Aircraft that float because of lighter-than-air gas and that have power sufficient to direct their course of flight.

Dirigibles, like balloons, are often referred to as lighter-than-air (LTA) craft, in contrast with airplanes and helicopters, which are heavier-than-air (HTA) craft. The term “dirigible” is a shortened form of “dirigible balloon,” meaning directable, or steerable, balloon. Buoyancy is the key to dirigible flight. The ancient mathematician Archimedes stated that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. For dirigibles, two LTA gases, hydrogen and helium, are combined to provide the buoyancy that lifts the dirigible and any payload.

Typically, hydrogen lifts 60 pounds per 1,000 cubic feet. Helium lifts 14 percent less (53 rather than 60 pounds) per 1,000 cubic feet. Helium has a major safety advantage over hydrogen, in that it does not burn, whereas hydrogen can ignite explosively.

Unfortunately, helium did not become available until the 1920’s, and even then, the U.S. government, which controlled most of the world’s supply, was slow to allow exports. Consequently, dirigibles manufactured outside the United States flew using highly flammable hydrogen, which caused many catastrophic fires. A third LTA gas, hot air, has only one-third the amount of lift of hydrogen, meaning the propulsion unit must have proportionately more thrust.

Another dirigible concern is that the density and pressure of the surrounding air decreases with altitude. Hence, there is less lift available per unit volume, so the craft must be larger to carry a given payload to higher altitudes. Consequently, dirigibles with heavy payloads tend to be limited to low altitudes of a few thousand feet. For higher altitudes, designers can compensate for decreased lift per unit volume by using lighter payloads, such as remotely controlled instruments instead of people.

Dirigibles are divided into three categories: nonrigid, semirigid, and rigid. Nonrigids are essentially large, streamlined cylindrical balloons, nicknamed “blimps,” supposedly for the sound made by a finger thumping into the side of the envelope, or gas bag. Nonrigids, such as the Goodyear blimps, get their shape from the gas within a single gas bag or envelope. The engines and a car or a gondola hang below the gas bag.

The design of nonrigid dirigibles simplifies cost and minimizes structural weight, which in turn reduces the net lifting capability. However, nonrigids are limited in size, because an unsupported gas bag may bend unpredictably under heavy loads or strong winds. In a worst-case scenario, a partially deflated gas bag may flop over the gondola or propellers. Conversely, the gas bag cannot be filled too tightly, lest it burst. The one gas bag is a single point of failure that could cause a crash, although the large size of dirigibles means that operations could continue for some hours, even with significant leaks. Another method to compensate for pressure loss in the gas bag is the use of an inner ballonet, which can be inflated with outside air.

Semirigid dirigibles have a keel on the bottom to support a larger gas bag, and the keel can hold the gondola and engines, at the cost of additional weight. The risks associated with a single gas bag also apply to semirigid dirigibles. The most famous semirigid was the airship Norge, which made the first transpolar flight from Spitsbergen Island to Alaska.

Rigid dirigibles have a framework to support an outer skin and individual gas bags. Although the term “zeppelin” is sometimes used to describe any rigid dirigible, the name legally applies only to the type of craft manufactured by the Luftschiffbau Zeppelin company of Germany.

In rigid dirigibles, an individual gas bag can fail without damaging the aerodynamic integrity of the craft, and there are usually sufficient reserves among the other cells to maintain buoyancy. Those advantages cost additional weight. However, greater weight can be compensated for by greater size. There is theoretically no limit to a rigid’s size. The German passenger airship Hindenburg had an LTA gas capacity of 7,000,000 cubic feet, and designs of twice that size have been proposed.

Beginning in the 1790’s, balloons made true humankind’s dream of the possibility to drift like clouds. Like clouds, however, balloons drifted wherever the wind blew. Thus, inventors realized their craft must be directable as well as lighter than air. The key to this directional ability was generating sufficient power while remaining light enough to fly. Repeated attempts in the first half of the nineteenth century showed that human power was insufficient against even slight winds. A number of inventors flew models powered by springs or clockworks during that time, but none of the models’ mechanisms could be sufficiently scaled up to power a craft carrying a person.

