Musk's Hyperloop

[tom66]

I read the document today, if anyone has heard of it, it is a proposal for a high speed transit system based on essentially very low pressure air.

The physics behind it seem solid (but I don't know that much to be certain), but I do not believe it can be done for the $6bn claimed. Especially given some of the clearances in the tube are on the few-mm scale. What about welds between tube sections?

The document is here, for anyone wanting to read up on some of the ideas:
http://www.spacex.com/sites/spacex/files/hyperloop_alpha-20130812.pdf

[EEVblog]

Resistant to Earthquakes

How? 

[tom66]

Did you read the full document? He goes into how the axis dampers are supposed to help.

[EEVblog]

Did you read the full document? He goes into how the axis dampers are supposed to help.

Ah, didn't get that far yet. Yes, mentioned, but dampeners aren't going to help with "the big one". Or I suspect even a reasonable sized one.

BTW, am I the only one picturing the gun crazy yanks getting drunk and taking pot shots at the tube to see if they can puncture it? 

[c4757p]

Nope. I'm related to some of those Yanks.

[ElectroIrradiator]

Ohh, this is lovely. Let me see if I get this right...

The passenger pods each have a frontal area of 1.4 m², and their maximum travel speed is 1220 kph. For a start they will be launched at 2 minutes interval. So average distance in meters between any two pods, traveling at 1220 kph, is going to be:

Sep(m) = 1220 km/h * 1000 m/km * 2 min / 60 min/h = 40700 m.
Total tube volume in front of each pod is then 1.4 m² * 40700 m ~ 57000 m³.

Additionally, the operational pressure in the tube is 100 Pa, slightly less than 1/1000th of normal atmospheric pressure. Pressure this low cannot be survived without a full body pressure suit. So in case a pod has a major seal leak at the (outward facing, gullwing) door, their suggested fallback is to pressurize the tube, and have the pods putter to the end of the line on retractable wheels.

One m³ of air at 1 atmosphere of pressure weighs roughly one kg.

Question: How fast and how long can each pod travel on those retractable wheels, when they now have to accelerate and overcome the wall friction of the fiftyseven metric tonnes of air in front of them?

The full length of the tube is 560 km / 350 mi.

BTW, am I the only one picturing the gun crazy yanks getting drunk and taking pot shots at the tube to see if they can puncture it? 

The tube has a 20+ mm wall thickness, which begs the question: Are 50 cal Barrett M107s legal to own in CA? 

[tom66]

Indeed. But isn't the situation with rapid depressurisation similar to that of an aircraft, or spacecraft?  In both of those situations, there is absolutely no way out. Oxygen may deploy, but that isn't a long term solution, and doesn't help with low pressure. In the hyperloop situation, the wheels would possibly be electrically driven at a comparatively low speed -- approx 60mph for example -- which the large battery pack could easily achieve. Of course, since depressurisation events are incredibly rare in aircraft and spacecraft, the only real additional danger is from idiots with guns.  I don't know how that can be solved.

[ElectroIrradiator]

Indeed. But isn't the situation with rapid depressurisation similar to that of an aircraft, or spacecraft?  In both of those situations, there is absolutely no way out. Oxygen may deploy, but that isn't a long term solution, and doesn't help with low pressure. In the hyperloop situation, the wheels would possibly be electrically driven at a comparatively low speed -- approx 60mph for example -- which the large battery pack could easily achieve. Of course, since depressurisation events are incredibly rare in aircraft and spacecraft, the only real additional danger is from idiots with guns.  I don't know how that can be solved.

Well, that is what I am trying to say. After pressurizing the tube you are not facing the same situation, as you do when you drive your car on the open highway. In the open your car literally pushes the air out of its way, as there is lots of room for the air to move sideways into.

In an enclosed tube the situation is dramatically different. Here you either have to squeeze the air sufficiently to allow it to pass along the narrow opening between the tube and the pod (if indeed it is narrow?). Or you literally have to push the full weight of the air column in front of you, since it cannot get out of your way.

Conversely, even if we only need an overpressure of, say, 0.5 atmosphere to squeeze through the air, this would add a force of 7000 N to overcome.

