1. What is an Airship?
An airship is a type of lighter-than-air aircraft with propulsion and steering systems; it is used to carry passengers and cargo. It obtains its buoyancy from the presence of a lighter-than-air gas such as hydrogen, helium, or hot air, based on archimedes' principles. The first airship was developed by the french; called a "ballon dirigible" (steerable balloon), it could be steered and also be flown against the wind, which is not possible with a simple balloon.
Vehicles such as airships belong to the category of aerostats because of their ability to stand in the air. Airships and balloons are the two subcategories of aerostats. there are three types of airships, rigid, semirigid and nonrigid. Hot air airships can be counted as a part of the nonrigid category.
a) Rigid Airships (zeppelins)
As its name implies, a rigid structure, traditionally an aluminum alloy, holds up the form of a rigid airship, which is also called a zeppelin, after its inventor Ferdinand Graf von Zeppelin. This structure resembles a cage that encloses a series of balloons called gas cells. these cells are tailored to fit the cylindrical space and are secured in place by a netting that transmits the lifting force of their gas to the structure. Each gas cell has two or more valves, which operate automatically to relieve pressure when the gas expands with altitude; the valves can also be operated manually so that the pilot can release gas whenever desired.
Also on board is a ballast system that uses water as ballast. On the ground this ballast serves to make the airship heavier than air. When part of it is discharged, the airship ascends to a cruising altitude where the engines supply propulsion, and further ballast can be discharged to gain more altitude. As fuel is consumed, the airship becomes lighter and tends to climb. This is countered in hydrogen-inflated airships by simply releasing gas into the atmosphere. The method is uneconomical, however, with helium-inflated airships, and they are therefore equipped with ballast generators, apparatuses that condense moisture out of the engines' exhaust gases to compensate for fuel that is consumed. But this ballast-generating equipment is expensive, complex, heavy, and difficult to maintain and is thus one of the most serious disadvantages of airships filled with the safer but more expensive helium.
In general, rigid airships are only efficient when longer than 120 m because a good weight to volume ratio is only achievable for large airships. For a small airship the solid frame would be too heavy. However, the use of composite materials is changing this at this very moment with a project called the Zeppelin NT (see 4. The Present and the Future).
b) Nonrigid Airships (Blimps)
Nonrigid airships, also known as blimps, are the most common form nowadays. They are basically large gas balloons. In contrast to the rigid airship, the nonrigid airship has no internal structure to maintain the shape of its hull envelope. Its shape is maintained by internal overpressure. The only solid parts are the gondola and the tail fins.
Inside the gas space of the hull of modern, high-altitude blimps are two or more air diaphragms called ballonets that are kept under slight pressure, either by blowers or by air that is forced through scoops as a result of the forward motion (ram effect). The ballonets in turn exert pressure upon the gas, which fills the envelope, and this pressure in turn serves to stiffen the shape of the envelope and create a smooth flying surface. On takeoff the ballonets are almost fully inflated, but as the airship gains altitude and the gas expands, air is bled from the ballonets while a constant pressure is maintained throughout the envelope. When the gas contracts upon descent, air is pumped back into the ballonets.
All the airships currently flying for publicity use are nonrigid: the Goodyear blimps, the Budweiser and the Metlife blimps, airship "Shamu" in the USA, and the Fuji blimp in Europe.
c) Semirigid Airships
Semirigid airships were more popular earlier this century. Inventors sought to combine the best features of zeppelins and blimps in a semirigid type, but it met with only limited success. They usually comprised a rigid lower keel construction and a pressurized envelope above that. The rigid keel could be attached directly to the envelope or hung underneath it.
The airships of the Brazilian aeronaut Alberto Santos-Dumont and of the German engineer Otto Prill were semirigid. One of the most famous representatives of the type was Italia, the airship which General Umberto Nobile used on his attempt to reach the north pole.
d) Hot Air Airships
Hot air airships might be counted as a fourth kind although they are part of the nonrigid class. Hot air or thermal airships derive from traditional hot air balloons. Early models were almost like balloons with an engine and tail fins added. Very soon envelopes were lengthened and the tail fins and rudders were pressurized by air from the wash of the propeller. Only newer hot air airships maintain their shape with internal overpressure in the whole envelope. Older models did not have that feature, exactly like traditional hot air balloons.
2. The Past
The history of the zeppelin started when Graf Ferdinand von Zeppelin, a young cavalry general of the German army, returned to Germany from a journey to America. He brought with him the ideas of a rigid balloon as the world had never seen it before, the zeppelin.
It took years before Graf Zeppelin was able to realize his dreams. He did not get tired of trying to persuade the technical world to realize his plans of an aerostat. Finally, in the year 1899, the German emperor Wilhelm II granted Graf Zeppelin a complex near Friedrichshafen, which became the Luftschiffbau Zeppelin Ltd., where on July 3, 1900, the first zeppelin lifted off the ground.