Henri Giffard of France had the first partial success on September 24, 1852, with a dirigible powered by a steam engine. The engine, advanced for its time, produced 3 horsepower and weighed as much as two large men. Giffard’s aerial steamer, as it was known, launched from the Paris Hippodrome and hissed sedately to a landing 17 miles away. In a later flight, Giffard circled around Paris. However, because his craft’s top speed was only 6 miles per hour, it was not steerable against even a breeze.

Fortunately for dirigible designers, the development of metallurgy and power plants advanced in the second half of the nineteenth century. In 1886, an electrolytic process was invented for producing aluminum inexpensively enough so that it could be used to replace the heavier steel in dirigible support structures. In 1876, German engineer Nikolaus August Otto began marketing a four-stroke, internal combustion engine yielding more power per unit weight than the external-combustion steam engines. In 1885, another German engineer, Gottlieb Daimler, patented significant improvements to the internal combustion engine and offered it for use in dirigibles.

On November 12, 1897, an airship built by Austrian David Schwartz sported a 10-horsepower Daimler motor. Before it was ready to launch, a gust of wind pulled the craft loose from its moorings and toward nearby buildings. The pilot panicked and valved out so much gas that he crashed on the field. Despite its misfortune, this ship represented the first rigid dirigible, with a solid structure and a thin aluminum skin around the gas bag.

By this time, both airships and balloons had developed a bad reputation. Fortunately, public relations assistance and superb flying skill arrived from Brazil in the form of wealthy experimenter Alberto Santos-Dumont, who took a single-cylinder engine from each of two tricycle automobiles to make a single, 66-pound, two-cylinder engine delivering 3.5 horsepower, roughly five times the power-to-weight ratio that had been available to Giffard. Santos-Dumont launched this eighty-two-foot nonrigid craft with 64,000 cubic feet of gas volume, along with himself and a basket.

On September 20, 1898, Santos-Dumont began flying around Paris in his airship, usually flying low enough to greet people on the streets. As both his flying skills and his dirigibles became progressively more advanced over the next several months, he aroused tremendous public interest, especially because he commuted around Paris in his compact dirigibles, mooring his craft above the spots to which he traveled.

Santos-Dumont engendered so much interest in flying that a prize was offered to the aviator who could fly a seven-mile course to the Eiffel Tower and back within in thirty minutes. After several heroic attempts, Santos-Dumont won the prize. He became a global celebrity and inspired many others built nonrigid airships.

In Germany, Count Ferdinand von Zeppelin built a large rigid dirigible, Luftschiff Zeppelin Number 1, or LZ-1, which was 420 feet long and 42 feet in diameter with a gas volume of 400,000 cubic feet, sixty times greater than that of Santos-Dumont’s model number 1. LZ-1, which first flew in July, 1900, had seventeen separate gas cells held together by an aluminum framework and covered with fabric.

However, the two 15-horsepower engines gave LZ-1 a top speed of only 16 miles per hour, still insufficient to fly against moderate winds. Zeppelin raised more money to build LZ-2 and LZ-3, both of which had two 65-horsepower engines. LZ-2 was destroyed at its mooring by winds, but successful flights of LZ-3 led the German government to offer payment for a still-larger craft, if it could stay aloft for twenty-four hours.

On August 4, 1908, the LZ-4 began a majestic tour from its home base on the Swiss border, heading north along the Rhine River. People along the way cheered the giant airship. LZ-4 flew for eleven hours, as far as Mainz, Germany, and had begun its return when one engine failed. Rather than press on in the dark with only one engine, Zeppelin set LZ-4 down near the town of Echterdingen. That night a storm pulled the craft loose and destroyed it.

Yet Zeppelin’s story continued, as envelopes of cash began arriving from all over Germany. The so-called Miracle of Echterdingen supplied his company with more money than the German government had offered. The count continued to build, and by 1910, Zeppelin dirigibles had begun carrying sightseeing passengers and mail. By 1914, a number of Zeppelin dirigibles were in regular service.

