Last June, Boeing's Phantom Eye unmanned air vehicle lifted off its takeoff trolley at Edwards AFB, Calif., under the power of two modified Ford automotive engines burning hydrogen fuel. After a first flight, the UAV returned to a lake-bed landing using a lightweight skid under its belly. Its landing skid dug into the landing surface and the UAV was damaged. It is now being prepared for a second flight attempt.

The mishap followed the April 2011 loss at Edwards of AeroVironment's Global Observer, also hydrogen-powered but using a single generator serving eight electrically powered propellers. A second demonstrator had been nearly completed, but funding for further tests had run out.

First-flight blues likewise afflicted Lockheed Martin's High-Altitude Long Endurance Demonstrator (HALE-D), a prototype for a solar-powered high-altitude airship. Its inaugural sortie from Akron, Ohio, in June 2011 ended up with the limp envelope tangled in Pennsylvania treetops, 3 hr. into a planned two-week mission.

As recently as 2010, the U.S. was funding eight ultra-long-endurance UAV projects through flight-testing and, in many cases, operational evaluation. Two have been canceled outright, two have no funding, two have been cut back to technology demonstrations and two more survive on minimal money.

The Phantom Eye demonstrator, built using company funds, performed a 40-kt taxi test Feb. 6 in preparation for another flight attempt “later this year.” The trolley and skid landing gear have been modified. One timing issue, Boeing says, is the variability of conditions at Edwards, particularly for lake-bed operations, and pressure on the flight schedule there, where major U.S. Air Force programs take precedence over an experimental program.

Even though the demonstrator has a 150-ft. wingspan, it is not considered large enough for operational use: The baseline operational vehicle discussed before the accident would have spanned 250 ft. and carried twice as much fuel, offering a 10-day endurance with a 1,000-2,000-lb. payload, and the company had talked about a 350-ft.-span monster with a payload up to 10,000 lb.

Distantly related to Phantom Eye is Aurora Flight Sciences' Orion. Aurora was a subcontractor to Boeing and the aerodynamic and structural designs are similar. However, Orion has an entirely conventional propulsion system—two commercial Austro diesel engines that, like its flight-control system, it shares with Aurora's Centaur optionally piloted vehicle. Unlike most ultra-long-endurance types, it is designed to operate at medium altitude, and it was funded under a Joint Capabilities Technology Demonstration (JCTD) program as a carrier for the Gorgon Stare wide area surveillance system. The vehicle was completed but had not flown before funding was exhausted, and the USAF has asked the contractor not to talk about it.

AeroVironment's Global Observer was being developed as a JCTD, funded by no fewer than six U.S. government agencies, and was to fly with the Air Force's Joint Aerial Layer Network (JALN) payload, designed to allow multiple tactical communications systems to talk to one another and connect to distant command centers. AeroVironment says it is still talking to its customers.

The U.K.'s solar-powered Qinetiq Zephyr was developed through a series of larger and more ambitious prototypes, and set a number of flight-endurance records in 2010—including a 336-hr. flight—after which nothing more was announced publicly. Qinetiq says it has no further progress or plans to report.

A much more ambitious solar-electric program, the Defense Advanced Research Projects Agency's (Darpa)Vulture, was cut back last April from a flight demonstration to research into critical technologies, 18 months after an $89 million contract award to a Boeing team. Vulture was designed to demonstrate a five-year operational endurance: one of the original competitors envisaged a vehicle that would be carried to altitude by an aerostat and recovered by parachute after a single mission, with its components being recycled as possible. However, the program is now focusing on solar-cell materials and fuel-cell technology.

Lighter-than-air craft have been faring no better, as contractors fail to deliver on their promises because of technical problems, schedule delays and cost overruns.

Of four Pentagon programs launched to develop and demonstrate long-endurance surveillance airships, one has crashed, one has been terminated, one scaled back substantially and one survives on only a trickle of funding. All have come to grief in the process of developing the air vehicle itself.

The first of the four airship programs to spring a leak was HALE-D, begun by the Missile Defense Agency in 2003 and taken over by Army Space & Missile Defense Command (SMDC) in 2008. At 232 ft. long, HALE-D was a subscale demonstrator for an unmanned airship designed to stay aloft at 65,000 ft. for more than 30 days. But after the prototype's crash, there was no more money, and the program ended.