[tom66]

Perhaps they could add periodic release/escape valves along the tube? Such devices would open on failure, meaning the car is pushing only against normal atmospheric pressure. That's not mentioned in the document though. 

[Phaedrus]

BTW, am I the only one picturing the gun crazy yanks getting drunk and taking pot shots at the tube to see if they can puncture it? 

The tube has a 20+ mm wall thickness, which begs the question: Are 50 cal Barrett M107s legal to own in CA? 

Nope. Though depending on what the walls are made of, several hits from a .308 in the same place might be enough to do the job.

The gun (and explosive) problem could possibly be solved by just slabbing the whole thing with AR500 armor plate, at an extra $1bn cost. 

[tom66]

If only it were placed on 50 ft+ pylons to stop the average idiot from firing at it...
(won't stop a trained marksman)

[Phaedrus]

Most guns have accurate ranges exceeding 50ft.  And most shooting is done at 75ft - 300ft

[tom66]

This shows precisely how much I know about guns. Maybe an argument for more sensible gun control, can understand one having a handgun (well I don't see it but it could be useful), but have no idea why you would need a 50cal to defend yourself or go hunting.
(*please do not let thread turn into gun control debate*)

[bbarbour]

I think guns are the least of the problems.  We're not all cowboys firing guns in the air all day long... that's just what the news media wants everyone to think.  After all, guns could take down subways, trains, cars, planes, etc... and you don't hear about people randomly shooting existing infrastructure too often.  Certainly not with heavy artillery.

This is nothing more than the daydreamings of an extreme thinker.  It's great to be discussing it but we won't be travelling around the globe in a few hours any time soon.  I once worked on a project to extend a highspeed rail just under a hundred miles and it was quoted at a little under $3bn.. A measurable portion of that was just the environmental impact studies and land acquisitions alone.  I applaud Musk for exciting the news media and getting the world to discuss, but clearly it's quite premature.  I'd wager the hardest challenge would be to get enough acceptance to make it financially sustainable before the money runs out.  I sure as hell won't be riding it! .

[FJV]

Don't know.

What I find hard to believe is that basically 4 iron bars with wood and filling underneath will be more expensive than 2 raised tubes on pylons.
At the very least the last option uses more material and residential real estate under a pylon will lose some value, savings in that area may be less than hoped for.

Also what precision of tube is needed for the air bearings to work? And they want to keep the tube below atmospheric pressure, that also adds to the cost.

You would need a long report full of math formulas to answer all such questions in detail.
A report which would be costly to make.

[nctnico]

I read the document today, if anyone has heard of it, it is a proposal for a high speed transit system based on essentially very low pressure air.

The physics behind it seem solid (but I don't know that much to be certain), but I do not believe it can be done for the $6bn claimed. Especially given some of the clearances in the tube are on the few-mm scale. What about welds between tube sections?

Elon must have seen the same James Bond movies I've seen 

[ElectroIrradiator]

Well, I realize Mr. Musk considers this version of the project description to be an Alpha release. However, given his track record, I had hoped for something I couldn't poke holes in, just by using a bit of high school physics.

The proposed cost, $68.4bn, for the high speed rail link did strike me as high, compared to what $5bn buys you in Denmark. Admittedly I don't know anything about the local conditions in CA, so this price may or may not be realistic over yonder.

[Phroon]

The proposed cost, $68.4bn, for the high speed rail link did strike me as high, compared to what $5bn buys you in Denmark. Admittedly I don't know anything about the local conditions in CA, so this price may or may not be realistic over yonder.

That's what, around 20km?  The high speed rail is on the 560km scale.

30 times bigger for 14 times the cost.  The estimate might actually be low.

[KTP]

I predict the Hyperloop would work really fastl at transferring cash from taxpayers to Musk.

If we are going to build something off the wall crazy, I would rather it be a space elevator

[mswhin63]

I wonder how much induction the human body can handle while sitting inside the pod! Pacemakers not accepted.

[free_electron]

Ohh, this is lovely. Let me see if I get this right...

