Yesterday, Vladimir Putin presented his country with a belated Christmas present: the Avangard hypersonic missile. According to Russian media, it's capable of reaching Mach 20. And if its ability to conduct evasive maneuvers at high velocity is as good as the Russian president boasted back in March, it would render missile defense systems effectively useless.
Cold War recidivists aren't the only ones hoping hypersonic technology will deliver a futuristic throwback. Last month marked the 15-year anniversary of the Concorde’s final flight, but right now, a handful of aerospace outfits are working to leapfrog supersonic travel and launch straight into the Mach 5 world of hypersonic propulsion.
'Hypersonic' isn’t just buzzy reboot jargon for ‘supersonic.’ It’s a word scientists and engineers use to generally describe air travel between Mach 5 and Mach 10 (that’s 3,836 and 7,673 mph for you sticklers). Aircraft travelling faster than the speed of sound need all sorts of heat shielding and aerodynamic redesigns. But really, all that stuff is secondary to propulsion—without speed, there is no need. Standard jet engines won’t cut it. The rotating detonation engine, though, just might.
Turbofan engines are great for most commercial travel, because they can get a plane going up to around 600 mph while burning fuel really efficiently. North of that, they burn through fuel like a Powerball winner with 50 second cousins. Also, they don’t have the muscle to take an aircraft too far past Mach 1. The Concorde got around that latter problem by using turbofans to get up to sub-Mach speed, then kicking in a set of turbojet afterburners for the rest of the way through the sound barrier, settling into cruising speed at just above Mach 2. But the Concorde was an expensive aircraft to fly, and modern airlines are all about value.
The rotating detonation engine, though, might someday offer both high velocity and decent fuel economy. The engine’s awesome name pretty much describes how the thing works. The engine’s detonation chamber is essentially a thin, hollow cylinder (actually, it’s the thin, hollow space between two concentric cylinders, if you want to get specific). The engine sets off a detonation using the usual means—fuel, oxygen, pressure, heat—which sends a shockwave chasing itself through the cylindrical loop. Imagine a movie scene where the heroes are running away from an explosion then get knocked forward by the shockwave. A rotating detonation engine traps that shockwave in an endless loop, using it to repeatedly jumpstart new detonations.
If you’re wondering how a shockwave detonates something, consider how explosions happen: Pressure. Heat is important, but it’s really just a side-effect of molecules being forced close to one another. Force enough of the right kind of molecules close together and they react. Here, the shockwave slams into oxygen molecules and fuel molecules with so much force that they compress, excite, and detonate. Each subsequent detonation keeps the shockwave going, and the engine keeps those detonations coming by feeding the chamber carefully timed injections of fuel and oxygen.
"What this allows the engine to do is burn fuel at a much higher rate compared to conventional combustion engines," says Narendra Joshi, the chief engineer of propulsion technologies at GE Research. This higher burn rate creates more thrust, which is how these engines will (theoretically, one day) push aircraft into hypersonic speeds.
But wait, isn’t burning fuel at a higher rate contradictory to the whole efficiency thing? In this case, higher rate doesn’t necessarily mean more. See, the combustion chamber—that thin space between the two metal cylinders—is about 10 times smaller than the chamber in conventional turbine engines. That means it is burning fuel at a much higher pressure than the competition. Internal combustion (or detonation) type engines produce work by compacting fuel. The higher the pressure, the more work the engine gets out of the molecules once they explode. “We estimate a 5 to 10 percent improvement in gas mileage,” says Stephen Heister, a propulsion engineer at Purdue University whose research includes rotating detonation engines. (That’s compared to conventional turbines, jet engines, even rockets.) Also, because this engine isn’t purging a bunch of combustion byproducts that happen in each cycle, it’s far more efficient with the fuel it does burn.