If you ask Admiral Mike Mullen, the days of manned fighter jet piloting are numbered. A couple of years back, the chairman of the U.S. Joint Chiefs of Staff argued that Lockheed Martin's (NYSE: LMT) F-35 Lightning II warplane could be "the last manned fighter" the U.S. ever builds. Recent developments suggest the admiral may have jumped the gun.

According to an April story on SpaceDaily.com, the lithium-ion batteries that power today's smaller flying robots aren't optimized for military performance. They need to last longer, and cost less. Because their weight drags down the UAVs that must haul them into the sky, it would be nice if they weighed a bit less as well, to improve flight times. And that's just the beginning.

What's the frequency, Kenneth?
According to DefenseNews.com, the more UAVs get put to real-world use, they more they're revealing an Achilles heel: vulnerability to jamming. Dependent on GPS to tell them where they are, and radio transmissions to tell them what to do, remotely piloted aircraft are especially susceptible to interference by tech-savvy foes.

According to DN, the Air Force is beginning to worry about how well UAVs can operate in "GPS-denied environments" and "comm-out environments." One analyst even mused that "for operations inside defended airspace, manned aircraft would be the preferred option until a solution is found."

Speak of the devil
UAVs only recently joined the fight in Libya, but they may already reflect the Air Force's concerns. Combat operations began in Libya on March 19 with a volley of Tomahawk missiles, followed by human-piloted fighter bombers -- striking ground targets. It was more than a month  before President Obama deemed Libyan airspace safe enough to authorize deployment of unmanned Predator drones.

This raises the troubling prospect that we may need piloted aircraft to make the battlefield safe for robots -- which would kind of defeat the purpose of inventing pilotless aircraft in the first place.

Foolish takeaway
Defense contractors Textron (NYSE: TXT), Boeing (NYSE: BA), United Technologies (NYSE: UTX), Honeywell (NYSE: HON), and even Lockheed itself have collectively invested billions in inventing the UAV industry. But unless they figure out a way to make UAVs survivable in contested airspace, the whole darn shootin' match could be rendered obsolete.

Personally, I think that means Lockheed's F-35 just got a new lease on life.

Fool contributor Rich Smith has no position in any company named above, but The Motley Fool owns shares of Textron and Lockheed Martin. Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.