In July of last year, Tesla and SpaceX founder Elon Musk let slip that he was working on the Hyperloop — an ultra-fast mode of transportation that will get you from downtown Los Angeles to downtown San Francisco is under 30 minutes. This is a distance of roughly 340 miles, and would require speeds of around 700 mph, or almost the speed of sound. Perhaps most importantly, though, Musk said the Hyperloop will only cost around $6 billion — compared to the $60 billion of the proposed high-speed rail link connecting the two cities. So far, so good, except for one niggling issue: Musk still hasn’t told us how he intends to build it.
Some of the world’s brightest minds have speculated that a vacuum tube is the only way to do it — but before that idea could even get off the ground, Musk said that the Hyperloop isnot based on an evacuated tunnel. With that possibility ruled out, there aren’t actually that many ways of safely and economically propelling carriages at 700 mph (1126 kph). Furthermore, when you factor in Musk’s comments that the Hyperloop “can never crash,” has no need for rails, and is “immune to weather,” the architecture of the system becomes a real head-scratcher. Oh, did I mention that Musk envisions the entire system being self-powered by solar panels, and that it somehow stores energy inside the system itself, without the need for batteries?
How, then, might the Hyperloop work? One possibility is by acoustic levitation. At that speed, the biggest enemy is always going to be air resistance, which is why a vacuum tunnel is usually the favored solution: In a vacuum there is no air resistance (drag), and thus you can essentially move as fast as you like — much like a spacecraft barreling through the great black expanse. But it isn’t an evacuated tube, so it must be something else. Not to mention, a vacuum tunnel would definitely not fulfill the “can never crash” factor; poke a hole in a vacuum tube, and the results would be very messy indeed.
What we need is another way of efficiently reducing drag. Just recently, we wrote about a research group that levitated arbitrarily shaped objects in acoustic waves. This technique involves an acoustic phenomenon called standing waves — essentially, waves that are held in place by interference. If you imbue these waves with enough power (volume) and hit just the right frequency, you can levitate an object. Standing waves, as the name implies, don’t move — but Björn Smedman and Charles Alexander both theorize that, if you pump these waves into a loop (which we assume the Hyperloop is), and change up the acoustic parameters slightly, then it might be possible to carry vehicles on the edge of these waves as they travel around the loop.
It turns out that, by hitching a ride on the peak of a sound wave, you only really have to deal with drag caused by air density (linear), which is much less than drag caused by air velocity (square).If you pump enough power into the acoustic wave (i.e. increase the amplitude), the air density increases but the relative air velocity drops. In effect, the vehicle in the wave is stationary, in reference to its surroundings. Eventually, as the sound wave gets stronger and stronger, you achieve almost adiabatic travel — travel that loses no energy at all to the environment via drag or friction. In theory, this process is so efficient that solar panels on top of the loop (a very large surface area!) can power the system. The acoustic waves, traveling continuously around the loop, would effectively act as energy storage.
While acoustic waves neatly solve the traveling-at-almost-the-speed-of-sound bit, they don’t explain how you would embark and disembark from the Hyperloop. The best guess at the moment is that there will be an extra section at each end of the loop for managing acceleration and deceleration. To board the Hyperloop, you will hop into a carriage at the San Francisco or Los Angeles terminus, and then be accelerated up to speed using arailgun before entering the main loop. At the other end, you will be gently decelerated before disembarking. This neatly ties in with Musk’s comments that the Hyperloop will be a“cross between a Concorde and a railgun and an air hockey table.”
Via Dr. Stefan Gruenwald