An airship or dirigible is a type of aerostat or lighter-than-air aircraft that is propelled through the air by engine power. An aerostat stays aloft by having a large "envelope" filled with a lifting gas that is less dense than the surrounding air. In early dirigibles, the lifting gas used was hydrogen, due to its high lifting capacity.
Helium gas has almost the same lifting capacity and is also nonflammable, but is rare and relatively expensive. Significant amounts were first discovered in the United States and, for a while, helium was rarely used for airships outside the United States. Most airships built since the 1960s have used helium, though some have used hot air.[note 1]
Besides the main envelope, an airship also has engines and crew accommodation, typically in a gondola hung beneath the envelope.
The main types of airship are non-rigid, semi-rigid, and rigid. Non-rigid airships, often called "blimps", rely on internal pressure to maintain the shape of the airship. Semi-rigid airships maintain the envelope shape by internal pressure, but have some form of supporting structure, such as a fixed keel, attached to it. Rigid airships have an outer structural framework which maintains the shape and carries all structural loads, while the lifting gas is contained in one or more internal gas bags or cells. Rigid airships were first flown by Count Zeppelin and the vast majority of rigid airships built were manufactured by the firm he founded. As a result, all rigid airships are sometimes called zeppelins.
Airships were the first aircraft capable of controlled powered flight, and were most commonly used before the 1940s, but their use decreased over time as their capabilities were surpassed by those of aeroplanes. Their decline was accelerated by a series of high-profile accidents, including the 1930 crash and burning of British R101 in France, the 1933 storm-related crash of the USS Akron and the 1937 burning of the hydrogen-filled Hindenburg. From the 1960s, helium airships have been used in applications where the ability to hover in one place for an extended period outweighs the need for speed and manoeuvrability such as advertising, tourism, camera platforms, geological surveys, and aerial observation.
A total of 4,700 airships now exist across the world.
- 1 Classification
- 2 Terminology
- 3 Construction
- 4 History
- 5 Modern airships
- 6 Current design projects
- 7 Comparison with heavier-than-air aircraft
- 8 Safety
- 9 See also
- 10 Notes
- 11 References
- 12 External links
Airships are classified, according to their method of construction, into rigid, semi-rigid and non-rigid types.
A rigid airship has a rigid framework covered by an outer skin or envelope. The interior contains one or more gas bags, cells or balloons to provide lift. Rigid airships are typically unpressurised and can be made to virtually any size. Most, but not all, of the German Zeppelin airships have been of this type.
A semi-rigid airship has some kind of supporting structure but the main envelope is held in shape by the internal pressure of the lifting gas. Typically the airship has an extended, usually articulated keel running along the bottom of the envelope to stop it kinking in the middle by distributing suspension loads into the envelope, while also allowing lower envelope pressures.
Non-rigid airships are often called "Blimps". Most, but not all, of the American Goodyear airships have been blimps.
A non-rigid airship relies entirely on internal gas pressure to retain its shape during flight. Unlike the rigid design, the nonrigid airship's gas envelope has no compartments. However it typically has smaller internal bags or "ballonets" containing air. At sea level, the ballonets are filled with air. As altitude is increased, the lifting gas expands and air from the ballonets is expelled through valves to maintain the hull shape. To return to sea level, the process is reversed. Air is forced back into the ballonets by both scooping air from the engine exhaust and using auxiliary blowers.
During the pioneer years of aeronautics, terms such as "airship", "air-ship", "air ship" and "ship of the air" meant any kind of navigable or dirigible flying machine. In 1919 Frederick Handley Page was reported as referring to "ships of the air," with smaller passenger types as "Air yachts." In the 1930s, large intercontinental flying boats were also sometimes referred to as "ships of the air" or "flying-ships". Nowadays the term "airship" is used only for powered, dirigible balloons, with sub-types being classified as rigid, semi-rigid or non-rigid.
Airships were originally called dirigible balloons from the French (dirigible, meaning steerable or navigable. This came to be shortened to "dirigible" and this term is still sometimes used to mean an airship. In modern usage, balloon usually refers to an unpowered aerostat.
A blimp is a non-rigid aerostat. In American usage it refers specifically to a non-rigid type of dirigible balloon or airship. In British usage it refers to any non-rigid aerostat, including barrage balloons and other kite balloons, having a streamlined shape and stabilising tail fins.
The term zeppelin is a genericized trademark that originally referred to airships manufactured by the German Zeppelin Company, which pioneered the use of very large airships in the early years of the twentieth century. The initials LZ, for Luftschiff Zeppelin (German for "Zeppelin airship"), usually prefixed their craft's serial identifiers.
In technical usage, rigid airship is the term used for all aircraft of this type, with zeppelin referring only to aircraft of that manufacture.
The two main parts of an airship are its gas-containing envelope and a gondola or similar structure slung beneath and containing crew and other equipment. The engines may be mounted in the gondola or elsewhere off the envelope.
The basic structure of an airship may be rigid, semi-rigid or non-rigid, as described.
The envelope itself is the outer surface, usually surrounding one or more gas-bags and/or ballonets within it.
Fins at the rear of the envelope stabilise the airship, allowing it to fly straight. On some smaller designs these fins are themselves part of a gas bag and gain their shape only when inflated.
A few airships have been metal-clad, with rigid and nonrigid examples made. Each kind used a thin gastight metal envelope, rather than the usual rubber-coated fabric envelope. Only four metal-clad ships are known to have been built, and only two actually flew: Schwarz's first aluminum rigid airship of 1893 collapsed, while his second flew; the nonrigid ZMC-2 built for the US Navy flew from 1929 to 1941 when it was scrapped as too small for operational use on anti-submarine patrols; while the 1929 nonrigid Slate Aircraft Corporation City of Glendale collapsed on its first flight attempt. Both nonrigid ships nevertheless had strong metal monocoque envelopes which, while they maintained their shape uninflated, required an overpressure during flight.
Early airships used hydrogen as their lifting gas, which is the lightest available. Typically it was generated during the filling process, by reacting dilute sulphuric acid with metal filings. The first hydrogen balloon in 1783 used iron filings, while the British Nulli Secundus of 1907 used zinc.
Later, the USA began to use helium because it is non-flammable and has 92.7% of the buoyancy (lifting power) of hydrogen. Following a series of airship disasters in the 1930s, and especially the Hindenburg disaster where the airship burst into flames, hydrogen fell into disuse.
