Hyperloop

[Norbert]

He was talking about the air flow friction of traveling that fast in a tube. Think of how hot a rifle barrel gets after a bullet is fired. That heat would have to be dealt with.

Sure, that's how I understood it. However the air should still be able to take away heat, at the same time. As it does with airplanes, which in addition produce a lot of heat themselves. If he even mentions the water tank...

EDIT: (The friction can't be that bad since otherwise it would slow down the pod too quickly.)

 

[DonPedro]

Thermal expansion/contraction is clearly an interesting issue.

Assuming a fixed point at the midpoint between LA and SF, the station at each end would need to pick up the delta length from a 175 mile segment of steel tube. Applying Larry's assumptions from above, that means that the location of the end of the steel tube will move by ~801 feet at each station. I don't understand the station infrastructure sufficiently to propose a solution, but it should be possible at reasonable cost and functionality.

I also think that, due to the low bending radii involved, the steel would have sufficient elasticity to accommodate the bending from expansion/contraction. However, the designers have assumed that they can have sharper bends at the low velocity sections near the stations, and that is where this bending is most critical. This is easy to visualize this if you picture that there is a straight section pulling in towards the station, and then a bend when it gets there. When the steel expands, basically you would have to push the straight section of steel into the bend, and the bent steel would go in to the (presumably) straight platform area. Such movements would also create lateral moments on the pylons (which are loads they don't like) and could also create issues of steel fatigue. All of this has to be examined, and could range from "cost increase" to "show stopper".

The inside curvature of the tube is also going to be slightly affected. The diameter will be ~0.1 percent smaller in cold weather than in hot. The air hockey skis are going to be finely manufactured to match the geometry of the tube. They need to verify that it will be able to cope with this variation.

 

[RDoc]

I was responding to RDoc who said

My calculations show 25mm, not a fraction of a mm. And if the expansion joints were not distributed, you would end up with 1 inch per hundred feet at the stations. That boggles the mind (at least mine.)

The environment is 1/6 of Mars which is 1% of Earth. So as far as the forces acting on the structure, it might as well be a hard vacuum.

The slop would be (1inch/100ft)*5280ft/mi*100mi= 5280in= 440ft per 100mi.

You are correct, it is on the order of cm, not fractions of a mm. but I confess I misread the pdf where it says there are XY adjustments to allow for thermal expansion. I now believe what was meant was to account for changes in turn radius because of thermal expansion, not the linear expansion of the tube. As was pointed out, the design is to allow the tube to slip and take all the expansion up at the terminal stations.

Assuming the expansion is taken up at the two ends as the design states and assuming 80C swings, that gives a total difference of about 650m, or 325m at each end. These are described as bellows type expansion sections, so they might be a total of a km or so long at the very end just before the platform. This section of the track wouldn't have to be at low pressure or support high speeds since the car has to come to a stop at atmospheric pressure anyway, so having it first slow down, then pass through the airlock before getting to the expansion area at the very end of the line seems pretty reasonable. At atmospheric pressure, and very low speed, it's pretty easy to just have the part of the track needed for the wheels and linear motor slide in and out under the base of the station and platform.

 

[kevin99]

A good article contains two other good articles:

• Elon Musk's Ingenious, Implausible Hyperloopat Bloomberg(Tue 1:04PM EDT)

"
Broadly, the technology world responded with exuberant enthusiasm, the transportation world by noting the plan's serial implausibilities.
"

 

[DonPedro]

Junctions will be challenging if delta length due to thermal expansion/contraction is only taken out at the stations. If there is only one junction, that can be the fixed point of the system and the tubes expand from there. If there is more than one junction, however, the distance between them will vary according to weather, possibly be several hundred feet. There would need to be a non-trivial support system there to allow the tubes at the junction to move around in space - probably some sort of platform which it could slide on.

- - - Updated - - -

What this project need now is maybe a project web site, similar to the one that is used in open source software projects? I do not think that EM and his team have the capacity to absorb all the discussions and contributions available from the thousands and thousands of enthusiasts that could easily be harnessed to work on this.

 

[RDoc]

Thermal expansion/contraction is clearly an interesting issue.

Assuming a fixed point at the midpoint between LA and SF, the station at each end would need to pick up the delta length from a 175 mile segment of steel tube. Applying Larry's assumptions from above, that means that the location of the end of the steel tube will move by ~801 feet at each station. I don't understand the station infrastructure sufficiently to propose a solution, but it should be possible at reasonable cost and functionality.

I also think that, due to the low bending radii involved, the steel would have sufficient elasticity to accommodate the bending from expansion/contraction. However, the designers have assumed that they can have sharper bends at the low velocity sections near the stations, and that is where this bending is most critical. This is easy to visualize this if you picture that there is a straight section pulling in towards the station, and then a bend when it gets there. When the steel expands, basically you would have to push the straight section of steel into the bend, and the bent steel would go in to the (presumably) straight platform area. Such movements would also create lateral moments on the pylons (which are loads they don't like) and could also create issues of steel fatigue. All of this has to be examined, and could range from "cost increase" to "show stopper".

