NASA 

National Aeronautics and 

Facts

Space Administration 

Dryden Flight Research Center 

P.O. Box 273

Edwards, California 93523 

AC 661-276-3449 

FAX 661-276-3566 

pao@dfrc.nasa.gov FS-1998-10-0054 DFRC 

Solar-Power 

Research and 

Dryden 

Since 1980 AeroVironment, 

Inc. (founded in 1971 by the 

ultra-light airplane innovator—Dr. Paul MacCready) 

has been experimenting with 

solar-powered aircraft, often in conjunction with 

NASA’s Dryden Flight Research Center, Edwards, 

California. Thus far, 

AeroVironment, now headquartered in Monrovia, Calif., has achieved several altitude records with its Solar 

Challenger, Pathfinder, and 

Pathfinder-Plus aircraft. It 

expects to exceed them with Pathfinder-Plus, which flew to a record altitude of 80,201 feet. NASA photo EC98 44621-247 the newer and larger solarpowered Centurion and its successors in NASA’s Environmental Research Aircraft and Sensor Technology (ERAST) 

program, the Helios. 

Solar Challenger set an initial altitude record of 14,300 feet. More spectacularly, on July 7, 1981, the solar-powered aircraft 

flew 163 miles from Corneille-en-Verin Airport north of Paris across the English Channel to Manston Royal Air Force Base 

south of London, staying aloft 5 hours and 23 minutes. 

At the time, AeroVironment was headquartered in Pasadena, Calif. Dr. MacCready, a former gliding champion, whose 

Gossamer Albatross crossed the English Channel using human power in 1979, saw solar power as a way to “help business and 

government recognize and meet their environmental and energy objectives.” MacCready remains the Chairman of the Board 

at AeroVironment. 

Following the success of Solar Challenger, AeroVironment received funding in 1981 from the U.S. government for a classified 

program to look into the feasibility of long-duration, solar-electric flight above 65,000 feet. The firm designed an airplane 

designated HALSOL (High-Altitude Solar Energy), built and test flew three subscale models and a final prototype. The 

HALSOL proved the aerodynamics and structures for the approach, but subsystem technologies, principally for energy 

storage, were inadequate for the intended mission. The HALSOL was mothballed for ten years but later evolved into 

Pathfinder, which first flew at Dryden in 1993 under the auspices of the Ballistic Missile Defense Office for potential use in 

an anti-ballistic-missile defense role. 

1
When funding for this program ended, Pathfinder became part 

of NASA’s ERAST program to develop remotely piloted, 

long-duration aircraft for environmental sampling and sensing at altitudes above 60,000 feet. On September 11, 1995, 

Pathfinder exceeded Solar Challenger’s altitude record for 

solar-powered aircraft by a long margin when it reached 

50,500 feet at Dryden. 

After moving its flight operations to the U.S. Navy’s Pacific 

Missile Range Facility (PMRF) on Kauai, Hawaii, in April 

1997, an upgraded Pathfinder took advantage of the improved 

angle of sunlight to set a new record of 71,530 feet for solarpowered aircraft on July 7, 1997. 

Further modified with longer wings, improved motors, and 

more efficient solar array, Pathfinder-Plus (as it was now 

called) flew to still another record of 80,201 feet at the PMRF 

on August 6, 1998. In the process, it stayed above 70,000 feet 

for almost three and a half hours while carrying 68 pounds of 

test instrumentation and other payload. 

Background 

The first flight of a solar-powered aircraft took place on 

November 4, 1974, when the remotely controlled Sunrise II, 

designed by Robert J. Boucher of AstroFlight, Inc., flew 

following a launch from a catapult. 

Following this event, AeroVironment took on a more ambitious project to design a human-piloted, solar-powered aircraft. The firm initially took the human-powered Gossamer 

Albatross II and scaled it down to three-quarters of its previous size for solar-powered flight with a human pilot controlling it. This was more easily done because in early 1980 the 

Gossamer Albatross had participated in a flight research 

program at NASA Dryden in a program conducted jointly by 

the Langley and Dryden research centers. Some of the flights 

were conducted using a small electric motor for power. 

Gossamer Penguin 

The scaled-down aircraft was designated the Gossamer Penguin. It had a 71-foot wingspan compared with the 96-foot 

span of the Gossamer Albatross. Weighing only 68 pounds 

without a pilot, it had a low power requirement and thus was 

an excellent test bed for solar power. 

