Unmanned aircraft have been billed as the ultimate in efficiency, a model for the Pentagon’s vision of substituting technology for labor.

But don’t be fooled, Pentagon officials warn. Unmanned aerial vehicles are great for spying over enemy territory, but they are a huge drain on human resources. In fact, a combat air patrol that relies on UAVs demands more operators and support staff than if it were conducted by conventional aircraft with a pilot in the cockpit.

“Over 150 people are needed to do one combat air patrol” with U.S. Air Force Predator or Reaper drones, says Reginald Brothers, deputy assistant secretary of defense for research.

“That’s a lot of people,” he adds. “These unmanned systems require more people to operate than tactical manned aircraft,” Brothers tells an industry conference Feb. 14 hosted by Aviation Week & Space Technology.

How to make unmanned systems less labor intensive was cited by Brothers as one the Pentagon’s pressing technological challenges. “We are seeing increasing reliance on UAVs” but the military cannot afford the soaring personnel demands associated with these systems, he says.

One reason why so-called “autonomous” vehicles require so much human oversight is that machines and humans are not able to share a “common perception” of what is happening, he explains. As a result, machines are not yet fully trusted to conduct missions independently.

“The challenge is having shared situational awareness, a shared perception between the machine and the human operator,” Brothers says.

While the search for more intelligent machines continues, the military continues to beef up its unmanned vehicle fleet. The Air Force today has 128 Predators, 30 Reapers and 13 Global Hawks deployed in combat operations, Brothers says. Predators and Reapers alone generate more than 1,000 hours of full-motion video each day. Global Hawk produces more than 50 hours of imagery intelligence per month. The Air Force expects to own about 700 drones by next year, and the Army will have an additional 500 Shadow and Gray Eagle UAVs. The Defense Department also has purchased 6,000 ground robots for use by deployed troops.

The U.S. Navy, perhaps more so than the other services, is betting on the success of autonomous UAVs. Naval Air Systems Command officials believe that there are few technological barriers to having combat aircraft flying autonomously, but worry that policies stand in the way. And there is no clear consensus on what level of autonomy vehicles should have given the politically sensitive issues that have arisen in recent years as the U.S. military and the CIA have escalated a campaign of drone strikes that, while aimed at terrorist leaders, have on several occasions killed civilians.

For the foreseeable future, UAVs will require giant crews. The Air Force in fact decided to cut its future purchases of Reaper aircraft in half — from 48 to 24 — because it cannot provide enough manpower to operate and process the data from more aircraft.

“It didn't make sense to have the production out that far ahead of our ability to actually do the processing and exploitation and dissemination function,” says Deputy Assistant Secretary of the Air Force for Budget Marilyn Thomas. “We're actually stretching the production [of Reapers] to sync it up better with our ability to produce crews.”