As a follow-on to the Centurion (and earlier Pathfinder and Pathfinder-Plus) aircraft, the solar-powered Helios Prototype is the latest and largest example of a slow-flying ultralight flying wing designed for long-duration, high-altitude Earth science or telecommunications relay missions in the stratosphere. Developed by AeroVironment, Inc., of Monrovia, California, under NASA's Environmental Research Aircraft and Sensor Technology (ERAST) project, the unique craft is intended to demonstrate two key missions: the ability to reach and sustain horizontal flight at 100,000 feet altitude on a single-day flight in 2001, and to maintain flight above 50,000 feet altitude for at least four days in 2003, with the aid of a regenerative fuel cell-based energy storage system now in development. Both of these missions will be powered by electricity derived from non-polluting solar energy.

The Helios Prototype is an enlarged version of the Centurion flying wing, which flew a series of test flights at NASA's Dryden Flight Research Center in late 1998. The craft has a wingspan of 247 feet, 41 feet greater than the Centurion, 2 1/2 times that of its solar-powered Pathfinder flying wing, and longer than the wingspans of either the Boeing 747 jetliner or Lockheed C-5 transport aircraft.

The remotely piloted, electrically powered Helios Prototype went aloft on its maiden low-altitude checkout flight Sept. 8, 1999, over Rogers Dry Lake adjacent to NASA's Dryden Flight Research Center in the Southern California desert. The initial flight series was flown on battery power as a risk-reduction measure. In all, six flights were flown in the Helios Protoype's initial development series.

In upgrading the Centurion to the Helios Prototype configuration, AeroVironment added a sixth wing section and a fifth landing gear pod, among other improvements. The additional wingspan increased the area available for installation of solar cells and improved aerodynamic efficiency, allowing the Helios Prototype to fly higher, longer and with a larger payload than the smaller craft. In addition, project engineers added a differential Global Positioning Satellite (GPS) system to improve navigation, an extensive turbulence monitoring system payload to record structural loads on the aircraft both in the air and on the ground, and radiator plates to assist in cooling the avionics at high altitudes where there is little air to dissipate heat.

During 2000, more than 65,000 solar cells in 1,800 groups were mounted on the upper surface of Helios' wing. Produced by SunPower, Inc., these bi-facial silicon cells are about 19 percent efficient in the flight regime in which the helios is designed to operate, converting about 19 percent of the solar energy they receive into electrical current. The entire array is capable of producing a maximum output of about 35 kw at high noon on a summer day.

The mission to reach and sustain flight at 100,000 feet in 2001 requires use of all 14 motors and minimal ballast to save weight, with the aircraft weighing in at only a little more than 1,600 lbs. The four-day mission above 50,000 feet envisioned for the Helios Prototype in 2003 will see only eight motors powering the craft and the addition of the regenerative energy storage system now in development. The system will increase the Helios Prototype's flight weight to a little over 2,000 lbs. Fewer motors are needed for the long-endurance mission due to the lesser altitude requirements, and the excess electrical energy generated by the solar arrays during the daytime will be diverted to the hydrogen-oxygen fuel cell energy storage system, which will release the electricity to power the Helios after dark.

With other system reliability improvements, production versions of the Helios are expected to fly missions lasting months at a time, becoming true "atmospheric satellites."

The Helios Prototype is the latest and largest example of a slow-flying ultralight flying wing designed for high-altitude, long-duration Earth science or telecommunications relay missions. A follow-on to the Pathfinder and Pathfinder-Plus solar aircraft, the Helios Prototype soared to 96,863 feet altitude in August 2001, setting a new world record for sustained altitude by winged aircraft, powered only by energy from the sun.

Developed by AeroVironment, Inc., of Monrovia, Calif., under NASA's Environmental Research Aircraft and Sensor Technology (ERAST) project, the unique craft was designed to demonstrate two key missions: the ability to reach and sustain horizontal flight near 100,000 feet altitude on a single-day flight, and to maintain flight above 50,000 feet altitude for almost two days, the latter mission with the aid of an experimental fuel cell-based supplemental electrical system now in development.

The Helios Prototype is an enlarged version of the Centurion flying wing that flew a series of test flights at Dryden in late 1998. The craft has a wingspan of 247 feet, 41 feet greater than the Centurion, 2 1/2 times that of the Pathfinder flying wing, and longer than the wingspans of either the Boeing 747 jetliner or Lockheed C-5 transport aircraft.

The remotely piloted Helios Prototype first flew during a series of low-altitude checkout and development flights on battery power in late 1999 over Rogers Dry Lake adjacent to NASA's Dryden Flight Research Center in the Southern California desert.

In upgrading the Centurion to the Helios Prototype configuration, AeroVironment added a sixth wing section, a fifth landing gear pod and a differential Global Positioning Satellite (GPS) system to improve navigation, among other improvements. The additional wingspan increased the area available for installation of solar cells and improved aerodynamic efficiency, allowing the Helios Prototype to fly higher, longer and with a larger payload than the smaller craft.

During 2000, more than 62,000 bi-facial silicon solar cells were mounted on the upper surface of Helios' wing. Produced by SunPower, Inc., these solar arrays convert about 19 percent of the solar energy they receive into electrical current and can produce up to 35 kw at high noon on a summer day.

The second milestone established by NASA for its development Ð a long-endurance demonstration flight of almost two days and nights Ð required development of a supplemental electrical power system to provide power at night when the solar arrays are unable to produce electricity. AeroVironment developed an experimental fuel cell-based electrical energy system combining advanced automotive fuel cell components with proprietary control technology designed for the harsh environment above 50,000 feet altitude.

The first version of this system combines gaseous hydrogen from two pressurized tanks mounted on Helios' outboard wing sections with compressed oxygen from the atmosphere via a series of proton-exchange membrane fuel cell "stacks" mounted in the central landing gear pod. The system produces more than 15 kW of direct-current electricity to power Helios' motors and operating systems, with the only by-product being water vapor and heat. The system will increase the Helios Prototype's flight weight by about 800 lb to about 2,400 lb.

Two other versions of the system are contemplated: One, employing liquid hydrogen, would enable the Helios to fly for up to two weeks in the stratosphere anywhere around the Earth, not limited to temperate or equatorial latitudes. Another version, a closed or "regenerative" system, uses water, a fuel cell, and an electrolyzer to form a system similar in function to a rechargeable or "secondary" battery, but with much greater efficiency than the best rechargeable battery systems.

A production version of the Helios with the regenerative fuel cell system is of interest to NASA for environmental science, the military and AeroVironment for various roles, primarily as a stratospheric telecommunications relay platform. With other system reliability improvements, production versions of the Helios are expected to fly missions lasting months at a time, becoming true "atmospheric satellites."