Airships : A Second Coming

Mention airship and the image that pops to mind is of a small boy at a party darting around, pin in hand, eliminating all balloons within reach. Could airships be as vulnerable? Not really. A modern airship’s envelope is made of tough synthetic material and contains many independent balloons. A couple of winged warriors armed with spears could attack an airship but fail to inflict critical damage. Slice an airship in half with a giant knife and both portions would probably remain airborne independently.

Phobia about airships is rooted in the dramatic mishaps involving the British R-101, the USS Akron and the German Hindenburg in the 1930s. The last of these caught fire while attempting to land in front of thousands of spectators, killing 35 of the passengers and crew aboard. What few remember is that there were 62 survivors. Contrast this with the thousands who have died in air travel over the years—sometimes a few hundred in a single crash, and no survivors. Yet, in public perception, airliners are not unsafe. Most airships of the 1930s were filled with hydrogen—a highly inflammable gas—and fashioned out of flammable materials. Modern airships are made of flame-resistant materials and contain helium, one of the most inert gases in nature.

A more rational apprehension is bad weather. Airships, especially those with rigid structures, are liable to be twisted apart by the severe wind shear experienced in a violent storm. Some of the most spectacular airship crashes of the last century were due to bad weather. However, with modern weather forecasting, radar and satellite communications, airships can avoid threatening storms. A modern airship is usually a poor target for lightning strikes since it is constructed mainly from composite materials. Built-in lightning protection devices also minimise the risk of damage.

Airships Ahoy
Safety concerns notwithstanding, there are two major factors currently driving a renewed interest in airships. First, the cost of oil. Though oil prices have tumbled from their peak of July 2008, a rise is perhaps inevitable in the future. This should hasten the return of airships at least for short haul flights, which are the most fuel inefficient for fixed wing aircraft. Given the large surface area of an airship, the maximum speed attainable would be around 130 to 160 km/hour. However, practical speeds are currently in the region of just 80 to 100 km/hour, and a good, strong tailwind is much valued. Therefore, airships are most useful if speed is not critical. An airship could transport a given load using a tenth of the fuel burnt by conventional aircraft. If this is not compelling economic logic, what is?

Second, climate change. Even in the unlikely event that the price of oil were to stay low for a few years, green pressures on aviation are intensifying by the day. Teams of researchers are feverishly searching for ways to cut aviation’s contribution to greenhouse gas emissions, but the historical trend line of improvement in aircraft fuel efficiency is just 1 to 2 per cent per year. Using airships, at least for freight delivery, would be one way to achieve dramatic progress in quick time.

Airships, balloons, dirigibles, zeppelins and blimps are all members of the lighter-than-air family of aircraft. Balloons are filled with hot air or helium. They drift where the wind takes them and have no real commercial application today. Fit an engine to a balloon and it can be purposefully steered—it becomes an airship. A zeppelin is a brand of airship named after its inventor, Ferdinand Graf von Zeppelin of Germany, who built rigid airships from around 1900 to 1939. Rigid airships are also called dirigibles. Blimp is used particularly in the US as a colloquial term for all kinds of airships. Strictly speaking, however, blimp describes only non-rigid airships.

Airships are of three main types. Non-rigid airships that have no interior skeleton or supporting structure and maintain their shape through the pressure of the gas inside the envelope are the most common variety today. Semi-rigid airships have a rigid keel structure beneath the airship envelope or a rigid partial structure inside the envelope. Rigid airships have an exterior form determined by an inflexible skeletal structure of large rings fastened to longitudinal girders and are fairly unpopular because of the massive weight of the structure.

The Physics of Static Lift
Under standard conditions (0°C, 1 bar) air has a density of 1.28 kg/m3. Any gas with a lower density (weight per cubic metre) is, therefore, lighter than air, and could be used to provide static lift. The envelope of an airship, when inflated, is aerodynamically shaped by the gas within. The pressure differential from inside to outside the envelope is very low, about 0.0045 of a standard atmosphere, equal to that of a column of water 4 to 5 cm high. This low differential means that a small hole in the envelope would result only in a very slow leak, taking hours or even days to affect the airship’s performance.

Hydrogen—one of the lightest elements and relatively easy to produce—was the lifting gas of choice to begin with. However, mixed with oxygen or air it becomes highly explosive, and has long been abandoned in favour of helium. Helium is an inert gas found in the Earth’s crust and is a by-product of natural gas extraction. At 0.18 kg/m3, its density is twice that of hydrogen, but still much less than air. The small size of the helium atom makes the manufacturing of a gas-tight envelope more difficult. However, with modern materials it is now possible to better confine the diffusion within the envelope material. Helium is also rather scarce and costly—to fill an airship could cost between £1 million and £3 million (Rs 7 crore and Rs 22 crore)—and is getting more expensive. But it doesn’t need to be refilled frequently, and normal operating loss is negligible.

At sea level, helium takes up only a small part of the envelope, while the rest is filled with air in one or more ballonets. Ballonets are flexible bags containing air, which are inflated or deflated to maintain constant pressure inside the envelope. This permits the helium to contract and expand correspondingly. As the airship ascends, the helium expands due to the reduced external pressure, and air is pushed out and released from a downward valve. When the ballonet is completely empty of air and the envelope is full of helium, the airship is said to be at its pressure height. Depending on the airship, this lies at around 2,000 m. At this altitude, the airship’s spring-loaded automatic valves open to relieve the pressure and prevent the gas cell or envelope from bursting. In addition to the lift provided by helium, modern airships obtain aerodynamic lift from the shape of the envelope as it moves through the air, much as an aeroplane does. Maximum payload capacity may be achieved by making a running takeoff, like an aeroplane. To descend, the buoyancy of the airship is reduced by pumping air into the ballonets, thus compressing the helium.

According to the Fédération Aéronautique Internationale, the current absolute airship world record for speed stands at 115 km/hour; for altitude it is 8,180 m; for distance 6,384.50 km and for airborne duration 71 hours.