A small aerobatic biplane from the 1930s hangs in the National Air and Space Museum’s Steven F. Udvar-Hazy Center outside Washington, DC. This particular Bücker Bü-133 Jungmeister, which was flown for decades at US airshows, has a unique distinction. It was flown across the Atlantic as disassembled, crated cargo stuffed aboard the Zeppelin Hindenburg in 1936.
Less than a year later, the burned-out remains of the Hindenburg at NAF Lakehurst, New Jersey, signaled the end of the era of large, hydrogen-filled rigid airships. Today, the term airship usually refers to a 190-foot-long, helium-filled non-rigid blimp carrying TV cameras.
But in 2013, an idea once old will become new again when the captain of the first SkyTug issues the traditional command: “Up ship.” Then, this hybrid airship – a vehicle nearly the length of a football field that is part blimp, part aircraft and is capable of carrying twenty tons of cargo – will lift off for the first time. And by 2016, the captain of an even larger airship—one nearly the length of three football fields and capable of carrying one million pounds of cargo—could issue the same command before starting a trip across the Atlantic.
The revived interest in airships can be directly linked to global commerce. More than 36 billion tons of cargo—everything from flowers to food to Formula 1 race cars—are shipped around the world annually. But in the last twenty years, the military has also seen a need to quickly move large quantities of everything from relief supplies to tanks into areas that often don’t have adequate—if any—infrastructure.
“There is a big gap between using a ship and using an aircraft to haul cargo in both the commercial and the military worlds,” notes Dr. Bob Boyd, the hybrid airship program manager at the Lockheed Martin Skunk Works, the company’s advanced technology development organization in Palmdale, California.
“A ship can carry a lot and is inexpensive, but it’s also slow. It costs a commercial shipper between six and ten cents per ton mile—moving one ton of cargo one mile—to transport goods by ship. However, a ship takes two to three weeks to get from China to the United States,” observed Boyd. “A 747 air freighter can get cargo around the world in hours, but the cost is fifty or sixty cents per ton mile, and space is limited. With the hybrid airship, we’re looking at transporting a lot of goods in about three days for around twenty-five cents per ton mile. We’re trying to fill that middle niche.”
Completely New Design
Development of new airship technologies started in 1990 when Skunk Works engineers looked at buoyant lift system vehicle concepts—that is, lifting by helium alone—for a commercial shipping company. In 1999, Skunk Works launched an internally funded effort to develop a revolutionary aeronautical concept called AeroCraft, a hybrid airship that not only gets lift from the nonflammable helium inside its envelope, but also from its aerodynamic shape as well.
“A number of issues are associated with non-hybrid airships,” Boyd noted. “They’re very large, which makes them difficult to steer; they don’t land on runways; they’re difficult to land in a crosswind; and they always land with their nose into the wind. They usually need a large circle of concrete to land on with a mooring mast to tether them. Since the idea was to have only minimal crew for economic reasons, the vehicle has to operate with a low crew workload.”
The key to the control issue was found by using movable engines and propellers that provide not only propulsion but also act as control surfaces working in conjunction with the airship’s rudders. The engines have to be economical to operate—smaller and burn less fuel than the ones on current aircraft—for the entire airship concept to be viable.
The development work done on the AeroCraft program showed a three-lobed envelope design flies better than the cigar shape of historic airships or the roughly oval profile of blimps. “This design gets about eighty percent of its lift from buoyancy and the other twenty percent from aerodynamics,” Boyd noted. “The airship needs to be going about twelve knots to get airborne. There has to be wind over the body.”
The critical element to making a practical airship was the development of an air cushion landing system, or ACLS. The ACLS allows the airship to operate like a hovercraft over unimproved ground or even over water.
The landing pads are two interlocked ovals. A fan inflates the outer ring, which provides a shape and a semi-solid surface to land on. When parked, the fan in the inner ring runs in the opposite direction and creates a low-pressure cushion, essentially becoming a suction cup that keeps the airship from being blown around by the wind. This system eliminates the need for a mooring mast or acres of concrete.
“All our development work showed the need to build a demonstrator,” said Boyd. “We had to prove a hybrid airship could be built. We had to show we could park it, and it wouldn’t blow away. We had to show the entire system would work.”
P-791
At the Skunk Works, projects are designated with randomly chosen numbers so its nature or sequence is not revealed. The hybrid airship demonstrator was given the identifier P-791. The company-funded demonstrator, which is 123 feet long, fifty-three feet wide, and thirty feet high, was built in 2005. First flight came on 31 January 2006.
“P-791 was designed using a Mr. Potato Head approach,” Boyd continued. “All of the major components—cockpit, engines, fuel lines, control surfaces, ACLS pads—are attached on the outside of the envelope.” The only structures inside the envelope are two nose-to-tail curtains that run down the seam lines of the envelope’s three lobes to provide shape and strength and two air bags, called ballonets, that expand and contract to equalize pressure as the helium expands and contracts with altitude.