The term "gondola" is used to describe a crew car of an airship, slung beneath the centre of the envelope. These may be short, for cockpit and landing gear alone, or longer to provide passenger space. Early gondolas were open structures slung beneath the envelope, later ones were enclosed and hung directly from the internal framing. A nonrigid blimp carries all of its passengers within a gondola. Rigid airships may have further passenger or cargo space inside the envelope. The large airship Graf Zeppelin was noted for its distinctively short passenger gondola, mounted far forward so as to improve ground clearance. The majority of crew accommodation and cargo holds were placed inside the envelope.
Propulsion and control
Small airships carry their engine(s) in their gondola. Where there were multiple engines on larger airships, these were placed in separate nacelles, termed power cars or engine cars. To allow asymmetric thrust to be applied for maneuvering, these power cars were mounted towards the sides of the envelope, away from the centre line gondola. This also raised them above the ground, reducing the risk of a propeller strike when landing. Widely spaced power cars were also termed wing cars, from the use of "wing" to mean being on the side of something, as in a theater, rather than the aerodynamic device. These engine cars carried a crew during flight who maintained the engines as needed, but who also worked the engine controls, throttle etc., mounted directly on the engine. Instructions were relayed to them from the pilot's station by a telegraph system, as on a ship.
While elevators and swivelling propellers provide fine control of altitude, larger changes of height used to be controlled by either venting gas to lose altitude or dropping ballast to gain altitude. Large airships typically carried several water tanks fore and aft, allowing them to adjust longitudinal trim as well as height. Some modern designs instead pump lifting gas between the gas bags and storage cylinders.
In 1670 the Jesuit Father Francesco Lana de Terzi, sometimes referred to as the "Father of Aeronautics." published a description of an "Aerial Ship" supported by four copper spheres from which the air was evacuated. Although unrealisable since external air pressure would cause the spheres to collapse unless their thickness was such as to make them too heavy to be buoyant, the basic principle was sound.
A more practical dirigible airship was described by Lieutenant Jean Baptiste Marie Meusnier in a paper entitled "Mémoire sur l’équilibre des machines aérostatiques" (Memorandum on the equilibrium of aerostatic machines) presented to the French Academy on 3 December 1783. The 16 water-color drawings published the following year depict a 260-foot-long (79 m) streamlined envelope with internal ballonnets that could be used for regulating lift: this was attached to a long carriage that could be used as a boat if the vehicle was forced to land in water. The airship was designed to be driven by three propellers and steered with a sail-like aft rudder. In 1784 Jean-Pierre Blanchard fitted a hand-powered propeller to a balloon, the first recorded means of propulsion carried aloft. In 1785 he crossed the English Channel in a balloon equipped with flapping wings for propulsion and a birdlike tail for steering.
The 19th century saw continued attempts to add methods of propulsion to balloons. The Australian Dr William Bland sent designs for his "Atmotic Airship" to the Great Exhibition held in London in 1851, where a model was displayed. This was an elongated balloon with a steam engine driving twin propellers suspended underneath. The lift of the balloon was estimated at 5 tons and the car with the fuel to weigh 3.5 tons, giving a payload of 1.5 tons. Bland believed that the machine could be driven at 80 km/h (50 mph) and could fly from Sydney to London in less than a week.
In 1852, Henri Giffard became the first person to make an engine-powered flight when he flew 27 km (17 mi) in a steam-powered airship. Airships would develop considerably over the next two decades. In 1872, the French naval architect Dupuy de Lome launched a large limited navigable balloon, which was driven by a large propeller and the power of eight people. It was developed during the Franco-Prussian war, as an improvement to the balloons used for communications between Paris and the countryside during the siege of Paris, but was completed only after the end of the war.
In 1872 Paul Haenlein flew an airship with an internal combustion engine running on the coal gas used to inflate the envelope, the first use of such an engine to power an aircraft. Charles F. Ritchel made a public demonstration flight in 1878 of his hand-powered one-man rigid airship, and went on to build and sell five of his aircraft.
In 1883 the first electric-powered flight was made by Gaston Tissandier, who fitted a 1.5 hp (1.1 kW) Siemens electric motor to an airship. The first fully controllable free-flight was made in 1884 by Charles Renard and Arthur Constantin Krebs in the French Army airship La France. The 170 ft (52 m) long, 66,000 cu ft (1,900 m3) airship covered 8 km (5.0 mi) in 23 minutes with the aid of an 8.5 hp (6.3 kW) electric motor, and a 435 kilograms (959 lb) battery. In 1884 and 1885, it made seven flights.
In 1888 the Campbell Air Ship, designed by Professor Peter C. Campbell, was made by the Novelty Air Ship Company. This was lost at sea in 1889 while being flown by Professor Hogan during an exhibition flight.
In 1888–97 Dr. Frederich Wölfert built three airships powered by Daimler Motoren Gesellschaft-built petrol engines, the last of which caught fire in flight and killed both occupants in 1897. The 1888 version used a 2 hp single cylinder Daimler engine and flew 10 km (6 mi) from Canstatt to Kornwestheim.
In 1897 an airship with an aluminum envelope was built by the Hungarian engineer David Schwarz. It made its first flight at Tempelhof field in Berlin after Schwarz's had died. His widow, Melanie Schwarz, was paid 15,000 marks by Count Ferdinand von Zeppelin to release the industrialist Carl Berg from his exclusive contract to supply Schwartz with aluminium.
Alberto Santos-Dumont was a wealthy Brazilian who lived in France and had a passion for flying. He designed 18 balloons and dirigibles, including the first practical dirigible airship, before turning his attention to fixed-winged aircraft. On 19 October 1901 he flew his airship Number 6, a small semi-rigid with a detached keel, from the Parc Saint Cloud to and around the Eiffel Tower and back in under thirty minutes. This feat earned him the Deutsch de la Meurthe prize of 100,000 francs. Many inventors were inspired by Santos-Dumont's small airships and a veritable airship craze began worldwide. Many airship pioneers, such as the American Thomas Scott Baldwin, financed their activities through passenger flights and public demonstration flights. Stanley Spencer built the first British airship with funds from advertising baby food on the sides of the envelope. Others, such as Walter Wellman and Melvin Vaniman, set their sights on loftier goals, attempting two polar flights in 1907 and 1909, and two trans-Atlantic flights in 1910 and 1912.
In 1902, the Spanish engineer Leonardo Torres Quevedo published details of an innovative airship design in Spain and France. With a non-rigid body and internal bracing wires, it overcame the flaws of these types of aircraft as regards both rigid structure (zeppelin type) and flexibility, providing the airships with more stability during flight, the capability of using heavier engines and a greater passenger load. In 1905, helped by Captain A. Kindelán, he built the airship "España" at the Guadalajara military base. Next year he patented his design without attracting official interest. In 1909 he patented an improved design which he offered to the French company Astra, who started mass-producing it in 1911 as the Astra-Torres airship. The distinctive three-lobed design was widely used during the Great War by the Entente powers.