AstroFlight, Inc., of Venice, Calif., provided the power plant 

for the Gossamer, Penguin, an Astro-40 electric motor. Robert Boucher, designer of the Sunrise II, served as a key 

consultant for both this aircraft and the Solar Challenger. The 

power source for the initial flights of the Gossamer Penguin 

consisted of 28 nickel-cadmium batteries, replaced for the 

solar-powered flights by a panel of 3,920 solar cells capable 

of producing 541 Watts of power. 

The battery-powered flights took place at Shafter Airport near 

Bakersfield, Calif. Dr. MacCready’s son Marshall, who was 

13 years old and weighed roughly 80 pounds, served as the 

initial pilot for these flights to determine the power required 

to fly the airplane, optimize the airframe/propulsion system, 

and train the pilot. He made the first flights on April 7, 1980, 

and made a brief solar-powered flight on May 18. 

The official project pilot was Janice Brown, a Bakersfield 

school teacher who weighed in at slightly under 100 pounds 

and was a charter pilot with commercial, instrument, and 

glider ratings.She checked out in the plane at Shafter and 

made about 40 flights under battery and solar power there. 

Wind direction, turbulence, convection, temperature and radiation at Shafter in mid-summer proved to be less than ideal 

for Gossamer Penguin because takeoffs required no crosswind and increases in temperature reduced the power output 

from the solar cells. 

Consequently, the project moved to Dryden in late July, 

although conditions there also were not ideal. Nevertheless, 

Janice finished the testing, and on August 7, 1980, she flew a 

public demonstration of the aircraft at Dryden in which it went 

roughly 1.95 miles in 14 minutes and 21 seconds. 

This was significant as the first sustained flight of an aircraft 

relying solely on direct solar power rather than batteries. It 

provided the designers with practical experience for developing a more advanced, solar-powered aircraft, since the Gossamer Penguin was fragile and had limited controllability. This 

necessitated its flying early in the day when there were 

During the early part of 1980, 

in a joint project with NASA’s 

Langley Research Center and 

AeroVironment, NASA’s 

Dryden Flight Research Center 

gathered data on a Gossamer 

Albatross in which the lightweight aircraft flew under three 

conditions: using human 

power, using a small electric 

motor, and in a towed condition 

with the propeller removed. 

NASA photo ECN 12558 

2
Gossamer Penguin in flight above Rogers Dry Lakebed at 

Edwards, Calif., showing the solar panel perpendicular to the wing 

and facing the sun. NASA photo ECN 13413 

minimal wind and turbulence levels, but the angle of the sun 

was also low, requiring a panel for the solar cells that could be 

tilted toward the sun. 

Solar Challenger 

Using the specific conclusions derived from their experience 

with Gossamer Penguin, the AeroVironment engineers designed Solar Challenger, a piloted, solar-powered aircraft strong enough to handle both long and high flights 

docile” aircraft that dropped in a steady, wing-level attitude 

when stalled and rapidly regained unstalled flight. 

AstroFlight, Inc., again provided the motor, and the DuPont 

Company, which produced many of the advanced materials 

for the Gossamer Albatross, Gossamer Penguin, and Solar 

Challenger, sponsored the project. Janice Brown remained 

one of the pilots, but she was joined by the slightly heavier 

Stephen R. Ptacek (almost 150 pounds), who brought to the 

project over 4,600 hours of flight in a variety of aircraft. 

The pilots flew the aircraft, first with batteries and then under 

solar power, at the Santa Susana, Shafter, and El Mirage 

airports in California before moving to Marana Airpark 

northwest of Tucson, Ariz., in late 1980 and early 1981. With 

some modifications, the Solar Challenger showed itself to be 

an effective aircraft. This was proved to the world during the 

cross-Channel flight on July 7, 1981, with Ptacek at the 

controls. 

Pathfinder 

Growing out of the post-1983 development of HALSOL, 

Pathfinder was modified with additional solar arrays and 

other upgrades. It was then brought back to Dryden for 

another series of developmental flights in 1995. On Sept. 11, 

1995, Pathfinder reached an altitude of 50,500 feet, setting a 

new altitude record for solar-powered aircraft. The National 

Aeronautic Association presented the NASA-industry team 

with an award for one of the “10 Most Memorable Record 

Flights” of 1995. 

when encountering normal turbulence. As compared 

with the Penguin’s 71-foot wingspan, Solar Challenger 

horizontal stabilizer and a large enough wing area to 

accommodate 16,128 solar cells. 

