ERAST: High-Altitude, Long-Endurance Science Platforms

In the early 1990s, NASA’s Earth Science Directorate received a solic­itation for research to support the Atmospheric Effects of Aviation project. Because the project entailed assessment of the potential envi­ronmental impact of a commercial supersonic transport aircraft, measurements were needed at altitudes around 85,000 feet. Initially, Aurora Flight Sciences of Manassas, VA, proposed developing the Perseus A and Perseus B remotely piloted research aircraft as part of NASA’s Small High-Altitude Science Aircraft (SHASA) program.

The SHASA effort expanded in 1993 as NASA teamed with industry partners for what became known as the Environmental Research Aircraft and Sensor Technology project. Goals for the ERAST project included development and demonstration of unpiloted aircraft to perform long – duration airborne science missions. Transfer of ERAST technology to an emerging UAV industry validated the capability of unpiloted aircraft to carry out operational science missions.

The ERAST project was managed at Dryden, with significant contri­butions from Ames, Langley, and Glenn Research Centers. Industry part­ners included such aircraft manufacturers as AeroVironment, Aurora Flight Sciences, General Atomics Aeronautical Systems, Inc., and Scaled Composites. Thermo-Mechanical Systems, Hyperspectral Sciences, and Longitude 122 West developed sensors to be carried by the research air­craft.[1022] The ERAST effort resulted in a diverse fleet of unpiloted vehi­cles. Perseus A, built in 1993, was designed to stay aloft for 5 hours and reach altitudes around 82,000 feet. An experimental, closed-system, four – cylinder piston engine recycled exhaust gases and relied on stored liquid oxygen to generate combustion at high altitudes. Aurora built two Perseus A vehicles, one of which crashed because of an autopilot malfunction. By that time, the airplane had only reached an altitude of 50,000 feet.

Aurora engineers designed the Perseus B to remain aloft for 24 hours. The vehicle was equipped with a triple-turbocharged engine to provide sea-level air pressure up to 60,000 feet. In the 2 years following its maiden flight in 1994, Perseus B experienced some technical diffi­culties and a few hard landings that resulted in significant damage. As a result, Aurora technicians made numerous improvements, including extending the wingspan from 58.5 feet to 71.5 feet. When flight oper­ations resumed in 1998, the Perseus B attained an unofficial altitude record of 60,280 feet before being damaged in a crash in October 1999. Despite such difficulties, experience with the Perseus vehicles provided designers with useful data regarding selection of instrumentation for RPRVs and identifying potential failures resulting from feedback defi­ciencies in a ground cockpit.[1023] Aurora Flight Sciences also built a larger UAV named Theseus that was funded by NASA through the Mission To Planet Earth environmental observation program. Aurora and its

partners, West Virginia University and Fairmont State College, built the Theseus for NASA under an innovative, $4.9-million fixed-price contract. Dryden hosted the Theseus program, providing hangar space and range safety. Aurora personnel were responsible for flight-testing, vehicle flight safety, and operation of the aircraft.

With the potential to carry 700 pounds of science instruments to alti­tudes above 60,000 feet for durations of greater than 24 hours, the Theseus was intended to support research in areas such as stratospheric ozone depletion and the atmospheric effects of future high-speed civil transport aircraft engines. The twin-engine, unpiloted vehicle had a 140-foot wing­span and was constructed primarily from composite materials. Powered by two 80-horsepower, turbocharged piston engines that drove twin 9-foot – diameter propellers, it was designed to fly autonomously at high altitudes, with takeoff and landing under the active control of a ground-based pilot.

Operators from Aurora Fight Sciences piloted the maiden flight of the Theseus at Dryden on May 24, 1996. The test team conducted four addi­tional checkout flights over the next 6 months. During the sixth flight, the vehicle broke apart and crashed while beginning a descent from 20,000 feet.[1024] Innovative designers at AeroVironment in Monrovia, CA, took a markedly different approach to the ERAST challenge. In 1983, the com­pany had built and tested the High-Altitude Solar (HALSOL) UAV using battery power only. Now, NASA scientists were anxious to see how it would perform with solar panels powering its six electrically driven pro­pellers. The aircraft was a flying wing configuration with a rectangular planform and two ventral pods containing landing gear. Its structure consisted of a composite framework encased in plastic skin. In 1993 and 1994, researchers at Dryden flew it using a combination of battery and solar power, in a program sponsored by the Ballistic Missile Defense Organization that sought to develop a long-endurance surveillance plat­form. By now renamed Pathfinder, the unusual craft joined the ERAST fleet in 1995, where it soon attained an altitude of 50,500 feet, a record for solar-powered aircraft.[1025] After additional upgrades and checkout flight at Dryden, ERAST team members transported the Pathfinder to the U. S. Navy’s Pacific Missile Range Facility (PMRF) at Barking Sands,

