Definition: Craft that float in the sky because of lighter-than-air (LTA) gas, including both balloons that float with the winds and dirigibles that can propel themselves and direct their course.
Significance: Although LTA craft have been supplanted by heavier-than-air (HTA) for most tasks, many techniques and technologies later adopted for HTA were developed with LTA craft. LTA craft provide a number of niche functions, such as weather sampling, advertising, telecommunications repeaters, high-altitude science platforms, and heavy-cargo transporters.
Design Principles
A balloon is a fabric container for LTA gas that allows the balloon to float. Usually, a balloon also lifts a payload (often called a gondola) hanging beneath the balloon. A dirigible, which is a shortened form of the term “dirigible balloon” (meaning directable balloon), has one or more balloons plus the propulsion system and payload. Balloons and dirigibles are called LTA craft to compare them with airplanes and helicopters, which are heavier-than-air (HTA) craft that stay in the sky because of the application of some form of propulsion.
Buoyancy is the key factor for LTA craft. Archimedes (287-212 b.c.e.) derived the principle stating that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. The LTA uses gases including warmed air (which has expanded and is thus lighter than the surrounding air) or gases with densities less than air.
Two low-density gases widely used to provide buoyancy are hydrogen and helium. Typically, hydrogen lifts 60 pounds per thousand cubic feet. Helium lifts 14 percent less (53 rather than 60 pounds) per thousand cubic feet, but helium has the major safety advantage that it does not burn, while hydrogen can ignite explosively.
Unfortunately, helium was not available until the 1920′s, and even then the United States government (which had most of the world’s supply) was slow to allow exports. Consequently, most LTA craft until the late 1930′s flew with hydrogen, and there were many catastrophic fires.
Hot air gets only 17 to 20 pounds of lift per thousand cubic feet, about one third that of hydrogen. Thus, hot-air balloons must be three times larger to lift the same pay-load, which makes hot-air dirigibles very inefficient. However, for balloons, the lesser complexity and cost of avoiding hydrogen or helium is a major advantage.
Heating air for buoyancy is usually done by burning propane or kerosene. Heat is constantly drained away at the surface of the balloon, so hot-air balloons require frequent firings of their burners. Consequently, they tend to have shorter range than balloons using low-density gas. However, the rapid changes in the buoyancy of hot-air balloons do allow their pilots to ascend or descend to catch different winds and thus get some control of their craft’s direction.
LTA craft pilots can decrease buoyancy to drop lower or land by valving out some of the lifting gas. They can increase buoyancy by dropping ballast (water, sand, or other material carried along for dropping as needed). In extreme conditions, balloonists have dropped everything in the gondola and even the gondola itself.
There are several more-sophisticated methods of modifying buoyancy, particularly for craft on long-duration and/or high-altitude flights, where warmth during the day causes the craft to rise too high and cold at night causes it to sink too low. Shiny upper surfaces, reflecting sunlight that would cause the balloon to expand and rise too high, and transparent lower surfaces, absorbing infrared radiation (heat waves) from the ground at night, often help. The Rozier balloon has a hot-air balloon, providing buoyancy variation, beneath a low-density gas balloon, providing endurance.
Conversely, super pressure balloons maintain buoyancy by having an envelope strong enough to keep the same volume even if the gas inside them expands. This comes at a cost of additional weight compared to zero-pressure balloons, which expand and contract with changes in surrounding pressure.
Another aspect of buoyancy is that the density and pressure of the surrounding air decreases with altitude. Hence, there is less lift available per unit volume, so LTA craft must be larger to carry a given payload to higher altitudes. Consequently, LTA craft 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 to operate the craft instead of people.
Light materials are vital for LTA construction. The best material for the early balloons was light, strong, and expensive silk. By the mid-twentieth century, synthetic materials, such as polyester and polyethylene-coated nylon, improved on silk’s performance at a lower price. By the beginning of the twenty-first century, composites of a number of synthetic materials allowed even greater strength and lighter weight. Similarly, the electrolytic process for purifying aluminum, invented in 1886, allowed structures light enough to fly dirigible structures and pressurized gondolas carried by balloons. Composites in the late twentieth and early twenty-first centuries allowed all of these structures to become lighter still.
