Definition: Fabric containers holding a lighter-than-air gas so that the containers and any payload are buoyed up and float in the sky.
Significance: One of humanity’s oldest dreams has been to float in the sky. Balloon flights first transformed that dream into adventure, and they continue to do so. For more than a century after the first balloon flight, balloons were the cutting edge of science and aviation technology, and they remain the best craft for scientific missions operating in altitudes of roughly 10 to 30 miles, which are too high for airplanes and too low for orbiting vehicles.
Nature and Use
The term “balloon” may refer to the gas bag, or envelope, or to the balloon and any additional objects attached to it, which are usually hung below. Objects are attached to smaller balloons by a single line, but larger balloons require netting to spread the load over the entire gas bag. A large cargo below is called a gondola or basket, often a large wicker basket.
Buoyancy is the key to balloon 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 balloons, a lighter-than-air (LTA) gas provides buoyancy to lift the balloon containing it as well as any payload. LTA gases include gases with densities lower than that of air and warmed air that has expanded and is thus lighter than the surrounding air. The two low-density gases used for balloons are hydrogen and helium, which require the balloon to be sealed so that they do not mix with the heavier air.
Air is usually heated for buoyancy by burning propane or kerosene. Warmed air rises through an open base, and air that has cooled drains out of that same orifice. Warmth 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 with low-density gas.
More importantly, hot air has less lifting capacity than hydrogen or helium. Typically, hydrogen has a net lift of 60 pounds per 1,000 cubic feet, but hot air provides only 17 to 20 pounds of lift. Thus, hot-air balloons must be three times larger to lift the same payload. However, hot-air balloons are less expensive to operate, because they do not have to accommodate the complexities of hydrogen and helium.
Although helium lifts 14 percent less than hydrogen (53 rather than 60 pounds) per 1,000 cubic feet, it has the major safety advantage of being nonflammable, whereas hydrogen can ignite explosively.
As a balloon increases its altitude, the density and pressure of the surrounding air decreases, meaning there is less lift available per unit volume, so the balloon must be larger to carry a given payload to higher altitudes. A partially compensating factor is that the buoyant gas also grows less dense as the pressure decrease allows it to expand, but the trend is toward miniscule lift per unit volume, as most of the atmosphere is left below. Balloon builders can compensate with lighter payloads, such as remotely controlled instruments, but at some point, the weight of balloon fabric alone matches the lift from the gas volume, and even the largest balloons can go no higher.
Balloonists have two other ways to vary the buoyancy of their craft. They can descend by decreasing buoyancy or land by valving out some of the lifting gas. They can increase buoyancy by dropping ballast, which is water, sand, or other material carried along for that purpose. In extreme conditions, balloonists have dropped all articles in the gondola and even the gondola itself.
History
For centuries, the Chinese made toy hot-air balloons of a design that could and might have been scaled up to carry passengers. There are accounts from twelfth century b.c.e. China of people in balloons, but the records are too old and incomplete to be confirmed. Likewise, drawings on pottery associated with the Nazca Lines, constructed more than two thousand years ago in southern Peru, suggest that these massive earthen line drawings were made with overhead direction from hot-air balloons.
Confirmable accounts begin in the eighteenth century. In 1782 and 1783, Joseph-Michel and Jacques-Etienne Montgolfier, two French brothers, flew hot-air balloons larger than toys, with animals as their first passengers.
Ironically, in their first balloons, the Montgolfiers had wanted to use hydrogen, which British chemist Henry Cavendish had discovered in 1776, noting in his experiment reports that this “inflammable air” was lighter than ordinary air. The French Academy in Paris was working toward a rubberized or varnished fabric to contain the troublesome gas that seeped through ordinary fabrics and escaped. When Joseph-Michel Montgolfier experienced the same problem, he noted that scraps of paper in a fireplace rose up the chimney. Paper, which the Montgolfier family manufactured, could contain smoke, so the Montgolfiers’ made hot-air balloons and successfully flew three animal passengers: a rooster, a sheep, and a duck. King Louis XVI’s permission was required for people to fly because it was not known whether leaving the ground might be harmful to people.
Jean-Francois Pilatre de Rozier, a young doctor who wanted to take the risk of human flight, recruited the Marquis Francois d’Arlandes to serve as copilot and, more importantly, to secure the king’s permission. On November 21, 1783, having obtained permission, the two men flew over Paris for twenty-five minutes while desperately stoking their lifting fire and sponging out fires in their rigging caused by sparks. Below them, nearly the entire populace of Paris watched.