Pre-World War 1
In July 1900 the Luftschiff Zeppelin LZ1 made its first flight. This led to the most successful airships of all time: the Zeppelins, named after Count von Zeppelin who began working on rigid airship designs in the 1890s, leading to the flawed LZ1 in 1900 and the more successful LZ2 in 1906. The Zeppelin airships had a framework composed of triangular lattice girders covered with fabric which contained separate gas cells. At first multiplane tail surfaces were used for control and stability: later designs had simpler later cruciform tail surfaces. The engines and crew were accommodated in "gondolas" hung beneath the hull driving propellers attached to the sides of the frame by means of long drive shafts. Additionally, there was a passenger compartment (later a bomb bay) located halfway between the two engine compartments.
Other airship builders were also active before the war: from 1902 the French company Lebaudy Frères specialized in semirigid airships such as the Patrie and the République, designed by their engineer Henri Julliot, who later worked for the American company Goodrich; the German firm Schütte-Lanz built the wooden-framed SL series from 1911, introducing important technical innovations; another German firm Luft-Fahrzeug-Gesellschaft built the Parseval-Luftschiff (PL) series from 1909, and Italian Enrico Forlanini's firm had built and flown the first two Forlanini airships.
In Britain, the Army built their first dirigible, the Nulli Secundus, in 1907. The Navy ordered the construction of an experimental rigid in 1908. Officially known as His Majesty's Airship No. 1 and nicknamed the Mayfly, it broke its back in 1911 before making a single flight. Work on a successor did not start until 1913.
World War I
The prospect of airships as bombers had been recognized in Europe well before the airships were up to the task. H. G. Wells' The War in the Air (1908) described the obliteration of entire fleets and cities by airship attack. The Italian forces became the first to use dirigibles for a military purpose during the Italo–Turkish War, the first bombing mission being flown on 10 March 1912. It was World War I, however, that marked the airship's real debut as a weapon. The Germans, French and Italians all used airships for scouting and tactical bombing roles early in the war, and all learned that the airship was too vulnerable for operations over the front. The decision to end operations in direct support of armies was made by all in 1917.
Many in the German military believed they had found the ideal weapon with which to counteract British naval superiority and strike at Britain itself. More realistic airship advocates believed the zeppelin's value was as a long range scout/attack craft for naval operations. Raids on England began in January 1915 and peaked in 1916: following losses to the British defenses only a few raids were made in 1917-8, the last in August 1918. Zeppelins proved to be terrifying but inaccurate weapons. Navigation, target selection and bomb-aiming proved to be difficult under the best of conditions and the cloud cover that were frequently encountered by the airships reduced accuracy even further. The physical damage done by airships over the course of the war was insignificant, and the deaths that they caused amounted to a few hundred. Nevertheless the raid caused a significant diversion of British resources to defense efforts. The airships were initially immune to attack by aircraft and antiaircraft guns: as the pressure in their envelopes was only just higher than ambient air, holes had little effect. But following the introduction of a combination of incendiary and explosive ammunition in 1916 their flammable hydrogen lifting gas made them vulnerable to the defending aeroplanes. Several were shot down in flames by British defenders, and many others destroyed in accidents. New designs capable of reaching greater altitude were developed, but although this made them immune from attack it made their bombing accuracy even worse.
Countermeasures by the British included sound detection equipment, searchlights and anti-aircraft artillery, followed by night fighters in 1915. One tactic used early in the war, when their limited range meant the airships had to fly from forward bases and the only zeppelin production facilities were in Friedrichshafen, was the bombing of airship sheds by the British Royal Naval Air Service. Later in the war, the development of the aircraft carrier led to the first successful carrier-based air strike in history: on the morning of 19 July 1918 seven Sopwith 2F.1 Camels were launched from HMS Furious and struck the airship base at Tondern, destroying the zeppelins L 54 and L 60.
The British Army had abandoned airship development in favour of aeroplanes by the start of the war, but the Royal Navy had recognized the need for small airships to counteract the submarine and mine threat in coastal waters. Beginning in February 1915, they began to develop the SS (Sea Scout) class of blimp. These had a small envelope of 1,699-1,982 m³ (60–70,000 ft³) and at first used aircraft fuselages without the wing and tail surfaces as control cars. Eventually more advanced blimps with purpose built gondolas were built. The NS class (North Sea) were largest and most effective nonrigid airships in British service. These had a gas capacity of 360,000 cu ft (10,000 m3), a crew of 10 and an endurance of 24 hours. Six 230 lb (100 kg) bombs were carried, as well as three to five machine guns. British blimps were used for scouting, mine clearance, and convoy patrol duties. During the war, the British operated over 200 nonrigid airships. Several were sold to Russia, France, the United States, and Italy. The large number of trained crews, low attrition rate and constant experimentation in handling techniques meant that at the war's end Britain was the world leader in nonrigid airship technology.
The Royal Navy continued development of rigid airships until the end of the war. Eight rigid airships had been completed by the armistice, (No. 9r, four 23 Class, two R23X Class and one R31 Class), although several more were in an advanced state of completion by the war's end. Both France and Italy continued to use airships throughout the war. France preferred the nonrigid type, whereas Italy flew 49 semirigid airships in both the scouting and bombing roles.
Aeroplanes had essentially replaced airships as bombers by the end of the war, and Germany's remaining zeppelins were destroyed by their crews, scrapped or handed over to the Allied powers as war reparations. The British rigid airship program, which had mainly been a reaction to the potential threat of the German airships, was wound down.
The interwar period
A number of nations operated airships between the two world wars. Britain, the United States and Germany were the main operators of rigid airships, with Italy and France using them to a lesser extent. Italy, the Soviet Union, the United States, and Japan mainly used semirigid airships.
Under the terms of the Treaty of Versailles Germany was not allowed to build airships of greater capacity than a million cubic ft. Two small passenger airships, LZ 120 Bodensee and its sister-ship LZ 121 Nordstern, were built immediately after the war but were confiscated following the sabotage of the wartime Zeppelins that were to have been handed over as war reparations: Bodensee was given to Italy and LZ 121 Nordstern to France. On May 12, 1926, the Italian semirigid airship Norge was the first aircraft to fly over the North Pole.