The inside curvature of the tube is also going to be slightly affected. The diameter will be ~0.1 percent smaller in cold weather than in hot. The air hockey skis are going to be finely manufactured to match the geometry of the tube. They need to verify that it will be able to cope with this variation.

I agree that the lateral motion caused by the length expansion changing the curve radius is definitely something to consider, but this seems to me to look a lot like a push rod cable like system since it's so long and thin, although some of those pylons may have to be designed for lateral loads. Based on their discussion of this issue in the paper, I'm assuming they've done the math.

- - - Updated - - -

Junctions will be challenging if delta length due to thermal expansion/contraction is only taken out at the stations. If there is only one junction, that can be the fixed point of the system and the tubes expand from there. If there is more than one junction, however, the distance between them will vary according to weather, possibly be several hundred feet. There would need to be a non-trivial support system there to allow the tubes at the junction to move around in space - probably some sort of platform which it could slide on.

- - - Updated - - -

What this project need now is maybe a project web site, similar to the one that is used in open source software projects? I do not think that EM and his team have the capacity to absorb all the discussions and contributions available from the thousands and thousands of enthusiasts that could easily be harnessed to work on this.

It would also need some really vicious moderators. :wink:

 

[DonPedro]

I tried to estimate the total steel weight of the tube. The circumference of the tube is 7 meters, it is ~597,000 meters long and 0.0215 meters thick. Multiplying these I get 89,849 m3 of steel. Assuming a density of 7.9 I get approx. 710,000 metric tons of steel. Costing this at $650 million means approx. $915 per ton. This is a low figure, but in the right order of magnitude. It depends quite a bit on the required steel grade. Here it needs to be rolled and fabricated to fine tolerances, which increases the cost. Curvature is also significantly more expensive than cylindrical sections.

- - - Updated - - -

It would also need some really vicious moderators. :wink:

You betcha! :-D

I wonder how this is done in open source software. The challenge is different, because the contributors produce code not designs. But still, maybe it could be done.

Probably there would need to be sub-fora for the different components of the system. These would work independently within the framework of the alpha design, but would also be allowed to output suggested modifications to the general concept. These would be discussed in a separate forum for the general concept. The general concept would be allowed to fork, so that several alternatives could co-exist. Likewise, the component designs could also fork. Then some sort of Darwinian principle would have contributors leave the inferior designs to rather work on the most promising ones that would emerge.

Would be wicked.

 

[IceWendigo]

"What this project need now is maybe a project web site, similar to the one that is used in open source software projects? I do not think that EM and his team have the capacity to absorb all the discussions and contributions available from the thousands and thousands of enthusiasts that could easily be harnessed to work on this."

Agreed. Also consider potential collaboration from several Universities' Tech department/faculties from around the world.

On another topic, what are the (speculated) differences between the about 1% air pressure and some other level (ex:5% or 10%) on the requirements for the tube/pipe and pumps, and on the pod speed ?

 

[ElSupreme]

Bathroom issue solved. Next.

View attachment 28180

And as a backup this.

 

[Norbert]

I don't buy it. He talks about "too many similarities", but only mentioned 3 interesting ones (apart from coincidences such as in the cost number 6.8 million):

1. evacuated tube (well there've been a lot of those ideas around, including ET3 which Elon's paper does mention explicitly)
2. Maglev trains (this is even wrong! Elon's proposal does *not* use magnetic levitation, it uses air cushion)
3. ground? support (I'm not sure about the word "ground", and have no idea what else it could be, but that is hardly a unique idea for trains.)

So as far as I can tell that leaves only the "evacuated tube" as a real idea in common, and Elon's paper already says that his proposal is not the first with evacuated tubes. However, in an interview he mentions that he has not seen "low-pressure" tubes before (as opposed to "vacuum" or near-vacuum, as perfect vacuum is not achievable in any case).

Also, quickly googling Frankel only revealed that he has been credited with the idea to run trains in evacuated tubes, but nothing else was mentioned in common with Elon's proposal. Only some cooperation with MagLev ideas (which do not apply here). Frankel can hardly claim credit for running trains between "two cities" as a unique idea.

My impression is that Frankel hasn't even examined Elon's proposal in detail. Certainly, the CNBC report fails to identify anything which could back up his complaint in any meaningful way.

To add to that: According to Wikipedia, the history of the "vactrain" goes back to 1910, and Frankel isn't even mentioned in the midst of all the evacuated tube and maglev developments.