Dirigibles are of three types: nonrigid, a streamlined balloon with the car and engines below; semirigid, the same with a strengthening keel below so the craft can be larger; and rigid, an enclosed structure holding any number of gas bags so the size can be very large.
History of LTA Flight
In 1782 and 1783, Joseph-Michel and Jacques-Etienne Montgolfier, two French brothers, flew hot-air balloons with animals as their first aeronauts. On November 21, 1783, they were ready for a human crew. Their pilot, Jean-Francois Pilatre de Rozier, and another man flew over Paris for twenty-five minutes while desperately stoking their lifting fire and sponging out fires in their rigging caused by sparks from the lifting fire.
Only a few days later, on December 1, 1783, Jacques-Alexander-Cesar Charles, of the French Academy, flew a hydrogen balloon. The flight illustrated the advantages of hydrogen balloons over hot air. Because hydrogen is more buoyant than hot air, the balloon could be one-third the size of a comparable hot-air balloon. Rather than just twenty-five minutes, Charles flew for two-and-a-half hours, dropped off his passenger at sunset, and then rose high enough to be the first person to see the sun set twice in one day.
Shortly thereafter, balloonists began attempting not just to fly, but to go places. Jean-Pierre Blanchard, another Frenchman, and John Jeffries, an American, were the first aeronauts to fly across the English Channel to France on January 7,1785. However, their flight illustrated the major problem of balloons as transportation. They had to drop all their cargo to reach land, and their destination could be only roughly planned—they could have no more specific intention than to land somewhere in France. That vagueness increased as balloonists made longer flights. Inventors tried vainly for decades to make their balloons steer-able, but they always failed because engines powerful enough to move a craft against strong winds were too heavy to be lifted.
Still, balloon flights in the nineteenth century supplied entertainment, scientific data, and observation data for armies. Balloon rides and balloon-borne fireworks were connected with most major celebrations. For scientists, balloonists discovered that the atmosphere grew thinner and cooler with increased altitude but that the magnetic field retained its strength. For armies, tethered balloons allowed observers to see several miles beyond the enemy’s lines. Such balloons were first used during the French Revolution in 1793, and again in the American Civil War (1861-1865). By the end of the nineteenth century, observation balloons were in wide use.
Dirigibles, Balloons, and Their Competition
As with HTA aircraft, dirigibles only became practical when light and powerful internal combustion engines were developed. On September 20, 1898, Brazilian Alberto Santos-Dumont first used a 3.5-horsepower, 66-pound motor to propel himself and Number 1, an 82-foot nonrigid craft with 64,000 cubic feet of gas volume, around Paris. Santos-Dumont made steady improvements over the next several years, inspiring many other nonrigids.
Events in the History of Lighter-Than-Air Craft
1782-1783: Joseph-Michel and Jacques-Etienne Montgolfier fly hot-air balloons with animals as their first aeronauts. 1793: French armies use tethered balloons to see several miles beyond enemy lines during the French Revolution. 1861-1865: Balloons are used for observation of enemy positions during the American Civil War.
1870-1871: The French use observation balloons during the Franco-Prussian War.
1899-1902: The British use balloons and kites for observation purposes during the Boer War.
1900: The first zeppelin is built.
1904-1905: During the Russo-Japanese War, the role of balloons leads to later military systems for employing aircraft for reconnaisance and artillery spotting.
1920′s-1930′s: The U.S. government, which controls most of the world’s supply of helium, operates four rigids for long-range reconnaissance.
May 6, 1937: The hydrogen-filled German airship Hindenburg explodes at Lakehurst, New Jersey, convincing the public that large dirigibles are unsafe.
Meanwhile, in Germany, Count Ferdinand von Zeppelin built a large rigid dirigible, Luftschiff Zeppelin Number 1 or LZ-1, which translates as “airship number one.” It was 420 feet long and 42 feet in diameter, with a gas volume of 460,000 cubic feet, sixty times greater than Santos-Dumont’s Number 1. The LZ-1, which first flew in July, 1900, had seventeen separate gas cells held together by an aluminum framework and covered with fabric. After ten more years of work, von Zeppelin had dirigibles in commercial service carrying sightseeing passengers and mail.
