# Elon Musk's "Hyperloop": LA to SF in 30 Minutes



## Lenny (Aug 13, 2013)

After a year of teasing, Elon Musk (of PayPal, Tesla Motors, and SpaceX fame) has released a 56-page document detailing the technical specifications of what he calls the *Hyperloop* - a high speed transit system that could transport passengers from Los Angeles to San Francisco, and other city pairs less than 1,000 miles apart, in around half an hour.

The Hyperloop would consist of capsules, each capable of carrying 28 people, travelling at up to 760mph through tubes built atop pylons twenty-feet high, and spaced up to 100 feet apart. When the concept was first announced, experts came to the conclusion that the system would work either by forcing air through the tubes at very high speeds (much like pneumatic tube systems), or by creating a total vacuum within the tubes to eliminate friction. 

Musk's proposal actually sits between the two: the tubes would have a very low air pressure (about a tenth of the atmosphere on Mars), and the capsules would ride on air sucked through the capsule nose (which would also reduce drag). Acceleration (and deceleration at the destination end) is achieved using linear induction motors spaced throughout the tube (much like a railgun). The tubes would have solar panels on top of them to power the entire system, with extra energy being stored in lithium-ion battery packs for travel during the night and periods of extended cloudy weather.

One of the more incredible parts to this is that the technology that could accomplish such a system already exists - Tesla cars use similar induction motors and battery packs, and the alloys necessary for the tubes (they have to be able to withstand the internal pressure) are used in SpaceX's rockets.

There is a summary of the project at the start of the document that Musk released, followed by fifty pages of technical engineering and physics. Ars Technica also have a good summary article.

Ars Technica - Hyperloop
http://www.spacex.com/sites/spacex/files/hyperloop_alpha-20130812.pdf

Rather than keeping the Hyperloop specification secret, and patenting every part of it, Elon Musk has released the entire thing publicly, with the hope that people will read it, find any errors, and improve upon the design. Musk has also said that whilst building the Hyperloop is not his priority, he will probably build a prototype, and that he may actually build the entire project if nothing has happened in three to four years.

Exciting times!


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## Brian G Turner (Aug 13, 2013)

760mph?? How would the system deal with hypersonic pressures? Also, hope they accelerate slowly.


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## Lenny (Aug 13, 2013)

I know nothing about aerodynamics or fluid mechanics, so I can only guess that hypersonic pressure would be the build-up of air in front of the capsule as it moves (the document compares the capsule moving through the tube as behaving "like a syringe", pushing the entire column of air through the tube ahead of it). That's where the compressor fan on the nose of each capsule comes in:



> Nature’s top speed law for a given tube to pod area ratio is known as the Kantrowitz limit. This is highly problematic, as it forces you to either go slowly or have a super huge diameter tube. Interestingly, there are usually two solutions to the Kantrowitz limit – one where you go slowly and one where you go really, really fast.
> 
> . . .
> 
> ...







Section *4.1 Capsule* (page 11) of Musk's document details the capsule design, and there's a section on the compressor fan (page 17, I think).

There's also a discussion on the geometry of the tube, and how that will also decrease pressure in section 4.2.1 (page 25).

---

The acceleration is described in section *4.3 Propulsion* (page 32):



> The propulsion system has these basic requirements:
> 
> 1. Accelerate the capsule from 0 to 300 mph (480 kph) for relatively low speed travel in urban areas.
> 2. Maintain the capsule at 300 mph (480 kph) as necessary, including during ascents over the mountains surrounding Los Angeles and San Francisco.
> ...


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## Null_Zone (Aug 13, 2013)

It might just be my taste in 1970s disaster movies but building this is just asking for California to fall into the pacific/asteroid impact/killer bea infestation. Probably whilst a deadly virus has been released onboard.


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## JoanDrake (Aug 13, 2013)

An average jet moves 500mph, so I'm not sure an increase to just 760 mph would be worth it. 

We've had the Concorde, which was about that fast or a little over, and they discontinued it. It seems we may be at the upper limit for useful speed in our present society, though that could change, I guess.


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## Lenny (Aug 13, 2013)

JoanDrake said:


> An average jet moves 500mph, so I'm not sure an increase to just 760 mph would be worth it.



That's also briefly touched upon in the document. 

Air travel is quick (and the in-air time for the journey from LA to SF, chosen by Elon Musk because it's a trip he makes regularly, is around half an hour), but you have, on top of the in-air time, at least an hour of waiting around in the airport (for you can't just rock up five minutes before your plane), so your total journey time is closer to two hours. The Hyperloop, which could have capsules departing from the station as often as every thirty seconds (estimated so that every capsule has at least five miles of empty tube between it and the next capsule), would do the entire journey in half an hour, and could ferry up to 840 passengers an hour.

The reason the Hyperloop has been designed for use between city pairs less than 1,000 miles apart is because at greater distances, supersonic jets can make the journey quicker. At short distances, the jet will spend most of its time ascending and descending, as it needs to be at a high altitude to reach supersonic speeds, and will only be able to hit full speed for a couple of minutes (the atmosphere low down is too thick). Obviously, the greater the travel distance, the more time a supersonic jet can stay at full speed, and the faster it will make a journey.


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## Abernovo (Aug 13, 2013)

It all looks interesting, and I like that he's put it out in the public domain.

Brian, the report I saw today said the acceleration at the beginning would be 1G, approximately equivalent to the take-off acceleration of a passenger jet. It would last a few minutes, longer than in a plane, but it's not outside of the comfort zone for most people.

Joan, there's been a wry comment or two that it's faster to travel from London to Paris on Eurostar (train) through the Channel Tunnel than it is to fly, despite the speed difference. The train takes you from city centre (ish) to city centre and there's much less waiting around. Airports are often outside the cities and the check-in restrictions (it was 1 hour minimum before take-off in UK) put extra (unproductive) time on the journey.


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