The envelope is woven Vectran, a lightweight, high-strength material similar to Kevlar. It was assembled using heat seal bonding or chemical bonding on the seams. “Airships are very low pressure—about 1/10th of one psi,” Boyd noted. “The Goodyear blimps get shot at all the time by idiots trying to bring them down. But the gas flow is so slow that the crew can land and patch the leak. We weren’t shot at, but if we had been, the envelope could have been repaired using a contact patch that’s just peel and stick.”
The engines and propellers are mounted on aluminum rings that allow vectoring up and down and left and right for climbing, descending, and steering. The forward engine mounts are lashed to the envelope, while the two aft engines are attached with a composite, wishbone-shaped assembly that fits along the contours of the envelope. The engine/propeller/mount combinations, called thrusters, give P-791 a top speed of about thirty knots.
Company test pilot Eric Hansen made the first two flights and was followed by Bill Francis. The pilots sat side by side in an enclosed gondola with flight engineer Tim Blunck. “It is difficult to detect small changes in the wind in a vehicle this large,” Boyd noted. “We developed a closed-loop digital flight control system that uses an ultrasonic anemometer pressure sensor to gather data. This system allows the pilot to simply put a line on the moving map display showing where he wants to go and the system reacts, adjusts to conditions, and takes care of everything else.”
A total of six flights, each about thirty minutes long, were carried out in the P-791 test program. The tests were mostly ground handling demonstrations. When out of ground effect, the airship crew remained in the traffic pattern at Palmdale, staying below 2,000 feet.
“Everything worked as planned,” Boyd recalled. “We conducted engine-out operations and the ship responded well. We even accidentally tore one of the landing pads and were able to land without incident. We ended up with a very strong design. We then started putting a business plan together for larger airships.”
Big, Bigger, Really Big
A year after going on contract, Aviation Capital Enterprises, based in Calgary, Alberta, Canada, announced on 17 March 2011 that it had teamed with the Skunk Works to develop, build, test, and certify to US Federal Aviation Administration standards a family of hybrid airships designed for heavy lift, economical cargo transport. Aviation Capital will market the hybrid airship to the commercial market, while Lockheed Martin retains rights to the military market.
The first variant, called SkyTug, will be like a super-sized helicopter, only at about one-tenth the cost. SkyTug will have a range of up to 1,000 nautical miles at a cruise speed of sixty knots. It will be used on four- to eight-hour missions to support remote drilling sites or mines where there is no infrastructure for aircraft. The 290-foot-long SkyTug is scheduled to be flown in early 2013. The two airships called for in the initial contract will be built in Palmdale, as will any additional SkyTugs in the future.
The second variant, called SkyFreighter, is a regional lifter capable of carrying seventy tons of cargo. “An airship this size could be an enabler for countries to expand their economies,” Boyd noted. First flight of a 400-foot-long SkyFreighter is expected to come in 2014.
Both SkyTug and SkyFreighter will be powered by piston diesel engines that will only vector up and down, as it was determined from P-791 testing that rudder turns only were sufficient for steering, so side-to-side slewing of the engines wasn’t necessary. A market exists for as many as fifty of the smaller airships.
The third and largest variant is called SkyLiner. This behemoth, expected to be flown in 2016, will measure 800 feet long and be capable of lifting 500 tons of cargo. SkyLiner, which will be powered by six turboprop engines each turning a large-diameter propeller, will be used on international or transoceanic routes. The ACLS pads on SkyLiner, which will measure eighty feet long and thirteen feet tall, will retract to reduce drag.
“SkyLiner will be ideal for goods like electronics, vegetables, or sports cars that have a short time/value exchange and are heavy and expensive to put on an aircraft,” Boyd said. The projected market could support a fleet of several hundred SkyLiners.
All three variants will have long, rectangular cargo containers attached underneath the envelope. The flight deck is located in front. Each box will have a roll-on/roll-off capability and will accommodate containers or pallets. The SkyLiner cargo box is 300 feet long, fifty feet wide, and thirty feet tall with an upper and a lower deck.
“The flight deck on SkyLiner will look more like a ship’s bridge than a cockpit. The SkyLiner will have room for galleys, lavatories, and bunks,” noted Boyd. “Like on a container ship, the minimal crew is there to make sure everything is running. Honestly, there won’t be a whole lot for the airship crew to do. The airship is going to be traveling in a straight line at about 100 knots for several days, so we are looking at ways to stream ESPN or HBO to the crew.
“The airship’s big advantage is it gets away from an airport,” Boyd continues. “A hybrid airship that can land anywhere only needs a cleared field next to a trucking terminal. Cargo comes off the airship and is hauled away in waiting trucks without needing an intermediate stop. Airships represent the next stage in the consolidation of the global transportation system.”
The P-791 demonstrator, now inflated only with air, sits in a hangar at the Skunk Works. One day, it may be seen as the progenitor of a revolutionary new generation of airships that span the globe. If these airships prove to be a commercial, military, and humanitarian success, P-791 could one day find itself in another large hangar—at the National Air and Space Museum.