Using in-house computer programs, AeroVironment 

engineers Peter Lissaman and Bart Hibbs designed the 

unusual wings and stabilizers, which they made flat on 

top to hold the solar cells. Hibbs developed the aerodynamic design for the propeller with another in-house 

Artist’s conception of Solar Challenger (left). 

Preliminary design of Solar Challenger 

(above). Photos courtesy of Du Pont Co. 

had only a 46.5-foot wingspan, but it had a huge 

computer program. The result was a “smooth and 

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After additional upgrades and one checkout flight at Dryden 

in late 1996, Pathfinder was transferred to the U.S. Navy’s 

Pacific Missile Range Facility (PMRF) at Barking Sands, 

Kauai, Hawaii, in April 1997. Kauai was chosen as an 

optimum location for testing the solar-powered Pathfinder 

due to the high levels of sunlight, available airspace and radio 

frequencies and the diversity of terrestrial and coastal ecosystems for validating scientific imaging applications. While in 

Hawaii, Pathfinder flew seven high-altitude flights from 

PMRF, one of which reached a world altitude record for 

propeller-driven as well as solar-powered aircraft of 71,530 

feet. 

to useful electrical energy to power the craft’s motors, 

avionics and communication systems. That compares with 

about 14 percent efficiency for the older solar arrays that 

cover most of the surface of the middle and outer wing panels 

from the original Pathfinder. Maximum potential power was 

boosted from about 7,500 Watts on Pathfinder to about 

12,500 Watts on Pathfinder-Plus. 

In addition, the Pathfinder-Plus was powered by eight electric 

motors, two more than had powered the previous version of 

Pathfinder. Designed for Centurion, the motors are slightly 

more efficient than the original Pathfinder motors. 

Pathfinder silhouetted by the morning sun on Rogers Dry Lakebed, Edwards, Calif. NASA photo 

Centurion 

Centurion, like its immediate 

predecessors Pathfinder and 

Pathfinder-Plus, is a lightweight, solar-powered, remotely piloted flying wing 

aircraft that is demonstrating 

the technology of applying 

solar power for long-duration, 

high-altitude flight. It is considered to be a prototype technology demonstrator for a 

future fleet of solar-powered 

aircraft that could stay airEC96 43817-2 

Pathfinder “Plus” 

During 1998, the Pathfinder was modified into the longerwinged Pathfinder-Plus configuration. On Aug. 6, 1998, the 

modified aircraft was flown to a record altitude for propellerdriven aircraft of 80,201 feet on the third of a series of 

developmental test flights from PMRF on Kauai. The goal of 

the flights was to validate new solar, aerodynamic, propulsion and systems technology developed for the Pathfinder’s 

successor, the Centurion, which is designed to reach and 

sustain altitudes in the 100,000-foot range. 

Essentially a transitional vehicle between the Pathfinder and 

the follow-on Centurion, the Pathfinder-Plus is a hybrid of the 

technology that was employed on Pathfinder and developed 

for Centurion. 

The most noticeable change is the installation of a new 44-

foot-long center wing section that incorporates a high-altitude airfoil designed for Centurion. The new section is twice 

as long as the original Pathfinder center section and increases 

the overall wingspan of the craft from 98.4 feet to 121 feet. 

The new center section is topped by more-efficient silicon 

solar cells developed by SunPower Corp., Sunnyvale, Calif.; 

they can convert 19 percent of the solar energy they receive 

borne for weeks or months 

on scientific sampling and imaging missions or while serving as telecommunications relay 

platforms. 

Although it shares much of the design concepts of the Pathfinder, the Centurion has a wingspan of 206 feet, more than 

twice the 98-foot span of the original Pathfinder and 70 

percent longer than the Pathfinder-Plus’ 121-foot span. At the 

same time, it maintains the eight-foot chord (front to rear 

distance) of the Pathfinder wing, giving the Centurion wing 

an aspect ratio (length-to-chord) of 26 to 1. 

