A new generation of airships – bigger than any ever built before – will soon be taking off from an airfield in Germany. Called CargoLifters, these giant zeppelins are being developed to transport plant and machinery – 160 tonnes of it at a time – around the world at speeds of up to 100 km/h. A fleet of up to 50 airships is planned by 2010, with the first expected to leave the factory by 2003.
These giant, CL160 blimps will measure a staggering 65 m in diameter and 260 m in length. To build airships the size of small ocean liners clearly needs a large building, and a new hangar has had to be designed to house the factory at Brand. When construction is completed next month, the hangar will be the world’s first large-scale airship manufacturing plant in 70 years, and it will form the biggest enclosed space in Germany. The airships will be built inside in pairs, side by side, like two beached whales.
CargoLifter is a new venture and money is tight, and the hangar’s design is fundamental to the scheme’s success. “The hangar had to minimise the total enclosed space so as to keep cladding and heating costs to a minimum,” says Rüdiger Lutz, director of Ove Arup & Partners, the project’s structural engineer. Up to 250 people will work inside the hangar, which will be heated to above 17ºC even when the temperature outside drops to -20ºC in the depths of the German winter.
Before work could start on designing the hangar, however, its orientation had to be decided. “Airships don’t live in garages,” explains Lutz. Instead, they are kept outside, tethered to masts. “Getting the airship from the hangar to the mast is the most dangerous day of its life, which is why the hangar’s orientation is so important.” The hangar has to be in line with the prevailing wind direction when the airship is eased out of its protective cocoon. If not, cross-winds could force its delicate fabric into contact with the side of the hangar, damaging it irreparably. In Brand, the wind blows primarily in an east-west direction, so the hangar follows this axis.
Once the orientation was established, the hangar’s shape had to be defined. “This was a simple exercise in geometry to produce the minimum internal volume,” says Lutz. In plan, the building’s shape is like a running track, its two flat sides capped by semicircular ends where the doors are fitted. In section, it is semi-circular.
The closed doors form the rounded ends of the tubular hangar. Each set of doors consists of two fixed sections and six moving sections mounted on rails and powered by an electric drive motor at each corner of their base. All eight sections are fixed to a central pivot at the top. The doors fan out from the middle with the moving sections sliding behind the fixed section at each side. The individual sections are huge: 168 m tall and 42 m wide at their base, tapering to the pivot.
“The design of the doors was a challenge as they lean against the hangar’s main structure,” says Lutz. This meant they had to be as light as possible to minimise the structure needed to support them, and in turn keep costs down. To achieve this, the engineer opted for a shell structure covered with lightweight metal cladding. Other solutions were considered, including inflatable doors that would rise up out of the ground “like a Japanese fan”, says Lutz. However, the ditch to house the fan would have had to be about 10 m wide and would have needed a bridge across it, both of which would have added to the expense. The shell construction adopted allows for rigid doors that are only 240 mm thick, which has the added advantage that when the three sections slide back on each side of the hangar, their combined depth will still be only about 1 m.
The doors lean against the hangar at both its ends, the weight of one set balancing the forces of the other. To ensure the force from the doors is always in balance, both sets have to be opened simultaneously. This matches perfectly the method that will be used to get the airships out of the hangar.
Before a completed airship can take to the sky, it will have to be slowly and carefully eased out of the building. For this to happen smoothly, both sets of hangar doors will need to be opened to allow the wind to blow through from east to west; the wind speed must also be less than 10 m/s. If one set of doors remains closed, pulling the airship out of the hangar will be like pulling a piston out of a cylinder, says Lutz. The wind movement would be unpredictable, and the airship could pop out like a cork from a bottle.
With the door design resolved, Arup turned its attention to the hangar’s main structure. This had to provide a clear span of 225 m over a length of 360 m – equivalent to about eight football pitches – and a height of 107 m. “The volume could not be disturbed by the structure,” says Lutz. The engineer settled on a barrel vault-type structure consisting of five huge steel arched-trusses to span the floor.
Various alternatives were assessed. These included a portal frame, but this would have been more expensive with a larger volume to heat and an increased surface area to clad. A cable-stayed structure, such as the Millennium Dome’s, was also considered, but the weight of the doors leaning on both ends of the hangar meant that this, too, was unfeasible. In the end, the arched solution proved suitable, “taking into consideration the architecture, functional and financial considerations,” says Lutz.
The arched trusses are constructed from tubular steel and are 8 m deep to “take up the out-of-balance loads”, says Lutz, such as those imposed by the wind. A ridge beam links all five arches. This transfers the loads from the doors at one end to those at the other, helping to cancel the horizontal loads imposed by each, and keeping the structure in equilibrium.
The structure is enclosed using a stressed fabric membrane stretching between the arches. The membrane is being installed from the ridge beam, sliding in grooves down each of the arches using a luffing-type detail similar to the way a sail is attached to the mast on a yacht. The fabric panels are being fitted in four separate sheets because the maximum size that can be manufactured is only about 1000 m². Polyester was used for the fabric rather than PTFE simply because “it was about one-tenth the price”, says Lutz. With just over a month to go before the hangar is due to be completed, the final cladding sections are now being installed.
With the construction side of the scheme almost complete, the structural engineer can relax. But building the hangar is only the start of the CargoLifter project. The next stage will be to build the giant airships; over to the aeronautical engineer.
Check on the hangar’s progress at the live webcam on www.cargolifter.de