Fuel Cell Technology Soars
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Recent advances in fuel cell technology offer the potential for small tactical unmanned aerial systems (UASs) to operate up to 24 hours. The Experimental Fuel Cell (XFC) UAS being developed by the U.S. Naval Research Laboratory (NRL) currently can fly for up to six hours and carry a variety of sensor payloads. |
A small unmanned aerial vehicle powered by a fuel cell soon may be soaring over distant battlefields. Lightweight tactical robot aircraft are vital for supplying ground forces with immediate reconnaissance information, but their battery-powered engines limit their operational time. New advances in fuel cell technology will allow smaller, lighter robotic aircraft to stay aloft for 24 hours or more to supply commanders with continuous data.
Developed by the U.S. Naval Research Laboratory (NRL),
The XFC weighs 16.5 pounds with payload. Its asymmetric wings are designed to unfold into an X shape and lock into place after launch. The payload, which consists of an electro-optic camera, can be removed quickly and replaced by a variety of sensors. The UAS is fully autonomous after launch. It operates with minimal user guidance, primarily to control the sensor package.
Designed for storage and launch in a transport tube, the XFC uses a process called electronically assisted takeoff (ETO). The ETO system consists of a battery-powered electric motor running two counter-rotating propellers. This package is attached to the nose of the UAS. During launch, the batteries use all of their charge in several seconds, but they allow the electric motor to pull the XFC out of its cargo tube and reach flying altitude. Once the aircraft is at altitude and its wings unfold, the ETO engine is jettisoned and the fuel cell engine is activated. The ETO system allows the XFC to deploy from a variety of places such as the deck of a ship or the back of a pickup truck, and it eliminates the need for traditional launch assist technologies such as rockets and explosive charges.
The XFC program began in fall 2006. Warren W. Schultz, associate superintendent of the NRL’s chemistry division, notes that the program began from scratch with a first-generation 300-watt fuel cell and the aircraft concept. The entire platform went from design to first flight in eight months. “The design is proof that the technology is mature,” Foch maintains.
The XFC’s body is made of fiberglass reinforced with carbon fiber spars. Foch notes that the design reflects tradeoffs between cost and weight. He adds that the aircraft has internal antennas, which require the skin to be transparent to radio waves. Carbon fiber composites can block radio waves, so the aircraft’s wings are a thin outer layer of fiberglass sandwiching an internal foam core. The fuselage also is a thin shell of fiberglass. Foch explains that using materials such as fiberglass eliminates the need for internal ribs and other support structures, which frees more internal space for fuel tanks and sensors.
Foch notes that the NRL has been developing new UAS technologies since the 1970s. This early research focused on developing UASs as ship-launched decoys to draw away enemy missiles. The requirements were for inexpensive, autonomous, high-performance platforms that could unfold their wings after launch and quickly distract incoming missiles by transmitting a false electromagnetic signature. He notes that these capabilities were not adopted as requirements by the wider UAS community for many years.
Harnessing rapidly evolving fuel cell technology for tactical UAS platforms is the heart of the XFC program. Prior to the XFC, the NRL conducted some initial proof-of-concept testing to study the feasibility of fuel cells in small tactical UAS platforms. The first proof-of-concept aircraft was powered by a 90-watt fuel cell. This soon was replaced by the 300-watt fuel cell installed on the XFC. Schultz adds that the NRL currently is developing and test-flying a 500-watt fuel cell.
He relates that when the NRL began its initial experiments with the 90-watt fuel cell, the Navy did not anticipate such rapid development. “We’re even a bit surprised of where we are now,” he says, adding that the NRL recently began work on a 1.5-kilowatt fuel cell.
Foch is excited about the speed and development of fuel cell research for UAS applications. He says this is the first program he has worked on where the power-to-weight ratio has improved by a factor of five over the past five years. “The fact that it’s not ramping over yet is tremendous. We could barely make an incredibly flimsy model airplane fly four years ago—just enough to set a world record. Now we can build a tube-launched tactical airplane that can fly a military mission,” he relates.
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The XFC is equipped with a 300-watt fuel cell. NRL researchers are working on a 500-watt fuel cell to provide the aircraft with additional endurance and power. |
“In three and a half years, we’ve gone from barely able to fly to mission practical,” Schultz adds. He notes that instead of the typical incremental development pattern for airborne platforms, the fuel cell research has been revolutionary.
The additional power of the new fuel cells increases many of the XFC’s capabilities, such as endurance and dash speed. The 500-watt fuel cell also is the same size as the 300-watt version. Foch explains that in aircraft design, an optimal amount of power is required to keep an airplane airborne. After this basic requirement, additional power is needed to lift the aircraft above bad weather and to carry payloads. He notes that by designing a UAS to fly with 300 watts, the engineers are really making an aircraft that can fly on 100 watts, with an additional 100 watts each for payload and altitude.
Hydrogen offers a variety of advantages as a fuel. Foch explains that the Navy likes hydrogen because ships can hydrolyze it directly from seawater. Foch, who has worked on more than 50 aviation/UAS programs for the NRL, notes that hydrogen is the safest fuel the NRL has ever used. He adds that the lightweight carbon fiber fuel tank is designed to fail in a specific manner and safely disperse the pressurized gas.
Range and endurance are key elements of the NRL’s research. Small hand-launched tactical UASs used by Army and Marine Corps infantry units have relatively short range and endurance. But systems launched from Navy ships must cross many miles to carry out missions that cannot be accomplished by battery power. Foch notes that endurance is a new requirement for tactical UAS systems, which often are viewed as short-range platforms.
No operational requirements existed for long endurance tactical UASs prior to the development of the XFC, Foch says. But with fuel-cell-powered aircraft soon able to fly up to 24 hours, new sets of requirements—such as the need for day and night cameras—will become necessary. As the technology matures, he explains, it will force engineers to change their thinking about how different systems are used. The NRL is working with other groups such as the Marine Corps Warfighting Laboratory to help develop tactical doctrine and new operational ideas.
Another advantage of fuel cells is that they are very efficient power sources for small airborne platforms. Foch notes that the NRL’s XFC fuel cell is 40 percent efficient. But a small reciprocal gasoline engine of the type used on military UASs is only about 7 to 12 percent efficient. He points out that in the tactical UAS category, fuel cells soon will exceed the endurance of the gasoline and heavy fuel engines in use today.
An additional benefit of fuel-cell-powered aircraft, something they share with their battery-powered cousins, is silence, which is very useful for low-altitude surveillance missions. “If you don’t mind handling the hydrogen, you get a bit of everything—low IR [infrared signature], low RF [radar signature], low acoustic signature and long endurance,” Foch says. “But you’ve got the logistics of handling the fuel [to consider],” he adds.
Although hydrogen is combustible, the aircraft use only small amounts of the gas. It was the NRL’s first fuel cell prototype that set a record for fuel-cell-powered aircraft by flying for three and a half hours on 15 grams of fuel. Schultz observes that that weight is roughly the equivalent of a quarter coin. Hydrogen also is not explosive when it is compressed. It must be mixed with air to become explosive.
The program is in Phase Two, or technology transition. In fiscal year 2010, the goal is to transition the program to industry for manufacture. Schultz notes that warfighters in the services are aware of and interested in the XFC and its capabilities. NRL researchers traditionally develop new technologies and then approach the Navy with their potential. “Our job is to establish that baseline of technologies that the fleet can reach into, so they can propose requirements,” Foch says. The NRL is working with the Navy’s PMA 263 office as its acquisition sponsor. The program’s research also is supported by the Office of Naval Research, the Defense Department’s Rapid Reaction Technology Office and the department’s Technology Transition Office.
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