Other visible changes from its predecessor include a modified 

wing airfoil designed for flight at extreme altitude and four 

underwing pods to support its landing gear and electronic 

systems, compared with two such pods on the Pathfinder. The 

flexible wing is primarily fabricated from carbon fiber and 

graphite epoxy composites and kevlar. It is built in five 

sections, a 44-foot-long center section and middle and outer 

sections just over 40 feet long. All five sections have an 

identical thickness that is 12 percent of the chord, or about 

11.5 inches, with no taper or sweep.

Solar arrays that will cover most of the upper wing surface 

will provide up to 31 kilowatts of power at high noon on a 

4
One quarter-scale model of AeroVironment’s Centurion ultra-highaltitude flying wing, flown as a low-cost, low-risk proof-of-concept 

vehicle before going ahead with the full-scale prototype. NASA 

photo EC97 43965-4 

summer day to power the aircraft’s 14 electric motors, avionics, communications and other electronic systems. Centurion 

also has a backup lithium battery system that can provide 

power for between two and five hours to allow limitedduration flight after dark. Initial low-altitude test flights at 

Dryden in 1998 are being conducted on battery power alone, 

prior to installation of the solar cell arrays. 

Centurion flies at an airspeed of only 17 to 21 mph, or about 

15 to 18 knots. Although pitch control is maintained by the use 

of a full-span 60-segment elevator on the trailing edge of the 

wing, turns and yaw control are accomplished by applying 

differential power—slowing down or speeding up the motors—on the outboard sections of the wing. 

Helios 

AeroVironment envisions Helios as the ultimate solar aircraft 

that can offer virtually eternal flights in the stratosphere. It 

will build upon the technologies developed by Pathfinder and 

Centurion but will add an energy storage system for nighttime 

flying. From 25 to 50 percent larger than Centurion, the 

Helios will store up to two-thirds of the energy received by its 

solar array during the day and will 

use this stored energy to maintain 

its altitude overnight. Because it 

will renew its energy every day 

from the sun, the Helios will have 

flight endurance limited only by 

the reliability of its systems, meaning a practical limit of perhaps six 

months on station. 

Because of this long duration of 

flight, the Helios will be extremely 

economic in operation. However, it will have to be perhaps 

the most reliable aircraft ever built, with each flight lasting 

longer than the time between overhaul for a typical, general 

aviation aircraft. As a consequence, much of the Helios 

design will involve a minimum of moving parts, high redunAircraft Specifications 

• Wingspan: Solar Challenger, 46.5 feet (14.8 meters); 

Pathfinder, 98.4 feet (29.5 meters); 

Pathfinder-Plus, 121 feet (36.3 meters) 

Centurion, 206 feet (61.8 meters) 

• Length: Solar Challenger, 30.3 feet (9.22 meters); 

Pathfinder, Pathfinder-Plus, and 

Centurion, 12 feet (3.6 meters) 

• Wing chord: Solar Challenger, 5.8 feet (1.78 meters); 

Pathfinder, Pathfinder-Plus, and 

Centurion, 8 feet (2.4 meters) 

• Gross weight: Solar Challenger, about 336 pounds

 (152.8 kg); 

Pathfinder, about 560 pounds (252 kg.); 

Pathfinder-Plus, about 700 pounds

 (315 kg.) 

Centurion, varies depending on power

 availability and mission profile;

 approximately 1,900 pounds for a

 mission to 80,000 feet altitude. 

• Payload: Solar Challenger, weight of pilot, up to 150

 pounds 

Pathfinder, up to 100 pounds (45 kg.); 

Pathfinder-Plus, up to 150 pounds (67.5 kg.) 

Centurion, varies depending on altitude;

 about 100 pounds. to 100,000 ft.,

 600 pounds. to 80,000 feet. 

• Airspeed: Solar Challenger, approx. 25-34 mph cruise 

Pathfinder, Pathfinder -Plus. approx. 17-20

 mph cruise 

Centurion, approx. 17-21 mph cruise 

• Power: Arrays of solar cells, max. output: 

Solar Challenger, 2,700 Watts 

Pathfinder, about 7,500 Watts; 

Pathfinder-Plus, about 12,500 Watts 

Centurion, 31,000 Watts 

• Motors: Solar Challenger, one electric motor, 2.7 kW 

Pathfinder, six electric motors, 1.25 kW each 

Pathfinder-Plus, eight electric motors,

 1.5 kW maximum each

Centurion, 14 electric motors, 2.2 kW each 

• Manufacturer: AeroVironment, Inc. 

