Residents of Caribou, Maine, who happened to glance up at the skies over the former Loring Air Force Base recently got a glimpse of the future—although they might have thought they were looking at something out of the past. Engineers from my company, Science Applications International Corp. (SAIC), in McLean, Va., have been conducting test flights of a new type of lighter-than-air vehicle.
In appearance, the Skybus 80K bears the same oblong shape as the Goodyear Blimp, and it's based on the same flight principles that have governed airships since the 1800s. But this airship, one of a number of commercial and military vehicles now under development, represents a distinct break from tradition. Unlike their dirigible cousins of past centuries, these new vehicles are being designed to lift heavy payloads, remain aloft for weeks or even months at a time, and fly without pilots—all while expending far less energy than a conventional airplane or unmanned aerial vehicle. The Predator UAV, for instance, can carry a payload of 340 kilograms on a typical mission of up to 40 hours. SAIC's Skybus 1500E pilot-optional airship is being designed to carry a payload three times that size and stay aloft for up to 21 days.
The renewed investment in airships comes at a time when the energy footprint of all modes of transportation is being scrutinized. Some aviation visionaries now argue that we can't continue using exclusively petroleum-based fuels to power our aircraft. Such concerns have prompted new research into jet biofuels and energy-efficient jet engines. We've also begun to understand that not every flight has to be made at eight-tenths the speed of sound. For certain tasks, airplanes just can't compete with airships.
Modern airship designers are targeting two pressing needs: intelligence, surveillance, and reconnaissance missions and the transporting of multiton payloads to locations unreachable by conventional transport. For example, airships are ideal for continuously monitoring sites where improvised explosive devices or rocket launchers may be deployed. They also excel at scanning for distant airborne threats. That's why, in June, the U.S. Army awarded a US $517 million contract to Northrop Grumman and British firm Hybrid Air Vehicles to build three airships, each as long as a football field, to monitor trouble spots in Afghanistan. Cargo airships, meanwhile, are especially attractive for places that have poor roads and for remote regions that have no roads at all. At a transportation conference I recently attended in Canada's Northwest Territories, mining company executives and community leaders expressed strong support for using airships to shuttle equipment and supplies to distant mining outposts and villages. Such needs are driving the reinvention of the airship.
An airship flies primarily by Archimedes's principle, which describes the buoyancy of a body submerged in a denser fluid. That is, an airship operates more like a submarine than an airplane or a helicopter. Those aircraft have to generate 100 percent of their lift from the flow of air over their wings or rotor blades. An airship, however, employs a lighter-than-air nonflammable gas such as helium to give it buoyancy. When the lifting gas displaces a volume of air that weighs more than the entire airship (including fuel and payload), the airship floats. That resultant lift is what's known as the airship's static buoyancy. For instance, to lift 1 kilogram at sea level, the airship needs approximately 1 cubic meter of helium gas. Airships weigh considerably more than that, of course; the Skybus that recently flew in Maine tipped the scales at 1600 kg unfilled.
The lifting gas is contained within the airship's outer skin, a large fabric bag or envelope that is aerodynamic, lightweight, and rugged. Inside the envelope are one or more smaller bags, called ballonets, which hold ordinary air. On the ground, electric fans pump air into the ballonets until the pressure of the helium surrounding the ballonets exceeds atmospheric pressure by a very slight margin of about 480 pascals. The ballonets occupy between 25 and 50 percent of the airship's total gas volume. Bleeding off a measured amount of air through valves in the ballonets provides room inside the envelope for the helium to expand as the ship rises.
As the airship ascends, the decreasing atmospheric pressure causes the helium inside the airship to expand steadily. Once all the air in the ballonets is gone, the airship cannot ascend higher without either bursting or venting its helium. This point is known as the airship's pressure altitude. To descend, the airship uses its electric fans to blow air back into the ballonets. This gas-management system must constantly keep the helium at a pressure that's slightly higher than the surrounding atmosphere, to preserve the aerodynamic shape of the envelope.
If ascending and descending were all an airship did, this combination of gases and ballonets would be sufficient. But an airship also needs a certain amount of power and propulsion, to run the onboard navigation and communications systems and any electronics in the payload, and to maneuver to different locations. Most current airships use traditional gasoline engines, but increasingly designers are looking at alternative power and propulsion systems. One idea is a regenerative system incorporating photovoltaics and fuel cells, in which hydrogen fuel cells produce water vapor. The solar power could be used to separate the water back into its component gases; the hydrogen would then be fed back into the fuel cells.
Almost all airships flying today are of a nonrigid design, which means the ship's shape comes only from the pressure of the gases inside. By contrast, the giant airships of the 1930s, the Hindenburg being the most iconic example, had rigid internal skeletons made of aluminum or wood. Inside this cage were a dozen or more gas-filled lifting bags. Those days also saw the development of semirigid designs, which typically had a stout aluminum keel running lengthwise from the nose to the tail, providing a convenient mounting point for the individual gas cells and distributing the lift of each cell evenly. The only semirigid airships flying today are the Zeppelin NT series, which began operations in the late 1990s and are used primarily for sightseeing and advertising.