Volume 9, Number 6 • November/December 2001 • Aerospace Technology Development

Solar Aircraft Sets Altitude Record

The unique Helios Prototype solar-powered flying wing, developed by AeroVironment, Inc. for a NASA program managed by Dryden, reached an altitude of 96,863 feet during a maximum-altitude flight on August 13 from the US Navy’s Pacific Missile Range Facility (PMRF) on the Hawaiian island of Kauai.

Although short of the 100,000-foot altitude that project officials hoped for, the altitude is the highest ever flown by a non-rocket-powered aircraft in sustained horizontal flight and well above the current world altitude record of 85,068 feet for sustained horizontal flight by an aircraft, set by a US Air Force Lockheed SR-71A reconnaissance aircraft in July 1976. It also surpassed the existing altitude record for propeller-driven aircraft, 80,201 feet, set by the Helios Prototype’s predecessor, the Pathfinder-Plus, in August 1998. The 96,863-foot record altitude remains unofficial, however, until certified by the National Aeronautics Association’s Board of Records and Standards.

“This is like going to the Olympics and setting a new world record for engineers,” added John Del Frate, solar aircraft project manager at Dryden. “This achievement did not come easily. Thousands of things had to work right for something like this to come together.”

Due to shortening hours of daylight, a reduced sun angle as the summer waned in the northern hemisphere and low cloud cover during the morning, which delayed take-off by about 36 minutes, the Helios Prototype’s time-to-climb was limited to about seven hours. The aircraft reached its maximum altitude shortly after 4:00 PM, when the sun angle had already dropped to less than 45 degrees, and Helios’ climb rate dropped to zero. Even with the reduced angle, however, the solar arrays atop the Helios’ wing were still producing about 24 kilowatts, or about 70 percent of their maximum rated output of 35 kilowatts at mid-day.

The Helios Prototype flew for more than 40 minutes above a 96,000-foot altitude before beginning its descent. It was in the air for almost 17 hours on the record flight, having lifted off the PMRF runway at 8:48 AM and landed at 1:43 AM the following morning after a 9 1/2-hour descent. Electrical power for post-sunset flight was provided by the generating capability of the motors using the windmill effects of the propellers as the aircraft glided down.

Project officials report that Helios’ propulsion, avionics, environmental and flight control systems worked flawlessly during the flight. Temperatures encountered ranged from 80¡ F at take-off to a low of minus 85¡ F at 58,000 feet, but all systems stayed within their temperature limits.

The remotely operated aircraft had reached 76,200 feet during its first checkout flight from PMRF under solar power a month earlier on Saturday, July 14. The Helios Prototype flew six low-altitude airworthiness validation flights on battery power at Dryden in the fall of 1999.

Since then, the aircraft underwent major upgrades, including the installation of high-efficiency solar cell arrays across the wing, navigation and emergency lights, and improved avionics. AeroVironment technicians also completed upgrades to the ground control station and the tracking antennas, and updated operational procedures. More recently, AeroVironment developed a new propeller design that is both stronger and more efficient than the propellers that drove the Helios Prototype during its earlier test flights. The new propellers were used on both the record flight in August and the earlier checkout flight in July.

The Helios Prototype aircraft in the early stages of its record-breaking high-altitude flight. Photo provided by Dryden Flight Research Center.

The maximum-altitude flight was one of two major flight milestones set for the craft by NASA—the other being a four-day non-stop endurance demonstration flight above 50,000 feet planned for 2003. Development of a regenerative hydrogen-oxygen energy storage system that would make the multi-day continuous flight possible is progressing at AeroVironment. The system will use excess power generated by the solar arrays during the daytime to run an electrolyzer that separates water into its component parts, hydrogen and oxygen, which are then stored under pressure in specially designed tanks. At night, the hydrogen and oxygen will be recombined by the fuel cells, producing electricity as a by-product to power Helios’ motors and systems.

Production variants of Helios might see service as long-term Earth environmental monitors, disaster monitoring, as well as communications relays, reducing dependence on satellites and providing service in areas not covered by satellites. The record-altitude flight also provided NASA and AeroVironment with information on how an aircraft would fly in a Mars-like atmospheric condition, since the atmosphere at that altitude above the Earth is similar to the atmosphere near the Martian surface.

The 247-foot-span ultra-light flying wing’s development is being funded and managed under NASA’s Environmental Research Aircraft and Sensor Technology (ERAST) project. Q

For more information, contact John Del Frate at NASA Dryden Flight Research Center, 661/276-3704, John.DelFrate@dfrc.nasa.gov. Please mention you read about it in Innovation.

NASA Official: Jonathan Root • Web Design: Printing & Design Office, NASA Headquarters • Credits