Only a few days later, on December 1, 1783, Jacques-Alexander-Cesar Charles, of the French Academy, flew a hydrogen balloon. The preparation required the production of large quantities of hydrogen gas and the careful varnishing of cloth to render it relatively airtight. The flight illustrated the advantages of hydrogen balloons over hot-air balloons. Because hydrogen is more buoyant than hot air, the hydrogen balloon could be a third the size of a comparable hot-air balloon. Charles flew for two and one-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 only to fly but also to reach destinations. Jean-Pierre Blanchard, another Frenchman, and John Jeffries, an American, decided to be the first aeronauts to fly across the English Channel to France, which they did on January 7, 1785. However, they were somewhat humbled upon arrival, because they had jettisoned most of their clothes, along with the gondola and the articles within it, in order to avoid falling into the Channel.
A rivalry ensued with the French wanting to have a flight from France back to England. Pilatre de Rozier, who had piloted the first hot-air balloon, had another balloon made that advanced balloon technology. This hybrid de Rozier balloon had a hydrogen balloon that rode over a hot-air balloon. The pilot could vary the balloon’s buoyancy, and thus its altitude, by adjusting the fire under the hot-air balloon instead of valving out hydrogen gas or dropping ballast, neither of which could be replenished. OnJune 15,1785, Pilatre de Rozier and his copilot floated in this balloon toward England.
Unfortunately, Pilatre de Rozier fell prey to two problems of early balloonists. He had no reliable weather reports, and when the wind changed, he floated back toward France. Worse, his hydrogen lifting gas was very flammable and the varnished cloth only slightly less so. As the audience who had joyously witnessed the launch watched, the balloon caught fire, lost buoyancy, and plummeted the two aeronauts to their deaths. After Pilatre de Rozier’s death, hot-air balloons fell out of favor until experiencing a renaissance in the 1960′s.
Similar problems plagued ambitious balloon flights for the next century. Attempts at crossing the Atlantic Ocean lost credibility as balloonists waited vainly for suitable weather. Balloon flights crossed the Alps in Europe, and John Wise crossed a third of North America, but the final destination of long-distance flights was always a surprise, and disaster was always just a spark away.
Despite its shortcomings, the balloon went to war in 1793 when revolutionary France was attacked by a number of neighbors. At the Battle of Fleurus in 1794, the fledgling French balloon corps fielded a single reconnaissance balloon tethered on a line several hundred feet above the ground. Observers in the balloon, who could see several miles past the line of battle, provided tactical reports via notes dropped from the gondola. The most important observation was that the attacking Austrian army had pitched an empty tent city in an effort to overawe the French commander into retreating. Because of that vital bit of intelligence, the French did not retreat but rather fought on with their exotic new technology looming over and unnerving their opponents, until they eventually won the battle.
Aerial reconnaissance was reinvented in the American Civil War (1861-1865), in which several groups operated observation balloons. The most successful was inventor Thaddeus Sobieski Coulin court Lowe, who organized an aeronautic corps of balloon observers for the Union. Although balloon technology had not advanced tremendously, communications technology had. Lowe’s observers transmitted their reports either by signal flags or by telegraph wire running down to the ground.
The potential of Lowe’s reports is shown by accounts of one 1862 engagement, in which Lowe directed Union cannon fire at an area invisible to Union guns because it was behind a hill. When the Confederate horsemen rode away from the shell impacts, Lowe had the guns redirect their fire. After the war, Confederate accounts revealed that one of the horsemen was showered by so much dirt from a near-miss that his colleagues feared he had been hit. That horseman was Jefferson Davis, the Confederate president. Nearby, also in danger, was Robert E. Lee, Davis’s commander of all Confederate armies. Despite Lowe’s successes, a change in the Union Army high command caused Lowe to fall from favor and come under stringent control by an unsympathetic regular officer. Lowe ultimately resigned from his post, and his entire corps withered away.
Military ballooning was next reinvented by the French during the Franco-Prussian War of 1870-1871. The Prussians smashed the regular French army and surrounded the French capital city of Paris. The plucky Parisians responded by raising a militia to hold off attacks and launching balloons to carry observers and send messages out of the city, rallying the countryside. Fifty-four of sixty-two balloons got through, carrying one hundred people and two and one-half million pieces of mail. Although France eventually accepted harsh peace terms, the utility of war balloons was established.
By the time World War I began in 1914, observation balloons were in use by both sides. By the end of the war, they were being replaced by heavier-than-air airplanes.
Surprisingly, small balloons did evolve into an important military and civilian use during World War II. The development of small radio transmitters combined with remotely operating weather instruments made possible balloon-borne radiosondes that reported temperature, pressure, and relative humidity. Angle data from antennas tracking the radiosondes yielded the more important factors of wind speed and direction at different heights. The use of weather balloons has continued into the twenty-first century to help predict weather, to plot sky conditions for aircraft, and to fire artillery more accurately.
