Hyperloop: riding sound’s density peak to exploit the drag equation?
charlesalexander2013.wordpress.com
charlesalexander2013.wordpress.com
The "density front" travels at exactly the speed of sound, but it consists of nearly static particles (not static from the frame of reference, from the frame of reference they are moving backward at nearly the speed of sound), these static particles are the relevant V.
The sparse particles just in front of the dense ones are accelerating to get more sparse. From the perspective of the reference frame, they are therefore moving 'backward' at slightly less than the speed of sound, making there relative velocity closer to that of the reference frame. So lower density and lower velocity. and that doesn't even account for additional compression which I don't even know enough to do armchair reasoning on.
So, I'm the farthest thing from an expert, but this is why it doesn't seem to make sense to me.
The difference between the dense particles just behind the craft with maximal drag and sparse particles just in front, means a pressure differential pushing the vehicle forward
http://www.acs.psu.edu/drussell/Demos/waves/Lwave-v8.gif
In all seriousness though I'm going through your post to see if you've found a gaping problem =). I don't think there can be different density velocity ratios at any point of displacement in the wave's reference frame because the same net number of particles has to pass through each point or you start making a permanent buildup on one side or the other of that point, which you can't do because in the reference frame of the still air that point is moving at the speed of sound.
A moving point passes by the same number of unique particles at the speed of sound as it would if it were going the speed of turtle (if you ignore thermal dispersion).
Agreed: the corollary to the "the sparse particles just in front of the dense ones" is the "sparse ones just behind the next wave of dense ones", which are, in fact, decelerating, or moving backward even closer to the speed of sound than the dense ones relative to the reference frame. So individual particles oscillate back and forth, but net effect is no movement (to the reference frame of the average speed of the particles, which is a third reference frame :-) )
Shouldn't you put the front of the craft at the pressure minimum and the back of the craft at the pressure maximum? Then the wave would be pushing you along.
As time goes on he keeps getting more and more specific with his deadline for going public though. First it was "eventually", then it was "sometime after Tesla is profitable", then it was "sometime later this year", now we've actually got a date to look forward to. I now think that he actually is thinking of something.
"A tube, a car, a revolving fan! Little more is required. The ponderous locomotive, with its various appurtenances, is dispensed with, and the light aerial fluid that we breathe is the substituted motor!" -- http://www.columbia.edu/~brennan/beach/chapter2.html
Trains in tubes and promises of dramatic changes in the way people think of transportation! People thought they would be building tubes all over the countryside to move trains around... and then they went and invented the electric-multiple-unit subway train which took off instead. :P
Better luck this time?
[The Segway] will be to the car what the car was to the horse and buggy (Dean Kamen)[1]
John Doerr, predicted Segway would rack up $1 billion in sales faster than any company in history [1]
Let's hope the Hyperloop fares better.
A system allowing to travel LA to SF in 30 minutes, without the hassle involved with air travel, WOULD be revolutionary, regardless of how good or bad is the marketing department of its company.
http://www.paulgraham.com/segway.html
No one predicted this would be the problem, and indeed most people still haven't come to Graham's conclusion. In that sense, concrete metrics are irrelevant. The hyperloop is just as likely a priori to have some sort of similar fatal flaw. This is the problem with getting too excited about any new particular device; there's too much that can go wrong between zero and revolutionary.
Doerr has been equally bullish about Google Glass. Still, Glass is fast developing a stigma, and it’s not even on the market yet: It’s been called a “Segway for your face.” [1]
[1] http://pandodaily.com/2013/05/23/john-doerr-doesnt-think-goo...
> But when he rides the Segwell, they shout abuse from their cars: "Too lazy to walk, ya fuckin homo?"
> Why do Segways provoke this reaction? The reason you look like a dork riding a Segway is that you look smug. You don't seem to be working hard enough.
What is wrong with people?
Where do they travel in European cities? Bicycle paths?
Regarding seeing them being used by tourists etc. I imagine the prohibition exists when framed in 'this will change everything - everyone will have one!' context, which is different from the reality of 'these are actually rubbish - almost nobody wants one'.
The first mobile phone users looked like smug jerks and show offs as well (What, are you too lazy to walk to a wired phone ? Are you so important that people must be able to contact you at any time ?) , yet the functionality offered was so useful that in the end everybody followed suit.