• Primary materials: Composites, plastic, foam. 

Three-view drawing of 

Pathfinder-Plus 

5 
dancy, low temperatures, and 

solid-state control systems. The 

Helios will also be able to diagnose degradation of its control 

systems and reconfigure the autopilot while in flight. As a result, AeroVironment expects the 

Helios to perform as a non-polluting, re-configurable “atmospheric satellite” in the new 

millenium. 

The ERAST Program 

Centurion is and Helios will be 

one of a number of remotely 

piloted aircraft being evaluated 

under NASA’s Environmental 

Research Aircraft and Sensor 

Technology (ERAST) program. 

The ERAST program is one of 

NASA’s initiatives designed to 

develop the new technologies 

needed to continue America’s 

leadership in the highly competitive aerospace industry. 

The primary focus of ERAST is 

on the development of slowflying, remotely-operated aircraft that can perform long-duration science missions at very 

high altitudes above 60,000 feet. These missions could include in-situ atmospheric sampling, tracking of severe storms, 

remote sensing for earth sciences studies, hyperspectral imaging for agriculture monitoring, and serving as telecommunications relay platforms. The most extreme mission envisioned for solar-powered aircraft such as the Centurion, 

Centelios and Helios would reach altitudes of 100,000 feet. 

A parallel effort is developing lightweight, microminiaturized sensors that can be carried by these aircraft. Additional 

technologies considered by the joint NASA-industry ERAST 

Alliance include lightweight materials, avionics, aerodynamics, and other forms of propulsion suitable for extreme 

altitudes and duration. 

The ERAST program is sponsored by the Office of Aeronautics and Space Transportation Technology at NASA Headquarters, and is managed by the NASA Dryden Flight Research Center. Sensor technology development is headed by 

NASA Ames Research Center, Moffett Field, Calif. 

Three-view drawing of Centurion with top views of Pathfinder and 

Pathfinder-Plus for comparison. 

6
Sources 

AeroVironment Web site, URL: http:// 

www.aerovironment.com/overview/history/history.html and 

linked sites; 

Benedek, Ladislao, Solar Energy and the Aeronautics Industry, translation of “Energía Solar y la Industria Aeronautica,” 

thesis, Olivos, Argentina, Oct. 9, 1985 (Washington, DC: 

NASA TM-77957, 1985): 24-33; 

Cowley, Martyn, “Wings in the Sun: the evolution of Solar 

Challenger,” Flight International (Jun. 13, 1981): 1865-

1868; 

Documents relating to Solar Challenger, Dryden Historical 

Reference Collection, including 3-view photograph and various background papers and fact sheets; 

Dornheim, Michael A., “Pathfinder Plus Exceeds 80,000 Ft.; 

Centurion Assembled,” Aviation Week & Space Technology 

(Aug. 17, 1998): 33; 

Fact Sheets on Pathfinder and Centurion, NASA Dryden 

Flight Research Center, prepared by Alan Brown, Sept. 1998, 

and coordinated with ERAST and AeroVironment personnel; 

Flittie, K., and Curtin, B., “Pathfinder Solar-Powered Aircraft 

Flight Performance,” paper delivered at AIAA Atmospheric 

Flight Mechanics Conference & Exhibit, Aug. 10-12, Boston, 

Mass. (AIAA 98-4446); 

“Gossamer Penguin Makes First Sustained Flight,” X-Press, 

vol. 23, Issue 6 (Aug. 22, 1980): 2; 

MacCready, Paul B., et al., “Sun Powered Aircraft Design,” 

paper delivered at the AIAA 1981 Annual Meeting and 

Technical Display, Long Beach, Calif. (AIAA-81-0916); 

Penner, Hellmut, Sun Worshiper [sic]. McCready’s [sic] 

Solar Challenger Flies Over the English Channel, translation 

of “Sonnenanbeter. McCreadys Solar Challenger flog über 

den Armelkanal,” Flug Revue (No. 9, Sept. 1981): 67-69 

(Washington, DC: NASA TM-77327, 1983). 

7