Kauai, HI, in April 1997. Predictable weather patterns, abundant sun­light, available airspace and radio frequencies, and the diversity of terres­trial and coastal ecosystems for validating scientific imaging applications made Kauai an optimum location for testing. During one of seven high – altitude flights from the PMRF, the Pathfinder reached a world altitude record for propeller-driven as well as solar-powered aircraft at 71,530 feet.[1026] In 1998, technicians at AeroVironment modified the vehicle to include two additional engines and extended the wingspan from 98 feet to 121 feet. Renamed Pathfinder Plus, the craft had more efficient sil­icon solar cells developed by SunPower, Corp., of Sunnyvale, CA, that were capable of converting almost 19 percent of the solar energy they received to useful electrical energy to power the motors, avionics, and communication systems. Maximum potential power was boosted from about 7,500 watts on the original configuration to about 12,500 watts. This allowed the Pathfinder Plus to reach a record altitude of 80,201 feet during another series of developmental test flights at the PMRF.[1027] NASA research teams, coordinated by the Ames Research Center and including researchers from the University of Hawaii and the University of California, used the Pathfinder/Pathfinder Plus vehicle to carry a vari­ety of scientific sensors. Experiments included detection of forest nutri­ent status, observation of forest regrowth following hurricane damage, measurement of sediment and algae concentrations in coastal waters, and assessment of coral reef health. Several flights demonstrated the practical utility of using high-flying, remotely piloted, environmentally friendly solar aircraft for commercial purposes. Two flights, funded by a Japanese communications consortium and AeroVironment, empha­sized the vehicle’s potential as a platform for telecommunications relay services. A NASA-sponsored demonstration employed remote-imaging techniques for use in optimizing coffee harvests.[1028] AeroVironment engi­neers ultimately hoped to produce an autonomous aircraft capable of flying at altitudes around 100,000 feet for weeks—or even months—at a time through use of rechargeable solar power cells. Building on their experience with the Pathfinder/ Pathfinder Plus, they subsequently devel­oped the 206-foot-span Centurion. Test flights at Dryden in 1998, using only battery power to drive 14 propellers, demonstrated the aircraft’s

capability for carrying a 605-pound payload. The vehicle was then modi­fied to feature a 247-foot-span and renamed the Helios Prototype, with a performance goal of 100,000 feet altitude and 96 hours mission duration.

As with its predecessors, a ground pilot remotely controlled the Helios Prototype, either from a mobile control van or a fixed ground station. The aircraft was equipped with a flight-termination system— required on remotely piloted aircraft flown in military restricted air­space—that included a parachute system plus a homing beacon to aid in determining the aircraft’s location.

Flights of the Helios Prototype at Dryden included low-altitude eval­uation of handling qualities, stability and control, response to turbu­lence, and use of differential motor thrust to control pitch. Following installation of more than 62,000 solar cells, the aircraft was transported to the PMRF for high-altitude flights. On August 13, 2001, the Helios Prototype reached an altitude of 96,863 feet, a world record for sus­tained horizontal flight by a winged aircraft.[1029]

During a shakedown mission June 26, 2003, in preparation for a 48-hour long-endurance flight, the Helios Prototype aircraft encoun­tered atmospheric turbulence, typical of conditions expected by the test crew, causing abnormally high wing dihedral (upward bowing of both wingtips). Unobserved mild pitch oscillations began but quickly
diminished. Minutes later, the aircraft again experienced normal tur­bulence and transitioned into an unexpected, persistent high wing – dihedral configuration. As a result, the aircraft became unstable, exhib­iting growing pitch oscillations and airspeed deviations exceeding the design speed. Resulting high dynamic pressures ripped the solar cells and skin off the upper surface of the outer wing panels, and the Helios Prototype fell into the Pacific Ocean. Investigators determined that the mishap resulted from the inability to predict, using available analysis methods, the aircraft’s increased sensitivity to atmospheric disturbances, such as turbulence, following vehicle configuration changes required for the long-duration flight demonstration.[1030] Scaled Composites of Mojave, CA, built the remotely piloted RAPTOR Demonstrator-2 to test remote flight control capabilities and technologies for long-duration (12 to 72 hours), high-altitude vehicles capable of carrying science payloads. Key technology development areas included lightweight structures, science payload integration, engine development, and flight control systems. As a result, it had only limited provisions for a scientific payload. The D-2 was unusual in that it was optionally piloted. It could be flown either by a pilot in an open cockpit or by remote control. This capability had been demonstrated in earlier flights of the RAPTOR D-1, developed for the Ballistic Missile Defense Organization in the early 1990s.

D-2 flight tests began August 23, 1994. In late 1996, technicians linked the D-2 to NASA’s Tracking and Data Relay Satellite system in order to demonstrate over-the-horizon communications capabilities between the aircraft and ground stations at ranges of up to 2,000 miles. The D-2 resumed flights in August 1998 to test a triple-redundant flight control sys­tem that would allow remotely piloted high-altitude missions.[1031] General Atomics of San Diego, CA, produced several vehicles for the ERAST pro­gram based on the company’s Gnat and Predator UAVs. The first two, called Altus (Latin for "high”) and Altus 2, looked similar to the compa­ny’s Gnat 750. Altus was 23.6 feet long and featured long, narrow, high aspect ratio wings spanning 55.3 feet. Powered by a rear-mounted, tur­bocharged, four-cylinder piston engine rated at 100 horsepower, the vehicle was capable of cruising at 80 to 115 mph and attaining altitudes of up to 53,000 feet. Altus could accommodate up to 330 pounds of sensors and scientific instruments.

