It goes without being said that stealth is paramount to any special operations forces (SOF) mission. It is inherently advantageous to be able to move from point A to point B while drawing as little attention as possible. Thanks to The Defense Advanced Research Projects Agency (DARPA), the transportation abilities of U.S. SOF just got a lot easier. Soon, the soldiers will have access to the so-called Silent Hawk and Nightmare, which are leaps and bounds ahead of current models.
According to Defense One:

The Silent Hawk by Logos and the Nightmare from LSA Autonomy could be cousins. DARPA has funded both to phase two development under a small business innovative research award, essentially two non-competing grants to develop the technology. Both bikes feature cutting-edge hybrid multi-fuel engines that can burn a variety of combustibles like JP-8, Jet A-1, gasoline, propane, etc.. “If it’s gasoline, tell it it’s gasoline, tell it it’s something else. It will figure it out,” said Alex Dzwill, and engineer with Logos. Could you run it on lipids, olive oil? “Theoretically, yeah,” said Dzwill.

The key difference with these bikes is the ability to switch into an electric mode, which is much more tactful than standard gas. Popular Science detailed the difference in the engines in a report on its site.

The combustion engines are for when the bikes don’t need to be quiet. Running on fuel, they reach 80 decibels — somewhere between a vacuum cleaner and a garbage disposal. Running on just electric power, the engines quiet down to 55 decibels — roughly as loud as an office. That silence becomes stealth, allowing the bicycling Green Berets to travel unheard, provided there is any other ambient sound at all.

The full Popular Science report on the two bikes can be read on its website here.