The British R33 and R34 were near-identical copies of the German L 33 which had come down almost intact in Yorkshire on 24 September 1916. Despite being almost three years out of date by the time they were launched in 1919, they became two of the most successful airships in British service. The creation of the Royal Air Force (RAF) in early 1918 created a hybrid British airship program. The RAF was not interested in airships and the Admiralty was, so a deal was made where the Admiralty would design any future military airships while the RAF would handle manpower, facilities and operations. On 2 July 1919, R34 began the first double crossing of the Atlantic by an aircraft. It landed at Mineola, Long Island on 6 July after 108 hours in the air. The return crossing began on 8 July and took 75 hours. This feat failed to generate enthusiasm for continued airship development, and the British airship program was rapidly wound down.
America then started constructing the USS Shenandoah, designed by the Bureau of Aeronautics and based on the Zeppelin L 49. assembled in Hangar No. 1 and first flown on 4 September 1923  at Lakehurst, New Jersey. It was the first airship to be inflated with the noble gas helium, which was then so scarce that the Shenandoah contained most of the world's supply. A second airship, USS Los Angeles, was built by the Zeppelin company as compensation for the airships which should have been handed over as war reparations according to the terms of the Versailles Treaty but had been sabotaged by their crews. This construction work saved the Zeppelin works from the threat of closure. The success of the Los Angeles, which was flown successfully for 8 years, encouraged the US Navy to invest in its own, larger airships. When the Los Angeles was delivered, the two airships had to share the limited supply of helium, and thus alternated operating and overhauls.
In 1922 Sir Dennistoun Burney suggested a plan for a subsidised air service throughout the British Empire using airships (the Burney Scheme). Following the coming to power of Ramsay MacDonald's Labour government in 1924, the Burney scheme was transformed into the Imperial Airship Scheme, under which two airships were built, one by a private company, and the other by the Royal Airship Works under Air Ministry control. The two designs were radically different. The "capitalist" ship, the R100, was more conventional, while the "socialist" ship, the R101, had many innovative design features. Construction of both took longer than expected, and the airships did not fly until 1929. Neither airship was capable of the service intended, though the R100 did complete a proving flight to Canada and back in 1930. However, on 5 October 1930 the R101, which had not been thoroughly tested after major modifications, crashed on its maiden voyage at Beauvais in France killing 48 of the 54 people aboard. Among the dead were the craft's chief designer and the Secretary of State for Air. The disaster put an end to furher British airship development.
Meanwhile Germany was building the Graf Zeppelin (LZ 127), the largest airship that could be built in the company's existing shed, and intended to stimulate interest in passenger airships. The Graf Zeppelin burned blau gas, similar to propane, stored in large gas bags below the hydrogen cells, as fuel. Since its density was similar to that of air, it avoided the weight change as fuel was used, and thus the need to valve hydrogen. The Graf Zeppelin was a great success and had an impressive safety record, flying over 1,600,000 km (990,000 mi) (including the first circumnavigation of the globe by air) without a single passenger injury.
The US Navy experimented with the use of airships as airborne aircraft carriers, developing an idea pioneered by the British. The USS Los Angeles was used for initial experiments, and the USS Akron and Macon, the world's largest at the time, were used to test the principle in naval operations. Each carried four F9C Sparrowhawk fighters in its hangar, and could carry a fifth on the trapeze. The idea had mixed results. By the time the Navy started to develop a sound doctrine for using the ZRS-type airships, the last of the two built, USS Macon, had been lost. The seaplane had become more capable, and was considered a better investment.
Eventually the US Navy lost all three American-built rigid airships to accidents. USS Shenandoah flew into a severe thunderstorm over Noble County, Ohio while on a poorly planned publicity flight on 3 September 1925. It broke into pieces, killing 14 of its crew. USS Akron was caught in a severe storm and flown into the surface of the sea off the shore of New Jersey on 3 April 1933. It carried no life boats and few life vests, so 73 of its crew of 76 died from drowning or hypothermia. USS Macon was lost after suffering a structural failure offshore near Point Sur Lighthouse on 12 February 1935. The failure caused a loss of gas, which was made much worse when the aircraft was driven over pressure height causing it to lose too much helium to maintain flight. Only 2 of its crew of 83 died in the crash thanks to the inclusion of life jackets and inflatable rafts after the Akron disaster.
The Empire State Building was completed in 1931 with a dirigible mast, in anticipation of passenger airship service. Various entrepreneurs experimented with commuting and shipping freight via airship.
In the 1930s the German Zeppelins successfully competed with other means of transport. They could carry significantly more passengers than other contemporary aircraft while providing amenities similar to those on ocean liners, such as private cabins, observation decks, and dining rooms. Less importantly, the technology was potentially more energy-efficient than heavier-than-air designs. Zeppelins were also faster than ocean liners. On the other hand, operating airships was quite involved. Often the crew would outnumber passengers, and on the ground large teams were necessary to assist mooring and very large hangars were required at airports.
By the mid-1930s only Germany still pursued airship development. The Zeppelin company continued to operate the Graf Zeppelin on passenger service between Frankfurt and Recife in Brazil, taking 68 hours. Even with the small Graf Zeppelin, the operation was almost profitable. In the mid-1930s work started to build an airship designed specifically to operate a passenger service across the Atlantic. The Hindenburg (LZ 129) completed a very successful 1936 season carrying passengers between Lakehurst, New Jersey and Germany. 1937 started with the most spectacular and widely remembered airship accident. Approaching the mooring mast minutes before landing on 6 May 1937, the Hindenburg burst into flames and crashed. Of the 97 people aboard, 36 died: 13 passengers, 22 aircrew, and one American ground-crewman. The disaster happened before a large crowd, was filmed and a radio news reporter was recording the arrival. This was a disaster which theater goers could see and hear the next day. The Hindenburg disaster shattered public confidence in airships, and brought a definitive end to the "golden age". The day after Hindenburg crashed, the Graf Zeppelin landed at the end of its flight from Brazil, ending intercontinental passenger airship travel.
Hindenburg's sister ship, the Graf Zeppelin II (LZ 130), could not perform commercial passenger flights without helium, which the United States refused to sell. The Graf Zeppelin flew some test flights and conducted electronic espionage until 1939 when it was grounded due to the start of the war. The last two Zeppelins were scrapped in 1940.
Development of airships continued only in the United States, and to a smaller extent, the Soviet Union. The Soviet Union had several semirigid and nonrigid airships. The semirigid dirigible SSSR-V6 OSOAVIAKhIM was among the largest of these craft, and it set the longest endurance flight at the time of over 130 hours. However, it crashed into a mountain in 1938, killing 13 of the 19 people on board. While this was a severe blow to the Soviet airship program, they continued to operate nonrigid airships until 1950.
