Elon Musk is going to reveal a new “fifth mode” of transport today, something akin to Concorde crossed with a train and possibly a hovercraft. Or rather he’s not going to reveal a working hyperloop…he’s merely going to reveal his ideas for how one would work. Let’s pop on our physicist hats and have a good hard think about this, eh?
From Leonardo da Vinci through Arthur C. Clarke to Christopher Cockerell to Pons and Fleischmann (the cold fusion chaps, remember them?) smart thinkers have unveiled seemingly whacky ideas for centuries, each claiming they’ll revolutionize the world somehow. Sometimes–and we really do mean sometimes–they’re proven right, either swiftly or by the cold ticking clock of history. Da Vinci’s parachute and machine gun came real centuries after his death. Arthur C. Clarke (yes, really, that Arthur C. Clarke) came up with a notion of geostationary communications satellites, and pretty much changed the world, even though only scientists know that these orbits are still sometimes called the “Clarke orbits.” Cockerell invented the hovercraft, and though it never took off as a personal mode of transport, pun intended, at least you can now buy a hover golf cart, and the Marines love the things. Pons and Fleischmann claimed to have discovered cold fusion, which could be the secret to virtually limitless, safe, clean electrical energy. History has so far judged these two chaps rather differently to the famous names above. They are, more or less, cranks.
So will Elon Musk’s idea hold water? Or will he be another madman in a footnote in history (at least in terms of this new idea)?
We don’t know just yet exactly how the Hyperloop will work, but Musk has dropped some big clues:
- It involves a travel tube but not a vacuum tube.
- It travels very near to the speed of sound.
- It may be solar powered and can potentially store energy to be used shortly afterward.
- It could be installed above or below ground.
- It doesn’t involve rails.
- It can never “crash”!
- It involves a nearly frictionless transportation system.
So how do the laws of physics deal with this sort of claim?
To start with, let’s look at the frictionless claim. That’s a biggie. Considering the clues he’s dropped, Musk almost certainly means frictionless in terms of wheels on the ground. Air friction, since he’s not talking about a vacuum tube, is going to be a limiting factor here. It’s a force you probably don’t pay much attention to, but it’s critical for the design of aircraft and high-speed vehicles from trains to Formula 1 cars. Put simply, if you move a solid object through the air very fast it’s resisted by air friction, which tries to slow it down. The heating action of air friction on a very fast-moving vehicle (like an SR-71 Blackbird) is very pronounced, and requires all sorts of clever materials or engineering to get around. To keep the then-revolutionary Concorde cool at twice the speed of sound the engineers had to pump its own fuel around the plane’s body as a coolant to keep its passengers from boiling in their seats. Friction along with the well-known aerodynamic craziness that happens near the speed of sound is probably what’s limiting the Hyperloop.
A frictionless air bearing, however, may come into the design, and that’s different. Think of the effortless way that an air hockey puck skates across a table: That’s thanks to the pressurized cushion of air pumped beneath it through all those little holes. Then remember that we mentioned Cockerell earlier on. A hovercraft works in a similar way to an air hockey table, and it is indeed a very efficient mode of transport because it floats on a cushion of very slippery air. This exerts much less friction on the motion of the hovercraft than a road would on a wheeled vehicle or perhaps rails on the wheels of a train would. A similar effect makes the Russian ekranoplan work. Essentially a ground-based aircraft, technically known as a ground effect vehicle, an ekranoplan squeezes a cushion of air beneath it as it flies very close to the ground, creating its own flying air cushion. Google “caspian sea monster” and you might have a post Cold War shock when you learn the size of these things.
So it’s possible that Musk’s hyperloop craft is like an ekranoplan trapped in a tube. It could float away from the tube walls by pumping high pressure air between its body and the tube, like a hovercraft, or by compressing air around it as it moves quickly, a little like an ekranoplan. This frictionless cushion could enable some very high speed “flights,” and deliver a very low cost and efficient transport system providing the scale of the vehicle makes sense commercially and that a suitable power source can be found.
That last bit is the trick. Could solar power be harnessed to generate electricity to accelerate the hyperloop up to speed like a rail gun (again, Google this for a distracting hour of video fun), and then it would “fly” along using some other means? Possibly. But we’re talking lots and lots of energy here–think about how many gallons of diesel fuel are burned by a big rig truck getting up to cruising speed on the freeway, and you’re thinking the right sort of physics.
And then there’s the “no crashing” thing. A vehicle that relies on something mechanical like its own progression through a tube to compress air into a “cushion” to fly on sounds safer than one that uses machines to compress air into one–those machines could break unexpectedly. At near Mach 1 speeds it will be important to stop foreign objects like birds from hitting the vehicle, which suggests the tube would be totally enclosed from end to end and act as a sterile environment. Think bird strike on a plane, but on a train in a tube, and you see the problem.
The energy storage claim Musk makes has us foxed, unless he’s talking about some clever system that converts energy from a decelerating hyperloop vehicle into a stored form, perhaps in flywheels or compressed air or some form of chemical battery. After all, flywheel subways seem to work, so why not scale that tech up?
So there we have it. It’s not beyond the pale that a hyperloop could work. Providing you could make a rigid an impenetrable tube with little to no air gaps between L.A. and San Francisco; that you could solve the problem of air friction in a way that wouldn’t waste energy in flying the vehicles along; that you could come up with an energy-efficient way to accelerate a vehicle up to speed and then slow it down; that you could solve the problem of crashes meaningfully; and finally that you could perfect a means of storing vast amounts of energy at will, and releasing it on demand without losing too much in the process to not make it worth your while.
The problems, assuming Musk has actually solved them, would place him somewhere above Cockerell and below Robert Goddard, the father of the rocket, in terms of revolutionary ideas for transport. But perhaps there’s more of a Pons and Fleischmann fate ahead of Musk as the harsh laws of physics crash into his ideas, robbing them of energy and meaning (though we expect he’d’ve been able to pay a scientist or two to have a think about Hyperloop’s issues before today’s news).