That year, World War I began. At first, the dirigibles dominated the skies, and the competing airplanes posed no threat to them. In 1915, German rigid dirigibles conducted the first long-range bombing attacks against targets in Great Britain, with little effective resistance from airplanes. However, faster and larger airplanes were soon able to catch the dirigibles, which proved to be large, slow targets. A single incendiary round of fire passing through a hydrogen gas cell could transform an airship into a fireball. In order to escape the airplanes, the Germans piloted their dirigibles to higher altitudes, where at 20,000 feet, water froze in the crews’ canteens. The airplanes, however, were improved enough reach the dirigibles. By the end of the war, large airplanes had replaced rigids for long-distance bombing. The only dirigibles successful throughout the war were two hundred nonrigids the British used to guard convoys against submarines.

The long flights made by rigid dirigibles during the war suggested that dirigibles might be used for intercontinental passenger service, or even as flying warships. Continued research was conducted by four countries: France, Great Britain, the United States, and Germany.

France had a number of smaller nonrigids, as well as one large rigid taken from Germany as part of its war reparations. The airship was renamed the Dixmude and flew for several years, making a record-breaking flight over the North African desert. After the airship exploded in flight during a storm in 1923, France abandoned large dirigibles.

During World War I, the British had built an R (for rigid) series of dirigibles, which the British continued to develop after the war by reverse engineering from a captured German dirigible. On July 2, 1919, the R-34 left England, and, four days later, it had completed the first east-to-west aerial crossing of the Atlantic Ocean.

In 1924, the British government started two competing programs to build dirigible airliners. The R-100, built with private funding, was known as the capitalist ship, and it flew well on a demonstration flight to Canada and back.

The R-101, built by the government, was known as the socialist ship and was heavy with safety features. To increase lift, the builders cut the ship in half and inserted an additional gas bag. They also loosened wire netting around the gas cells, so they could be expanded. Unfortunately, this adjustment allowed the cells to rub against the framework, causing many small leaks. Because officials wanted to use the airship for a prescheduled demonstration flight to India, the major changes were not flight-tested. The R-101 launched from England on October 4, 1930, and early the following morning, it crashed into a hillside and exploded 40 miles northwest of Paris; forty-eight of the fifty-four people aboard died. As a result of this accident, Great Britain abandoned passenger airships and even scrapped the successful R-100.

In the 1920’s and 1930’s, the U.S. government operated four rigids as military ships intended for long-range reconnaissance. Two of the airships, the USS Akron and the USS Macon, actually carried their own fighter planes for defense. Because the United States held most of the world’s helium supply and used helium for its LTA gas, none of these craft exploded. However, three were lost in storms, and the United States abandoned the giant rigids after the last, the Macon, broke up in a storm and went into the sea off Point Sur, California, on February 12, 1935.

The Luftschiffbau Zeppelin company of Germany, with its experience in building more than one hundred rigids and its thorough design details, had the best safety record of any dirigible manufacturing company. For several years after World War I, Germany was forbidden by the Treaty of Versailles from possessing dirigibles larger than 1,000,000 cubic feet. However, in 1922, the U.S. Navy placed an order for a dirigible, which was named the Los Angeles. Zeppelin’s brilliant manager, Hugo Eckener, flew the craft to the United States. After the size limit on German dirigibles was lifted in 1925, Eckener organized construction of the Graf Zeppelin. Beginning in 1928, the Graf Zeppelin circled the world, flew regularly to Brazil and North America, made an Arctic expedition, and traversed one million miles before being retired.

The last and greatest rigid was the Hindenburg, launched in 1936. The Hindenburg was 803 feet long and 135 feet in diameter. Its 7,000,000 cubic feet of gas allowed it to carry fifty passengers and sixty crew in absolute luxury at a speed of 84 miles per hour for a range of 11,000 miles. The Hindenburg and the older Graf Zeppelin, represented great profits for Luftschiffbau Zeppelin and good propaganda for Germany’s Nazi regime.

Then disaster struck. Although Luftschiffbau Zeppelin was negotiating with the U.S. government for helium, it still employed hydrogen in its airships. As the Hindenburg was docking at Lakehurst, New Jersey, on May 6, 1937, several crew members noticed a small fire in one gas cell. Within one minute, the craft had exploded into a ball of fire and lay on the ground, a smoldering wreckage. Although many theories proposed causes such as lightening, leaking gas, and anti-Nazi sabotage, filmed footage of the event convinced the public that large dirigibles were unsafe.