World War II
While Germany determined that airships were obsolete for military purposes in the coming war and concentrated on the development of aeroplanes, the United States pursued a program of military airship construction even though it had not developed a clear military doctrine for airship use. When the Japanese attacked Pearl Harbor on 7 December 1941, bringing the United States into World War II, the U.S. Navy had 10 nonrigid airships:
- 4 K-class: K-2, K-3, K-4 and K-5 designed as patrol ships built from 1938.
- 3 L-class: L-1, L-2 and L-3 as small training ships, produced from 1938.
- 1 G-class built in 1936 for training.
- 2 TC-class that were older patrol ships designed for land forces, built in 1933. The US Navy acquired them from the United States Army in 1938.
Only K- and TC-class airships were suitable for combat and they were quickly pressed into service against Japanese and German submarines which were then sinking American shipping within visual range of the American coast. U.S. Navy command, remembering airship's anti-submarine success in World War I, immediately requested new modern antisubmarine airships and on 2 January 1942 formed the ZP-12 patrol unit based in Lakehurst from the four K airships. The ZP-32 patrol unit was formed from two TC and two L airships a month later, based at NAS Moffett Field in Sunnyvale, California. An airship training base was created there as well. The status of submarine-hunting Goodyear airships in the early days of World War II has created significant confusion. Although various accounts refer to airships Resolute and Volunteer as operating as "privateers" under a Letter of Marque, Congress never authorized a commission, nor did the President sign one.
In the years 1942–44, approximately 1,400 airship pilots and 3,000 support crew members were trained in the military airship crew training program and the airship military personnel grew from 430 to 12,400. The U.S. airships were produced by the Goodyear factory in Akron, Ohio. From 1942 till 1945, 154 airships were built for the U.S. Navy (133 K-class, 10 L-class, seven G-class, four M-class) and five L-class for civilian customers (serial numbers L-4 to L-8).
The primary airship tasks were patrol and convoy escort near the American coastline. They also served as an organization centre for the convoys to direct ship movements, and were used in naval search and rescue operations. Rarer duties of the airships included aerophoto reconnaissance, naval mine-laying and mine-sweeping, parachute unit transport and deployment, cargo and personnel transportation. They were deemed quite successful in their duties with the highest combat readiness factor in the entire US air force (87%).
During the war some 532 ships without airship escort were sunk near the US coast by enemy submarines. Only one ship, the tanker Persephone, of the 89,000 or so in convoys escorted by blimps was sunk by the enemy. Airships engaged submarines with depth charges and, less frequently, with other on-board weapons. They were excellent at driving submarines down, where their limited speed and range prevented them from attacking convoys. The weapons available to airships were so limited that until the advent of the homing torpedo they had little chance of sinking a submarine.
Only one airship was ever destroyed by U-boat: on the night of 18/19 July 1943, a K-class airship (K-74) from ZP-21 division was patrolling the coastline near Florida. Using radar, the airship located a surfaced German submarine. The K-74 made her attack run but the U-boat opened fire first. K-74's depth charges did not release as she crossed the U-boat and the K-74 received serious damage, losing gas pressure and an engine but landing in the water without loss of life. The crew was rescued by patrol boats in the morning, but one crewman, Aviation Machinist's Mate Second Class Isadore Stessel, died from a shark attack. The U-Boat, submarine U-134, was slightly damaged and the next day or so was attacked by aircraft, sustaining damage that forced it to return to base. It was finally sunk on 24 August 1943 by a British Vickers Wellington near Vigo, Spain.
Some US airships saw action in the European war theater. In 1944-45, the U.S. Navy moved an entire squadron of eight Goodyear K class blimps (K-123, K-130, K-109, K-134, K-101, K-112, K-89, & K-114) with flight and maintenance crews from Weeksville Naval Air Station in North Carolina to Naval Air Station Port Lyautey, French Morocco. Their mission was to locate and destroy German U-boats in the relatively shallow waters around the Strait of Gibraltar where magnetic anomaly detection (MAD) was viable. PBY aircraft had been searching these waters but MAD required low altitude flying that was dangerous at night for these aircraft. The blimps were considered a perfect solution to establish a 24/7 MAD barrier (fence) at the Straits of Gibraltar with the PBYs flying the day shift and the blimps flying the night shift. The first two blimps (K-123 & K-130) left South Weymouth NAS on 28 May 1944 and flew to Argentia, Newfoundland, the Azores, and finally to Port Lyautey where they completed the first transatlantic crossing by nonrigid airships on 1 June 1944. The blimps of USN Blimp Squadron ZP-14 (Blimpron 14, aka The Africa Squadron) also conducted mine-spotting and mine-sweeping operations in key Mediterranean ports and various escorts including the convoy carrying United States President Franklin D. Roosevelt and British Prime Minister Winston Churchill to the Yalta Conference in 1945. Airships from the ZP-12 unit took part in the sinking of the last U-Boat before German capitulation, sinking U-881 on 6 May 1945 together with destroyers Atherton and Mobery.
Other airships patrolled the Caribbean, Fleet Airship Wing Two, Headquartered at NAS Richmond, Florida, covered the Gulf of Mexico from Richmond and Key West, FL, Houma, Louisiana, as well as Hitchcock and Brownsville, Texas. FAW 2 also patrolled the northern Caribbean from San Julian,[clarification needed] the Isle of Pines (now called Isla de la Juventud) and Guantanamo Bay, Cuba as well as Vernam Field, Jamaica.
Navy blimps of Fleet Airship Wing Five, (ZP-51) operated from bases in Trinidad, British Guiana and Paramaribo, Suriname. Fleet Airship Wing Four operated along the coast of Brazil. Two squadrons, VP-41 and VP-42 flew from bases at Amapá, Igarape Assu, Sao Luiz,[disambiguation needed] Fortaleza, Fernando de Noronha, Recife, Maceió, Ipitanga (near Salvador, Bahia), Caravellas, Vitoria and the hangar built for the Graf Zeppelin at Santa Cruz, Rio de Janeiro.
Fleet Airship Wing Three operated squadrons, ZP-32 from Moffett Field, ZP-31 at NAS Santa Ana, and ZP-33 at NAS Tillamook, Oregon. Auxiliary fields were at Del Mar, Lompoc, Watsonville and Eureka, CA, North Bend and Astoria, Oregon, as well as Shelton and Quillayute in Washington.
From 2 January 1942 until the end of war airship operations in the Atlantic, the airships of the Atlantic fleet made 37,554 flights and flew 378,237 hours. Of the over 70,000 ships in convoys protected by blimps, only one was sunk by a submarine while under blimp escort.
The Soviet Union used a single airship during the war. The W-12, built in 1939, entered service in 1942 for paratrooper training and equipment transport. It made 1432 runs with 300 metric tons of cargo until 1945. On 1 February 1945, the Soviets constructed a second airship, a Pobeda-class (Victory-class) unit (used for mine-sweeping and wreckage clearing in the Black Sea) which crashed on 21 January 1947. Another W-class - W-12bis Patriot - was commissioned in 1947 and was mostly used for crew training, parades and propaganda.