The passenger pods each have a frontal area of 1.4 m², and their maximum travel speed is 1220 kph. For a start they will be launched at 2 minutes interval. So average distance in meters between any two pods, traveling at 1220 kph, is going to be:

Sep(m) = 1220 km/h * 1000 m/km * 2 min / 60 min/h = 40700 m.
Total tube volume in front of each pod is then 1.4 m² * 40700 m ~ 57000 m³.

Additionally, the operational pressure in the tube is 100 Pa, slightly less than 1/1000th of normal atmospheric pressure. Pressure this low cannot be survived without a full body pressure suit. So in case a pod has a major seal leak at the (outward facing, gullwing) door, their suggested fallback is to pressurize the tube, and have the pods putter to the end of the line on retractable wheels.

One m³ of air at 1 atmosphere of pressure weighs roughly one kg.

Question: How fast and how long can each pod travel on those retractable wheels, when they now have to accelerate and overcome the wall friction of the fiftyseven metric tonnes of air in front of them?

The full length of the tube is 560 km / 350 mi.

BTW, am I the only one picturing the gun crazy yanks getting drunk and taking pot shots at the tube to see if they can puncture it? 

The tube has a 20+ mm wall thickness, which begs the question: Are 50 cal Barrett M107s legal to own in CA? 

you didn;t read the explanation.
the nose of each capsule contains a turbofan creating a pressurized bubble around the main cabin. as the capsule travels forward the fan sucks in the air volume from front , pressureises it , sends it around the main capsule ( so the capsule floats ) and uses it as ehaust in the back to propel the capsule.

if a capsule fails the system comes to a halt and the system essentially repressurizes itself to atmospheric. it kinda works like a peristaltic pump. the capsule acts as the rotary vane and the turbofan in th enose of the capsule creates the pressuredifferential across the vane ( the capsule) low pressure up front , high pressur behind , so the damn thing essentially propels itself.

it's like sticking jet engines in a long pipe. they simply propel themselves forware. use a maglev construction to float the cabin in rest , while moving the air pressure takes over so less maglev power is required. the maglev provides contactless power to the motor driving the turbofan in the nose as well as the slife support systems for the humans on board.

[AG6QR]

He wants to have a cruise speed of 760 mph, or roughly 1220 kph for anyone outside the USA.  That's about ten times the speed limit on I-5, the highway which he wants to use for the route.  He claims the pylons can be put in the median between the northbound and southbound parts of the highway.  That's going to be one crazy ride.  When cornering, acceleration is proportional to the square of velocity.  So the cornering forces will be approximately 100 times what the route was designed for.  That highway has lots of bends.  It's gentle to drive at the speed limit, and I suspect it would be comfortable at double the speed limit.  But ten times?  Nope.  The route will need some serious straightening.  But his cost estimate is based on putting the pylons on the highway median, thus avoiding having to buy land or relocate things.  Good luck with that.  This won't be a "coffee and laptop on your tray table" style ride, but more like the wildest, darkest, longest-lasting roller coaster you've ever ridden.  Or else he'll have to allow for the cost of acquiring land for a straighter route, which won't be cheap.

And no, Dave, you're not the only one to think of the gun thing; the thought entered my mind, too.  That would be prime high value target for vandals and terrorists, and it only takes one to make a big mess.  Also, having driven that road more times than I can count, I know about the heavy truck traffic, lousy weather, and frequent accidents on that highway route.  If the pylons are in the middle of the highway, it's only a matter of time before a big truck will knock out a pylon.  Yes, we already have deadly accidents on the highway, and they're catastrophic for the victims, but this type of accident could not only have mass casualties (how fast can those air-suspended cars brake to a stop from cruise speed?), but when the tube is damaged, the entire route will be down for quite awhile until repairs are completed.

[GeoffS]

A bit of info on the power source can be found here 

[bbarbour]

It's gentle to drive at the speed limit, and I suspect it would be comfortable at double the speed limit.

I can attest that the I-5 route at 2x the speed limit is not very comfortable on the bends.  Forces start to become a bit strong at around 110-120mph, especially when on the inside of a corner.  Naturally the tube would have to bend far more gradually... I wonder how the farmers would feel about that.  Horizontal isn't the only issue, vertical becomes an issue towards Los Angeles.. you have to go through the mountains!

