

Speculating about the Hyperloop - btilly
http://bentilly.blogspot.com/2012/11/speculating-about-hyperloop.html

======
stcredzero
_> People go into vehicles that I'll call cars, even though they aren't really
cars. These cars can be fired by a railgun to match speeds with the tube_

Most railguns I've read about produce extreme g-loads. Most people would come
out as super-gross wonder-jelly if fired out of one. I think what you want is
some kind of electromagnetic accelerator, but not necessarily a railgun in
particular.

 _> On its own that space would heat up due to the friction on the gas, but
you can put a heat sink (eg a block of ice) in the car and keep it comfortable
inside._

The cabin of a vehicle is quite small, and the ride time is quite short, so a
large block of ice probably would be feasible. But you are talking about
having a car leave every second, so for each loop you would have to have a
plant capable of freezing something like 2 gallons of water per second, as
well as a means of refilling the ice container in each car. That's going to
add quite a bit to the expense and energy use of the system you describe.
Refrigeration systems become quite a bit more efficient as they become larger,
so probably this is manageable.

Depending on how quickly you can turn around each car, each loop might require
quite a few additional cars. (But still a fraction of the total number)

------
btilly
A random quick note.

The first public discussion of the Hyperloop was in
[http://pandodaily.com/2012/07/12/pandomonthly-presents-a-
fir...](http://pandodaily.com/2012/07/12/pandomonthly-presents-a-fireside-
chat-with-elon-musk/) starting about 43 minutes in. A revealing bit of the
conversation a minute later went like this:

Elon: _I have a name for it, the Hyperloop._

Interviewer: _Is that like a Jetson's tunnel?_

Elon: _Something like that, yeah._

Interviewer: _You just get in, it whisks you?_

Elon: _Um, yeah. Yeah._

He later on said, _You're guessing in the right direction._

This suggests to me VERY STRONGLY that his technology proposal MUST be a tube
of some sort. He also said that it could store enough energy to run 24x7
without using batteries. That implies that he's got a lot of material going
very, very fast.

Interestingly this is the time that he made his 30 minutes downtown to
downtown comment, and his solar panel comment, but said nothing indicating no
right of way issues. Later on he said that he came up with a way to make it
much cheaper, and to get rid of right of way issues, but hasn't been repeating
the other comments that I know of.

This suggests to me that in July he might not have had right of way figured
out, and suggests to me that getting rid of the right of way issues by putting
it in the ocean might not be a bad guess as to the insight he had over the
summer that dramatically reduces costs.

My proposal may not be on the right track. But I think it is safe to conclude
from what Elon has said that it is a tube. It has a lot of mass moving. And I
don't think it infeasible that he would be thinking about putting this in the
ocean to solve the rights issue.

~~~
jlgreco
Tubes in the ocean, whether sunken or floating, seems like a _nightmare_ when
you consider ships. Ships snagging fiber cables with their anchors is one
thing, but something with people in it? Great care would probably have to be
taken with this sort of thing.

------
btilly
One point that just occurred to me.

If you put this tube inside of another tube, and then pumped most of the air
out of the second tube, the vacuum in the inner tube would improve and the
whole system would become more efficient. I believe it is linear. For instance
if the outer tube reduced atmosphere to 1/3 air pressure, then the energy
losses due to pushing air around drop to 1/3 of what they would have been.
However local compromise of the outer tube does not significantly compromise
the vacuum in the inner tube, which avoids major problems that normally exist
in maintaining an evacuated tube.

A lot depends on the flaps that let air out but (mostly) not in. When a
plunger goes by, you need it to be open for a very brief period to let air
out. I'd be curious whether it is better to have a lot of little flaps that
can open/shut fast because they are little, or to connect the flaps to a
computer system that can open the flap just in time and close it again
promptly.

The smart system in theory can be more efficient, but the straight mechanical
system would be massively easier to design and build.

I'm also curious what breakthrough Elon had over the summer that dramatically
reduced costs. The things that I'm thinking of involve the design of the
plunger. Possibly a spike creating chaotic flow at the right place. Or a
plunger design that more efficiently pushes air out. Or some clever design of
the behind of the plunger so that the air that is left is not following at
high speed. Or even the idea I put above about having 2 shells, with partial
evacuation in sections of the outer one.

~~~
jacquesm
Hey Ben,

That's pretty clever, so you're using the 'plungers' (carriages?) as the means
of partial evacuation and propulsion at the same time...