Balloons, were used during the American Civil War and other nineteenth century wars for military observation.
Finally, in the 1990′s, tethered balloons returned to service as aerostats, providing platforms at altitudes as high as 10,000 to 15,000 feet for radar stations and communications repeater stations.
Scientific Applications
Some have said that balloons are a pacifist technology. They are big, slow, and cannot be piloted accurately, particularly when the wind changes. Yet, balloons do a number of things well. They move gently and can carry large payloads that would not fit in an airplane fuselage. Most importantly, they can reach high enough altitudes to perform many of the research tasks generally performed by spacecraft. However, balloons can accomplish this research more cheaply and quickly than can spacecraft, and without the vibration and acceleration forces of a rocket launch into space.
In the twentieth century, a number of supporting technologies radically improved, allowing the balloon to become much more practical for research applications. Most importantly, helium became widely available as a nonflammable lifting gas. Synthetic fibers, such as nylon, polyethylene, and Kevlar, supplied fire-resistant materials with the lightness of silk and strengths approaching steel. Vulcanized rubber allowed light, cheap, disposable balloons. Virtually all measuring instruments shrank in size. Finally, worldwide weather databases and communications links made it more possible to guide balloons on long voyages.
The quest for altitude began as both an adventure and a science. The first balloonists had no idea whether the atmosphere continued indefinitely or became lethal a short distance above the ground. They soon discovered that pressure and temperature decreased with increasing altitude. Those who attempted altitude records discovered temperatures tens of degrees below the freezing point of water. However, the greatest risk was hypoxia, or oxygen deprivation, which causes weakness, shakiness, mental confusion, and eventual death. To deal with these problems, balloonists developed oxygen-supply systems and learned to use equipment that would not freeze in the bitter cold.
However, even the breathing of pure oxygen was found to be insufficient above altitudes of49,000 feet. Swiss balloonist Auguste Piccard surmounted this problem with a pressurized cabin that essentially represented the first space capsule. On May 27,1931, Piccard and an assistant launched from southern Germany and reached 51,793 feet, making them the first fliers ever to reach the stratosphere. More importantly, they discovered that cosmic rays increased with altitude, proving that these rays came from somewhere in space and not from radioactivity within the earth.
From 1933 to 1935, the governments of the United States and the Soviet Union foreshadowed the space race that would begin nearly a quarter-century later. Balloon flights carried personnel and instruments to steadily greater heights and developed many technologies that were later used in the space race. For example, on May 4, 1961, the American Stratolab High V balloon reached a world-record-breaking altitude of 113,600 feet, with an open gondola so that the two pilots could test space suits in near-space conditions for the Mercury orbital-flight program.
In retrospect, the best high-altitude science data began to be collected in the 1960′s, after improved robotic instrumentation allowed shedding the weight of the balloonists and their life-support gear. Over 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, the National Aeronautics and Space Administration (NASA) began using super-pressure balloons for relatively small payloads of several tens of pounds. Balloons called zero-pressure balloons expand when warmed by the sun and contract at night when cooled. When warmed at high altitude, they must vent excess helium to prevent bursting. This gas loss limits mission duration to only several days. Superpressure balloons, in contrast, keep the same maximum shape when the balloon is warmed. Because no gas is lost, such balloons can operate for weeks or months, and some of these balloons have circled the globe one or more times. By the early twenty-first century, NASA had begun flying large super-pressure balloons in a program called the Ultra Long Duration Balloon (ULDB). These large balloons could carry several tons of instrument payload for weeks at a time. Less well documented are flights of small super pressure balloons by U.S. intelligence agencies since the 1950′s.
Recreational Applications
Although ballooning is no longer the world’s primary means of aviation, a balloon ride remains a beautiful and awe-inspiring experience. Balloonists enjoy panoramic views that float by below them and sounds that float up from the ground.
However, a recreational balloon ride was a rare experience until the so-called renaissance of hot-air ballooning, which was started by American balloonist Edward Yost. While Yost was developing high-altitude balloons for the U.S. government in the 1950′s, it occurred to him that polyethylene-coated nylon would be a lighter, less flammable material than that used for the Montgolfiers’ balloons. He used an acetylene welding torch as a less labor-intensive source of hot air than that used by the Montgolfiers. 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. Hot-air balloons have been flown, primarily for advertising, in whimsical shapes, including those of sparkplugs, light bulbs, human faces, and even a mansion.
A combination of Yost’s hot-air technology, lightweight insulating material lining the gas bag, and helium made the de Rozier balloon practical for more ambitious, 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 it possible to make balloon flights across the Atlantic and 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.