Vactrain - Wikipedia, the free encyclopedia

 

[IceWendigo]

Toilet

Solution 1: Small Restroom Cabin (in larger version of the loop, the one that could have cars could probably have a intercity bus like toilet)
Solution 2: Depends and Clorox
Solution 3: Have a Station located around 15 min mid point (like a subway line with stations about every 250-300 miles [or whatever the 15 min mark ends up being in miles])

About Solution 3:

Time: Instead of LA to SF in 30 minutes, with one 2 min stop mid way, you would have what, 16min+2min+16min, something like 35-40 minutes, still much better than trains and planes?
Cost: How much more would it cost to have a smaller mid point station?

Would most people prefer a 30 min non-stop and no possible stop in a tube (that cost less per ticket?), to a 40-ish minute with possible stop every 16-ish minutes?

 

[gregincal]

Junctions will be challenging if delta length due to thermal expansion/contraction is only taken out at the stations. If there is only one junction, that can be the fixed point of the system and the tubes expand from there. If there is more than one junction, however, the distance between them will vary according to weather, possibly be several hundred feet. There would need to be a non-trivial support system there to allow the tubes at the junction to move around in space - probably some sort of platform which it could slide on.

You could have expansion sleeves at the junctions as well, but the paper doesn't really talk about how the junctions would work.

 

[raymond]

USA Today article has an expert that claims it won't work. Says the high velocity will build up too much heat for the capsules. He said he didn't see how the water tank could provide enough cooling for the trip.

Although I really am interested in what experts say, I haven't yet figured out how to filter out the experts that said one couldn't sell EVs or couldn't start a private space company.

 

[Norbert]

Sure, that's how I understood it. However the air should still be able to take away heat, at the same time. As it does with airplanes, which in addition produce a lot of heat themselves. If he even mentions the water tank...

EDIT: (The friction can't be that bad since otherwise it would slow down the pod too quickly.)

P.S.: As far as I can tell, the water tank is used to deal with the high temperature of the compressed air (not, or not so much, for any friction heat).

 

[Lloyd]

I would envision the junctions just being a double or triple redundant sliding o-ring on a teflon sleeve. Not too hard to envision or manufacture.

 

[AudubonB]

COMPRESSED AIR:

Cowboys and cupcakes, aren't most of you forgetting what happens at the back of each capsule? Decompressing that air? And you all do know what happens to T when that occurs, don't you?

The overall effect is quite negligible, and easily should be addressed by the water coolant.

 

[Norbert]

COMPRESSED AIR:

Cowboys and cupcakes, aren't most of you forgetting what happens at the back of each capsule? Decompressing that air? And you all do know what happens to T when that occurs, don't you?

The overall effect is quite negligible, and easily should be addressed by the water coolant.

Right, I think the issue in question was the friction heat created by the high speed.

 

[RDoc]

My reading of the paper is that it's quite clear that the water is used to cool the output of the compressor which is used to pressurize the passenger compartment and also to levitate the car.

1.
Tube air
is compressed with a compression ratio of 20:1 via an axial
compressor.
2.
Up to 60% of this air is bypassed:
a.
The air travels via a narrow tube near bottom of the capsule to
the tail.
b.
A nozzle at the tail expands the flow generating thrust to mitigate
some o
f the small amounts of aerodynamic and bearing drag.
3.
Up to 0.
44
lb/s (0.2 kg/s) of air is cooled and compressed an additional
5.2:1 for the passenger version with additional cooling afterward.
a.
This air is stored in onboard composite overwrap pressure
vessels.
b.
The stored air is eventually consumed by the air bearings to
maintain distance between the capsule and tube walls.
4.
An onboard water tank is used for cooling of the air.
a.
Water is pumped at 0.
30
lb/s (0.1
4
kg/s) through two intercoolers
(6
39
lb or
2
90
kg total mass of coolant).
b.
The steam is stored onboard until reaching the station.
c.
Water and steam tanks are changed automatically at each stop.

 

[Lloyd]

The friction at that altitude (eff 150,000 ft) is very diminished. very little from the air bearings. A decompression, however would be devastating. Not sure how they are going to certify the pressure vessel and safety equipment required. Consider certifying an aircraft the cruises at 150,000 feet! Almost like a commercial spacecraft.

 

[RDoc]

The friction at that altitude (eff 150,000 ft) is very diminished. very little from the air bearings. A decompression, however would be devastating. Not sure how they are going to certify the pressure vessel and safety equipment required. Consider certifying an aircraft the cruises at 150,000 feet! Almost like a commercial spacecraft.

The paper discusses this with the plan varying from using the on-board reserves of air with masks to stopping all the cars and rapidly re-pressurizing the entire tube. This does seem like an area the needs some serious design work, although it also seems possible to handle since there's lots of air at 1 atm pretty close by, unlike an aircraft at high altitude.

It seems to me though that a bigger issue is what happens if the tube is suddenly breached and a wave of 1 atm air propagates down the tube at close to Mach 1? The oncoming cars would rapidly go from Mach .9 to almost Mach 2 which would cause very high deceleration and possibly structural damage.

Note however, that a derailment at 130 mph would be pretty catastrophic as well.