FAQ: All Your Hyperloop Questions Answered

By Wesley Fenlon

At a proposed $6 billion, the Hyperloop is actually tens of billions of dollars cheaper than California's high speed rail, and more than three times as fast.

New York to Los Angeles in 45 minutes? Hold on there, tiger. Elon Musk's seemingly radical plan for the Hyperloop, which would blast people across the entirety of the United States in under an hour, is out. Sort of. On Monday, Musk elaborated on the Hyperloop proposal he first revealed in July, toning down his original estimate to focus on a slower 30 minute ride from San Francisco to Los Angeles.

That slower ride, however, is still faster than any bullet train on the planet, at a near-supersonic 700 miles per hour. The fastest train moves along at a pokey top operating speed of 368 miles per hour or so. Musk believes that the SF-LA Hyperloop could be built above ground, powered by solar panels, and cost a mere $6 billion to build. Which sounds like a lot, until you realize that California's in-progress High Speed Rail system, which will travel at speeds of less than 200 miles per hour, is projected to cost about $70 billion.

By comparison, the Hyperloop sounds absolutely amazing. And cheap. And now the whole system has been laid out in a white paper, which Musk hopes will raise public interest and elicit public feedback that will make the Hyperloop even better. He doesn't plan to spearhead the project himself--at least not right now--because he's too busy with Tesla Motors and SpaceX.

But if someone did decide to built the Hyperloop, here's how it would work:

For starters: Is this just a really fast bullet train?

Not exactly, though it uses a pod kind of like a train car. But the Hyperloop is in a tube, and the pods ride on skis rather than rails--the tube itself is designed to be as simple as possible to keep costs down.

Is it pressurized? Or a vacuum?

Neither. It's easy to compare the Hyperloop to pneumatic tubes, which push canisters around using air pressure inside a sealed system. But Musk writes "the friction of a 350 mile long column of air moving at anywhere near sonic velocity against the inside of the the tube is so stupendously high that this is impossible for all practical purposes."

A vacuum's no good, either. Once you spring a small leak, your pressurized system stops working. And we're talking about tubes that cover hundreds of miles of terrain. Maintenance would be impossible.

If it's not high pressure or a vacuum, how does the propulsion work?

The Hyperloop uses a low pressure system. The big obstacle to overcome, as the white paper explains, is getting around the Kantrowitz limit: "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." It's possible to overcome this limit with a slow pod or a very fast pod, but neither solution works in this case--very fast would be supersonic, and exert too much g-force pressure on passengers.

So here's Musk's proposed solution:

"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...

It 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. 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.

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.

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. This would accelerate the pod to high subsonic velocity and provide a periodic reboost roughly every 70 miles. The linear electric motor is needed for as little as ~1% of the tube length, so is not particularly costly."

How do these air bearings work, exactly?

The white paper has a technical section that adds more detail for the engineering-minded. Here's an excerpt about the suspension: "Hyperloop capsules will float above the tube’s surface on an array of 28 air bearing skis that are geometrically conformed to the tube walls. The skis, each 4.9 ft in length and 3.0 ft in width, support the weight of the capsule by floating on a pressurized cushion of air 0.020 to 0.050 in. off the ground. Peak pressures beneath the skis need only reach 1.4 psi to support the passenger capsule (9% of sea level atmospheric pressure). The skis depend on two mechanisms to pressurize the thin air film: external pressurization and aerodynamics.

The aerodynamic method of generating pressure under the air bearings becomes appreciable at moderate to high capsule speeds. As the capsule accelerates up to cruising speed, the front tip of each ski is elevated relative to the back tip such that the ski rests at a slight angle of 0.05º. Viscous forces trap a thin film of air in the converging gap between the ski and the tube wall. The air beneath the ski becomes pressurized which alters the flow field to satisfy fundamental laws of mass, momentum, and energy conservation. The resultant elevated pressure beneath the ski relative to the ambient atmosphere provides a net lifting force that is sufficient to support a portion of the capsule’s weight.

However, the pressure field generated by aerodynamics is not sufficient to support the entire weight of the vehicle. At lower speeds, very little lift can be generated by aerodynamic mechanisms. Temperature and density in the fluid film begin to rise more rapidly than pressure at high speeds, thus lift ceases to increase as the capsule accelerates into the transonic regime.

Lift is supplemented by injecting highly pressurized air into the gap. By applying an externally supplied pressure, a favorable pressure distribution is established beneath the bearing and sufficient lift is generated to support the capsule."

Why isn't the system below ground?

Solar power is the big one. The white paper says "the short answer is that by placing solar panels on top of the tube, the Hyperloop can generate far in excess of the energy needed to operate. This takes into account storing enough energy in battery packs to operate at night and for periods of extended cloudy weather."

Building a 350 mile tunnel probably wouldn't be practical, either, and the white paper claims that building the tracks on pylons allows them to design the system to withstand earthquake damage: "By building a system on pylons, where the tube is not rigidly fixed at any point, you can dramatically mitigate Earthquake risk and avoid the need for expansion joints. Tucked away inside each pylon, you could place two adjustable lateral (XY) dampers and one vertical (Z) damper."

What would it be like to ride in the capsule?

Capsules would take off every couple minutes, or as often as every 30 seconds during peak hours. Each pod would have seats for 28 passengers and would be traveling at up to 760 miles per hour. Despite that speed, it would be pretty comfy. After the pressure of acceleration, which would feel similar to takeoff in a plane, the Hyperloop would exert a maximum of half a G on passengers. The white paper also includes a proposal for a larger capsule, which would include space for up to three cars.

So how much would it cost me to ride the Hyperloop from LA to San Francisco?

Twenty bucks. Well, that's the white paper's (optimistic) estimate, but the final cost could be pretty close. The paper elaborates "amortizing [the $6 billion] cost over 20 years and adding daily operational costs gives a total of about $20 USD (in current year dollars) plus operating costs per one-way ticket on a passenger Hyperloop. Those operating costs could add to the price, but a $20 base price for a ticket is still pretty amazing.

Why abandon the New York to LA idea?

While a longer Hyperloop still seems possible, making the trip quickly enough would require supersonic travel. Interestingly, the white paper claims that for cities more than 900 miles apart, "supersonic air travel ends up being faster and cheaper. With a high enough altitude and the right geometry, the sonic boom noise on the ground would be no louder than current airliners, so that isn’t a showstopper. Also, a quiet supersonic plane immediately solves every long distance city pair without the need for a vast new worldwide infrastructure."