While I'd love a Segway, it's usefulness is much less than my cellphone to me. Maybe if cities had been redesigned around it, things would be different
For me, the appeal is that I live a 20 minute walk from the train station I use for my commutes. A lot of the time I don't mind that walk, or I take the bus. I don't have a car. A Segway would make the bus uninteresting except in really bad weather - it'd be about as fast and not tie me to the bus schedule. It'd also make dragging shopping around easier.
But those are slight conveniences, and conveniences that are even less of a deal for someone with a car (who'd just drive their shopping home), and that has become less important as time has passed: These days I have a shortcut to a countdown display for my local bus stops on my phone, so while I sometimes have to wait, when leaving home I can wait in my living room and walk out just in time for the bus, and when returning I always know if it's worth waiting for the bus or starting to walk or flag down a taxi. And we get the shopping delivered to our door step, in a one hour time slot every week, and if we forget to order, they automatically put together an order that is ok based on our usual order pattern, so grocery shopping is totally off my radar.
I still think it'd be great if Segway's get to the point where they become accepted, and maybe they still can (around London, there's tons of people on silly looking kick scooters, some of which are motorized, so clearly some people are prepared for alternatives...).
But they'd first become revolutionary if they reach a sufficient level of use displacing cars that it'd change city cores, and that, I'm afraid would take a miracle or a lot of time.
This just isn't true. The reason that Americans give up cycling as they get older is because it's inconvenient. Plenty of cities in other countries make cycling convenient, so a lot more people do it. Sure, there's a substantial portion of the population (15-20%?) who aren't physically capable of cycling, but that doesn't mean that getting closer to 40% of people commuting on bikes like Amsterdam does is impossible.
https://secure.flickr.com/photos/16nine/sets/721576183922194...
It was only hyped because it was "cool".
If the hyperloop, however it works, can duplicate the capabilities of a high speed rail system in terms of passenger throughput, speed, and waiting time and do so safely at much lower cost then even if was the uncoolest thing in the world it would still be of immense practical benefit.
If it's using the air-hockey principle, I'd expect reduced forward pressure to below the limit on which the vehicle can float, so it starts to skid along the ground, hopefully with enough inter-car spacing that you don't get a pileup.
A less catastrophic situation might be, what happens if the pipe has a relatively minor leak due to subsidence or whatever? Just a loss of efficiency, or loss of function?
You may have a different opinion. It gets brought up because not all of the world shares your opinion.
Generally the TSA x-rays my bags looking for weapons and makes me walk through a metal detector, then I go on my way. Sometimes the line is 15-20 minutes, though usually it's less than 5. Sometimes there's a millimeter wave body scanner that some might find intrusive - personally I couldn't care less if a TSA employee sees my junk, and those that object can opt for a pat down. Those that object to being touched might be better served by a therapist than a plane ride.
This objection to flying because of the TSA is a psychological problem on your part and has nothing to do with reality. Unless you are carrying prohibited items - knives, liquids, etc - you will almost never encounter a problem with the TSA. Sure, the rule may be arbitrary and ridiculous, but they are not particularly onerous or oppressive either. I certainly make comments on bad policies and advocate for change. The system is nowhere close to the gestapo crazies seem to make it out to be.
Of course, you're likely psychotically obsessed so this will mean nothing to you, but get some help.
Your claim that they do not treat people as criminals because you don't mind or encounter trouble is laughable. Some of us don't have a case of Stockholm Syndrome to sooth the abuse.
Of course as someone who sympathizes with and advocates for your aggressors you won't understand where I am coming from, but get some help.
"I'm going to feel with my glove inside your waistband now..."
"It's rather clear you have no idea what criminals are treated like."
TSA have always treated me and everyone I know decently. Airport security is basically the same as it was 30 years ago. It's really disgusting and irresponsible to attack the TSA for carefully and respectfully enforcing regulations. I have no problem attacking idiotic regulations and regulators.
And, of course, there's the taxpayer money wasted—if we're going to employ that many people it may as well be doing something useful rather than attempting to stop all possible methods of bringing down an explosive tin can tens of thousands of feet in the air.
Metrics are like statistics, in that they can also mislead. It's really about the experience in specific contexts.
> It was only hyped because it was "cool".