The world's largest thermal airship (300,000 cubic feet/8,495 m³) was constructed by the Per Lindstrand company for French botanists in 1993. The AS-300 carried an under-slung raft, which was positioned by the airship on top of tree canopies in the rain forest, allowing the botanists to carry out their treetop research without significant damage to the rainforest. When research was finished at a given location, the airship returned to pick up and relocate the raft.
In the spring of 2004, Lindstrand Technologies supplied the world's first fully functional unmanned airship to the Ministry of Defense in Spain. This airship carried a 42 kilograms (93 lb) classified payload and its surveillance mission was also classified. Four years later, this airship, which is designated GA-22, still flies on an almost daily basis.
In June 1987, the US Navy awarded a US$168.9 million contract to Westinghouse Electric and Airship Industries of the UK to demonstrate whether a blimp could be used as an airborne platform to detect the threat of sea-skimming missiles, such as the Exocet. At 2.5 million cubic feet, the Westinghouse/Airship Industries Sentinel 5000 (Redesignated YEZ-2A by the U. S. Navy) prototype design was to have been the largest blimp ever constructed. However, additional funding for the Naval Airship Program was killed in 1995 and development was discontinued.
The CA-80 airship, which was launched in 2000 by Shanghai Vantage Airship Manufacture Co., Ltd., had a successful trial flight in September 2001. This model of airship was designed for the purpose of advertisement and propagation, air-photo, scientific test, tour and surveillance duties. It was certified as a grade-A Hi-Tech introduction program (№ 20000186) in Shanghai, China. The CAAC authority granted a type design approval and certificate of airworthiness for the model CA-80 airship, which has been published in the Jane's All the World's Aircraft for five times (2003–08).
In the 1990s, the Zeppelin company reentered the airship business. Their new model, designated the Zeppelin NT, made its maiden flight on 18 September 1997. As of 2009, there were four NT aircraft flying, a fifth completed in March 2009 and an expanded NT-14 (14,000 cubic meters of helium, capable of carrying 19 passengers) under construction. One was sold to a Japanese company, and was planned to be flown to Japan in the summer of 2004. Due to delays getting permission from the Russian government, the company decided to transport the airship to Japan by ship. One of the four NT craft is in South Africa carrying diamond detection equipment from De Beers, an application at which the very stable low vibration NT platform excels. The project included design adaptations for high heat operation and desert climate, as well as a separate mooring mast and a very heavy mooring truck. NT-4 belongs to Airship Ventures of Moffett Field, Mountain View in the San Francisco Bay Area, and provides sight-seeing tours.
Blimps are used for advertising and as TV camera platforms at major sporting events. The most iconic of these are the Goodyear Blimps. Goodyear operates three blimps in the United States, and The Lightship Group operates up to 19 advertising blimps around the world. Airship Management Services owns and operates three Skyship 600 blimps. Two operate as advertising and security ships in North America and the Caribbean. Airship Ventures operates a Zeppelin NT for advertising, passenger service and special mission projects. They are the only airship operator in the U.S. authorized to fly commercial passengers.
Skycruise Switzerland AG owns and operates two Skyship 600 blimps. One operates regularly over Switzerland used on sightseeing tours. The Switzerland-based Skyship 600 has also played other roles over the years. For example, it was flown over Athens during the 2004 Summer Olympics as a security measure. In November 2006, it carried advertising calling it The Spirit of Dubai as it began a publicity tour from London to Dubai, UAE on behalf of The Palm Islands, the world's largest man-made islands created as a residential complex.
In May 2006, the US Navy began to fly airships again after a hiatus of nearly 44 years. The program uses a single American Blimp Company A-170 nonrigid airship, with designation MZ-3A. Operations focus on crew training and research, and the platform integrator is Northrop Grumman. The program is directed by the Naval Air Systems Command and is being carried out at NAES Lakehurst, the original centre of U.S. Navy lighter-than-air operations in previous decades.
In November 2006, the U.S. Army bought an A380+ airship from American Blimp Corporation through a Systems level contract with Northrop Grumman and Booz Allen Hamilton. The airship started flight tests in late 2007, with a primary goal of carrying 2,500 lb (1,100 kg) of payload to an altitude of 15,000 ft (4,600 m) under remote control and autonomous waypoint navigation. The program will also demonstrate carrying 1,000 lb (450 kg) of payload to 20,000 ft (6,100 m) The platform could be used for Multi-Intelligence collections. In 2008, the CA-150 airship was launched by Vantage Airship. This is an improved modification of model CA-120 and completed manufacturing in 2008. With larger volume and increased passenger capacity, it is the largest manned nonrigid airship in China at present.
Hybrid designs such as the Heli-Stat airship/helicopter, the Aereon aerostatic/aerodynamic craft, and the CycloCrane (a hybrid aerostatic/rotorcraft), struggled to take flight. The Cyclocrane was also interesting in that the airship's envelope rotated along its longitudinal axis.
In 2005, a short-lived project of the US Defense Advanced Research Projects Agency (DARPA) was Walrus HULA which explored the potential for using airships as long-distance, heavy lift craft. The primary goal of the research program was to determine the feasibility of building an airship capable of carrying 500 short tons (450 t) of payload a distance of 12,000 mi (19,000 km) and land on an unimproved location without the use of external ballast or ground equipment (such as masts). In 2005, two contractors, Lockheed Martin and US Aeros Airships were each awarded approximately $3 million to do feasibility studies of designs for WALRUS. Congress removed funding for Walrus HULA in 2006.
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In 2010, the US Army awarded a $517 million (£350.6 million) contract to Northrop Grumman and partner Hybrid Air Vehicles, to develop a Long Endurance Multi-Intelligence Vehicle (LEMV) system, in the form of three HAV 304's. The project was cancelled in February 2012, due to it being behind schedule and over budget; also the forthcoming US withdrawal from Afghanistan where it was intended to be deployed.