With the beginning of World War I, rigid dirigibles did well at first, staging the first long-range bombing attacks in 1915. However, airplane technology rapidly improved, and pilots found the rigids to be large, slow, highly flammable targets. Likewise, airplanes replaced observation balloons because the airplanes could cover more territory and also attack targets. The only dirigibles successful throughout the war were nonrigids used to guard convoys against submarines.
Still, the long flights by rigid dirigibles during the war suggested that intercontinental passenger service, or even flying warships, might develop. All these dreams eventually crashed. France abandoned large rigids when the Dixmude exploded in 1923. Great Britain abandoned large rigids when the R-101 crashed and burned in 1924.
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 Akron and Macon, carried their own fighter planes for defense. Because the United States had most of the world’s helium supply and used helium for its LTA gas, none of these craft exploded. However, three of them were lost in storms, and the United States abandoned the giant rigids after the third, the Macon, went down in a storm at sea in 1935.
The Lufftschiffbau Zeppelin company in Germany had the best safety record because it had built more than a hundred rigids and had thoroughly worked out the design details. In 1928, the company’s Graf Zeppelin began a commercial flight life that circled the world, made regular flights to Brazil and North America, made an Arctic expedition, and flew one million miles before being retired.
The last and greatest rigid was the Hinden-burg: 803 feet long and 135 feet in diameter. Its seven million cubic feet of gas allowed it to carry fifty passengers and sixty crew in absolute luxury at a speed of 84 miles per hour and a range of 11,000 miles.
Unfortunately, the Luftschiffbau Zeppelin company still flew with hydrogen, and the doped-cloth skin was also quite flammable. Lightning, leaking gas, or anti-Nazi sabotage caused the Hindenburg to catch fire while preparing to land at Lakehurst, New Jersey, on May 6, 1937. Within one minute, the craft was destroyed, and filmed footage of the event convinced the public that large dirigibles were unsafe.
That left only nonrigids, which were again a major part of antisubmarine warfare in World War II. However, they were retired in the 1950′s when 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 flying advertising billboards and carrying television cameras for overhead views of sporting events. The only new application came in the 1990′s, when tethered balloons returned to service as aerostats, providing platforms at altitudes as high as ten to fifteen thousand feet for radar stations and communications repeater stations.
Balloons fared better than dirigibles. Development of small radio transmitters combined with remotely operating weather instruments made possible balloon-borne radiosondes to report temperature, pressure, and relative humidity. Angle data from antennas tracking the radiosondes yielded wind speed and direction at different heights. Use of radiosonde balloons continued into the twenty-first century, helping predict weather, plot sky conditions for aircraft, and fire artillery more accurately.
Larger balloons have carried science payloads and human crews to high altitudes for decades because they can reach altitudes as high as 30 miles, which airplanes cannot reach, carrying large payloads that would not fit in an airplane fuselage. From the 1930′s through the early 1960′s, balloons were the frontier of human-crewed aviation that led to higher flights by HTA craft and eventually to space capsules.
The greatest problem at high altitudes is low pressure, which Swiss balloonist Auguste Piccard surmounted with a pressurized cabin, essentially the first space capsule, and he suggested similar pressurized cabins for high-flying transports. On May 27, 1931, Piccard and an assistant reached 51,793 feet (9.8 miles), making them the first to reach the stratosphere. More importantly, they discovered that cosmic rays increased with altitude, proving that they came from somewhere in space rather than the other suggested source, radioactivity within the earth. Such flights carried personnel and instruments to steadily greater heights and developed many technologies that were later used in the space race. In fact, on May 4,1961, the American Stratolab Vballoon reached an altitude of 113,700 feet (21.5 miles) with an open gondola so the two pilots could test space suits in near-space conditions for the Mercury orbital-flight program.
After the 1960′s, scientific balloon flights using improved robotic instrumentation allowed balloons to shed the weight of the balloonists and their life-support gear. In the closing decades of the twentieth century, astronomic balloon-borne instruments conducted sky surveys in a number of frequency bands that cannot penetrate the lower atmosphere and provided valuable weather data from the lower stratosphere.