A total of 119 zeppelins were built , most of them during world war I, when 103 airships were delivered to the military, and about 40 of them were shot down while on air raids over London. However, the most famous zeppelin was the original "Graf Zeppelin", which between 1928 and 1937 made flights to the United States, the Arctic, the Middle East, and South America; it also made one flight around the world. The last zeppelin was the "Graf Zeppelin II", which was first flown on September 14, 1938. It could carry loads of 30 tons over transoceanic distances. It was scrapped in may 1940.
The british also made intermittent efforts to develop the rigid airship; They built eight during world war I and six shortly thereafter. The most noteworthy was the R-34, which in July 1919 made the first transatlantic round-trip flight. An effort to develop two airships for intercontinental air service came to grief in october 1930 when the R-101 crashed and burned in France. The R-100, which had made a successful flight to Canada earlier in the year, was subsequently scrapped.
In the United States, the development of rigid airships was undertaken by the navy, and only five were operated. The navy-built ZR-1 Shenandoah made its first flight on September 4, 1923, and was torn to pieces by a thunderstorm over southern Ohio on September 3, 1925. The ZR-2 was procured in England but crashed on August 24, 1921, before it could be delivered to the United States. The ZR-3 Los Angeles, built in Germany by Luftschiffbau Zeppelin, made its transatlantic delivery flight during October 11-15, 1924; it was flown successfully until decommissioned in 1931 and was scrapped in early 1940. The ZRS4 and ZRS5 were built by the Goodyear-Zeppelin Co. of Akron, Ohio. Their design was unique in that it provided for an internal space for five airplanes that could be launched and retrieved while the airships were in flight. The akron first flew on September 23, 1931 and was lost in a storm over the Atlantic on April 4, 1933; The macon first flew on April 21, 1933, and crashed in the pacific on February 12, 1935.
3. The Disaster of Lakehurst
Why were all the airships scrapped in 1940? - because of the Hindenburg!
Crowds of onlookers had come to Lakehurst on May 6, 1937, to watch the landing of the LZ 129 Hindenburg after a long winter break. The scene was not new, for the Hindenburg had landed and started in Lakehurst, New Jersey, before. Everything seemed to be routine, especially for the spectators who did not know about an airship captain's problems with the wind, temperature, and weight.
People at that time were used to airships flying over the atlantic ocean with 200,000 m3 of highly flammable hydrogen, and nobody thought of any danger; however, it was there. While maneuvering to land at lakehurst the airship's hydrogen was ignited and the Hindenburg was destroyed by the resulting fire. 35 of the passengers and crew died, along with one member of the ground crew. Claims and speculations that the Hindenburg had been the target of sabotage have never been supported by evidence. Recent studies by a NASA engineer show that the probable cause of the disaster was a new aluminum-lacquer on the envelope which was extremely flammable. The destruction of the Hindenburg marked the end of the use of airships in world air commerce.
4. The Present and the Future
Over the past few years the airship has begun to make a remarkable comeback. Many people already considered them as something belonging to the past, unsuitable for our modern times. However, the last 25 years have shown an increasing interest in nonrigid airships. In the early 1970's there were only three airships flying in the USA and now there are more than twenty in four continents, with many more on the stocks.
Graf Zeppelin's dream has become reality. The modern airship is here, it is flying already. It is safe and it is commercially viable. The future is, if carefully nurtured, now on a very solid foundation. New ships will be truly capable of transoceanic flights; after all the Graf Zeppelin flew from Tokyo to Los Angeles, non-stop, and that was in 1929. 50 years after the disaster of Lakehurst, a new zeppelin is presenting itself in Friedrichshafen, next to various other projects in the whole world.
On april 23, 1997, the future arrived with the presentation of the LZ N07 Zeppelin NT (new technology) at the AERO International Flight Fair in Friedrichshafen . It is the first rigid airship since 1939, built again by the original Luftschiffbau Zeppelin Ltd., which now calls itself Friedrichshafener Zeppelin Luftschiff Ltd.. Its size is comparable to a boing 747 jumbo jet with a length of 68.4m. The frame of the new zeppelin is made up of carbon fiber compounds and aluminum alloys. Because of a new propulsion system, it is very easily maneuverable by one of the two pilots. There are three swivel propellers of 150,000 W power each and a cross fan at the tail, used to further control the Zeppelin NT. The three swivel propellers help at takeoff and landing procedures. This reduces the ground crew to three men.
the Zeppelin NT weighs only about 7000 kg and has a payload of up to 2000 kg. In the gondola, there is room for two pilots and 12 passengers. Its cruising speed is 32 m/s, with a maximum speed of 40 m/s. The unit price is 12.5 million marks (about 7.5 million $). Delivery of the first Zeppelin NT should start this year, with the first customer a Swiss company called Skyship Cruise Ltd. 14 more zeppelins have already been ordered by various companies from all over the world.