COMEBACK OF THE AIRSHIPS

COMEBACK OF THE AIRSHIPS
By Christopher Hess
More than 60 years after the LZ 129 "Hindenburg" went up in flames in Lakehurst, such ending the short but glorius era of the Zeppelins, engineers worldwide are working again on applications for aerostatic aircraft.

For a long time, it looked as though winged aircraft had overtaken the airships for good. However, the catastrophic end of the LZ 129 was not the true reason behind this development. It was mainly due to the quantum leaps in the engine development that aircraft surpassed the airships in both, speed and punctuality.

Airships can not be fast because they are too voluminous. Also, they can not be on time because their operation very much depends on the weather. Furthermore, their lightweight construction makes airships very vulnerable during take-off and landing.

Meanwhile, the aerospace branch has recognized that there are again useful applications for airship designs, which do not require speed or punctuality or which require the airship to land.

Experts have high expectations from one application that has been under study for several years but has not had a chance to become realized until today because of the technological progress: "The use of airships as high-altitude platforms is an extremely useful application for airships", says Professor Bernd Krцplin, who heads the Institute for Statics and Dynamics of Aerospace Structures (ISD) at the University of Stuttgart. Krцplin is member of a team of researchers who was awarded the 1,5 million DM Koerber prize of the European Science in 1999. The team is planning to develop technological solutions for stratospheric airship platforms.

Such applications supposedly have a large market potential in the area of telecommunications. One high-altitude platform at 20 kilometers of altitude with a 40 degree viewing angle can cover a large metropolitan area such as London, Paris or Berlin.

The technological concept sees large airships as antennae platforms which operate autonomously. During their mission, which may last several years, the airships are generating the necessary energy from solar technology. During night the energy supply is ensured by the use of fuel cells.

Meanwhile the team of researchers around Prof. Krцplin has joint their efforts with the Space Infrastructure business of Germany's DaimlerChrysler Aerospace. Dasa has been studying the use of stratospheric airship platform since 1998 for the European Space Agency ESA. "ESA sees a high degree of cohesion between space and stratosphere", says Dasa's Reimund Kьke. However, the high-altitude platforms are not envisioned to replace satellites but rather to compliment them. Along with telecommunication, other applications such as observation, military recconnaissance or scientific long-term studies in-situ in the stratosphere are possible.

Dasa has worked out a stepwise approach to realizing such a European HALE platform (High Altitude Long Endurance) which would use small demonstrators to validate specific technologies. The first demonstrator would be 80 meters long, reach altitudes of up to 15 kilometers. Its flight would last a few days. The second vehicle would already have a length of 180 meters and fly for half a year.

According to Reimund Kьke, such a project would require an investment of approximately 80 to 100 million Euro, which would include a first pre-series airship with 220 meters of length. The goal would be to achieve mission times that are comparable to satellites, i.e. two to five years.

The HALE platform would have a mass of approximately 30 tons and would be designed as a blimp. This design principle differentiates it from the Stratospheric Platform Airship (SPA) which is currently being studied in Japan by the National Aerospace Laboratory in a project for the country's Science Agency and the Ministry for Post and Telecommunication. The Japanese airship platform is currently designed as a semi-rigid design which has a rigid keel structure.

Another stratospheric platform project is under study in the USA. In 1995, the former Secretary of Defence, General Alexander Haig teamed up with a group from the University of California and set up SkyStation Incorporated. According to the published data, SkyStation, like HALE, would be a blimp. However, the American design would omit all of the fins.

The extremely instable airship would be positioned and steered solely by its swivable propeller. Still, SkyStation has not yet gone beyond the concept phase. There are even rumors that the company does not operate any more. According to branch insiders, SkyStation has spent too much effort on marketing and fund raising and not enough effort on the absolutely essential technological developments.

The technological challenge of operating unmanned aerostatic platforms for years in the stratosphere is enormous. The light-built platforms must endure temperatures of minus 50 degrees Centigrade and high levels of ultraviolet radiation. The pressure in 20 kilometers of altitude is only one twentieth of the pressure at sea level. Worst, there are strong winds that change their intensity with the seasons and geographical location and can reach up to 50 m/s (180 km/h). Currently the HALE concepts is looking at a maximum airspeed of around 90 km/h, meaning that it would not be able to maintain a geostationary position in the strongest wind conditions, but would have to move into an airspace with more favorable conditions.

The application of the lighter-than-air technology to autonomous operating high-altitude platforms would require new flight control and attitude control concepts. Today, there is no autopilot available for airships. The reason: Airships have entirely different aerodynamical and flight mechanical characteristics than winged aircraft.

Since 1992, ISD in Stuttgart has been using the small solar-powered airship "Lotte" for airship technology research. Lotte will now be used to study the coherence between flight control inputs and flight characteristics. The results, which can be scaled without problems to larger airship designs (at least as far as the control laws are concerned), will than be available for the altitude platform design and also for the currently most spectacular airship project in Germany.

At the former military airfield in Brand, approximately 60 kilometers in the south of Berlin, the CargoLifter AG is building the world's largest self-supporting hangar. Next year, the 400 meter long, 70 meter high and 140 meter wide hangar will host the assembly of the first CargoLifter CL160. The 260 m long airship is designed to carry heavy loads of up to 160 tons over distances up to 10000 kilomters from point to point. The CargoLifter's semi-rigid design features are more than 200 meter long keel in composite/metal construction which holds a uniquely designed loading system along with the flight deck and the propulsion system.