A-NSE, a French company, manufactures and operates airships and aerostats. For 2 years, A-NSE has been testing its airships for the French Army. Airships and aerostats are operated to provide intelligence, surveillance, and reconnaissance (ISR) support. Many innovations are developed by A-NSE : water ballast take-off and landing system, variable geometry envelope, thrust–vectoring system etc.…
The US government has funded two major projects in the high altitude arena. The Composite Hull High Altitude Powered Platform (CHHAPP) is sponsored by US Army Space and Missile Defense Command. This aircraft is also sometimes called HiSentinel High-Altitude Airship. This prototype ship made a five-hour test flight in September 2005. The second project, the high-altitude airship (HAA), is sponsored by DARPA. In 2005, DARPA awarded a contract for nearly $150 million to Lockheed Martin for prototype development. First flight of the HAA was planned for 2008 but suffered programmatic and funding delays. The HAA project evolved into the High Altitude Long Endurance-Demonstrator (HALE-D). The U.S. Army and Lockheed Martin launched the first-of-its kind HALE-D on July 27, 2011. After attaining an altitude of 32,000 ft (9,800 m), due to an anomaly, the company decided to abort the mission. The airship made a controlled descent in an unpopulated area of southwest Pennsylvania. 
On 31 January 2006 Lockheed Martin made the first flight of their secretly built hybrid airship designated the P-791. The design is very similar to the SkyCat, unsuccessfully promoted for many years by the British company Advanced Technologies Group (ATG). Although Lockheed Martin is developing a design for the DARPA WALRUS HULA project, it claimed that the P-791 is unrelated to WALRUS. Nonetheless, the design represents an approach that may well be applicable to WALRUS. Some believe that Lockheed Martin had used the secret P-791 program as a way to get a head start on the other WALRUS competitor, US Aeros Airships.
In the 1990s, the successor of the original Zeppelin company in Friedrichshafen, the Zeppelin Luftschifftechnik GmbH, reengaged in airship construction. The first experimental craft (later christened Friedrichshafen) of the type ″Zeppelin NT″ flew in September 1997. Though larger than common blimps, the Neue Technologie (New Technology) zeppelins are much smaller than their giant ancestors and not actually Zeppelin-types in the classical sense. They are sophisticated semirigids. Apart from the greater payload, their main advantages compared to blimps are higher speed and excellent maneuverability. Meanwhile, several Zeppelin NT have been produced and operated profitably in joyrides, research flights and similar applications.
In June 2004, a Zeppelin NT was sold for the first time to a Japanese company, Nippon Airship Corporation, for tourism and advertising mainly around Tokyo. It was also given a role at the 2005 Expo in Aichi. The aircraft began a flight from Friedrichshafen to Japan, stopping at Geneva, Paris, Rotterdam, Munich, Berlin, Stockholm and other European cities to carry passengers on short legs of the flight. However, Russian authorities denied overflight permission so the airship had to be dismantled and shipped to Japan rather than following the historic Graf Zeppelin flight from Germany to Japan.
In 2008, Airship Ventures Inc. began operations from Moffett Federal Airfield near Mountain View, California and until November 2012 offered tours of the San Francisco Bay Area for up to 12 passengers.
In November 2005, De Beers, the diamond mining company, launched an airship exploration program over the remote Kalahari desert. A Zeppelin, loaded with high-tech equipment, is used to find potential diamond mines by scanning the local geography for low-density rock formations - so-called kimberlite pipes. On 21 September 2007, the airship was severely damaged by a whirlwind while in Botswana. One crew member, who was on watch aboard the moored craft, was slightly injured but released after overnight observation in hospital.
Several companies, such as Cameron Balloons in Bristol, United Kingdom, build hot-air airships. These combine the structures of both hot-air balloons and small airships. The envelope is the normal cigar shape, complete with tail fins, but is inflated with hot air instead of helium to provide the lifting force. A small gondola, carrying the pilot and passengers, a small engine, and the burners to provide the hot air are suspended below the envelope, beneath an opening through which the burners protrude.
Hot-air airships typically cost less to buy and maintain than modern helium-based blimps, and can be quickly deflated after flights. This makes them easy to carry in trailers or trucks and inexpensive to store. They are usually very slow moving, with a typical top speed of 25–30 km/h (15–20 mph, 6.7–8.9 m/s). They are mainly used for advertising, but at least one has been used in rainforests for wildlife observation, as they can be easily transported to remote areas.
Remote-controlled (RC) airships, a type of unmanned aerial system (UAS), are sometimes used for commercial purposes such as advertising and aerial video and photography as well as recreational purposes. They are particularly common as an advertising mechanism at indoor stadiums. While RC airships are sometimes flown outdoors, doing so for commercial purposes is illegal in the US. In particular, Docket FAA-2006-25714 states that: "The FAA recognizes that people and companies other than modelers might be flying UAS with the mistaken understanding that they are legally operating under the authority of AC 91-57. AC 91-57 only applies to modelers, and thus specifically excludes its use by persons or companies for business purposes." The same docket item identifies 14CFR121 as the appropriate certification basis for experimental unmanned aircraft, which would included unmanned airships operating for commercial purposes, so commercial use of an unmanned airship is not prohibited - instead it must be certified under part 121 not 91.
Current design projects
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Today, with large, fast, and more cost-efficient fixed-wing aircraft, it is unknown whether huge airships can operate profitably in regular passenger transport though, as energy costs rise, attention is once again returning to these lighter than air vessels as a possible alternative. At the very least, the idea of comparatively slow, "majestic" cruising at relatively low altitudes and in comfortable atmosphere certainly has retained some appeal. There have been some niches for airships in and after World War II, such as long-duration observations, antisubmarine patrol, platforms for TV camera crews, and advertising; these, however, generally require only small and flexible craft, and have thus generally been better fitted for cheaper blimps.
It has periodically been suggested that airships could be employed for cargo transport, especially delivering extremely heavy loads to areas with poor infrastructure over great distances. This has also been called roadless trucking. Also, airships could be used for heavy lifting over short distances (e.g. on construction sites); this is described as heavy-lift, short-haul. In both cases, the airships are heavy haulers. One recent enterprise of this sort was the Cargolifter project, in which a hybrid (thus not entirely Zeppelin-type) airship even larger than Hindenburg was projected. Around 2000, CargoLifter AG built the world's largest cantilever shop hall measuring 360 m (1,180 ft) long, 210 m (690 ft) wide and 107 m (351 ft) high about 60 km (37 mi) south of Berlin. In May 2002, the project was stopped for financial reasons; the company had to file bankruptcy. The enormous CargoLifter hangar was later converted to house the Tropical Islands Resort. Although no rigid airships are currently used for heavy lifting, hybrid airships are being developed for such purposes. John McPhee's The Deltoid Pumpkin Seed is the story of one company attempting this.
A metal-clad airship has a very thin metal envelope, rather than the usual fabric. The shell may be either internally braced or monocoque as in the ZMC-2 which flew many times in the 1920s, the only example ever to do so. The shell may be gas-tight as in a non-rigid blimp, or the design may employ internal gas bags as in a rigid airship. Compared to a fabric envelope the metal cladding is expected to be more durable.
