"I'll be back ... in about two months." If the U.S. Navy gets its way, this may be the big catchphrase from the next Terminator movie -- Terminator IV: Arnold Goes SCUBA Diving.

For a few months now, we've been hearing sonar rumblings of a big new Naval project to build a long-endurance, deep-diving robotic submarine. The project, now monikered "LDUUV" to denote the wished-for Large Displacement Unmanned Undersea Vehicle, aims to build a robot sub capable of operating away from base for 70 days at a time.

In August, I wrote about Boeing's (NYSE: BA) presumed entry into the competition, dubbed the Echo Ranger. In announcing its creation, Boeing noted that ER is an 18.5-foot-long vehicle weighing five tons, capable of making eight knots at sea, and diving to 10,000 feet. Crucially, the company promised that its sub would achieve the necessary 70-days' endurance "with the right power source." As it turns out, this is key.

You see, LDUUV will need to operate independently for long stretches of time. It must be smart enough to independently avoid obstacles such as fishing nets … and Greek islands. (iRobot (Nasdaq: IRBT) already has a small sub suited to that side of the task.) But LDUUV must also be able to operate far from base and not need to return for fuel for two months at a stretch. Right now, project managers are focusing on this latter task because, according to the Navy, "If you can't develop the energy we need, we can't do the mission."

So ... who can "develop the energy" they need?

The go-to choice for extended range underwater missions has historically been nuclear. But you can imagine the outcry if people discovered the U.S. was sending out fleets of potential Fukushima Daiichis to wander the seas unsupervised. Right now, speculation seems to be focusing on a fuel cell, or lithium-battery solution. It's not known which approach Boeing favors for its sub, but the fuel cell idea seems to suggest that Plug Power or Ballard Power could play a role here. On batteries -- well, many companies make rechargeable batteries. One company in particular that seems well suited to the task might be AeroVironment (Nasdaq: AVAV), which not only makes unmanned aerial vehicles already, but also has a fast-charging battery division that might be useful for "refueling" the LDUUV at sea.

The situation remains, shall we say, "fluid" -- but don't worry. Here at the Fool we'll keep an eye on it for you, and update you as developments happen.

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