Extreme endurance is a desirable quality for an unmanned aerial vehicle (UAV). Very-long-endurance UAVs reduce landings and takeoffs, reducing the need for ground crews. They can be based far from an area of operations, where crews are not subject to attack. Long endurance also translates into responsiveness, if the vehicle can be loitering for days close to an area of interest.

However, there are challenges. Few heavier-than-air vehicles have demonstrated true multi-day endurance—Boeing's Condor in the 1980s, QinetiQ's small solar-powered Zephyr series, Burt Rutan's round-the-world experimental aircraft and AeroVironment's solar vehicles.

Long endurance places a premium on reliability and ruggedness, which can conflict with the need for light weight and low cost. High efficiency is also associated with highly loaded propulsion systems and ultralight airframes.

One result is that ultra-long-endurance UAVs could be niche applications, which will have to be important if the cost of the UAV is to be justified. Communications relay is one of those. A high-altitude, persistent relay could be a revolutionary improvement for ground users, particularly in mountainous terrain.

In the longer term, long-endurance relay systems could allow deep-penetrating bombers, reconnaissance platforms and cruise missiles to communicate covertly with a global command network, without requiring heavy, complex, stealth-compatible satcom antennas. Another mission is missile warning—Boeing has linked Phantom Eye to the use of airborne infrared (ABIR) sensors for early missile detection.

Boeing is looking at solving the issue of providing weight and power for sensors by growing vehicle size to record levels. The Phantom Eye demonstrator is being readied for flight tests at Edwards AFB, Calif. Big as it is, with 150-ft. wings braced by flexible composite struts, the operational version will be larger.

Phantom Eye weighs 7,500 lb. empty and carries 1,850 lb. of liquid hydrogen (LH2) in two 8-ft.-dia., spherical tanks, made of aluminum with foam insulation and installed in the tadpole-shaped body.

Power comes from two Ford 2.3-liter engines, modified and fitted with three-stage turbocharger and intercooler systems, driving large-diameter propellers (made by the former Frontier Systems) via wide-ratio, single-stage reduction gears.

A redundant vehicle management system controls the complex fuel and propulsion subsystems, which include thermal management challenges. On takeoff, for instance, waste heat from the engines and turbochargers is needed to vaporize the LH2 before it is injected into the engines.

Inherited from Phantom Eye's shadowy 1980s ancestor, the Boeing Condor, is an unusual feature: Phantom Eye takes off from a four-wheeled trolley and lands on a lightweight nosewheel and skids. This is the kind of weight-saving shortcut that is a pain in normal operations, but tolerable if takeoffs and landings occur once a week.

The demonstrator is expected to fly for up to four days at 65,000 ft. The initial operational design will have similar engines, but a 250-ft. wingspan and 10-ft.-dia. tanks (with twice the volume), and is expected to deliver 10-day endurance. A cruising speed of 150 kt. makes it possible to achieve four days on station 10,000 nm from base, which translates to continuous presence with three air vehicles. Payload, Boeing says, will be 1,000-2,000 lb., endurance of up to 20 days being possible with lighter loads.

Beyond that, Boeing is looking at an even larger version, with a 350-ft. span (30 ft. more than the Hughes H-4, the current record holder), four engines and an 8,000-10,000-lb. payload.

The loss of the first AeroVironment Global Observer at Edwards AFB on its ninth test flight April 1, meanwhile, set that program back. A second air vehicle is partially complete, and parts have been produced for a third, but the investigation into the mishap is expected to take several months.

If Phantom Eye succeeds, it will be bad news for AeroVironment because the two vehicles are designed for the same performance regime. Global Observer, however, has a different propulsion system. Its turbocharged internal combustion engine drives a generator, which energizes four electrically driven propellers, and is backed up by a battery. One reason for this architecture is that it could facilitate a switch to a fuel cell at a later stage.

The third major heavier-than-air, ultra-long-endurance program in the U.S. is Aurora Flight Sciences' Orion. It differs from Phantom Eye and Global Observer in being designed for operations at lower altitude, and has a conventional propulsion system: the commercially certified Austro Engines AE300 turbodiesel. That engine is also used on the Diamond DA42 light aircraft, which Aurora modified as the Centaur optionally piloted vehicle. That allowed Aurora to adapt the flight and propulsion control hardware and software from Centaur to Orion.

Along with reusing airframe components from the earlier Orion high-altitude, long-loiter vehicle—an LH2-fueled demonstrator that Aurora had been developing with Boeing—this allowed Aurora to respond quickly to an urgent U.S. Central Command requirement: the Medium Altitude Global ISR and Communications (Magic) Joint Capability Technology Demonstration.

The numbers on Magic are simple—five days' endurance at 20,000 ft. with a 1,000-lb. payload, delivered within undisclosed but challenging schedule and cost limits. Behind those numbers is an operational requirement, driven by the development of wide-area airborne sensor (WAAS) systems.

WAAS systems are large, incorporating multiple cooled midwave IR sensors and powerful processors. Optimal operating altitude for best resolution is 20,000 ft. or less, and a slow-flying platform makes it easier to keep eyes on target. Together, Magic advocates have said, this makes a Reaper a less-than-ideal platform for WAAS: It is designed for higher speeds and altitudes.

Orion was unveiled in Starkville, Miss., in November 2010, but no progress has been reported since.

Alongside the slender-winged, heavier-than-air, long-endurance vehicles are at least four active lighter-than-air programs. The closest to being operational is USAF's Blue Devil 2, combining electro-optical (EO) and electronic surveillance measures packages on an optionally manned version of a commercially available TCOM Polar 100 airship. The integration is being performed by startup MAV6, whose CEO is former Air Force ISR chief David Deptula. The plan is to deploy it in February 2012.

The larger and more ambitious Long Endurance Multi-Intelligence Vehicle (LEMV), developed by a Northrop Grumman-led team, passed its critical design review in February and, according to the company, is on track to take part in an Army Joint Military Utility Assessment “in an operational environment” in early 2012. First flight should be imminent, leading to a long-endurance acceptance flight around year's end.

If the program is successful it will be a major accomplishment for a scratch team, headed not by an air-vehicle unit of Northrop Grumman but by ISR specialists, with a textile company fabricating the envelope to designs from a small U.K. company (Hybrid Air Vehicles).

A few more details of the concept were revealed in a briefing at the Paris air show. LEMV will carry nine operational systems: a sigint payload, Northrop Grumman's Vader (vehicle and dismount exploitation radar) with 360-deg. coverage; four EO sensors, and three communication channels.

The vehicle is intended to be free from the need for prepared runways, although it will need a 1,500-ft. ground run when fully fueled. Northrop Grumman claims 21-day mission endurance (at 20,000 ft. with a 3,500-lb. payload), and a 6,500-nm ferry range.

Stability and control, particularly on landing and takeoff, will be key issues. Lacking an air-cushion system, the vehicle will need to be tied down on the ground or to a mooring mast. (Without fuel, it will be lighter than air.) So far, testing of the hybrid air vehicle technology has been confined to a subscale test aircraft in the U.K.