Elon Musk followed up on his Tweet heard round the tech world today with published plans [link via Gizmodo] for the Hyperloop. The pod-and-tunnel design revolves mainly around a low-pressure tube system, a nose-mounted electric compressor fan, air bearings, and an external electrical motor. As the number of authors is unclear, all quotes from the plans will be attributed to Musk.
The SpaceX and Tesla entrepreneur took pains to make his paper understandable to outsiders, using accessible imagery and terminology, not to mention jokes. “Short of figuring out real teleportation, which would of course be awesome (someone please do this), the only option for super fast travel is to build a tube over or under the ground that contains a special environment,” he writes in his Hyperloop Alpha papers.
He explains using both pneumatic and vacuum tubes for infrastructure fails practicality tests. Air movement in pneumatic tubes on the scale Musk imagines would generate too much friction; and sustaining an airless vacuum just wouldn’t be pragmatic.
“However, a low pressure (vs. almost no pressure) system set to a level where standard commercial pumps could easily overcome an air leak and the transport pods could handle variable air density would be inherently robust,” he writes.
But air in the system brings its own trouble. “Whenever you have a capsule or pod…moving at high speed through a tube containing air, there is a minimum tube to pod area ratio below which you will choke the flow,” Musk writes. “What this means is that if the walls of the tube and the capsule are too close together, the capsule will behave like a syringe and eventually be forced to push the entire column of air in the system. Not good.”
Here come the physics: “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,” Musk writes. “Interestingly, there are usually two solutions to the Kantrowitz limit – one where you go slowly and one where you go really, really fast.”
While Musk acknowledges it’s tempting to pick the super-fast solution, he says it just won’t work for what the Hyperloop’s trying to do. And going thousands of miles an hour sounds exciting until you realize “even wide turns” bring “painful g loads.”
“The approach that I believe would overcome the Kantrowitz limit is to mount an electric compressor fan on the nose of the pod that actively transfers high pressure air from the front to the rear of the vessel,” he says. “This is like having a pump in the head of the syringe actively relieving pressure.”
Remember Musk’s first tantalizing description of the Hyperloop as part air hockey table? The compressor pump would help pods glide through a tunnel system a little like plastic pucks do..
“[The pump] would also simultaneously solve another problem, which is how to create a low friction suspension system when traveling at over 700 mph. Wheels don’t work very well at that sort of speed, but a cushion of air does,” he writes. “Air bearings, which use the same basic principle as an air hockey table, have been demonstrated to work at speeds of Mach 1.1 with very low friction. In this case, however, it is the pod that is producing the air cushion, rather than the tube, as it is important to make the tube as low cost and simple as possible.”
Musk then answers his own question about battery life and the viability of fueling a fan for a 700-mile trip. Following his description of the air bearings, he writes: “That then begs the next question of whether a battery can store enough energy to power a fan for the length of the journey with room to spare. Based on our calculations, this is no problem, so long as the energy used to accelerate the pod is not drawn from the battery pack.”
To top off the design, a battery-powered electric motor brings the system up to speed.
“This is where the external linear electric motor comes in, which is simply a round induction motor (like the one in the Tesla Model S) rolled flat,” he writes. “This would accelerate the pod to high subsonic velocity and provide a periodic reboost roughly every 70 miles.”
Faster than a speeding bullet train, the Hyperloop promises transit that is cheaper, better, faster, and safer, “compared to alternatives.” But for now, the system’s future design is as open as its open-source plans.
“Hyperloop is considered an open source transportation concept,” Musk writes. “The authors encourage all members of the community to contribute to the Hyperloop design process. Iteration of the design by various individuals and groups can help bring Hyperloop from an idea to a reality.”