By the late twentieth century, advances in fabrics allowed the U.S. National Aeronautics and Space Administration (NASA) to begin replacing zero-pressure balloons with super pressure balloons, which do not need to vent excess helium when warmed by the sun and which consequently can fly for weeks or months. By the early twenty-first century, NASA had begun flying large super pressure balloons with several-ton payloads in a program called the Ultra Long Duration Balloon (ULDB).
Ballooning for Fun and Adventure
Although the aviation frontier passed ballooning by, a balloon ride is still a beautiful and awe-inspiring experience. A panoramic view floats by below and sounds from the ground float up to balloonists.
This was a rare experience until the renaissance of hot-air ballooning, started by American Edward Yost. While developing high-altitude balloons for the United States government in the 1950′s, Yost realized that polyethylene-coated nylon is a lighter, less flammable material than that used in the Montgolfiers’ balloons. He used an acetylene welding torch as a less labor-intensive source of hot air than the Montgolfiers used. After some development, such as replacing the welding torch with a propane burner, Yost made the first modern hot-air balloon launch from Bruning, Nebraska, on October 10, 1960.
Beginning in the 1960′s, the new hot-air balloons radically reduced the cost and complexity of supplying buoyant gas. Thus were born ballooning clubs, competitions, and tour services. Also, for advertising, hot-air balloons have flown in shapes varying from spark plugs to human faces, and even a mansion.
For more ambitious flying, Yost’s hot-air technology (plus lightweight insulating material lining the gasbag, and helium) made the Rozier balloon practical for long distance flights. Varying the amount of heat in the inner balloon provides altitude control for hunting favorable winds. That capability, along with worldwide weather reports, made balloon flights possible across the Atlantic and then the Pacific Oceans. In March, 1999, another Piccard, Auguste’s grandson Bertrand, and Brian Jones spent twenty days flying 30,000 miles to make a complete circumnavigation of the globe.
For astronomical and meteorological observations, balloons are still a much cheaper alternative to spacecraft, with shorter turnaround times and without the vibration and acceleration of a rocket launch.
Dirigible Economics and Prospects
By the beginning of the twenty-first century, dirigibles were enjoying a resurgence in several niche markets. However, dirigibles will probably never recover aviation primacy from HTA’s for several reasons.
First there is a massive investment cost for building and developing dirigibles. Several factors make dirigibles more efficient as size increases. In particular, lifting volume increases by the cube while surface volume (and thus drag) only increases by the square. However, the large size makes the design and building of a dirigible as expensive as that of a ship. Large size also reduces the number of units made, so dirigibles have less chance to go down the learning curve toward lower costs and improved designs than 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 area by winds. Consequently, dirigibles must have their own special hangars rather than be casually parked on runways, as airplanes are.
Third, dirigibles are vulnerable to and limited by bad weather. The giant buoyant structures can be seized by freak gusts of wind on takeoff and landing, and they are more vulnerable than airplanes to icing. Zeppelin passenger flights were not scheduled in winter. In the sky, dirigibles are so large that winds may pull them in different directions and destroy them, as happened to the U.S. Shenandoah, Akron, and Macon. Moreover, unless specially designed for high altitude, dirigibles cannot readily climb above storms as jet-propelled airplanes can.
Fourth, due to the drag from the great size per unit mass of cargo, dirigibles are significantly slower than HTA competition. At best, they can obtain half the speed of propeller-driven planes and a 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. This longer time makes dirigibles uncompetitive in the passenger market.
Still, dirigibles have potential for certain markets because they can run quietly, run smoothly, linger for long periods, carry heavy and awkwardly large payloads, and land without runways. Lighter and more fire proof materials have increased these advantages. The number of advertising dirigibles increased steadily beginning in the 1980′s. At the start of the twenty-first century, the Zeppelin Company was marketing sightseeing semirigids a third the size of the Hindenburg. Cargo Lifter in Berlin was designing a cargo-carrying rigid larger than the Hindenburg.
Meanwhile, an entirely new concept was being developed: dirigibles in the lower stratosphere serving as high-altitude platforms. Such platforms could serve many functions of communications satellites and astronomical satellites at a fraction of the cost. However, as with most LTA tasks, there is competition from airplanes.
