About 10 months ago, Elon Musk, CEO of Tesla Motors and Space X, asked some engineers from both of these companies to help him invent a new form of transportation. Today he unveiled the design for what he calls the “hyperloop,” which would convey passengers from San Francisco to LA in about half an hour. It involves propelling 28-passenger “pods” through a tube at speeds of up to 760 miles per hour. The pods would be propelled by something called a linear motor, and they would ride on a cushion of air to minimize friction.
“It’s far-fetched,” says John Hansman, a professor of aeronautics and astronautics at MIT. “But Musk is a smart guy, so some of the things he’s doing make reasonable sense.”
Musk is proposing the system as an alternative to the high-speed rail system planned in California. He’s no expert on high-speed mass transit, but he’s gained prominence through a series of successes with electric cars and private space flight. He says that at $6 billion, his hyperloop would cost a tenth as much as the planned California project, and be much faster and safer. His cost estimate is a guess, of course, since the concept hasn’t even been demonstrated at a small scale.
“I don’t see anything that violates fundamental laws of physics,” Hansman says. But he says Musk’s cost estimates are too optimistic. “It would be enormously expensive. And I think there are a huge number of technical challenges with the vehicle,” he says. One of the biggest questions is how much energy the system will require. “My questions aren’t could you do it, but could you do it in a way that makes sense from an energy efficiency standpoint and makes sense from an economic standpoint,” he says.
Tube-based transport isn’t new. The idea of pushing pods along with air in a pneumatic tube has been around for at least 150 years. Alternatively, researchers at the Rand Corporation and MIT have proposed shooting trains through vacuum tubes. But vacuum tubes are hard to keep sealed. An air-filled pneumatic tubes create a lot of friction.
Musk’s idea is a hybrid of the two approaches. The system operates under low pressure, rather than a vacuum, which should be easier to maintain. “What he did that I think makes sense is he picked the sweet spot, where he’s trying to get the benefits of a vacuum but not go all the way to a hard vacuum,” Hansman says. But maintaining low pressures will still require substantial amounts of energy, and exactly how much will depend on things such as how well the system can be sealed, he says.
Because the system isn’t a complete vacuum, there is still air inside the tube that needs to be dealt with. To prevent air from building up in front of the pod and slowing it down, Musk’s plan is for each pod to suck air in through a duct in the front. The air would then be funneled to skis arranged on the outside of the pod so that the pod can ride along on a cushion of compressed air, making the vehicle a sort of air sled.
Musk says he could use electric motors from the Tesla Model S electric vehicle, powered by Model S battery packs, to suck in the air. But Hansman says it’s not clear, because of the complexity of the aerodynamics involved in pushing a vehicle at high speeds through a tube, whether those batteries can contain enough energy for the entire trip.
Unusual approaches to transportation like this one have, of course, had a difficult time getting implemented. In the early 1990s, researchers at MIT led by Ernst Frankel designed a vacuum-tube train system for a 45-minute trip from New York to Boston. He built a test loop around the playing fields at MIT. At the time, his team estimated that link between the cities would cost $6.8 billion, close to what Musk thinks his system will cost. He had discussions with Amtrak, but those went nowhere. At 89, he’s now an emeritus professor working from home, and was “totally surprised” when he started getting calls from the media about Musk’s system. “I hope we can do something,” he says. “We probably can’t get American government agencies and institutions to do it.”
One big problem is getting enough capital together to demonstrate and build untested technology. I reached Joseph Sussman, a professor of civil and environmental engineering at MIT just before the announcement today. He noted that the U.S. hasn’t even been able to build proven high-speed technology. “We’re behind the Japanese, the French, and everyone else. Given our inability to put together the package to do high-speed rail, which is proven technology, it’s hard to see how a chancy solution—given that it’s never been implemented—would fare,” he says.
If anything is to come of the idea, given the challenges involved, it might require funding from Musk, whose personal fortune has been essential for Tesla Motors. In recent days Musk has said that he was too busy to develop the hyperloop idea further. But on a conference call this afternoon he said that he’d likely build a demonstration version of the system himself to get the idea going. “I would like to see this come to fruition,” he said. “It will probably help to do a demonstration. I’ll probably end up doing that,” he said. But then, no doubt thinking of the challenges ahead of him with Tesla and Space X, he added, “But not immediately.”