What is the largest man-made object ever to have flown? Is it the Boeing 747 Jumbo? The Hindenburg - a German airship from the 1930s? Or the new 500-seat Airbus 380?
The answer is the Hindenburg - a lighter than air vehicle. It was colossal, to an extent that's difficult to imagine today, being comparable in overall size to the Titanic (it was just 24 metres shorter, at 245m). However, weight was a different matter, with the Titanic weighing in at 46,000 tons, and the Hindenburg at just 220 tons. The airship's overwhelming presence was enhanced by the stately nature of its progress. By comparison with the modern passenger jet that passes by at high speed, just a speck in the sky, Hindenburg cruised at 120kmh, 460m up, and people rushed outside to see.
With a possible non-stop range of 15,000km, transatlantic crossings were easily possible and in 1936 the ship established a regular service from Frankfurt. Ten round trips - two to three days each way - were made in 1936 to Lakehurst, New Jersey (for New York) and six to Rio de Janeiro. About 100 passengers paid $400 one way to travel in great luxury, with sleeping cabins, a promenade deck, a separate dining saloon and - amazingly - a smoking room.
By contrast, it would be the late 1930s before ordinary aeroplanes were capable of a regular transatlantic service. The story of the great airships is almost forgotten now, and yet there was a time, in the early 1930s, when they seemed to embody the future of long distance luxury travel. Airships grew from early experiments with hydrogen filled lighter than air balloons that dated back into the 18th century. By the mid 19th century it seemed clear to the pioneers that if you could add power to your balloon, and a means of steering it, then you had a viable means of transport. In 1852, more than half a century before the Wright Brothers got off the ground, Henry Giffard, in France, attached a steam engine and a propeller to an elongated balloon and managed a 25km journey in about three hours.
The German Zeppelin
Real progress came around the turn of the 20th century, when Ferdinand Zeppelin in Germany developed the first rigid-framed airship - a cigar shaped aluminium skeleton covered with fabric and filled with lifting gasbags. An airship floats because, according to the principle of buoyancy first defined by Archimedes, it weighs less than the air whose space it occupies. The gasbags provide enough lift to ensure that the ship - metal frame, engines, fabric covering, and all its passengers and cargo - float in the air. Forward motion is provided by engines carried outside the main structure. Zeppelin built a series of airships for the German armed forces, and his name became synonymous with them - they were just "Zeppelins", well known in Britain when they came over on bombing raids in the First World War (50 raids were made on London alone). After the war, thoughts turned to the development of passenger-carrying airships and the great airship era began. In early airships, crew and any passengers travelled in a "gondola"
hanging beneath the hull. Later ships were big enough to have one or more passenger decks inside the bottom part of the hull (alhough one unbuilt design also had a passenger lounge sitting on the very top of the hull, incorporating an open air "shelter deck" shielded by a windbreak.) Clearly, a commercial airship had to be big. It needed to carry about 100 passengers to pay its way. This, in turn, meant a sizeable crew. Then there were the engines - usually several, hanging from the hull - and all the fuel for the immense journeys which were envisaged. Zeppelin built a series of ships of increasing size before the Hindenburg. Britain, too, established an airship industry, as did the US. As the ships grew in size and luxury through the 1920s and 1930s, there was a real belief that here was the solution to safe and rapid long distance travel. The British Government, for example, was deeply interested in the idea that airships could link the Empire together, and through the 1920s invested heavily in their development, aiming to establish a service between Britain and India at a time when ordinary aeroplanes had limited range and carrying capacity.
Maintaining links to India was a government preoccupation in the late 19th and early 20th centuries (hence the jubilation at Disraeli's 1875 purchase of a controlling interest in the Suez Canal). The development of the airship seemed to offer a way of ferrying key people and documents in quantity at high speed.
So in 1924, the Imperial Airship Scheme was born. Two ships were commissioned, of similar size - about 235m in overall length. One, designated R100, was contracted to a private firm, a subsidiary of Vickers, at Howden in Yorkshire. The other, the R101, was to be built at the government's Royal Airship Works at Cardington in Bedfordshire. When the R101 first flew, in October 1929, it was the biggest flying machine ever.
With the slightly smaller R100, it was designed to fly a round trip to Karachi - then in India (Pakistan didn't exist) taking five days out and six days back against the prevailing Westerlies (at that time, Imperial Airways' aeroplane service, with overnight stops, took eight days one way.
