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Aviation History
1993
1993 - 2624.PDF
ANTI-MISSILE UMS GHT flight," says Tietz, describing the genesis of the Pathfinder solar-electric UAV — a parallel Raptor concept. "Through Lawrence Livermore National Laboratory [LLNL] we evolved a concept of something that can fly above 60,000ft. From there to around 85,000ft, there's a sort of global lull in the winds, a regime we wanted to target." The Pathfinder is a virtually transparent fly ing wing, which bears the unmistakeable im print of its design ori gins at the hands of Dr Paul MacCready, de signer of the human- powered Gossamer Con dor and Albatross air craft. It was produced by AeroVironment, a California-based engi neering and research company founded by MacCready in 1971. LONG-ENDURANCE The demonstrator was built in 1983, to explore technologies for an extremely long-endurance platform. Known as the Halsol (high-altitude solar) aircraft, it drew on AeroVironment's earlier experi- Performance comparisons Payload Grass Wt Altitude Loiter velocity Cell Area Cell Peak Power * transient Pathfinder 41kg-45kg 245kg 65,000ft plus* 31 kt 60m2 11.4kW Perpetual 66kg-68kg 470kg 60,000ft 4Skt 114m2 21.6kW ence with other solar-electric aircraft, the Gossamer Penguin and Solar Challenger. The Halsol's "spanloader" flying-wing configuration was chosen to minimise mass and required propulsive power gath ered from solar arrays. A decade later, new technology has been introduced dur ing a thorough upgrade carried out in conjunction with LLNL. Re-emerging as the Pathfinder, the UAV is ready for demonstration flights from Rogers Dry Lake bed at Edwards AFB, California. The Pathfinder is expected to succeed where the Halsol failed, because of tech nological advances. Improvements in power control and mo tors, coupled with ad vances in proton ex change membrane (PEM) fuel-cell technology (al lowing rechargeable fuel cells for storage of electri cal energy at less than half the weight of rechargeable batteries) are significant developments. Upgrades of the Pathfinder include a faster microprocessor, which allows more precise power tracking and load matching. A major aspect of the upgrade is the use of eight brushless DC electric motors, which have rare earth, permanent magnets and custom-designed inverters. Each 1.5kW motor has only three moving parts (armature and two bearings) and drives a fixed-pitch propeller. The most vital technological advance — Intercept j„ T- 80s . ff - Alt = 45km) ->&> • Axial + divert Axial + divert Event Time TBM launch 0 Raptor TBM ACQ 15s Power-up Talon Refine target track 19s Transfer missile and target states 19.5s TALON LAUNCH SEQUENCE Event Built-in test IMU warm-up complete Enable propulsion Begin cooldown Transfer missile and target Launch Talon states Time 15s 16s 17s 17s 19s 20 s Representative Raptor/Talon engagement Order of battle T he BMDO foresees formations of Raptor/Talons endlessly patrolling high over enemy TBM launch zones. A Raptor-to-Raptor distributed communi cations network will enable UAVs to handle multiple launches and pass on data about sightings. Once a launch is spotted and verified by several Raptors, a message is passed via the network to the nearest Raptor to intercept the mis sile. The Talon is fired from the UAV within 20s of the initial launch. Because of the short time in which fire/do-not- fire decisions will have to be made, ground commanders will have inhibit, rather than enable, authority following weapons release. the fully regenerative fuel cell — will allow the Raptor to cruise day and night. The 30m-span Pathfinder will be fol lowed by a 60m-span UAV, called the Perpeptual (or Helios to the AeroVi ronment design team). The larger UAV is the optimum size required to carry the sensor and weapons payload to the neces sary 60,000ft-plus patrol altitude. It will have 16 engines, on its leading edge. The Pathfinder will be remotely con trolled from a ground station. The radio control of the motor speed and elevon position — affecting attitude, speed and heading — will incorporate feedback to the ground controller. AeroVironment vice-president Ray Mor gan explains how the UAV will be ma noeuvred. "We will turn it by inducing drag and reducing engine power on one side, turning the [motors] into generators. This would allow it to drop out of high winds and into friendlier skies. Along the trailing edge are 26 elevators, each of which can be operated independently." . Redundancy is not limited to the con trol surfaces. "We can fly with up to four motors not working, but that's unlikely. The Pathfinder is designed for 2,000- 3,000h missions, which is longer than the mean time between failures of a general- aviation engine. It has to be the most reliable aircraft ever built." Morgan says that the flimsy-looking structure will easily withstand turbulence. "It will ride rough weather like an air mattress on the ocean — it will wobble around, "but it won't break." The circular section, composite main spar (Kevlar, carbonfibre re-rinforced plastic and Nomex honeycomb construction), is designed to take 5g loads in flight. The average unit cost of the Raptor is around $1.5 million and each Talon will cost between $500,000 and $1 million. The Pathfinder has had $2.5 million in funding so far and will need $25 million more to reach full day/night capability. n FLIGHT INTERNATIONAL 27 October - 2 November, 1993 37
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