A hybrid airship is a general term for an aircraft that combines characteristics of heavier-than-air (aeroplane or helicopter) and lighter-than-air technology. Examples include helicopter/airship hybrids intended for heavy lift applications and dynamic lift airships intended for long-range cruising. It should be noted that most airships, when fully loaded with cargo and fuel, are usually ballasted to be heavier than air, and thus must use their propulsion system and shape to create aerodynamic lift, necessary to stay aloft. All airships can be operated to be slightly heavier than air at periods during flight (descent). However, the term "hybrid airship" refers to craft that obtain a significant portion of their lift from aerodynamic lift or other kinetic means.
For example, the Aeroscraft is a buoyancy assisted air vehicle that generates lift through a combination of aerodynamics, thrust vectoring and gas buoyancy generation and management, and for much of the time will fly heavier than air. Aeroscraft is Worldwide Aeros Corporation's continuation of DARPA's now cancelled Walrus HULA (Hybrid Ultra Large Aircraft) project.
Comparison with heavier-than-air aircraft
The advantage of airships over aeroplanes is that static lift sufficient for flight is generated by the lifting gas and requires no engine power. This was an immense advantage before the middle of World War I and remained an advantage for long distance, or long duration operations until World War II. Modern concepts for high altitude airships include photovoltaic cells to reduce the need to land to refuel, thus they can remain in the air until consumables expire.
The disadvantages are that an airship has a very large reference area and comparatively large drag coefficient, thus a larger drag force compared to that of aeroplanes and even helicopters. Given the large frontal area and wetted surface of an airship, a practical limit is reached around 130–160 kilometres per hour (80–100 mph). Thus airships are used where speed is not critical.
The gross lift capability of an airship is equal to the buoyant force minus the weight of the airship. This assumes standard air temperature and pressure conditions. Corrections are usually made for water vapor and impurity of lifting gas, as well as percentage of inflation of the gas cells at liftoff. Based on specific lift (lifting force per unit volume of gas), the greatest static lift is provided by hydrogen (11.15 N/m3 or 71 lbf/1000 cu ft) with helium (10.37 N/m3 or 66 lbf/1000 cu ft) a close second. At 6.13 N/m3 (39 lbf/1000 cu ft), steam is a distant third. Other cheap gases, such as methane, carbon monoxide, ammonia and natural gas have even less lifting capacity and are flammable, toxic, corrosive, or all three (neon is even more costly than helium and merely has less lifting capacity). Operational considerations such as whether the lift gas can be economically vented and produced in flight for control of buoyancy (as with hydrogen) or even produced as a byproduct of propulsion (as with steam) affect the practical choice of lift gas in airship designs.
In addition to static lift, an airship can obtain a certain amount of dynamic lift from its engines. Dynamic lift in past airships has been about 10% of the static lift. Dynamic lift allows an airship to "take off heavy" from a runway similar to fixed-wing and rotary-wing aircraft. However, this requires additional weight in engines, fuel and landing gear, negating some of the static lift capacity.
The altitude at which an airship can fly largely depends on how much lifting gas it can lose due to expansion before stasis is reached. The ultimate altitude record for a rigid airship was set in 1917 by the L-55 under the command of Hans-Kurt Flemming when he forced the airship to 7,300 m (24,000 ft) attempting to cross France after the "Silent Raid" on London. The L-55 lost lift during the descent to lower altitudes over Germany and crashed due to loss of lift. While such waste of gas was necessary for the survival of airships in the later years of World War I, it was impractical for commercial operations, or operations of helium-filled military airships. The highest flight made by a hydrogen-filled passenger airship was 1,700 m (5,500 ft) on the Graf Zeppelin's around the world flight. The practical limit for rigid airships was about 900 m (3,000 ft), and for pressure airships around 2,400 m (8,000 ft).
Modern airships use dynamic helium volume. At sea level altitude, helium only takes up a small part of the hull, while the rest is filled with air. As the airship ascends, the helium inflates with reduced outer pressure, and air is pushed out and released from the downward valve. This allows an airship to reach any altitude with balanced inner and outer pressure if the buoyancy is enough. Some civil aerostats could reach 100,000 ft (30,000 m) without explosion due to overloaded inner pressure.
The greatest disadvantage of the airship is size, which is essential to increasing performance. As size increases, the problems of ground handling increase geometrically. As the German Navy changed from the P class of 1915 with a volume of over 31,000 m3 (1,100,000 cu ft) to the larger Q class of 1916, the R class of 1917, and finally the W class of 1918, at almost 62,000 m3 (2,200,000 cu ft) ground handling problems reduced the number of days the Zeppelins were able to make patrol flights. This availability declined from 34% in 1915, to 24.3% in 1916 and finally 17.5% in 1918.
So long as the power-to-weight ratios of aircraft engines remained low and specific fuel consumption high, the airship had an edge for long range or duration operations. As those figures changed, the balance shifted rapidly in the aeroplane's favour. By mid-1917, the airship could no longer survive in a combat situation where the threat was aeroplanes. By the late 1930s, the airship barely had an advantage over the aeroplane on intercontinental over-water flights, and that advantage had vanished by the end of World War II.
This is in face-to-face tactical situations. Currently, a High-altitude airship project is planned to survey hundreds of kilometres as their operation radius, often much farther than the normal engagement range of a military aeroplane.[clarification needed] For example, a radar mounted on a vessel platform 30 m (100 ft) high has radio horizon at 20 km (12 mi) range, while a radar at 18,000 m (59,000 ft) altitude has radio horizon at 480 km (300 mi) range. This is significantly important for detecting low-flying cruise missiles or fighter-bombers.
Today, airships are used primarily for command, control and as a communication platform; to establish and maintain reliable and secure connectivity among all forces, provide transparent data across the echelons; precisely locate friendly and enemy forces; detect targets on an extended battlefield at a minimal exposure to enemy forces; real time targeting; navigation assistance; battle management; monitor radio conversations, etc.
The most commonly used lifting gas, helium, is inert so presents no fire risk. Modern airships have a natural buoyancy and special design that offers a virtually zero catastrophic failure mode. A series of vulnerability tests were done by the UK Defence Evaluation and Research Agency DERA on a Skyship 600. Since the internal gas pressure was maintained at only 1–2% above the surrounding air pressure, the vehicle proved highly tolerant to physical damage or to attack by small-arms fire or missiles. Several hundred high-velocity bullets were fired through the hull, and even two hours later the vehicle would have been able to return to base. Ordnance passed through the envelope without causing critical helium loss. In all instances of light armament fire evaluated under both test and live conditions, the airship was able to complete its mission and return to base.
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