13 July 2001
13 July 2001
To reach the doorstep of space, engineers designed planes like the X-15, a rocket-powered aircraft that during the 1960s carried a hardy breed of test pilots to the atmosphere's edge and back at up to seven times the speed of sound.
This month, another crew hopes to reach similar heights. But it plans to do it at a stately 25 miles an hour with a craft that might make X-15 jockeys shake their heads in wonder.
Called the Helios Prototype, the craft is a flying wing whose tip-to-tip reach of more than 240 feet outspans the wings of an Air Force C-5 cargo jet. Built with carbon-fiber composites, Styrofoam, mountain-bike tires, plastic sheeting, and topped with solar cells to convert sunlight into electricity, the wing tips the scales at about 1,600 pounds. It is powered by 14 electric motors, each packing all the punch of a hand-held hair drier. The remotely piloted Helios is scheduled for a shakedown flight tomorrow from a US Navy test range in Hawaii.
Later this month, if all goes well, the crew will attempt to take the craft to 100,000 feet, shattering altitude records for propeller-driven planes and setting the stage for developing a fleet of aircraft that can fly for months at altitudes of 80,000 feet or higher.
"We call it the eternal airplane," quips Pete Jacobs, a spokesman for the project. Helios is a Goliath among unmanned aerial vehicles (UAVs), which range in size from Helios's 247-foot span to experimental craft with wingspans of six inches. The field has grown during the past two decades, largely driven in the US by demands from the Pentagon for safer ways to conduct battlefield reconnaissance, says Robert Michelson, an adjunct professor of aerospace engineering at the Georgia Institute of Technology in Atlanta, and past president of the Association for Unmanned Vehicle Systems International. Yet while hundreds of companies build UAVs, most "build only one or two of a kind and never sell them," he notes.
Helios's designers say they hope to change that picture. The $15 million craft represents a joint effort by the National Aeronautics and Space Administration and AeroVironment, Inc., a Monrovia, Calif., company that has been developing solar-powered aircraft since the 1970s. Both foresee craft like Helios emerging as cheap alternatives to satellites for a range of applications.
NASA's interest lies in the potential for such aircraft to be platforms for a range of earth- and atmospheric-science experiments, notes John Hicks, a NASA project manager for environmental-research craft. "Its big value is that it can do remote sensing and imaging in a continuous fashion, versus a satellite, which might make one pass every 90 minutes," he says. Moreover, the craft can monitor and sample atmospheric chemicals, such as stratospheric ozone, without leaving a trail of pollutants in its wake. Such samples now must be taken by jets like NASA's human-piloted ER-2, a converted U-2 spy plane.
He also notes that at 100,000 feet, the Earth's atmosphere has a density close to that of Mars, making that altitude region a perfect place to gather data on flight characteristics that could lead to tiny remotely piloted aircraft being added to future Mars missions. For its part, AeroVironment hopes to see Helios emerge as a key telecommunications link, as well as a remote-sensing tool to help monitor everything from crop-ripeness to forest-fire hot spots. "The first application we're pursuing is high-speed Internet access," says Tim Conver, AeroVironment's president and CEO. He explains that while the Internet's fiber-optic links "hold massive amounts of bandwidth potential," most users can't take advantage of that because they connect to the Web using copper wire, which can't carry as much data. He envisions a fleet of Helios-type aircraft acting as "11-mile high towers," linking users to the Web in a manner similar to satellite TV.
The key to the craft's endurance is a system - still under development - of fuel cells that convert hydrogen and oxygen into electricity and water. Mr. Hicks explains that during the day, the solar cells produce more than enough electricity to run the craft's motors. The excess is used to break down about seven gallons of water into oxygen and hydrogen. At night, the process reverses. The fuel cells use the oxygen and hydrogen to generate electricity and water. The Helios team hopes to add the fuel-cell system after this set of flights and set a high-altitude endurance record in 2003.