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.