After the Hindenburg disaster, only nonrigids remained, and they played a major role in the antisubmarine warfare of World War II. However, they were retired in the 1950’s, after it became clear that helicopters provided the same hovering capability with greater dash capability and easier storage. In the last third of the twentieth century, the few working nonrigid dirigibles were limited to use as advertising billboards and as vehicles for television cameras providing overhead views of sporting events.

Although dirigibles at the beginning of the twenty-first century enjoyed a small resurgence in several niche markets, they will probably never recover their primacy in aviation for five major reasons.

The first reason is the massive investment cost of building and developing dirigibles. Several factors make dirigibles more efficient as their size increases. However, the increase in size increases the cost of design and building. Large size also reduces the number of units made, so dirigibles have less chance for lower costs and improved designs than do HTA craft, which are typically made by the hundreds or thousands.

Second, hangar costs are high. Dirigibles are kept inflated because their helium lifting gas is expensive and would require too much time and effort to pump back into tanks. However, inflated dirigibles can easily be swept off their parking areas by winds. Consequently, dirigibles must be housed in their own special hangars instead of being parked on runways as airplanes are.

Third, dirigibles are vulnerable to bad weather, which limits their performance. The giant buoyant structures can be seized by gusts of wind on takeoffs and landings and are more vulnerable than airplanes to icing. Zeppelin passenger flights were not scheduled in winter. Dirigibles are so large that winds may pull them in different directions while they are in flight, destroying them. The USS Shenandoah, Akron, and Macon were all destroyed in this way. Moreover, unless they are specially designed for high altitude, dirigibles cannot readily climb above storms as jet-propelled airplanes can.

Fourth, because dirigibles’ great size causes more drag per unit mass of cargo, dirigibles are significantly slower than their HTA competition. They can at best obtain one-half the speed of propeller-driven planes and one-fifth that of jets. Thus, a jet with one-fifth of the cargo capacity of a dirigible can deliver the same cumulative mass of cargo. For the passenger market, shorter flight times are crucial.

Still, dirigibles have potential for certain markets because they can run quietly and smoothly, linger for long periods, carry heavy and awkwardly large payloads, and land without runways. These advantages have been increased by lighter and more fireproof materials. The number of advertising dirigibles increased steadily beginning in the 1980’s. At the start of the twenty-first century, the present-day Luftschifftechnik Zeppelin company marketed sightseeing semirigids one-third the size of the Hindenburg. A German-American company called CargoLifter designed a cargo-carrying rigid larger than the Hindenburg.

Meanwhile, an entirely new concept was being developed: the use of dirigibles in the lower stratosphere as high-altitude platforms. Such platforms could serve many functions of communications satellites and astronomical satellites at a fraction of the cost of spacecraft.

• Botting, Douglas. The Giant Airships. Alexandria, Va.: Time-Life, 1981. An exhaustive but readable history and technical description of the earliest dirigible attempts through the destruction of the Hindenburg.
• Cross, Wilbur. Disaster at the Pole. New York: Lyons Press, 2000. An historical account of the airship Italia’s disastrous mission of scientific research at the North Pole and the political backlash in Italy against the expedition’s commander and dirigibles in general.
• Hogenlocher, Klaus G. “A Zeppelin for the Twenty-first Century.” Scientific American 281, no. 5 (November, 1999): 104-109. A detailed description of the technical innovations of the “new technology” Zeppelin airships of the 1990’s.
• Kunzig, Robert. “Dirigibles on the Rise.” Discover 21, no. 11 (November, 2000): 92-99. A description of the new dirigible enterprises being developed at the end of the twentieth century, including new passenger craft and heavy cargo lifters.

Balloons

Blimps

Buoyant aircraft

Hindenburg

Icing

Lighter-than-air craft

Military flight

Reconnaissance

Alberto Santos-Dumont

Weather conditions

Ferdinand von Zeppelin