Anyways, another user pointed out that a cheap foam layer can reduce the daily flexing to arbitrarily low amounts, and give an arbitrarily amount of time for the tube temperature to equalize circumferentially.[2] Cheaper than either a temperature equalizing water jacket or an underground tunnel. I know you disagree in that other thread, but I now consider this problem solved.
That vacuum chamber has two sets of fifty foot tall doors. The hyperloop is an inch-thick welded steel tube. Pinholes are trivially detected by ultrasonic testers. There are no leaks. Even hydrogen and helium would take centuries to diffuse inside. The only problems are sealing the ends, which are a couple meters wide.
The hyperloop as written was very challenging, but the engineering aspects are not. The practical building (grinding the inside, transporting the tubes, etc.) are significant. The financing is insane. The theory is not even particularly hard, much less at the level of a space elevator.
Axial compressors aren't the only compressor design. Rotary vane compressors have no problem with those pressures.
The internal pressure was chosen explicitly because it was easy to maintain with simple, single stage mechanical vacuum pumps (see Figure 13 on page 22). Heck, the first result on Amazon for "vacuum pump" goes down to 5 Pa. https://www.amazon.com/dp/B012CFTYX4/
Compare the Hyperloop to true "vacuum train" designs, which need to run at 1/1,000,000th of an atmosphere to mitigate sonic booms (an alternate way to get around the Kantrowitz limit). This requires multi-stage pumping with mechanical roughing, followed by turbomolecular pumps and cryopumping. That 1000x harder vacuum takes 1000x as much pumping power (not because the differential pressure is meaningfully different, but because you expel 1000x less air per stroke).
Some colleagues of mine also did a short feasibility assessment of the original white paper, and came to some similar conclusions. The propulsion system also made some assumptions about compressor performance that seemed too optimistic compared to existing turbomachinery.
That said, I have not followed closely enough to know if the hyperloop startups out there are following the original conception or if it has significantly evolved or not. I hear that they've ditched the compressor altogether, but that's got to have a pretty big impact on expected speeds.
> 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].
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.
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).
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.
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.
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.
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.
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.
Weren't there published papers by NASA scientists proving, without a doubt, that it's impossible to construct a booster rocket with the structural stability to return to earth?
I'm willing to give this guy the benefit of the doubt considering his track record. It's also instructive to read the comments when the original Hyperloop idea was published. Even the people thinking Musk would fail were in the minority, because everyone was certain he had no plans of even trying.
> Weren't there published papers by NASA scientists proving, without a doubt, that it's impossible to construct a booster rocket with the structural stability to return to earth?
No. Going back to the 1960s, re-usable two-stage configurations were being drafted [1].
oh stop. People are excited about reusable rockets, a mainstay of science fiction since the beginning. Why do you have to be a wet blanket? "Elon Musk didn't think of reusing rockets, and he didn't personally build them, and he doesn't have a degree in rocketry!" You're fighting a Quixotic battle against a strawman.
I think reusable rockets are cool, and I think musk is impressive.
I'm just not a member of the cult of Elon. I think he follows a path of aim impossibly high, be happy with getting pretty far. People instead take it as "he's literally going to do everything".
I see many more people talking about "the cult of Elon" than people who actually think he's literally going to do everything.
It reminds me way more of the nickelback is terrible thing. It's more of a meme than the actual subject. It's fun to feel like a member of the enlightened in-group that doesn't follow Elon Musk like sheep. All those dumb people that think GM will go bankrupt next year.
We have oil pipelines stretching thousands of miles that are under far more stress. We can build a low-pressure tunnel that stretches without a problem. Nothing in the hyperloop is impossible - it's just incredibly expensive and unprofitable.
Solve the mass-scale construction and maintenance efficiency problem and we can get all kinds of cool things like hyperloops, space elevators, and fancy megacities.
To put that in perspective, the Boeing 787’s GEnx-2B67, the most powerful GEnx engine variant, generates a 43:1 pressure ratio [3]. To get a sense of the engineering differences between 6:1 and 1,000:1, look at NASA’s Space Power Facility [4].
The Hyperloop’s thermal issues are a hard enough problem that they alone put this in the domain of materials science. That’s the same category of problems separating us from a space elevator.
The Hyperloop always seemed like a transportation system not for this planet. The thermal issues, too, become trivial to solve on Mars: bury the tube. Mars does not appear to be too seismically active [5] and has no existing property rights to take into account [citation needed]. (The lack of water also makes steel more viable.)
[1] http://www.spacex.com/sites/spacex/files/hyperloop_alpha-201...
[2] https://en.m.wikipedia.org/wiki/Atmosphere_of_Mars
[3] https://en.m.wikipedia.org/wiki/General_Electric_GEnx
[4] https://en.m.wikipedia.org/wiki/Space_Power_Facility
[5] https://www.space.com/418-marsquakes-red-planet-rumble.html