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The Rejection of Big Dog

Ghost, and Jan Karpiel. January 1st, 2017

The US Marine Corps rejected Boston Dynamic’s Big Dog robot based on the fact that it was too damn loud. Big Dog is an impressive mechanical beast meant to march along with Marines and 4 mph, and carry up to 340 lbs. It is the product of 25 years worth of R&D, and untold millions in DARPA and Army funding. The machine itself is incredibly impressive, so what’s with the noise?

Jan’s Comment:

The robot is using a water-cooled two-stroke internal combustion engine that delivers about 15 hp. An engine like this is loud because of the pulse gas-flow characteristic in the exhaust. When the piston pushes out the exhaust it creates waves inside the pipe, and that gives it this incredible sound, a sound desired by all the dirt bike engine tuners.

To tune a dirt bike in the seventies and eighties, you’d buy a racing pipe from a high-end tuning company. These companies would take (for example) a standard Yamaha engine, analyze the existing kidney exhaust, and make some diameter and length adjustments. By building a more radical exhaust pipe, they’d gain as much as 10 hp over the standard factory versions. The term for this is “the engine comes on the pipe”.

I can see why Boston Dynamics would choose this engine because it’s compact, lightweight with good horsepower values, but the noise is always going to be a problem. The two-stroke puts out more horsepower than a four-stroke, and from a lab standpoint, it’s a perfect fit. I assume that this robot needs too much power to use an electric engine at this point, but that technology is not so far away that I’d call this a failure. It’s more like a justification for more research funding. Obviously, these guys are pros, and they know their own design challenges. Sometimes, the best way to get more funding support for a ground-breaking invention is to let it get rejected on a few technical points. You’ve proven that your prototype is almost there, and ultimately, someone is going to throw in to see it battle ready in the near future.


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