Airships for the 21st Century
Long-duration, heavy-lift designs breathe new life into the world's oldest aircraft technology
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.
Although nonrigid airships aren't weighed down by an internal framework, they still have to support the gases, fabric, and other components, as well as any payload. Obviously, the greater the airship's weight, the larger the volume of lifting gas needed and the bigger the envelope size. As the size increases, so does the vehicle's surface area and consequently the amount of aerodynamic drag during flight. These and other factors dictate the amount of power required to propel the airship through the sky.
Although people pilot most of today's airships, the newer designs are increasingly pilot optional, meaning that a crew can fly them during tests or initial deployments and then quickly switch them to remote operation. Several fully remotely operated airships are also in development. One of their chief uses right now is for battlefield surveillance. These airships carry various imagers and detectors to altitudes of 1500 to 5500 meters on missions lasting 24 hours or more. Guardian Flight Systems, based in North Carolina, developed the pilot-optional Polar 400 for the U.S. Department of Defense. In the fully pilotless category is SAIC's Skybus 80K airship, which so far has conducted more than 62 hours of flight tests in Maine. To date, the Skybus 80K is the only unmanned airship to hold an experimental designation from the U.S. Federal Aviation Administration. It has a gas volume of 2300 m3 (80 000 cubic feet) and is designed to carry a 230-kg surveillance payload as high as 3000 meters for up to 24 hours.
More ambitious is the U.S. Army's Long Endurance Multi-Intelligence Vehicle. The LEMV will carry a 1100-kg payload up to 6000 meters for as long as 21 days without refueling. Its first deployment is to be in Afghanistan in late 2011 or early 2012. A number of defense companies considered vying for the LEMV contract. But two months ago, the five-year contract—one of the largest airship contracts to be awarded since World War II—went to Northrop Grumman and Hybrid Air Vehicles.
To operate in the thin atmosphere at such high altitudes for extended periods of time, an airship needs to be light (at least compared with lower-flying counterparts) and have an efficient propulsion system that can function with little or no oxygen. Also essential is a design that minimizes aerodynamic drag, which is why high-altitude airships almost always have the familiar ellipsoidal shape. Among the power sources being considered for high-altitude airships are electric motors coupled with lithium-ion batteries, hydrogen fuel cells, and flexible-film photovoltaics, which would blanket the upper parts of the airship's huge surface. Any of these options would need to weigh less and be more efficient than standard engines.
To fly even higher and longer with heavier sensor payloads is the ultimate goal of military leaders who see the modern airship as an unblinking, ever-present eye in the sky. Under the Defense Department's $149 million High Altitude Airship program, Lockheed's Maritime Systems & Sensors Division in Akron, Ohio, is now exploring ways to build an airship capable of carrying a 230-kg sensor package into the stratosphere, as much as 18 kilometers up, where it would remain for a month at a time. At that altitude, one airship would be able to monitor a patch of ground 1200 km across. Just 11 of them could provide radar coverage of the coastal and southern borders of the continental United States, according to the North American Aerospace Defense Command.
If that sounds ambitious, consider the proposed high-altitude airship known as the Integrated Sensor Is Structure, or ISIS. Under this $400 million program jointly funded by the Defense Advanced Research Projects Agency and the U.S. Air Force, Lockheed's Skunk Works is building an unmanned stratospheric airship powered by solar cells and fuel cells that would be capable of operating at 21 kilometers' altitude for up to 10 years at a time. A one-third-scale prototype, itself longer than a football field, is scheduled to fly in 2013.
What makes ISIS unique is the integration of its mission sensors—a UHF radar for monitoring vehicles and soldiers on the ground and an X-band radar for tracking cruise missiles up to 600 km away—into the body of the airship. According to Raytheon, which is building the radars, the radar antennas form a cylinder in the center of the airship. By integrating the sensor system into the structural supports, the design reduces the airship's overall weight and adds structural stiffness. Even so, the demands of a 10-year high-altitude mission mean that the full-scale ISIS will need to be made of extremely durable, yet lightweight materials—materials that may not yet exist. In addition, its power system will need advanced photovoltaics and fuel cells capable of generating enough power to operate the radars, navigation system, communications gear, and the electric motors that will turn the airship's giant propellers. A lot of extreme engineering is going into today's airship designs.