No true vacuum, just enough to reduce drag to something manageable.

One thing that still bothers me about the whole concept (besides it being a
complete guess as to what Musk has in mind of course) is the stopping
properties, emergency shutdown.

If you would have to suddenly decelerate in the system I proposed you'd be
building up pressure in front of the car which would slow the car down until
some maximum pressure i s reached at which point you can breach the walls to
relieve it and keep it at that maximum. This provides a nice cushioning effect
for emergencies using very little besides the components already in the
system.

If you were to even partially evacuate the tube then that would no longer work
and you'd need active braking at all times.

Accelerating into the at a speed slightly higher than the loop speed backing
into a slug that has _just_ passed might be a way to mitigate this to some
extent, then you still have to worry about stopping the train but the braking
energy could be dispersed over a much longer distance due to closer packing.

I missed the end of the interview linked by someone else in the other thread
where Elon Musk says it has no 'right of way' issues so I think the whole
thing is moot but it was a fun exercise anyway.

~~~
btilly
I've seen his right of way comment. But if you've got something ground based,
that is 1200 km long, how can you avoid right of way issues? My guess is that
he means to build it in the ocean. There aren't a lot of people living there.
If so, it could look like anything you want and there would be no "right of
way issues". (Update, it just hit me! If you build this in the ocean, with
neutral buoyancy under water, then you don't have to worry about earthquakes
or weather! Built on land and able to handle earthquakes is a lot harder.)

But the stopping properties deserve more thought. California has regular
earthquakes. What happens if you've got large hunks of metal going 1200 km/h
over a section of loop that just was hit by one, what happens?

My suspicion is that you build regular emergency exit tubes. (Many of which
might also be good for regular tube service.) The cars are let out at the
nearest one. The machinery is at risk, the people are mostly OK.

You then try to stop the plungers with regenerative braking, dumping air in,
etc. Or you can build a fast emergency stop, such as having exit holes that
you can drop plungers into at high speed, which carries them far enough away
from the tube that they won't destroy it when they impact.

This has to be carefully worked through. But the first and most critical piece
is the ability to quickly and reliably dump passengers from most of the
system. And that shouldn't be that hard to do.

~~~
jacquesm
I read somewhere (can't find a quick reference) of a coating that resembles
the structure of shark-skin, this reduces drag enormously by creating local
turbulence which reduces the shear forces required to separate the layers of
fluid adhering to a surface.

A coating like that on the inside of the tube and the outside of the vehicle
would be an interesting component but the fluid dynamics of such a set-up are
way beyond my abilities to calculate.

This whole thing is all about energy budgets, you'd have to take the amount of
fuel it takes a passenger car or rail car to move from one point to another,
then take some reasonable savings estimate and then work backwards from there
to see how much energy you could expend on friction, drag and so on.

I have a hard time accepting that the friction in a tube while drafting would
be that much higher than friction losses to a body of air that is mostly
standing still and from playing around with a car and a friend of mine who is
a trucker (don't try this at home kids) I've seen how low fuel consumption can
go if you are properly in the slip-stream of another vehicle.

There has to be some way to make the numbers work on that. But unless 'no
right of way issues' means underground (or ocean bound, which means it would
only work between coastal cities and would have a pretty terrible failure
mode) the whole thing is off the table anyway.

I can't stop thinking about it though :)

------
jhuckestein
I'm greatly enjoying this entire debate, but I'd just like to point out one
thing: Elon said he doesn't know for sure how to do it yet or if it's even
feasible (although he thinks it is). This means

a) It's possible that not all of the statements he ever made about the
Hyperloop will be true for the final design (if any). I'd be especially
cautious about the comment about right of way issues. This might as well just
mean that he found an unused railroad track or something. And

b) Nobody knows if it can be done. Keep that in mind when debating the ideas.
As far as I'm concerned, all ideas are constructive at this point.

I'm not a physicist and it's very likely that this solution is completely
impossible, but let me ask anyway. If you just had a 1200 mile long train and
you somehow got the mass going in a circle at 1200 mp/h, how much energy would
you need to keep that momentum going? Could you cover that with solar panels
covering the 1200 miles of track?

~~~
btilly
Keeping mass in orbit about a point requires absolutely no energy. That is
because energy is force times distance (with times being a dot product), and
the force you apply is always at right angles to the motion.

It is a good thing that this is true, else the Earth could not manage to go
around the Sun indefinitely!

Therefore the energy to keep a system like this going is entirely what is
needed to replace various incidental losses like friction. With maglev
technology there some electromagnetic drag, but the bulk of losses are air
friction.

There is no way to keep a train like you describe going against air friction
at 1200 mph. If you eliminated air friction, it would be very doable.

