In late June, shortly after IEEE Spectrum’s recent feature on in-wheel motors for electric vehicles went to press, the Defense Advanced Research Projects Agency (DARPA) issued a press release describing various novel concepts for military vehicles, some of which make use of distributed electric motors. These prototypes were developed as part of DARPA’s Ground X Vehicle Technologies (GXV-T) program, which is intended to find ways to make military vehicles less vulnerable to attack. The usual technique for doing that is, of course, to add heavy armor. The GXV-T program explored another approach—to make vehicles nimbler and thus “improve survivability without up-armoring the vehicle,” according to program manager Amber Walker.
One of the technologies developed under the GXV-T program is an in-wheel motor developed by U.K.-based QinetiQ. Like the commercial in-wheel motors described in Spectrum’s recent feature article, QinetiQ’s motor can improve the handling of a vehicle by allowing each wheel to be powered separately. It differs from the commercial offering described in Spectrum's July issue, though, in that it has a built-in transmission. (The commercial unit makes do with no gearing at all.)
For a military vehicle, a key advantage of using in-wheel motors rather than a centrally mounted electric motor and transmission is that it eliminates the drive shafts or axles that you’d otherwise need under a vehicle—components that can become deadly projectiles in a military vehicle should it run over an explosive.
Another thrust of the GXV-T program was to develop technology that would allow vehicles to negotiate extremely rough terrain. Engineers from Pratt & Miller achieved that goal by constructing an odd-looking vehicle with a suspension system that allowed each wheel to be moved up and down over a range of almost 2 meters. This Multi-mode Extreme Travel Suspension (METS) system enabled the prototype to ride smoothly over enormous bumps and to keep its cab level while traversing steep slopes.
While the METS vehicle doesn’t appear to have true in-wheel motors, images of it suggest that it uses motors positioned near each drive wheel. GXV-T researchers have not yet been cleared by DARPA to share technical details of their designs, so I can't confirm that surmise, but that's certainly what it looks like. And it makes sense: Distributed electric motors offer vehicle designers more latitude than they are normally used to. And I bet they really needed that here.
The use of distributed electric motors for these military prototypes is reminiscent of a program sponsored by DARPA and the Office of Naval Research in the early 2000s that resulted in the development of the Shadow Reconnaissance, Surveillance, Targeting (RST) Vehicle. That hybrid electric vehicle used in-wheel motors in part for the flexibility they offered—literal flexibility. You see, one aim of that earlier program (which, incidentally, was managed at DARPA by Stephen Welby, currently IEEE’s Executive Director and COO) was to create a vehicle that could fit into the cargo hold of a V-22 Osprey tilt-rotor aircraft. To do so, the vehicle, which in normal operations had substantial ground clearance, needed to crouch to within 10 centimeters of the ground and pull its wheels inward. Using in-wheel electric motors helped make such contortions possible.
Although some once saw it as a possible replacement for the military’s High Mobility Multipurpose Wheeled Vehicle (better known as the Humvee), the Shadow never went beyond the demonstrator phase. The technologies recently developed in the GXV-T program could, of course, suffer the same fate. But even if the U.S. military doesn’t choose to follow through and put them into widespread use, I wouldn’t be surprised to see some of these technologies—the METS system in particular—commercialized, perhaps for next-generation electrically powered all-terrain vehicles. I suppose that could help save the lives of many young people—not on the battlefield but by making notoriously dangerous ATVs that much less likely to roll over.