Actually, it failed because it just looked dorky. (Which Steve Jobs predicted in his secret meeting with Dean Kamen.) Good tech badly marketed sometimes still doesn't sell.
"What does everyone think about the design?" asked Doerr, switching subjects.
"What do you think?" said Jobs to Tim. It was a challenge, not a question.
"I think it's coming along," said Tim, "though we expect—" "I think it sucks!" said Jobs.
His vehemence made Tim pause. "Why?" he asked, a bit stiffly.
"It just does."
"In what sense?" said Tim, getting his feet back under him. "Give me a clue."
"Its shape is not innovative, it's not elegant, it doesn't feel anthropomorphic," said Jobs, ticking off three of his design mantras.
"You have this incredibly innovative machine but it looks very traditional." The last word delivered like a stab.
...
"Screw the lead times. You don't have a great product yet! I know burn rates are important, but you'll only get one shot at this, and if you blow it, it's over."
It cost many times more than even high end bicycles, without being faster, without being able to be used on streets, without having as much cargo capacity, and while depriving you of exercise.
It cost many times more than a scooter, while being slower, having far less range, less cargo capacity, and not being usable on streets.
And so on.
If it had cost $200 it might have had a chance to find a niche, but at $3k or so it's just a toy.
It cost many times more than a scooter, while being slower, having far less range, less cargo capacity, and not being usable on streets.
What you are saying highlights the reason it failed, while missing the point of the device.
It was not for the bicycle use case. It was not meant to be faster or to be used on the streets. You can carry a couple of modest panniers worth of cargo, and touring-bike loads would again be besides the point. It was meant to deprive you of exercise.
The Segway was meant to create a "new" use case and market segment in the same way that the iPhone and iPad did. (You can find plenty of objections analogous to the ones you list for the iPhone and iPad.) In that kind of context, dorky is going to kill you.
at $3k or so it's just a toy.
What Steve Jobs understood was that it's okay to start out as a toy when creating a new market segment. However, you have to be cool enough while being cheap enough. The Segway had a too high a price point and a too low of a cool factor.
In this case, the wave (and the vehicle) would be moving at the speed of sound, but the air particles they hit wouldn't. They're basically still stationary. There is no magical force to bring them up to the speed of sound before they hit the vehicle. Hence, the drag is not reduced.
I'm a physicist, but by no means an expert on sound. There are more details on how sound actually works on this page: http://mysite.du.edu/~jcalvert/waves/soundwav.htm. A relevant quote: "v [the particle velocity] is much less than the phase [wave] velocity of sound" (note: phase and group velocity are equal for sound in air).
Edit: This criticism is not right; see below.
At no point is the air doing anything but apply drag to the vehicle's movement; there is just a sweet spot to minimize drag, but it isn't minimized to zero.
Read through this thread a bit and you'll see me talking about the exact same difference of propagation of a disturbance vs displacement/velocity of individual particles...
Why is this problematic? Well, once you have a shock wave (e.g., a wave crashing on the beach), the physics gets a lot more complicated. You are basically riding an explosion in a tube.
At this point, it no longer safe to me to neglect factors like dissipation. Your shock wave is going to heat up the air and eventually fizzle out. If you want to keep it going, you'll need to be continually supplying tremendous amounts of energy at exactly the right time along your tube. You'll also have to be continually cooling or cycling the air to relieve the waste energy.
To me, it seems a lot simpler and more reliable (not to mention safer) to simply evacuate your tube, which, I believe, is the original hyperloop idea.
When the wave is very strong the heat generated by compression isn't dissipated adiabatically the wave starts losing energy to waste heat; if you increase the wavelength you again move back to the adiabatic regime. How far can you take it? I don't know, I'd like to see some numbers and that's probably what I'd work on next if I keep looking into this.. probably won't until after Elon's announcement.
For extreme cases of high-amplitude waves where the particle velocity itself is getting close to the speed of sound, I definitely don't know what all happens, but somewhere in the write-up or comments here I at least had going that far as a bound on how far you could push things before you didn't really even have sound waves anymore.
Musk has said on the record that it's not an evacuated tube.
Hmmm... This should not be too tricky, since you are generating the original signal yourself, but if one of the exterior noise-cancellation speakers breaks down - it's gonna be LOUD.
