Larger modern airships rely on a semi-rigid design, in which engines and passenger compartments are suspended from a metal keel that runs along the bottom of the envelope and prevents it from distorting under the load. Since 1997 Zeppelin Luftshifftechnik, the direct descendant of the original Zeppelin company, has been testing the LZ-N07, a 75-metre semi-rigid design that can carry two tonnes of cargo, or 12 passengers. But this is a pygmy compared with the Zeppelins of old—so several other companies have been thinking bigger.

The furthest-advanced of the new airship companies, a German-American firm called CargoLifter, is planning a 260-metre-long airship that is intended to lift goods, not passengers. The CL160, a semi-rigid airship, will be the largest ever made, with a lifting capacity of 160 tonnes. Yet, though this is far more than any previous airship, it does not sound that impressive when compared with large cargo planes, such as the Ukrainian Antonov AN-225 which can carry as much as 250 tonnes. Airships might, however, come into their own carrying large (as well as heavy) things such as generators, turbines or components of oil refineries. These often have to be broken into pieces after construction and testing so as to be transported by road, rail and ship to their final destination, but an airship, acting as a “flying crane”, could transport such items directly and in one piece.

According to CargoLifter’s research, the market for such heavy lifting—cargo weighing more than 100 tonnes and measuring more than 25 metres in any one dimension—is worth $1 billion a year in America alone, and is growing by 12% annually, faster than any other part of the freight market. A study last year suggested that airships could easily capture 10% of this market.

So far, the company has built a one-eighth-size prototype to test its design, which consists of a carbon-fibre keel, an envelope made from modern composite materials, half-a-dozen engines, a passenger compartment, and a specially lightened crane. Tests have shown that the design could be scaled up to handle loads as heavy as 450 tonnes. When built, the full-sized airships (which will cost around $50m each) will have a crew of 12 and a top speed of around 100kph (60mph). Within the past few weeks, construction of a huge hangar, large enough to house two CL160s, has begun at a disused air base near Berlin. Once this is complete, fabrication of the first full-sized airship should begin next summer; test flights are planned for 2001.

Two other firms are taking a more traditional approach. The original Zeppelins were based on a rigid design in which separate gas-tight compartments were surrounded by a metal frame covered with a non-airtight fabric. The advantage of this arrangement is that it can be scaled up more easily than a semi-rigid design, since the envelope is not load-bearing. Proponents of rigid airships also claim that their babies are more resilient, thanks to the protective outer skin which shields the gas compartments.

Rigid Airship Design (RAD), a Dutch consortium, plans to build a 180-metre rigid airship on the blueprint of the original Zeppelins, but using modern materials. The engines, gas compartments and payload (either 30 tonnes of cargo, or 240 passengers) of the RA-180 will all be contained within the cigar-shaped rigid structure, which will be powered by four propellers and have a top speed of 150kph. The company has recently filed for planning permission to build a huge hangar at Lelystad in the Netherlands. It hopes to have its first airship (which will cost $30m to build) flying in 2001.

Hamilton Airship, a South African company, is taking a similar approach, but has updated the classical rigid-airship design with a novel twist. The superstructure is an “endoskeleton” built around a central spine, rather than a traditional “exoskeleton”-like frame. The skin is wrapped over a series of hoops suspended from the spine. This reduces both weight and cost, and also means that the design can easily be scaled up. A 44-metre-long prototype has been flying since last year, and the company is now trying to raise the $30m needed to build a larger, 160-metre version, which will have a capacity of 20 tonnes and a top speed of 145kph.

Both RAD and Hamilton plan to concentrate on the specialist freight and tourism markets. As well as carrying large items, they point out, airships are ideal for transporting light but bulky cargo such as flowers. And although they cannot compete with aeroplanes for speed, or with water-borne shipping for volume, they could cover in a few days a distance equivalent to a six-week journey by ship. They can even compete with road traffic. They should, for example, be able to deliver flowers from the Netherlands to London faster than lorries. The attractions for tourists would be strong too. Since they travel at low altitudes, airships offer spectacular views—raising the prospect of, say, airborne cruises around Europe.

Other companies have even more ambitious (though at this stage also more speculative) plans. Airships demonstrate a law of increasing returns: doubling the length increases the surface area (and hence weight) by a factor of four while the volume (and hence lifting capacity) goes up by a factor of eight. This has prompted researchers at the Illinois Institute of Technology to propose a 2¹/₂-km-long airship called the AeroCarrier that would be able to lift 35,000 tonnes and carry around 3,500 passengers.

Even more outlandish is the idea that has been dreamt up at Lockheed-Martin’s legendary Skunk Works, the research centre responsible for such curious aircraft as the F-117A “stealth” fighter. Its Aerocraft concept is not, strictly speaking, an airship at all, but an odd aeroplane-airship hybrid intended for rapid military airlifts. Half the lift would be provided by helium, and the craft would be shaped like a giant “flying wing” to provide extra lift when it is airborne. Four huge “tiltrotors” would make vertical take-off and landing possible, as well as providing propulsion in the air. The Aerocraft would have a cargo capacity of around 500 tonnes and a top speed of 180kph.

Evidently there is still plenty of life in the airship concept. Before his death in 1917 von Zeppelin was hailed, somewhat prematurely, as the greatest German of the 20th century. Perhaps, with a few new twists, the form of transport he pioneered will do better in the 21st.