~~~
ethanjlowry
What if the entire loop was in motion? Picture a maglev train 1200km long.
There is no "front" to the train, it's a loop, so air is not being compressed
as it whizzes along at 1200kmph. Most of the space inside the train could be
void, which could be filled with passenger cars as needed.

Would that eliminate enough of the air resistance to be feasible? Is a 1200km
long contiguous structure feasible?

~~~
btilly
I am fairly sure it would not help. If you're rubbing past air at that speed,
you'll have pretty serious friction issues.

------
dpark
I hate to be overly negative, but this is just a bunch of extra complexity
layered on top of an unworkable idea. The plungers aren't relevant. If you
could run this with self-propelled plungers that push cars, you could run this
with self-propelled cars.

The "flaps" are just trying to play with semantics. Either they work and this
is just an overly-complex evacuated tube or they don't work and this is no
different than Mattheij's idea. If you land in the middle where they "kind of"
work, then you likely get the drawbacks of an evacuated tube (not enough
oxygen to keep a human alive) with most of the drawbacks of Mattheij's idea
(too much friction to be feasible). Air becomes too thin to support human life
before air resistance becomes negligible.

This proposal also has the same right of way issues that any tunnel system
would have, issues that Musk specifically said Hyperloop would avoid.

~~~
btilly
I'd appreciate it if you put the same thought into the proposal that you did
into throwing out criticisms that it already addresses.

If you run this with cars only, the mass of cars that you'd need to
accelerate/decelerate is much higher. Also the heat issues that the cars need
to deal with is also much higher. Separating plungers and cars means you
accelerate/decelerate much less mass, and have far fewer heat issues to deal
with. The plungers themselves will get too hot for humans, but that's OK
because humans are insulated from them. (And I already said how to deal with
the heat in the cars.)

If the flaps work as proposed, the air pressure directly in front of the
plunger will significantly exceed the air pressure in the rest of the tunnel.
The cars sit in a bubble of air maintained by that shockwave. I have not tried
to do fluid dynamics calculations to estimate friction, but it is going to be
many times less than with Jacques' idea. I can believe it could be low enough
to make this system work.

I suspect that Elon plans to avoid right of way issues by building it in the
ocean. If he doesn't do that, then it is very difficult to see how he can do
anything like a “ground based Concorde” (his words) and still avoid right of
way issues.

~~~
dpark
> _I'd appreciate it if you put the same thought into the proposal that you
> did into throwing out criticisms that it already addresses._

I'm pretty sure you did not address my criticisms in your proposal.

> _If you run this with cars only, the mass of cars that you'd need to
> accelerate/decelerate is much higher._

If you run this with cars only, you probably wouldn't make them weigh 70 tons,
and then your acceleration costs drop dramatically.

> _Also the heat issues that the cars need to deal with is also much higher.
> Separating plungers and cars means you accelerate/decelerate much less mass,
> and have far fewer heat issues to deal with. The plungers themselves will
> get too hot for humans, but that's OK because humans are insulated from
> them._

You only have heat issues because you're trying to push a big column of air
for no reason. What is the value of this column of air? The air itself is just
creating friction. The passengers can't breath it, because it's too hot for
them to survive, and it's not enough to sustain life in the event of a
failure, because most of this hypothetical tunnel is a vacuum.

> _And I already said how to deal with the heat in the cars._

You proposed adding enough cold thermal mass to offset the heat they'd absorb
(literally a block of ice). You're just adding a bunch of extra mass that you
have to accelerate, and a bunch of mass you need to re-cool after every trip.

> _If the flaps work as proposed, the air pressure directly in front of the
> plunger will significantly exceed the air pressure in the rest of the
> tunnel. The cars sit in a bubble of air maintained by that shockwave. I have
> not tried to do fluid dynamics calculations to estimate friction, but it is
> going to be many times less than with Jacques' idea. I can believe it could
> be low enough to make this system work._

If the flaps work as proposed, then you have all the problems of an evacuated
tunnel and most of the problems of Mattheij's idea, as I said. You have an
inhospitable environment, need to deal with the pressure differential
(throughout most of the tunnel at least), tons of energy lost to friction (or
else it wouldn't be too hot for humans), etc. Even if your idea was
technically possible, it seems to have no advantages over an evacuated tunnel
except that you can say that it's not an evacuated tunnel.

> _I suspect that Elon plans to avoid right of way issues by building it in
> the ocean. If he doesn't do that, then it is very difficult to see how he
> can do anything like a “ground based Concorde” (his words) and still avoid
> right of way issues._

If you build far enough underwater to avoid right of way issues (aka, no boats
can get past), you're looking at worse engineering issues than an evacuated
tunnel anyway. At just 10m below the surface, you're already at 2 atmospheres
of pressure.

I'm also not sure that "underwater" counts as "ground based".

