One key advantage of unmanned vehicles is that their persistence is not limited by the human operator. This also means their size is set by the payloads and sensors that they carry, not by the need to accommodate a pilot or crew. But in many cases, this now means that unmanned systems run into an energy limit, both in terms of endurance and their ability to deliver power to radars, lasers and communications links.

Unmanned ground vehicles and small hand-launched UAVs continue to benefit from the fact that their electrical power requirements are not vastly different from those of consumer electronic devices, so they have been able to ride on a multibillion-dollar wave of investment. Larger systems face different challenges.

A number of efforts to develop multiday-plus endurance UAVs have foundered in the past year or so. The Defence Advanced Research Projects Agency's (Darpa) Vulture project for a huge solar-powered UAV was canceled earlier this year. Lockheed Martin's HALE-D solar-powered airship impaled itself on Pennsylvania treetops last July, on its first flight attempt. The U.S. Air Force let the gas out of the MAV6 Blue Devil 2 project with a stop-work order. AeroVironment's Global Observer was canceled after the first vehicle crashed at Edwards AFB, Calif., and the other U.S. hydrogen-powered UAV, Boeing's Phantom Eye, is under repair after a rough first-flight landing. The surviving airship program, the Northrop Grumman Long Endurance Multi-Intelligence Vehicle (LEM-V) is running late, to the surprise of almost no one.

The art and science of lighter-than-air control is not well understood, and even with hydrogen fuel, heavier-than-air ultra-long endurance requires light, long wings, unusual propulsion arrangements and aggressive weight-saving. Phantom Eye takes off from a trolley and lands on a skid-and-wheel combination, the latter being the source of its problems.

Power issues were one reason for the USAF's attempt (being opposed in Congress) to retire the RQ-4B Block 30 Global Hawk while retaining the U-2S. The Rolls-Royce AE3007H meets the RQ-4B's needs for efficient propulsion, but such engines are sensitive to generator loads at the edges of their envelope (high altitudes and low speeds) and cannot run the kinds of sensors that the U-2's fighter-based F118 can support.

One propulsion engineer notes that standard military specifications are part of the problem, requiring the engine to tolerate very rapid fluctuations in generator load—necessary in combat, less so in a typical UAV mission. But the problems involved are severe—and explain why the notion of a nuclear-powered UAV, explored recently in a Sandia-Northrop Grumman study, still gets serious consideration.

At the lower end of the UAV size scale, attempts to develop heavy-fuel engines (spark ignition or diesel) to replace gasoline-fueled reciprocating or rotary engines have been underway for decades. Advantages in efficiency and logistics (gasoline has disappeared from ground forces and is detested on ships) are undoubted, but the challenge of developing an HFE that is light, reliable and low in vibration (including torque vibrations that stress reduction gears) and that is supportable, is still daunting.

Underwater, battery technology is leading to some improvements as it is fostering the development of faster, longer-range, near-silent torpedoes. However, mobility and endurance are problems for unmanned undersea vehicles. The U.S. Navy wants systems that can move quickly—to get into position to screen the fleet, for example —but then stay on station, employing sensors, for days or weeks.

One radical solution is under discussion at the Naval Research Laboratory (NRL): turning ocean sediment into fuel for unmanned vehicles and unattended sensors. The idea is to create a microbial fuel cell that generates electricity using nothing more than organic material and oxygen that are readily available in the ocean. In theory, this could provide organic power for individual vehicles or power an unattended sea-bed “charging station” for unmanned undersea vehicles.

“Microorganisms have figured out how to acquire energy in this environment to satisfy their energy needs,” says Leonard Tender, a research chemist in the Center for Bio-Molecular Science and Engineering at the NRL. “What we have figured out is how to tap into the microbial processes to extrapolate enough electricity to power oceanographic sensors.”

The fuel cell works by harvesting the metabolic activity of microbial organisms that live in sediment—essentially mud on the ocean floor or at the bottom of estuaries—and then transferring that energy to a fuel cell, making it available to power the electronics on an unmanned system or sensor. This essentially creates a battery fueled by mud.

Turning ocean sediment into power is more than just a novelty for the Navy. Tender describes the need to power thousands of marine sensors as one of the “vexing problems” for the military. The idea is that instead of using sensors with batteries that run down, systems would be equipped with microbial fuel cells that could recharge themselves without human intervention.