Another stratospheric airship brought down to Earth is Darpa's Integrated Sensor Is Structure (ISIS), an ambitious program to integrate a dual-band active-array radar into a 1,000-ft.-long solar-electric airship capable of staying aloft for 10 years, tracking hundreds of air and ground targets from 70,000 ft. Started in 2004, ISIS encountered problems with the radar array and regenerative power system, and plans for Lockheed Martin to build a 510-ft.-long demonstrator capable of flying for a year were put on hold. For now, the program is focusing on radar and airship risk-reduction while Darpa reassesses the plan with the Air Force, says the Government Accountability Office (GAO).

Next to be punctured was the Air Force's Blue Devil 2 program, launched in 2010 to meet an urgent requirement to provide persistent multisensor surveillance in Afghanistan. The goal was to take a commercial airship, make it unmanned and install multiple video, radar and signals-intelligence sensors, communications links and onboard processing to create an ISR “fusion node.” The 370-ft.-long Blue Devil 2 was intended to fly for up to four days at 20,000 ft. with a payload up to 2,500 lb.

The Air Force set an aggressive schedule and awarded the integration contract to virtually unknown Mav6. Then the problems began. Based on a TCOM Polar 1000 airship, the program had problems with the envelope, overweight tail fins that failed structural testing, and flight-control software that did not scale up from a smaller version of the craft. The Air Force canceled Blue Devil 2 in June 2012, before the airship could fly, to avoid further delays and increasing costs, says the GAO.

Not yet canceled but cut back is the U.S. Army's Long-Endurance Multi-intelligence Vehicle (LEMV) program, also launched in 2010 to meet surveillance needs in Afghanistan. The system was based on promising but unproven hybrid airship technology. This combines buoyant, aerodynamic and propulsive lift for greater payload and endurance and easier ground-handling.

With Northrop Grumman as the integrator and the U.K.'s Hybrid Air Vehicles as airframe supplier, the 302-ft.-long LEMV is designed to fly for up to 21 days at 20,000 ft. carrying a 2,500-lb. payload. But the first airship is 12,000 lb. overweight, says the GAO, reducing its endurance and altitude capability to 16 days at 16,000 ft. Development was supposed to be completed in December 2011, but the LEMV did not fly until August 2012—and has yet to fly again.

Work continues but funding has been sliced. The core LEMV team is waiting while Army leadership debates where to spend money as the defense budget is cut. Even if funding is forthcoming, both manned and unmanned flight-testing has yet to be done and deployment looks unlikely without demand for Afghan operations.

And therein lies a cause of the halting progress of ultra-long-endurance UAVs: a lack of strong, consistent customer support. To a great degree, this is because user requirements for ISR and other functions have evolved in directions that make ultra-long-endurance platforms less attractive.

In the last half decade, many ISR users have called for sensor suites that provide situational awareness rather than just a video image of a small spot on the ground. One approach is to use electro-optical (EO) wide-area surveillance systems such as the U.S. Gorgon Stare and Vigilant Stare, and Israeli equivalents, and the other is multisensor, multisource integration using radar, communications-intelligence sensors and databases.

Wide-aperture EO systems need to operate at low-to-medium altitudes; radar requires rapidly increasing power with altitude and range; and multisensor systems add weight and drag, to which ultra-long-endurance vehicles are sensitive. Operators are looking for affordable, reliable medium-altitude sensor trucks. Most UAV procurement and export activity today is centered on the class of the GA-ASI Predator, IAI Heron and Elbit Hermes 900.

There are some missions for which high altitude, long range and endurance may be valuable even with a small payload, including communications relay (one target mission for both Global Observer and Zephyr) and carrying an EO sensor for ballistic missile defense operations (which involves forward operations close to launch sites). Both could be important for the USAF/Navy Air-Sea Battle concept, but those planners have higher funding priorities.

Finally, it was revealed last year that Sandia and Northrop Grumman had completed studies of a UAV propulsion system that would offer longer endurance and lower operating cost than with hydrogen or hydrocarbon fuel. According to an earlier study for the Pentagon's Office of Net Assessments, Sandia said its development “would not be an R&D project, but rather an engineering development effort that could culminate in a flight test within a decade.” However, the project was terminated (and practically buried) because its power source would be a nuclear reactor.

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