[Corporate666]

This shows precisely how much I know about guns. Maybe an argument for more sensible gun control, can understand one having a handgun (well I don't see it but it could be useful), but have no idea why you would need a 50cal to defend yourself or go hunting.
(*please do not let thread turn into gun control debate*)

Easy answer - the right to own guns in the USA is not premised on defense or hunting, therefore what one "needs" for those tasks is irrelevant.

[bbarbour]

This shows precisely how much I know about guns. Maybe an argument for more sensible gun control, can understand one having a handgun (well I don't see it but it could be useful), but have no idea why you would need a 50cal to defend yourself or go hunting.
(*please do not let thread turn into gun control debate*)

Easy answer - the right to own guns in the USA is not premised on defense or hunting, therefore what one "needs" for those tasks is irrelevant.

Very true.  The argument for guns is one based almost exclusively on principle.  It's something that will never be solved because both sides are correct and neither side is capable of admitting that.  A lovely modern day paradox really.

[Corporate666]

I remember several years ago thinking someone should invent some sort of ultra high speed tube that contains capsules and would be used for high speed transportation of packages.  I would think inanimate objects would be the best starting point before loading up humans.  I had thought of a vacuum pulled in a tube and using magnetics or air pressure to do the motion, but the turbofan idea is very neat!

I think the "people could shoot at it!" argument is a bit silly guys.  People can shoot at trains, planes and buses now, and it's not an epidemic in the USA by any stretch.  And before folks say "yeah but damaging the vehicle is different than damaging the infrastructure" - there are plenty of infrastructure targets out there that could be easily damaged by gun - power substations, water/gas/petroleum pipelines, and more.  And someone mentioned 20mm thickness earlier, I think?  20mm is not going to be penetrated by anything that's legal in CA.  I think you'd need a Barret .50 and armor piercing rounds for that.  IIRC, both illegal in CA, and even if someone smuggled one in from a less loony state, I doubt most "terrorists" are going to be spending tens of thousands of $$ to shoot a hole in a tube that gets patched and fixed quickly, not when the same $$ could be spent on causing real havoc.

Not sure why everyone is so paranoid about guns when anyone can very easily buy the materials to make thermite in any major city in the USA and have a much more effective "weapon of destruction" that could be used on gas pipelines or tossed into a power substation, etc, etc.

[GeoffS]

Please keep any discussions on guns to ones about ballistics and the affects on shooting at the Hyperloop and not about gun politics.

[vk6zgo]

I remember several years ago thinking someone should invent some sort of ultra high speed tube that contains capsules and would be used for high speed transportation of packages.  I would think inanimate objects would be the best starting point before loading up humans.  I had thought of a vacuum pulled in a tube and using magnetics or air pressure to do the motion, but the turbofan idea is very neat!

I think the "people could shoot at it!" argument is a bit silly guys.  People can shoot at trains, planes and buses now, and it's not an epidemic in the USA by any stretch.  And before folks say "yeah but damaging the vehicle is different than damaging the infrastructure" - there are plenty of infrastructure targets out there that could be easily damaged by gun - power substations, water/gas/petroleum pipelines, and more.  And someone mentioned 20mm thickness earlier, I think?  20mm is not going to be penetrated by anything that's legal in CA.  I think you'd need a Barret .50 and armor piercing rounds for that.  IIRC, both illegal in CA, and even if someone smuggled one in from a less loony state, I doubt most "terrorists" are going to be spending tens of thousands of $$ to shoot a hole in a tube that gets patched and fixed quickly, not when the same $$ could be spent on causing real havoc.

Not sure why everyone is so paranoid about guns when anyone can very easily buy the materials to make thermite in any major city in the USA and have a much more effective "weapon of destruction" that could be used on gas pipelines or tossed into a power substation, etc, etc.

I agree,there's not a lot of stuff out there which could make a hole in a wall that thick,& even if it could,the tube would not instantly go to atmospheric pressure through such a hole.

The possibility of idiots shooting at it?

I'm not so sure!
The latest fad among morons in Oz,is to drop rocks onto cars from pedestrian overpasses.
I'm quite sure if they could get hold of a gun,they would shoot at them as well.