~~~
btilly
Here is an exercise for you.

Go to [http://pandodaily.com/2012/07/12/pandomonthly-presents-a-
fir...](http://pandodaily.com/2012/07/12/pandomonthly-presents-a-fireside-
chat-with-elon-musk/). Jump to 43 minutes in. Listen for two minutes. Come
back and explain how to explain that interchange WITHOUT having a tunnel. If
you listen for slightly longer when he talks about storing energy in the
system so that it can run 24x7, it seems pretty obvious that whatever the
system is, it has a lot of weight moving very fast.

We also know, because Elon Musk has said so, that, _It is not an evacuated
tunnel._

If it is going to be reasonably efficient, it can't have a lot of air in it.

So those are the design constraints for what Elon is thinking of. What can you
propose?

Now why would you not want an evacuated tunnel? Well the common complaint
about an evacuated tunnel is how hard it is to maintain the vacuum, and what
to do if containment is breached. My proposal gives a way to maintain the
vacuum (the plungers). Minor breaches of containment are not that big of a
problem (the air that gets in is blown out again).

~~~
dpark
> _Come back and explain how to explain that interchange WITHOUT having a
> tunnel. If you listen for slightly longer when he talks about storing energy
> in the system so that it can run 24x7, it seems pretty obvious that whatever
> the system is, it has a lot of weight moving very fast._

Explain that exchange without it being a giant pneumatic tube like the
Jetsons. If we take it at face value, he basically agreed that it's a
pneumatic tube, but I'm pretty sure that's not really what he's thinking of,
and you seem to agree, given that your proposal is absolutely not a "Jetsons
tube".

I agree that storing energy without batteries likely means a lot of moving
mass. That doesn't mean it's a big weight flying through a tunnel. It could
also be a flywheel or something else. Or it could be that the "moving mass" is
an incorrect guess and it's something chemical, but not in a traditional
"battery" (e.g. a breakdown of water into component gases).

> _We also know, because Elon Musk has said so, that, It is not an evacuated
> tunnel._

We also know that Elon Musk tends to propose (and build) practical solutions.
It seems unlikely that he would be proposing what amounts to an evacuated
tunnel with unnecessary complexity tacked on. If he's stating that it's not an
evacuated tunnel, then it's presumably not just a semantic game.

> _If it is going to be reasonably efficient, it can't have a lot of air in
> it._

So he's designed pointless complexity into the system just for the sake of
saying it's not an evacuated tunnel. "It's not evacuated. It just doesn't have
a lot of air in it."

It's be a lot simpler if he just proposed a traditional partially evacuated
tunnel if that's what he wants.

> _So those are the design constraints for what Elon is thinking of. What can
> you propose?_

My lack of a proposal does not make yours feasible or sensible.

> _Now why would you not want an evacuated tunnel? Well the common complaint
> about an evacuated tunnel is how hard it is to maintain the vacuum, and what
> to do if containment is breached. My proposal gives a way to maintain the
> vacuum (the plungers). Minor breaches of containment are not that big of a
> problem (the air that gets in is blown out again)._

Your tunnel has all the same problems as an evacuated tunnel and more. A
standard evacuated tunnel also has a way to maintain the vacuum. It's called a
pump. This doesn't stop a breach from being problematic. You're ignoring the
fact that pushing all that air out of the way again is _hard_ , whether it's
being done with a pump or a piston. If there's a breach, your "plungers" will
run into huge pockets of air that have to be evacuated. This will slow them
down, potentially causing collisions depending on how large the breach is. It
will actually slow them much more than an evacuated tunnel, because they form
a plug in the tunnel, meaning they need to displace most of the air instead of
some of it.

Your plungers also have to form a tight seal in order for them to be
effective. I'm not even sure that's possible. 70 tons of mass flying through a
tunnel with a tiny gap all around. What happens when we hit that breach and
this mass wobbles a bit? How big does the gap have to be to ensure safety? How
big before the plunger isn't actually expelling gas anymore? And you admit
that the friction the plungers encounter will make them hot enough to be
deadly to humans. This is all wasted energy.

P.S. You also said this thing is going to be underwater. How on earth do you
imagine than an underwater breech would be minor? And how is this air being
expelled underwater anyway? Are you planning on building an exterior tunnel
big enough to hold an air gap plus the transit tunnel (plus the return tunnel)
all underwater? For a tenth of the cost of a bullet train?

