A project so purely weird as DARPA's flying Humvee will demand a unique--and no doubt powerful--engine. This thing is a Transformer! Surely it'll be propelled by some kind of cutting-edge power plant, right? Something with "fusion" in the name and a "top secret" stamp on the maintenance manual? Well, not quite.
After a long courtship, Pratt and Whitney has been asked to build an engine that'll power the heli-Humvee. (If the design succeeds, it'll be used in the end product.) The engine itself is surprising: It's a diesel. A rotary, Wankel-style diesel. It'll be a flight-weight version of the company's existing rotational engine technology, called EnduroCORE. Scott Claflin, director of Power Innovations for the company, told UPI:
We are honored DARPA chose Pratt & Whitney Rocketdyne to design an engine for a vehicle that will help our nation's troops effectively and safely carry out missions without being constrained by existing roadways and conventional landing zones. Since 2005, we have been talking to the U.S. Marines and DARPA about the propulsion needs for a Transformer Vehicle, and the EnduroCORE engine characteristics address those needs.
The best part? He actually calls the TX vehicle a Transformer. The world's most powerful engineers are nerds, just like us.
Anyway, this technology, which relies on a rotating, roughly triangular rotor that rotates and bounces inside of a lumpy, epitrochoidal housing, has been with us for a while. Some of the earlier combustion engines actually used similar systems, but only a handful--most memorably built by Mazda--were used in automobiles.
In aviation, rotary engines found a more comfortable home. Prop planes and small helicopters in particular benefit from the engines' naturally smooth operation and high rotation rates, so it's not that strange that DARPA's new half-chopper toy would use a rotary engine. The decision to use diesel fuel is most likely a consideration of practicality and convenience.
It'll remain to be seen if Pratt and Whitney can overcome the naturally low torque inherent in rotary engines, which could be problematic when the craft is in driving mode. But they've got time--the earliest date for TX vehicle deployment is 2015.