If such behaviour is rare in the USA,maybe you have less nuts per capita.

I doubt the practicality of such a system,though,due to the other issues raised in some of the earlier comments.

[tom66]

He wants to have a cruise speed of 760 mph, or roughly 1220 kph for anyone outside the USA.  That's about ten times the speed limit on I-5, the highway which he wants to use for the route.  He claims the pylons can be put in the median between the northbound and southbound parts of the highway.  That's going to be one crazy ride.  When cornering, acceleration is proportional to the square of velocity.  So the cornering forces will be approximately 100 times what the route was designed for.  That highway has lots of bends.  It's gentle to drive at the speed limit, and I suspect it would be comfortable at double the speed limit.  But ten times?  Nope.  The route will need some serious straightening.  But his cost estimate is based on putting the pylons on the highway median, thus avoiding having to buy land or relocate things.  Good luck with that.  This won't be a "coffee and laptop on your tray table" style ride, but more like the wildest, darkest, longest-lasting roller coaster you've ever ridden.  Or else he'll have to allow for the cost of acquiring land for a straighter route, which won't be cheap.

If you look at the design document he handles this, the route only follows the I5 where possible to maintain ~3mile turning curves. Musk claims that since the Hyperloop is essentially similar to electrical pylons (suspended) in that it can be built on existing land, without causing as much disruption as the proposed HSR.

[AG6QR]

If you look at the design document he handles this, the route only follows the I5 where possible to maintain ~3mile turning curves. Musk claims that since the Hyperloop is essentially similar to electrical pylons (suspended) in that it can be built on existing land, without causing as much disruption as the proposed HSR.

But do the math.  760mph around a 3 mile radius.  Convert to metric, that's 340 meters/sec around a curve of radius 4828 meters.  Acceleration is v^2/r,  for 23.9 m/sec^2.  That's 2.5g.  Certainly survivable, but not comfortable.  Especially since passengers will be entering and exiting those 2.5g turns every few seconds.

My point is that the I-5 alignment is too twisty for 760 mph.  The idea that the land costs will be negligible because it'll all be in the median of I-5 is wishful thinking.  Very little of the I-5 median can be used.  A straighter route will have to be made across the farmland in the Central Valley.  Cheaper to build on top of pylons than directly on the ground?  Maybe, but it's not obvious to me.  If that were the case, we should build more of our ordinary roads on elevated pylons.

[tom66]

I was reading the wrong part, the minimum bend radius is 14.6 miles at 760mph, which sets it closer to 0.5g.

But, you are right. While they can follow the median in parts, they will have to build over farmland. And the NIMBYers will have the same problem as with this, as they have with wind turbines being put on land. A wind turbine isn't particularly disruptive to farmland or to nature (probably about the same as a hyperloop pylon!), but don't tell that to anyone living in the countryside... they hate them. Presumably they'd rather see a big coal fired power station instead of wind turbines...

[ElectroIrradiator]

That's what, around 20km?  The high speed rail is on the 560km scale.

30 times bigger for 14 times the cost.  The estimate might actually be low.

Total length, including the new access roads and rails on land, is about 20 km, yep. About 14 km of the total is made of various bridges and tunnels, 3 in total. The cost then includes an 8km / 5mi dual track undersea railway tunnel, the third largest suspension bridge in the world plus the west box girder bridge. That one is actually two independent bridges of the same height, built right next to each other. The total width would have been impractically wide if it was built as a single construction.

So the project has about 28 km of bridge or tunnel included in the price tag.

Which is why I was wondering. If you just wanted some fairly straight and level high speed rail track, including foundation, power supply, signalling and stations, then you will get a *lot* of it for $68bn. Both the Germans, the French and the Japanese have decades worth of experience here, and at least the French and the Germans can assemble a high speed rail line pretty much in assembly line fashion.