~~~
btilly
Why are you inventing constraints that don't exist?

Elon's pauses make it clear to me, and presumably to you, that he was checking
what she said versus what he was thinking, and finding that, to a layman, the
two would seem similar. Even if underlying technologies are different.

His wording suggested strongly to me that his solution was a tube. Did it not
suggest that to you?

No matter how many times you say "pointless complexity", that doesn't mean
that there really is pointless complexity. (And we're disagreed on the
complexity.)

I'm quite aware of how hard it is to push air out of the way. A quick
comparison of the mass of the plunger and the mass of the air says that a
bubble of air will not actually slow the plunger down much. Being in air
constantly would, but a bubble of fixed size wouldn't. (You'd have to think
about what effects it might have on passengers inside.)

There is absolutely no need for a tight seal, as long as the flaps open and
shut at the right time. You can stand a long ways away from a moving truck and
still feel the wind pushing away. And the closer that it gets to Mach 1, the
farther away you can feel that gust.

The friction losses are hard for me to estimate. The upper limit on the
temperature of the plungers is caused by the heating of air that is suddenly
compressed to 1 atmosphere, and the friction from air moving past at hundreds
of miles per hour. However if only a little bit of air is involved, the
heating might happen at reasonable rates. Without parameters that I don't have
and a fluid dynamics simulation that I am not prepared to do, I can't tell.
That said, I can tell you that the rate of heating per volume (and therefore
the ease of dealing with it) goes down as you make the plungers larger, up as
you speed up, and down as you reduce air pressure. So if you have little
enough air and large enough objects moving through it, the heat generation
gets more and more reasonable.

And about your PS, I'm trying to guess at how Elon gets around right of ways.
Underwater is not part of my basic idea at all.

~~~
dpark
What constraints do you imagine I'm inventing? That it's practical?

Musk's pauses indicate to me that he was internally asking himself whether his
proposal is somehow like the Jetsons tubes. That doesn't necessarily mean he
was looking for literal parallels. And if he were, I don't see why "pneumatic
tube that sucks a single person up" would turn into just "tube" to a layman,
either. He's also mentioned the Concorde and a railgun. How do those fit into
your vision? Will a layperson see those in your car/tunnel system?

I'm not just saying "pointless complexity". I'm telling you why it's
pointless. Your system is certainly more complex than an evacuated tube, and
without a compelling reason to embrace such complexity, then it is indeed
pointless.

Let's step back a bit, though. You're saying that your system is easier to
implement than an evacuated tube. Let's assume that's true (though I don't
think it is). Is it also easier to implement than a partially evacuated tube?
Those are expected to travel at or past Mach I, and _don't need the car to
move nearly 100% of the air in the tunnel_. How is your proposal superior? You
talk about using the plungers to evacuate air, but the energy cost to remove
the air must still be paid. Your 70 ton slugs might carry more momentum, but a
car in a partially evacuated tube wouldn't need to displace all the air it
encounters, and so wouldn't need as much momentum. You're constantly paying
the cost to push around a bunch of air. That cost is largely avoided with a
more traditional design.

I'm not certain about your idea that you don't need a tight seal. Yes, at Mach
I a gust would blow pretty hard. But in a near-vacuum you'll not be blowing
much air. Remember that you're going to have to blow with >1 atmosphere of
pressure to push anything out and past the flaps. Also remember that this
"gust" is the result of resistance caused by the giant plug you're pushing
down the tube, resistance that another design might just avoid or at least
minimize. And any air not evacuated by the "gust" will contribute to further
drag as the plunger pushes past it.

------
axlerunner
It will be wonderful until the first time a plunger smashes a car with a
family to smithereens because the railgun timing was slightly off due to a
voltage irregularity.

