It’s now been three days since Elon Musk unveiled his alpha plans for the Hyperloop, the radical and ambitious high-speed transit line that promises passage from Los Angeles to San Francisco in just 35 minutes. And while Musk has been cagey about any intentions of actually building the system, the idea has already inspired widespread speculation: based on these plans, what would it take to actually build the Hyperloop? How plausible is Musk’s vision, really?

"Getting it that smooth won't be easy."

The biggest question mark is the tube itself, which has emerged as the most genuinely unprecedented part of the plan. By enclosing the track, the Hyperloop is able to sidestep worries about air friction and noise that usually limit the speed of trains to under 400mph, but the tube also presents a unique set of challenges. James Powell PhD, co-inventor of the maglev train, is particularly concerned about the smoothness of the inside of the tube. As Powell points out, the current design allows for just three hundredths of an inch between the tube wall and the skis encircling the pod. "Getting it that smooth won't be easy," says Powell, and might require a more expensive production process than the plans envision.

The small gap is crucial to the system’s overall design, allowing for a stable air cushion that keeps the pod hovering frictionlessly in the tube. But the small gap also requires great precision in tube construction. Powell thinks that a single bump, just three-quarters of a millimeter high, would trigger catastrophic damages, possibly even ripping the ski from the pod at top speed. Keeping the tubes straight can be done, but it won't be cheap. "It's going to be an arduous process," says Vinod Badani, an engineering consultant at E2 Consulting. "Quality control and measurement have to be very accurate." Musk's plans envision a specially designed device to smooth out the inside of the tube, but it presents a serious engineering problem for anyone thinking of building a prototype.

"In all our tests, we found people started to feel nauseous when you went above 0.2 lateral Gs."

Another concern is the straightness of the path. At 750mph, even a gentle curve jerks passengers to the side. In physics, it’s known as lateral G-force, and the human body can only take so much before motion sickness sets in. As a result, planners are always balancing the curviness of their route with their traveling speed and the level of G-force passengers can withstand. Musk's planned route is designed to limit lateral G-forces to a maximum of 0.5 Gs, which lets the proposed Hyperloop path follow I-5 at 760mph, and blaze through the first 14 miles outside of Los Angeles in just under three minutes. The 0.5 G limit lets Musk draw a windier path through California, but it's significantly higher than any existing transportation project.

According to Powell, that’s a problem: "In all our tests, we found people started to feel nauseous when you went above 0.2 lateral Gs." The closest comparison would be roller coasters, which usually top out around half a G — but the Hyperloop wouldn't just peak at 0.5; it would stay there for the duration of the curve. The result would be well short of blackout, which most studies peg around 4.7 lateral Gs, but it would make the Hyperloop challenging for the faint of stomach. A sick passenger might be less catastrophic than a crash but, given the tight passenger compartments, the results could still be fairly traumatic.

The design's biggest advantage is that most of the technologies involved have been around for decades. The Hyperloop's magnetic linear accelerator — the so-called "rail gun" in Musk's initial pitch — is based on a Rand Corporation patent from 1978. Versions of the same technology are often used for drilling or well-boring, and a comparable magnetic accelerator was used to power a generation of hovercraft designs in the 1970s. It might seem futuristic, but it's a technology many engineers take for granted.

"If you have enough surface area, you can float anything."

The "air hockey table" component of the design has also seen a lot of action. Hovercrafts are one obvious precedent, but as the technology has developed, air bearings have grown more powerful. When factories need to reduce friction, whether in a turbine, a semi-conductor or moving heavy loads, a pressurized air bearing is a common solution. In some cases, the bearings will float loads just thousandths of an inch off the ground. As long as there's no contact, there's little to no friction. "If you have enough surface area, you can float anything," says Bradley Engel, president of Nelson Air, a precision air bearing manufacturer. "It's just a matter of surface area and pressure."

Many riders might find the sharp curves too much for them

Engel has never seen air bearings deployed in a low-pressure tube like the Hyperloop, but he doesn't think the drop in atmospheric pressure will make much difference, since bearings typically need to create pressure that's five or six times higher than atmospheric pressure. In fact, the Hyperloop has the advantage of moving very, very fast, which helps the bearings sustain pressure. He compared the process to a paper airplane, which floats better when it flies faster. And if the pod did encounter a bump in the inside of the tube, that pressure would naturally soften the blow, hopefully limiting the damage from the crash scenarios that Powell envisioned.

The project is still in the very early stages, and the plans themselves acknowledge that more work will have to be done on station design and capsule control mechanisms before a prototype model can be built. But even with these admitted gaps in knowledge, one can already get a sense of what a finished Hyperloop might look like. It wouldn’t be perfect. Maintaining a smooth tube interior would be a constant issue, incurring delays and additional expense. Many riders might find the sharp curves to be too much for them. But it would be possible — and proving that, more than building or maintaining the system, may have been Musk’s goal all along.