What I don't know anything about are the local conditions in CA when it comes to geography and property rights, both of which are the big jokers here, of course.

you didn;t read the explanation.
the nose of each capsule contains a turbofan creating a pressurized bubble around the main cabin. as the capsule travels forward the fan sucks in the air volume from front , pressureises it , sends it around the main capsule ( so the capsule floats ) and uses it as ehaust in the back to propel the capsule.

if a capsule fails the system comes to a halt and the system essentially repressurizes itself to atmospheric. it kinda works like a peristaltic pump. the capsule acts as the rotary vane and the turbofan in th enose of the capsule creates the pressuredifferential across the vane ( the capsule) low pressure up front , high pressur behind , so the damn thing essentially propels itself.

it's like sticking jet engines in a long pipe. they simply propel themselves forware. use a maglev construction to float the cabin in rest , while moving the air pressure takes over so less maglev power is required. the maglev provides contactless power to the motor driving the turbofan in the nose as well as the slife support systems for the humans on board.

Actually I had fully read the complete document, when I wrote my previous post. I had considered the possibility of using the turbofan to create a virtual vacuum around the pod, thus reducing drag to nearly nothing (except for rolling and bearing losses in the wheels, of course).

Bottom line: Even at vastly reduced speed the energy budget doesn't add up, if the system is forced to run at atmospheric pressure during an emergency.

Some additional napkin math: In round numbers, then, if you slow the pods from 1200 to 100 kph (60mph) during an emergency, then the apparent air pressure in front of the pod drops to 1/12th of normal (high vacuum) conditions. However, atmospheric pressure is more than 1000 (one thousand) times higher than operational pressure. So during an emergency the total mass of the air, which has to be moved by the fan per second when moving at 100 kph, is 1000/12 ~ 80 times higher than usual. This is before we consider drag losses in the 'landing wheels'.

The different pressures means we probably cannot even get the high efficiency of the turbofan at atmospheric pressure, as compared to normal operation in a very low pressure. All the heat exchangers and tubing will ensure this. But let us for now assume that we 'only' need 80 times more energy per time to move the required amount of air at atmospheric pressure and under turbofan power.

Everything else being equal, this means we now move 12 times slower than maximum speed, and per time we need to expend roughly eighty times more energy to create our virtual vacuum bubble to eliminate air drag. Normal travel time is 35 minutes. Unfortunately during an emergency we will have pods, which has just entered the tube at one end when it is pressurized. So they have to travel about 560 km at 100 kph, taking 5.6 hours. As a fraction this increases the travel time by 5.6*60/35 = 9.3 times.

So the battery capacity has to be 80 * 9.3 ~ 750 times higher than what is required for completing the journey under low pressure conditions.

Which is probably a bit ineconomical.

...and then I haven't yet mentioned the problem with engine cooling during an emergency.

[edavid]

Bottom line: Even at vastly reduced speed the energy budget doesn't add up, if the system is forced to run at atmospheric pressure during an emergency.

It doesn't have to, they just have to run the pods to the next emergency exit.  They could also have tow pods/tugs.  Worst case, they cut open the tubes.   Emergencies have to be rare for the system to work.

[ElectroIrradiator]

It doesn't have to, they just have to run the pods to the next emergency exit.  They could also have tow pods/tugs.  Worst case, they cut open the tubes.   Emergencies have to be rare for the system to work.

The design document uses a fair amount of handwavium on the security issues, so I find it hard to judge whether numerous safety exits are included in the rough budget estimate. Thus I have to use the worst case scenario, until more detailed information is available.

[Phaedrus]

What I don't know anything about are the local conditions in CA when it comes to geography and property rights, both of which are the big jokers here, of course.

I don't know about property rights, but I do know the geography. Going north from LA there's about 20km of mountains, and then you're in the San Joaquin valley. Then it's a flat, straight shot all the way to San Francisco, with nothing but some hills at the very end.

[G7PSK]

I think this was tried in the 1880's 1890's in london for a passenger transport system it did not work then. A similar system is used some places to move money from till to accounts.

[Wytnucls]

For what it's worth, one or two bullet-size holes would not dramatically affect partial vacuum in the tube, just like an aircraft would not depressurize under the same condition. Of course, if the tube starts looking like a sieve, all bets are off.
One would imagine that a whole series of extracting pumps would be installed along the route and possibly some self sealing material would be included in the tube skin layers, if not prohibitively expensive.