Electromagnetic acceleration: That's the high technology behind the high-speed transit concept that billionaire Elon Musk calls the Hyperloop.
Musk — who already plays leading roles in the SpaceX rocket venture, the Tesla electric car company and the SolarCity solar-energy company — unveiled his vision of the Hyperloop on Monday.
The plan is aimed at cutting the travel time between San Francisco and Los Angeles to 35 minutes, at a price of $20 for a one-way trip.
"It would actually feel a lot like being on an airplane," Musk said.
Musk said the Hyperloop arrangement could be implemented between any pair of cities situated up to, say, 900 miles (1,500 kilometers) apart. For longer distances, air travel would probably be more efficient, he said.
In a blog posting and a 57-page PDF file about the Hyperloop, Musk said he came up with the plan out of frustration with the shortcomings of California's $68 billion high-speed rail project, which is just getting started. Musk estimated that about a dozen engineers from SpaceX and Tesla worked on the Hyperloop concept over the past year or so as a "background task."
How the Hyperloop would work
The Hyperloop would send travelers through low-pressure steel tubes in specialized pods that zoom at high subsonic speeds, reaching about 760 mph (1,220 kilometers per hour). That compares with typical speeds of 110 mph (for U.S. systems) to 300 mph (in China) for high-speed rail travel.
Musk's plan would rev up the pods from their stations using magnetic linear accelerators — and once they're in the main travel tubes, they'd be given periodic boosts by a linear induction motor built into the tube and the pods. "The moving motor element (rotor) will be located on the vehicle for weight savings and power requirements, while the tube will incorporate the stationary motor element (stator) which powers the vehicle," Musk wrote.
The pods would have electric compressor fans mounted on their noses to transfer high-pressure air from the front to the rear, getting around an aerodynamic limitation that would otherwise stymie near-supersonic travel in a tube. "We can make it work" with the current technology for electric motors and batteries, as implemented in the Tesla Model S sedan, Musk said.
The journey would be nearly frictionless, thanks to a cushion of compressed air between the cars and the tube's inner surface. Musk said the system could be scaled up to hold three full-size automobiles per pod, with passengers inside.
The whole system would be powered by solar panels installed onto the tubes. "By placing solar panels on top of the tube, the Hyperloop can generate far in excess of the energy needed to operate," Musk wrote.
The tubes would be elevated on pylons, and generally follow Interstate 5 between San Francisco and L.A. Musk said that would cut down on the cost of land acquisition and rights of way. He estimated that the whole system would cost around $6 billion to build. "Even several billion is a low number when compared with several tens of billion proposed for the track of the California rail project," he wrote.
This combination of technologies is what led Musk to describe the Hyperloop last month as a "cross between a Concorde, a rail gun and an air hockey table." The hints that he dropped along the way sparked a flurry of speculation, about schemes ranging from "Jetsons"-like people-movers to underground vacuum tunnels.
One of the closest guesses came from a self-described "tinker" named John Gardi, who laid out a plan for a turbine-driven pneumatic system. Gardi said Musk's system was even better. "Beautiful, elegant, efficient!" Gardi wrote in a Twitter update after Monday's big reveal. "The aerodynamic solution is brilliant, brings me to tears ... I can see why I missed it."
Who'll build the Hyperloop?
Musk has said he wouldn't be able to build the Hyperloop himself, due to his duties at SpaceX and Tesla. But he changed his tune slightly on Monday, during a news conference to discuss the idea. "I'm somewhat tempted to at least make a demonstration prototype," he told reporters. "I've sort of come around a little bit on my thinking here, that maybe I should do the beginning bit, create a subscale version that's operating, and then hand it over to somebody else."
However, Musk cautioned that such a demonstration wouldn't be immediate, and that it would serve as a technological test bed rather than a practical transit system. He compared the project to a rocket demonstration on SpaceX's test range in Texas.
Musk estimated that it could take seven to 10 years to build a working Hyperloop between San Francisco and Los Angeles.
The Hyperloop could be held back by technical as well as political and economic issues. Transportation policy experts say that high-speed transit in the United States has been stymied not so much by technological challenges as by the challenges of acquiring rights of way and getting enough money to build the required infrastructure. Despite the hurdles, high-speed transit projects are beginning to gain traction. California, for example, is continuing with its next-generation rail system, and other states are proceeding with their own high-speed rail initiatives.
Musk said he thought the California project should be put on hold, considering that the construction cost could balloon well past the current $68 billion estimate, and is likely to result in a rail system that's slower than taking an airplane. "That just doesn't seem wise for a state that was facing bankruptcy not that long ago," he said.
Emil Frankel, a former transportation official who is now a visiting scholar at the Bipartisan Policy Center, said he didn't know enough about Musk's freshly revealed concept to comment on its pluses and minuses. But he said anything that gave a boost to the debate over the future of transportation was a good thing.
"I think that the best way for us to spend our money on infrastructure, given scarce resources, is with incremental improvements, restoration of our existing systems and appropriate expansions," he told NBC News. "The analyses that have been done suggest that these kinds of incremental improvements to the efficiency and reliability of our existing systems provide the best benefits."
Correction for 7:15 p.m. ET: At one point I had a wildly high figure for the number of people working on the Hyperloop concept, based on something I misheard during Musk's news conference. It's a little over a "dozen," not a "thousand."
More about the Hyperloop:
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.