If you have sound moving through the tube, you have energy pushing the sound. Either you have to keep the whole tube 'humming' or you have to send people off in a blast wave. With the humming idea, you might be able to create vacuums that travel around the tube at the speed of sound (sound works by changing the density of air, oscillating from high pressure to low pressure), and then you can move vehicles around the tube at the speed of sound with very little drag.
So then you need some way to keep the vehicles floating, and you need to get them to the speed of sound in the first place (rail gun?). I'm going to pretend that we have a cheap way to do both of those things.
So, how expensive would it be to keep 100s of miles of hyperloop humming with a sound powerful enough to create vacuums large enough to fit transportation units in? Even if you are only using cars large enough to hold 1 person and a suitcase, you are going to need a lot of energy powering the sound waves throughout the tube.
You might be able to do some acoustics-like engineering to keep the sound loud at little energy cost, but I don't think that solar power above the tunnel is going to be enough. Using this model, Musk would need a handful of technological advances up his sleeve.
So I think the big 'secret' that makes a hyperloop viable is not this one. Then again, with the Tesla and his space adventures, he has had to deal with at least a handful of difficult obstacles. So maybe he is expecting to develop multiple technologies to make this viable.
If you read my post it has nothing to do with vacuums and instead somewhat counter-intuitively keeps the vehicle within the densest part of the sound wave the entire time.
Riding the pressure peak at exactly the speed of sound makes a lot more sense.
Now when Elon says it's not an evacuated tunnel, and also (indirectly) that you'll be traveling more or less exactly at the speed of sound, I'm leaning quite heavily towards the OP.
I can't find a link describing the calculation. This link talks about how the change in pressure is equivalent to "140 molecules for every million molecules" http://www.silcom.com/~aludwig/musicand.htm
So there is no way you are going to create a bubble of vacuum!
Also, the idea here isn't involving high frequency sound.
And you don't want low pressure, but high, so the air speed is low. Drag is proportional to the density, but proportional to the square of the speed.
But I suspect you'd always be better off by simply evacuating the tube as much as possible.
Or possibly there is a medium other than air being considered, one in which the speed of sound was much lower at a given density. Perhaps interesting things could happen if the working fluid was a refrigerant and the sound wave was intense enough to induce a phase change cycle.
Is the velocity in the drag equation relative to the vehicle, or to a stationary frame of reference?
Edit: Or is the point that the velocity of the air particles within the density peak is at its maximal forward-traveling rate before they start to loop back around and form their ellipse?
In that context, I don't really see a transformational improvement going from one 1 hour to 0.5 hours.
6 hours compared to 30 minutes (or more likely, 1.5 hours including the necessary boarding / unboarding) is a VAST difference.
It's amazing how easy it is to get in and out of different countries. Even getting on the ship is pretty easy.
I don't think there'll be much security theatre until a train gets blown up - most of the theatre so far seems to be reactive. "An idiot has explosives in his shoes. He is so incompetent that he tries to light it with a match" and now everyone has to take their shoes off.
You could perhaps... stop it?
And if you really want to cause havoc messing with trains, you go for cargo trains filled with hundreds of thousands of tons of toxic chemicals.
and then plowing into a strawberry field near Coalinga. Yeah, that's "a lot" of damage.
You are right that it would take less power.
... so we'll do it with trains because no one will stop us?
(faulty social analysis)
I'd like to think that if Elon were running it (doesn't look like he wants to though), it wouldn't be a problem. He's a brilliant engineer who designs systems to overcome difficult problems. The 'security theatre' is just a consequence of an extremely poorly-designed system. I'm pretty sure someone like Elon could exploit modern sensing equipment and AI/ML techniques to build a system that is demonstrably better than, say, the TSA, and doesn't require anyone's personal space to be invaded. Who knows - maybe we'll get lucky and 'secure, 90 second check-in' be an integral feature of his design?!
TSA security checkpoints suck, but IME they "only" represent about 15 or so minutes of the extra time required for airport lobby time (on average; sometimes more sometimes less. Which is at least 14 minutes too many, but hardly the whole 1-4 hour overhead often cited).