The quickest ships took four weeks each way).
The end of the dream
The vision - a fleet of airships ghosting through the sky across Europe and into the sunrise over India - was not to be. Within 10 years of the R101's first flight, and three years after the Nazis were proudly celebrating the success of the Hindenburg, airships were finished, those that remained broken up for scrap.
On the face of it, airship passengers could feel very safe. On landing, for instance, the ship slowed to a crawl, mooring ropes were dropped, and gangs of men physically manhandled the ship up to the mooring mast to which the nose of the ship was fastened. Departure was accomplished in the same careful way. To many passengers it all seemed preferable to the speed and drama of aeroplane travel, and there was the added bonus of ocean liner style comforts.
However, in truth they were always fatally flawed, and reality arrived with a vengeance first to the British Government and then to Hitler's. In October 1930 the British Government was keen to see a pioneering high-profile airship flight to Karachi, and the R101 was put to the task, carrying 50 or so of the great and the good, including the Air Minister, Lord Thompson. The ship left its mooring mast at Cardington in the evening of Saturday October 4: a maiden voyage made after a single trial trip of less than the scheduled 24 hours. By midnight, R101 was over Northern France travelling at 55kmh, just 460m above the ground. It was raining heavily, and the ship gradually lost height - the assumption is that rain, coming in through a tear in the fabric, damaged the forward gasbags.
Travelling very slowly, it struck rising ground near Beauvais. It was more of a landing than a crash, but a hot engine ignited leaking hydrogen and in the blaze that followed, 48 passengers and crew died. There were eight survivors. The R100 never flew in service and was broken up. The Government - and the public - had lost their appetite for airship travel.
The German Government, however, urged on by the Zeppelin company, did not immediately lose faith. The Hindenburg was built and put into successful transatlantic service. It flew uneventfully through 1936, and in May 1937 it made the first trip of the new season from Frankfurt to Lakehurst with 97 passengers. In the afternoon of May 6 the ship dawdled over Manhattan, providing spectacular views both from the ship and of the ship from the ground.
Then at 7.25pm she was creeping to her mast at Lakehurst, about 60m above the ground, when a flame was spotted at the tail end. Within seconds the ship was a mass of flames, and in just over half a minute she was a pile of hot wreckage on the ground. Astonishingly, 62 people survived, mostly by running away through the burning hull as the ship settled, though many were badly burned. It was assumed that leaking hydrogen was ignited by a spark, although conspiracy theories and alternatives still surface. Neither the R101 nor the Hindenburg would have burned had they been filled with non-inflammable helium gas (Hindenburg was built for helium, but the US controlled supplies and they weren't forthcoming).
There were, however, other problems that made airships risky, largely connected to their lightness and fragility in the face of bad weather - the US Navy's two big helium ships Akron and Macon were both destroyed by the weather. The coming of the Second World War, and the rapid development of passenger aeroplanes afterwards, put an end to further argument, and the passenger airship died with the Hindenburg.
A modern revival?
Lighter than air travel today is confined to hot air balloons for sport and leisure, and small non-rigid "blimps" for advertising. That said, there have been many attempts in modern times to revive the idea of the airship as an efficient heavy cargo lifter.
Modern materials for the frame, helium filling and up-to-date engines and controls, it's argued, make this a viable proposition. The case made by Millennium Airships for their proposed Skyfreighter - able to lift 500 tons and fly over 3,700km at 150kmh, with no need for a runway - is compelling.
It's clear that investors aren't always convinced. The Advanced Technologies Group based in Cardington, with plans for high-tech British airships (a small prototype flew extensively) went into administration in July 2005. No doubt there'll be many other attempts to keep the dream alive, and maybe one of those will succeed.
How does it fly?
An airship floats in the air because it weighs less - strictly, has less mass - than the lump of air whose space it occupies. It invokes the principle of buoyancy (the buoyant force acting on an object immersed in a fluid is equal to the weight of the displaced fluid) defined by Archimedes in about 250 bc.
There are three ways by which a container can theoretically be made lighter than the air which it displaces and so be capable of floating freely upwards when you let it go.