~~~
btilly
There are clearly a lot of details that you have to get perfectly right.
However getting them right is an engineering challenge, not a physics
challenge, and engineering for safety is something we as a society tend to do
pretty well. (As opposed to engineering for cost containment, which we as a
society are often horrible at.)

If I saw Elon Musk standing up there with cost projections, I'd be optimistic
that they were in the right ballpark. If I saw virtually anyone else there,
for a project of this type, I'd be assuming that they missed order of
magnitude cost issues.

------
coanda
You might want to read this: [http://www.daniel-irimia.com/2012/04/19/daryl-
oster-vs-henri...](http://www.daniel-irimia.com/2012/04/19/daryl-oster-vs-
henri-coanda-ett-vs-aerotub-express/). It's been done as a research project in
Romania. Not sure if it can be patented...

~~~
lnanek2
Transporting mining material, freight, etc. seems like an excellent proving
ground. Much less chance of a disaster killing someone and killing the
project. Often there's a large amount that has to go from one place to another
place, not people who have many different places to go. I guess the only
problem is that freight often doesn't have to move fast. If it takes your
mining output an extra 8 hours to get to processing it doesn't matter. If it
takes a person an extra 8 hours, they'll pay less or use other forms of
transport, or require stopping stations...

------
reubensutton
I feel quite certain that Elon Musk will be remembered as our generation's
greatest entrepreneur.

Even if this idea doesn't succeed, the scale of his ambition is breathtaking.

------
TeMPOraL
How about this:

<http://www.youtube.com/watch?v=0DfDOlUXEBo>

It's chinese concept of a train that doesn't stop - you have the main body in
constant motion (going around in a _loop_ ), and the passenger cars just
attach to it and detach from it via magnetic forces. No need to stop the big
train, you can catch on at any time, and this likely could work with -
properly outfitted - cars.

------
Lerc
What are the advantages of using air instead of liquid? Is it just the
breathablity aspect?

I posted a few comments in the past on my guess. A liquid filled loop with a
(super)hydrophobic inner surface. Energy stored as kinetic, solar accelerated.

I have been looking, and haven't been able to find much info on reduced energy
loss by using hydrophobic surfaces. It's a new enough tech that all the good
info is behind academic paywalls.

~~~
jlgreco
Would pressure differences throughout the tube caused by differences in
altitude be something to worry about? I'm not sure how, if at all, that would
effect the hydrodynamics.

Also, I would be concerned that with that much fluid moving through it that
fast, a possible failure mode could be "temporarily turns into a horizontal
launch loop". (which would be awesome ;)

~~~
Lerc
* Failure would look Awesome

Always a winning bullet point.

------
archenemy
Any hypothesis that does not include electric drivetrains (ala Tesla) powered
by solar (ala Solarcity) is going to be wrong. Also, the 'loop' part strongly
implies that it will store excess energy. I think the 'immune to weather + no
need for right-of-way' issue is the most intriguing here.

~~~
dpark
"No need for right of way" is also enough to know that none of these tube
proposals are correct. You can't build a tube from SF to LA without running
into right of way concerns.

~~~
socialist_coder
Yeah. That is my assumption too. But all these proposals just seem to gloss
over it.

edit: Jacques admitted it was an oversight.

Still, it's interesting reading these ideas even if they might not be what
Elon is thinking.

~~~
jacquesm
One of the central supports against patents is that engineers given a problem
will all come up with roughly the same solution. So given that Elon has
dropped a number of hints the question then becomes: can a bunch of engineers
come up with his proposed solution before he reveals what it is.

That's a very interesting puzzle to me and I think this may be a way in which
the patent office could do their reviews: ask a community of smart cookies to
come up with what is in the (secret to that point) patent application, the one
that gets it right gets a part of the filing fee and the patent is denied, if
nobody can crack it in some set time (say 4 weeks) then the patent is granted.

------
lazyjones
Why does everyone expect this "tube" to be filled with air? A liquid is more
interesting and it also stores a lot more energy when it moves ...

~~~
btilly
If you can find a liquid with low enough friction to be feasible, I'm all
ears.

The only ones that I'm aware of are thin gases or helium-II. The latter is
rather impractical to use for a great many reasons. But I'd be interested if
anyone has other suggestions.

~~~
lazyjones
Are you talking about friction between carriage and liquid or walls and
(moving) liquid?

The liquid will move at the desired travel velocity, so I believe the latter
will be important.