While the upcoming stratospheric surveillance airships will carry relatively small payloads, some airships now in development will lift a great deal more—payloads of hundreds of tons, albeit at lower altitudes. That presents an entirely different set of challenges.
An airship designed to carry 50 metric tons of cargo would be hundreds of meters long and weigh tens of tons lying empty of helium on the factory floor. The sheer size would make its assembly a daunting task. These new vehicles would likely be built in smaller subsections that would later be joined together in immense hangars.
A more critical issue is how to compensate for the sudden increase in the airship's static lift that occurs when a heavy payload is unloaded. The most straightforward remedy is to add onto the airship an amount of weight equal to the payload as the payload is removed.
Some heavy-lift designers are also developing hybrid vehicles. These incorporate the static lift of helium along with some form of dynamic lift, such as helicopter-style rotors or airplane-like wings. In most of these designs, the helium is sufficient to lift the vehicle's weight, while the dynamic lift is devoted to the payload's weight. This produces an aircraft that is slightly heavier than air and so is much less buoyant during cargo unloading.
Lockheed's Skunk Works first test-flew its P-791 proof-of-concept hybrid airship in 2006. The aircraft has two propulsion motors on the exterior of its envelope and two attached to its tail. This generates about 20 percent of the dynamic lift when the vehicle is flying forward. Other hybrid airships under development include Hybrid Air Vehicles' SkyCat, which will be the basis for the U.S. Army's LEMV; the Worldwide Aeros Corp.'s Aeroscraft, which was recently submitted to the FAA for design certification; and the proof-of-concept Dynalifter, being readied for test flight by Ohio Airships.
While these hybrids hold promise, they also have some inherent technical challenges. For one, the additional dynamic lift increases aerodynamic drag. To help with generating dynamic lift, they also typically have a flatter profile than conventional airships, but this shape gives them a higher ratio of envelope fabric to gas volume, increasing the airship's empty weight. Higher weight and drag, of course, mean more propulsive power and more fuel, both of which make the ship even heavier. And some hybrids employ multiple lobes in their design, which can create problems as the gases inside heat up from the sun's rays. Helium conducts heat six times as efficiently as air, so a multi-lobed hybrid may tend to list toward the side that's not exposed to the sun.
Perhaps the biggest issue, though, is the hybrid's potential to pitch nose up or down and to roll from side to side. A conventional, single-hulled airship avoids this problem because the majority of its gas volume is positioned well above its center of gravity, imparting what's known as pendulum stability. The higher up the center of lift is, the more stable the airship is; conversely, the closer the center of lift is to the center of gravity, the greater the tendency of pitching from wind gusts.
To get around these problems, Boeing and the Canadian company SkyHook International are collaborating on a different approach: a rotary-airship hybrid. It combines a conventional ellipsoidal envelope with four powerful helicopter rotor units, which are installed below the helium envelope. The helium is sufficient to support the weight of the vehicle itself, leaving the full power of the rotors to lift a 36-metric-ton payload. One of the first applications of the SkyHook is moving equipment and supplies for oil-drilling operations in northern Canada.
To spur further progress in heavy-lift designs, I and several other airship enthusiasts are setting up an international contest to promote the development of airships as a green, low-carbon form of cargo transport for commercial operations. The Zero Emissions Transport Airship Prize, or Z-Prize, similar to the more familiar X-Prize, will offer a large cash award for the successful development and flight test of a heavy-lift airship that meets the competition's criteria. We hope to entice airship developers to focus their efforts on designing cost-effective cargo airships that will have their greatest applications in developing regions—places where moving freight by conventional transport is difficult and hugely expensive or subject to disruption by criminals or terrorists. And by emphasizing airship designs with low carbon emissions, we hope also to encourage the creation of the first environmentally sustainable air-transport system.
It's an exciting time to be an airship engineer. These vehicles represent both the oldest and now the latest forms of aircraft. They're also an aviation technology that has yet to be fully exploited. While some naysayers may think the time of these leviathans is long past, in fact their day is just dawning.
This article originally appeared in print as "Airships Ahoy".
About the Author
Ron Hochstetler is director of lighter-than-air programs for Science Applications International Corp., in McLean, Va. For more about the author, see the Back Story, "Up in the Air."
To Probe Further
For more about modern airships, see the accompanying video, "Airship Renaissance."