NASA Dryden personnel initially operated the Altus vehicles as part of the ERAST program. The Altus 2, the first of the two aircraft to be com­pleted, made its first flight May 1, 1996. During subsequent developmen­tal tests, it reached an altitude of 37,000 feet. In late 1996, researchers flew the Altus 2 in an atmospheric-radiation-measurement study spon­sored by the Department of Energy’s Sandia National Laboratory for the purpose of collecting data on radiation/cloud interactions in Earth’s atmosphere to better predict temperature rise resulting from increased carbon dioxide levels. During the course of the project, Altus 2 set a single-flight endurance record for remotely operated aircraft, remain­ing aloft for 26.18 hours through a complete day-to-night-to-day cycle.[1032] The multiagency program brought together capabilities available among Government agencies, universities, and private industry. Sandia provided technical direction, logistical planning and support, data analysis, and a multispectral imaging instrument. NASA’s Goddard Space Flight Center and Ames Research Center, Lawrence Livermore National Laboratory, Brookhaven National Laboratory, Colorado State University, and the University of California Scripps Institute provided additional instru­mentation. Scientists from the University of Maryland, the University of California at Santa Barbara, Pennsylvania State University, the State University of New York, and others also participated.[1033] In September 2001, the Altus 2 carried a thermal imaging system for the First Response Experiment (FiRE) during a demonstration at the General Atomics flight operations facility at El Mirage, CA. A sensor developed for the ERAST program and previously used to collect images of coffee plan­tations in Hawaii was modified to provide real-time, calibrated, geo – located, multispectral thermal imagery of fire events. This scientific demonstration showcased the capability of an unmanned aerial system (UAS) to collect remote sensing data over fires and relay the information to fire management personnel on the ground.[1034] A larger vehicle called Altair, based on the Predator B (Reaper) UAV, was designed to perform a variety of ERAST science missions specified by NASA’s Earth Science enterprise. In the initial planning phase of the project, NASA scientists established a stringent set of requirements for the Altair that included mission endurance of 24 to 48 hours at an altitude range of 40,000 to

65,0 feet with a payload of at least 660 pounds. The project team also sought to develop procedures to allow operations from conventional air­ports without conflict with piloted aircraft. Additionally, the Altair had to be capable of demonstrating command and control beyond-line-of-sight communications via satellite link, undertake see-and-avoid operations relative to other air traffic, and demonstrate the ability to communi­cate with FAA air traffic controllers. To accomplish this, the Altair was equipped with an automated collision-avoidance system and a voice relay to allow air traffic controllers to talk to ground-based pilots. As the first UAV to meet FAA requirements for operating from conventional airports, with piloted aircraft in the national airspace, the aircraft also had to meet all FAA airworthiness and maintenance standards. The final Altair configuration was designed to fly continuously for up to 32 hours and was capable of reaching an altitude of approximately 52,000 feet with a maximum range of about 4,200 miles. It was designed to carry up to 750 pounds of sensors, radar, communications, and imaging equipment in its forward fuselage.[1035] Although the ERAST program was formally terminated in 2003, research continued with the Altair. In May 2005, the National Oceanic and Atmospheric Administration (NOAA) funded the UAV Flight Demonstration Project in cooperation with NASA and General Atomics. The experiment included a series of atmospheric and oceanic research flights off the California coastline to collect data on weather and ocean conditions, as well as climate and ecosystem moni­toring and management. The Altair was the first UAV to feature triple­redundant controls and avionics for increased reliability, as well as a fault-tolerant, dual-architecture flight control system.

Science flights began May 7 with a 6.5-hour flight to the Channel Islands Marine Sanctuary west of Los Angeles, a site thought ideal for exploring NOAAs operational objectives with a digital camera system and electro-optical/infrared sensors. The Altair carried a payload of instru­ments for measuring ocean color, atmospheric composition and tem­perature, and surface imaging during flights at altitudes of up to 45,000 feet. Objectives of the experiment included evaluation of an unmanned aircraft system for future scientific and operational requirements related to NOAA’s oceanic and atmospheric research, climate research, marine sanctuary mapping and enforcement, nautical charting, and fisheries assessment and enforcement.[1036] In 2006, personnel from NASA, NOAA, General Atomics, and the U. S. Forest Service teamed for the Altair Western States Fire Mission (WSFM). This experiment demon­strated the combined use of an Ames-designed thermal multispectral scanner integrated on a large-payload capacity UAV, a data link telem­etry system, near-real-time image geo-rectification, and rapid Internet data dissemination to fire center and disaster managers. The sensor system was capable of automatically identifying burned areas as well as active fires, eliminating the need to train sensor operators to ana­lyze imagery. The success of this project set the stage for NASA’s acqui­sition of another General Atomics UAV called the Ikhana and for future operational UAS missions in the national airspace.[1037]