Most of the overhead of plane travel in terms of airport and tarmac waiting time is due to deadlocking problems that occur in coordinating all the passengers into a single vehicle that is as full as possible (to maximize the airline's profit per trip), so you get a very uneven/lumpy demand curve based on when flights are scheduled, all sorts of complexity due to the system of having to check bags into a separate physical compartment, etc. A continually operating system where you could just show up whenever, your bags stay with you for the ride, etc could easily smooth that out. Solve the deadlocking issue (and most of the hyperloop guesses I've seen do) and even if the security checkpoint stuff stays mostly the same you'll still have a far more efficient embark/disembark system.
Times always add up like this - 0.5h to get to the airport, 1.5h on the airport, X h of flight and then up to 0.5h to leave the airport and another 0.5h to get to the city.
You suggest that we will tear up the hearts of LA and SF to build direct train-tubes, and then we'll let people breeze on board?
Currently with the only "fast" train in the US, you don't need to get on early, or go through special security. Why would this be different?
GIF that does a good job of explaining this: http://upload.wikimedia.org/wikipedia/commons/e/e4/Doppleref... That leading edge where all the circles are kind of meeting up? That is where the sonic boom is. It continuously sweeps across the landscape behind the source, as long as the source is moving faster than the speed of sound.
Going faster tightens that pressure wave cone, but there are a lot of other factors too. Vehicle geometry seems to be a pretty dominating factor for example, longer aircraft have less severe sonic booms. Depending on the shape of your aircraft you may or may not get several shockwaves originating from different points that after a distance combine to form a larger shockwave. Altitude is a biggie too, the higher you are the less intense it will be.
Take note of how the F-104 and Concorde compare (http://en.wikipedia.org/wiki/Sonic_boom#Measurement_and_exam...). The F-104 moves slightly slower, slightly lower, but has a much weaker sonic boom.
Why not go from Silicon Valley to a mountain area where you've bought a large area of land, ready to build on?
You could get investments from all over the tech industry, since it would lower living costs (and increase the maximum number) of the people that Silicon Valley can have (less infrastructure and living area "load").
Use the sales slogan "let's supersize the Silicon Valley!" for this new commuting solution.
Every time I hear this project spoken of, I have to hear people who live in the Bay Area talking about "SF to LA in half an hour"
I think what would be much more sensible would be "NY to Boston in 15 minutes" and then extending it to Philadelphia and DC. Where would the line go after it hit SF? Portland and Seattle? Denver? The populations of these places don't support the cost of the infrastructure. The east coast makes much more sense for this project.
The station cost will probably be high; it will be hard to have "side tracks". (No fan-out, you could say.)
So if there is a station in e.g. NY/LA, the housing costs for the other end station(s) ought to rapidly close with the big-city end station. It is just a way of opening a larger area for commuting.
So, there won't be that much comforts of New England.
My point was that with a HL the areas with high people pressure could continue to grow (I selected Silicon Valley because HN is a California-centric place, for good and bad. I'm in Europe.)
https://en.wikipedia.org/wiki/World%27s_busiest_air_routes#U...
I say this as a seasoned east coast corporate whore. Many, many business travellers are forced to just drive from New York to Boston due to the fact that the trip is 3 and a half hours long in a car. NYC to DC is about 3 hours 44 minutes by car. These are distances which are just short enough to make a company tell an employee to drive instead of fly, but long enough that nobody in their right mind would want to commute this way on a semi-regular basis.
http://www.wolframalpha.com/input/?i=population+of+metro+was...
Not counting Baltimore, the East Coast corridor has over 35 million people in the big city's metro area. If the price of this service is expected to be similar to air travel, then I get it (SF-LA), if this is simply about selling speed for a premium price...... but I think this is really about doing it faster AND cheaper. If its efficient enough, this could lead to a decoupling of workplace from home location, the way telecommuting was supposed to.
- Trains: the Acela is established and successful, already taking about half of the passengers on the Eastern corridor.
- Right of way: (to borrow one of your sentences) the population density of thses places [East Coast] drives up the cost of the infrastructure.
Another factor might be that the design is better optimized for 400 miles than for 200. The capsule acceleration phase is necessarily long and energy-intensive, so you want to optimize that over longer distances. It's the same reason that high-speed trains don't have stops every 20 miles.
Putting on my well-worn cynic hat, I wonder how much SV's anti-remote attitude is influenced by people whose inflated property values depend on SV's anti-remote attitude.
I seriously doubt that this is going to happen between SF-LA first. If this is feasible idea, he will be able to sell it to Japan, China and EU countries first.