"There would be no CargoLifter without that loading system", says Professor Krцplin who is also a member of the board of the company. The system allows the heavy duty airship to load or unload its cargo without having to land. While hovering in 100 meters above the ground, the cargo is lowered and is then anchored with six steel ropes at four anchor points on the ground. This allows the "flying crane" to stay in the wind and even to move slightly.

A complex propulsion string of four cruise and twelve maneuver engines is supposed to give the CL160 the required maneuverability. At least for the prototype airship, CargoLifter plans to use turboprop engines for all of the 16 propulsion units. Originally, the company had planned to use ship diesels to supply the cruise power. However, this might no be possible due to certification constraints. The airship will have a total power need of approximately 30000 hp.

The designers have already looked at 21 different materials for the airship hull. Because of CargoLifter, there has already been a quantum leap: "Today's hull material is five times better than earlier designs", says Krцplin. More than 60000 square meters of material will be needed for the hull and the ballonets. The hull material can not be too soft since the semi-rigid airship draws its shape from the confection. The hull, which will be filled with inflammable Helium, will have a pressure of 650 pascal.

The engineers are spending a considerable amount of effort on protecting the CargoLifter from lightning strikes. Just because of its size, the CL160 will definitely build up a potential that will generate discharges. "The question is not if the CargoLifter will be hit by lightning strike, but how often", says Ralph Maurer, the head of design of CargoLifter.

The biggest challenge for the CargoLifter designers is the computation of the airship's flight mechanical characteristics. The entire problem of the aerodynamical characteristics of such a voluminous body are not yet resolved: "One cannot find this in a book", says Professor Bernd Krцplin.

The first prototype of the CL160 is supposed to fly in 2002. Series production is supposed to start around 2004/2005. The new facility in Brand will have a capacity of four airships per year. The hangar has space for one airship in final assembly status and one airship in component status.

Until the Prototype is completed, CargoLifter will need approximately 500 million DM. Half of this is supposed to be generated once the company is officially starting to emit its stock on the market, which is due for the second quarter of 2000. Currently the company has more than 10000 stock holders.

In parallel to the airships, CargoLifter has already started to set up a worldwide network which is supposed to prepare the market for the gigantic airships. From an annual heavy duty transport demand of around 300 million tons, CargoLifter wants to capture ten percent. Just this would require a fleet of 200 CL160s, says CargoLifter Director of Production, Christoph von Kessel.

Competition for the German heavy duty airship could come from two projects which are currently in development in other companies. UK based Airship Technologies has plans for the so called SkyCat and the US concern Lockheed Martin is shaping the Aerocraft in the company's secret Skunk Works. Still, there is not much known about the two projects today beyond their name.

Another new airship project that has already left the design stage far behind, is the Zepplin New Technology (NT) which had its first flight already in 1997. The 75 meter long airship is a semi-rigid design which features a continous primary structure extending over its entire length. It consists of triangular frames and three rows of longerons which are braced with diagonal ties to form a rigid spatial framework. By using three engines with propellers capable of delivering vectored thrust, a high degree of maneuverability is achieved, enabling the airship to operate in heavy conditions.

After the flight test had a slow start, furthermore hampere by damaging the airship when it was towed out of its hangar during the last winter, the Zeppelin NT prototype has meanwhile accumulated more than 400 flying hours, 200 of which just in the time period since May of 1999. Until the airship will receive certification of the German aviation authority LBA, it will probably need another 300 hours. Most of these will be needed for documentation purposes, says Klaus Hagenlocher, the technical director of Zeppelin Luftschifftechnik in Friedrichshafen. The Zeppelin NT will probably be the first airship in the world to receive type certification in the commuter category.

Meanwhile the primary structure of the first production airship has been completed in the Hangar at the Friedrichshafen airport. Along with small design changes to the propulsion system, some of the structure was modified to improve its strength in withstanding the load peaks of up to 2,2 G at higher airspeeds.

The cockpit design with side sticks and fly-by-wire flight control system was not changed. However, half of the additional work time on the series airship was spent on the airship's lightning protection. The production airships will get a copper mash coating on some parts. Also, the data lines will be protected by lightning protection boxes. These are to reduce voltage peaks of up to 1600 V down to 300 V. The changes, which are now incorporated into the production design, will lateron applied to the prototype airship also.

Up until now, the company has logged preliminary contracts from four customers for five airships. The first Zeppelin NT will go to the Swiss company Skyship Cruise Ltd. which plans to operate it for touristic flights. The other ships are going to customers in Germany. Along with the traditional application as a touristic airship, the NT will also be offered as platform for observation and scientific missions - always together with advertisement. However, in the latter business area the NT would compete with the established blimps which are dominating todays airborn advertisment business.

A realistic market forecast is very difficult. Dr. Bernd Strдter, Zeppelin Luftschifftechnik's chairman sees a worldwide need of 50 to 100 airships in the NT category. Of these, Zeppelin plans to capture a 50 percent share.

A team of designers around the British airship expert Ian Alexander is much more optimistic. Ian Alexander is currently working on the design of a classic rigid airship. The 180 meter long Rigid Airship RA-180 will supposedly have a payload of up to 30 tons, either in freight or for up to 240 passengers. Alexander sees a market potential of up 1000 of these airships over the next 20 years.

There have been several new airship designs over the past years. "Only if there is an infrastructure and a market will a new airship project be successful", says Professor Bernd Krцplin.

From page 14 of FLUG REVUE 3/2000

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