First, you can pump all the air out of it. A vacuum has no mass at all and so is potentially the best possible lifting medium. The Jesuit priest Francesco de Lana de Terzi proposed, and drew, in 1670, a ship supported by four copper spheres from which all the air had been exhausted. He didn't realise, though, the fatal flaw of this idea which is that if the spheres are strong enough to withstand the pressure of the atmosphere, they are far too heavy to rise. Some of today's scientific dreamers have continued to speculate on the possibility of ships lifted by vacuum cells, using ultra-strong super-lightweight material that lies far in the future.
Second, fill it with a gas which is lighter than air. This gas must be light enough to lift not only itself, but its container and, to be practical, there must be enough extra lift to bear a load. Only two gases really fit the bill, hydrogen and helium. The latter is very slightly heavier than the former, but it has the overwhelming advantage of being non-inflammable.
Last, fill it with hot air - that's to say air so much hotter than the surrounding atmosphere that it's significantly less dense and so produces useful lift. Bottled gas, feeding burners to keep up the temperature of the air in the balloon, has made hot air ballooning into a relatively cheap and safe way to fly for sport and recreation
What is left today?
Travel south by train along the Midland Mainline and to the east, near Bedford, you can see the twin airship sheds at Cardington where the R100 and R101 were housed. They are immense buildings - a sizeable ocean liner could disappear into one of them.
* In October 1916 a fighter plane piloted by Lieutenant Wulstan Tempest shot down a Zeppelin which crashed in Potters Bar. There is a Tempest Avenue there and a Wulstan Park, but little local memory of the event.
* The crews of four Zeppelins shot down over Britain are buried in the War Cemetery on Cannock Chase in Staffordshire www.fylde.demon.co.ukcannock.htm
* The memorial to the victims of the R101 Disaster is in St Mary's church cemetery in Cardington.
* There's also a roadside memorial near the crash site at Beauvais
Hindenburg disaster www.unmuseum.orghindenburg.htm
The Airship Heritage Trust website has numerous pages of information and history.
The two websites below describe future airship projects www.21stcenturyairships.com
Hydrogen (H) The most abundant element in the universe, hydrogen was discovered by Henry Cavendish in 1776, who called it "inflammable air". It has only 114th the density of air, which means that one kilogram of hydrogen can lift a load of 13kg up through the air. Even when the weight of its container is accounted for, there's still a lot of lifting power available. It is, though, very inflammable. Hydrogen escaping from a balloon or airship gas bag, if mixed with air ignited by a spark, can destroy a whole craft in seconds.
Helium (He) Helium, a little heavier than hydrogen, is the second most abundant element in the universe. It is named for the Greek sun god "Helios", because it was first discovered on the sun. In 1870 the English astronomer Lockyer, using a spectroscope to analyse the light from the sun, identified it as a new element and gave it its name. It wasn't discovered on the Earth for another 30 years. Production of helium was limited until it was discovered in abundance in the natural gas fields of the US. For many years the US was by far the major producer but European countries couldn't easily buy helium, so hydrogen filled airships were used in Europe into the 1930s. Today, helium is produced in a number of gas-producing countries in Europe and North Africa, and it has numerous industrial uses.
1766 Cavendish discovers hydrogen.
1783-1784 Montgolfier brothers fly hot air balloons near Paris - unmanned at first, then with passengers. They believe they have discovered a new gas.
1785 First balloon crossing of the English Channel by Jean Pierre Blanchard and his US co-pilot, John Jefferies.
1852 Henri Giffard builds the first airship - steam powered.
1861 Tethered balloons used for observation in the American Civil War.
Balloons were used in this way until the First World War, when the arrival of fighter aircraft made them too dangerous.
1899 Santos-Dumont builds the first internal combustion engined airship.
1900 Ferdinand Zeppelin builds and flies the first rigid-framed airship.
1915-1916 Zeppelin airship raids on Britain.
1916 Short Brothers Engineering start an airship factory at Cardington, Bedfordshire.
1924 Imperial Airship Service announced. Large airships for travel across the Empire to be built at Cardington.
1928 German Graf Zeppelin demonstrates the practicality of airship travel with a series of long distance flights including one around the world.
1930 Loss of R101 ends the British airship programme.
1933-1935 US Navy airships Akron and Macon destroyed by storms, and US interest in airships wanes.
1936 German Hindenburg in service across the Atlantic.
1937 Loss of Hindenburg virtually closes the airship age.
1940 Remaining German airships broken up for scrap.