A fuel cell that can be autonomously recharged opens the door to more extensive use of sensors and unmanned systems at sea, allowing them to operate autonomously over long periods of time, according to Tender. Such microbial fuel cells could also someday be used to power underwater vehicles that recharge themselves while at sea.

All this is easier said than done, however. Tender has proved the microbial fuel cell concept can work in small-scale demonstration in the lab, and some limited tests in the field, but it's “very, very hard,” he says.

Getting the setup right requires implanting an electrode in just the right place below the surface sediment, which makes the lab's simulated environment critical to testing out the microbial fuel cell. “To get that arrangement is a real technological challenge,” says Tender.

But if it works, naval researchers will have created something that will please military officials and civilians alike: an environmentally conscious power source for unmanned systems and sensors.

Another solution to some UUV requirements—so long as persistence rather than speed is important—is to eschew normal propulsion completely. Underwater gliders are slow but steady robot submarines that can cross oceans and carry out missions lasting months. They are typically no more than 6 ft. long and weigh 100 lb. The U.S. had previously dominated glider technology, but China is catching up fast and may even pull ahead in the coming years. Few of the developments are explicitly military but this is very much a dual-use technology.

A “National Defense Key Laboratory of Autonomous Underwater Vehicle Technology” has been established at Harbin Engineering University. Published papers focus on sensing and navigation for autonomous underwater vehicles, and techniques for controlling fleets of them over a wide area.

“They're putting a lot of money, a lot of engineers into this field, “says Lyle Goldstein, strategic researcher at the U.S. Naval War College's China Maritime Studies Institute. “They're energized because they know there's a gap in underwater capability, and this is a chance to leapfrog ahead.”

Rather than having propellers or thrusters, a glider adjusts its buoyancy by pumping gas in or out of an external bladder, then glides a shallow path as it rises or falls. Average speed is typically just half a knot, but with the endurance to cross oceans. Since their development in the 1980s in association with the U.S. Navy, gliders have won acceptance. Three U.S. companies now offer different versions.

The Navy is the biggest user, with up to 150 vehicles on order over three years for the Littoral Battlespace Sensing-Glider program. Teledyne Webb is supplying the gliders, which will carry out a variety of missions including measuring underwater conditions affecting sonar and communications.

A U.S. Defense Security Service report claimed that in 2010 “East Asian and Pacific-affiliated collectors targeted underwater gliders specifically” for technology espionage, and China appears to be catching up in the glider race.

In 2011 China tested its first locally designed glider, the Sea Wing, developed at Shenyang Institute of Automation. This deep-water research vehicle reportedly carried out several successful missions in the Western Pacific, diving to 2,700 ft.

Many more designs are in the pipeline. The Petrel is a hybrid craft from Tianjin University, with a propeller as well as a buoyancy engine, using the propeller for rapid, short-range manoeuvring. Petrel is undergoing field trials at Fuxian Lake.

Other Tianjin projects are extending the gliders' endurance. The Dragon has a fuel cell rather than lithium batteries, with perhaps a mission duration measured in years. Another Tianjin glider will have a 'temperature difference engine.' This is similar to the prototype thermal glider pioneered by Teledyne Webb, which extracts energy from the temperature variation between sea water layers at different depths. It has the potential to power a glider indefinitely.

Researchers at Northwestern Polytechnic University at Xian are improving glider agility by giving them wings that can move independently. The researchers claim this design is faster and more efficient than fixed-wing gliders, as well as more maneuvrable.

A second Xian project takes the idea further, with flapping wings that act like a turtle's fins to provide extra propulsion. This model is now starting laboratory trials. China excels in the field of “robotic fish” with flapping fins. Gliders incorporating this technology could put on a burst of speed without compromising stealth, while retaining glider endurance for long missions.

The U.S. is carrying out R&D in many of the same areas, though not at a rapid pace; comparing the two is difficult. There may be more research in China not in the public domain. Papers from “No. 710 R&D Institute,” a facility at Yichang that carries out military research, mention studies of an underwater glider big enough to carry weapons. This would represent a dramatic new sea denial capability.

However, the Chinese have yet to prove that they can translate any of their advanced glider technology programs into a finished product that can outdo designs from the West.

“There has not been any Sputnik moment yet,” says Goldstein.

Goldstein warns that it's too early to say whether the Chinese navy will commit itself to gliders on a large scale. But while the progress of a new aircraft carrier or nuclear submarine under construction can be tracked over years, new gliders could be built rapidly and in secrecy in large numbers, and they are difficult to spot even when deployed. That Sputnik moment might yet arrive.