Ramon Lopez/WASHINGTON DC
The US military may have cooled to the concept of solar electric powered unmanned air vehicles (UAVs), but NASA has set new lofty goals for the emerging long endurance UAV.
In 1993, the US Ballistic Missile Defense Organization (BMDO) envisioned deploying Raptor (Responsive Aircraft Programme for Theatre Operations) UAVs, powered by stored solar energy. They would have fired lightweight hypervelocity missiles, called Talons (Theatre Applications Launch On Notice), at theatre ballistic missiles during the boost phase.
The first Raptor demonstrator to be flown was built by California based Scaled Composites. Designed by Burt Rutan, the high aspect ratio Raptor UAV was powered by a modified 60kW (80hp) Rotax 912 engine.
A parallel Raptor concept was the longer endurance, higher altitude, Pathfinder solar electric UAV produced by AeroVironment, also based in California. The 221kg 30m span flying wing has six electric motor driven propellers on the leading edge and 26 elevators on the trailing edge to control the aircraft. The upper surface of the wing is covered almost completely by thin silicon solar array panels. It has two small pods that extend below the wing's centre section for carrying scientific equipment.
The Pathfinder was first flown in 1983 under the name Halsol (High Altitude Solar) as part of a classified programmed sponsored by an unnamed Government agency. The vehicle successfully completed nine test flights, but the project was shelved because it lacked the required solar arrays.
PATHFINDER UPDATE
The Pathfinder was taken out of mothballs and updated thoroughly with BMDO funding, but, before it could demonstrate its value, the US Department of Defense shot down the Raptor/Talon project in favour of ground-based interceptors and the US Air Force's Airborne Laser system.
Fortunately for AeroVironment, says vice-president Ray Morgan, "NASA took pity on us" and began funding the programme as part of the Environmental Research Aircraft and Sensor Technology (ERAST) programme led by the NASA Dryden Flight Research Center.
ERAST aims to develop aeronautical technologies that will lead to a new family of long duration high altitude UAVs which could be used in upper atmospheric science missions lasting a week or longer. They might also assist weather forecasters through worldwide severe storm monitoring. The aircraft, the wingspan of which exceeds that of the Boeing 737-400, might be used to monitor major croplands and forests to provide early warning of crop damages or fires.
In 1995, 30 low and high altitude demonstration flights totalling 41.2h were conducted at Rogers Dry Lake bed at Edwards AFB. One flight lasted 11h, achieving 50,500ft (15,400m) in altitude. In November 1996, the Pathfinder had a successful low level checkout flight at Dryden to enable new systems to be verified for a series of high altitude flights in 1997 at the US Navy's Pacific Missile Range at Kauai, Hawaii.
Between June and November 1997, the Pathfinder underwent six flights totalling 77h from Hawaii. On 7 July, the Pathfinder flew to 71,500ft, a record for high altitude flight by a propeller driven aircraft. Before that, the record was 67,028ft set by Boeing's Condor UAV.
During fiscal year 1998, NASA and AeroVironment plan to conduct an 8h, 60,000ft, mission and fly to 80,000ft with a Pathfinder stretched by 6m span in the centre section and equipped with improved solar cells.
In FY1999, the goal is to fly to 90,000ft with the Centurion proof-of-concept testbed, now under construction. Wingspan will more than double, to 63m, allowing the UAV to carry a 91kg payload, says Morgan.
NASA and Aero Vironment hope to achieve 100,000ft during FY2000, using the final version of the Centurion. Morgan says that Aero Vironment needs an additional $5 million over the next two years beyond NASA funding to complete the work.
FUTURE PLANS
In FY2001, it is planned that the future Alliance 1 UAV will carry a 150kg payload to 85,000ft for more than 4h. The Alliance 1 project is led by AeroVironment with support from Scaled Composites, General Atomics and Aurora Flight Sciences.
During FY2002 and FY2003, the US space agency hopes to conduct flights at 60,000ft, lasting longer than four days, with the future AerVironment Helios, the wingspan of which could grow to 76m, says Morgan. With eight propellers and the latest solar cells, the Helios might in the future be able to undergo missions lasting for four months.
Morgan hopes that experience gained from the Pathfinder and the proposed Helios will "kickstart" development of commercially available solar powered drones that would each cost an estimated $3.5 million to build. Meanwhile, he hopes that the Pentagon will take another close look at solar electric powered UAVs, and he seeks to participate in US Navy wargames scheduled for later this year.
Source: Flight International
