At Home in the Sky.
In the early hours of a crisp June day a huge "pumpkin" rises from eastern New Mexico's high mountain desert. Slowly, gracefully, silently, this next generation of NASA's largest flight vehicles begins its inaugural test voyage. Being developed as a stable, high-altitude platform for scientific instruments, the Ultra Long Duration Balloon (ULDB) promises to open the way for applications ranging from steerable airships to military surveillance to inflatable, space-based solar panels.
But these applications were still in the future in June, as the more than 400-foot-tall "pumpkin" receded upward, becoming a white dot in the blue sky by the time it reached the top of the atmosphere at about 100,000 feet. This superpressure pumpkin-shaped balloon was to fly for more than 30 hours, including an impressive flight over a severe thunderstorm at night, before its flight was terminated with all objectives of the test met. The next balloon in the developmental series is scheduled to fly in mid-January 2001, weather permitting. It will be seven and a half times larger and carry more than two tons to the brink of space.
After helium-filled party balloons, most people, when asked to think of larger balloons, would probably mention either hot-air balloons (which are made of fabric) or weather balloons (made of rubber). In such a fashion, the public perception would overlook the important scientific balloon category. Scientific balloons are very, very large and are made of plastic. Carrying scientific instruments above most of Earth's atmosphere, they have claimed an important niche between surface-based and space-based instrument platforms.
Seeking to expand the role of scientific balloons, NASA is now approaching the home stretch for making its ULDB available to the scientific research community. NASA is developing this next generation of scientific balloons through its Balloon Program Office located at Goddard Space Flight Center's Wallops Flight Facility in Virginia. The ULDB program builds on strengths of the current NASA balloon program, while combining the latest advancements in design and materials. To some, it may seem paradoxical that such an apparently low-tech field as balloons would be impacted by high-tech developments. Yet such is the case.
What's a scientific balloon?
Ballooning provided man his first reach above the surface of the planet in 1783 with a short flight by France's Montgolfier brothers. Since that first flight, professional and amateur scientists around the world have used balloons as platforms for scientific instruments. In the late 1800s atmospheric scientists used balloons for measuring atmospheric temperature and moisture and even captured air samples at various heights. Early in the twentieth century, physicists used balloons to carry the instruments that made breakthrough discoveries of cosmic and gamma rays. Later, scientists used balloons for mapping the atmosphere as a series of layers. Driven by scientists' desire to raise their instruments ever higher above Earth's obscuring atmosphere, balloon technology steadily improved through the twentieth century.
The physics of balloon flight are simple to understand but challenging to harness. Most current scientific balloons are referred to as zero- pressure balloons. At their designed float altitude, they look like an upside-down teardrop. Although their single layer of 0.008-inch-thick polyethylene is similar in thickness to a dry cleaning bag, in quality and strength it is considerably better.
These large balloons are similar in construction (but not in color) to a beach ball made of gores (or panels) of different colors running vertically from top to bottom. For scientific balloons, each gore is essentially a long narrow sheet with a slight curvature along its edge. The balloon is constructed by heat sealing the edges of the gores together until they take on a three-dimensional shape. This polyethylene shell retains the balloon's helium, supports the gas's slight internal pressure, and even carries the payload weight. To carry the payload weight, zero-pressure balloons are constructed with polyester-fiber-reinforced load tapes that are heat sealed to the film and run like longitudinal lines from the top to the bottom of the balloon. The load tapes transfer the weight of the payload into the balloon film.
At launch these balloons are filled with a volume of helium that at surface level temperature and pressure is equal to only 1/200th to 1/100th of the balloon's final volume. The largest NASA balloons have a final float-level volume of 40 million cubic feet, which is equivalent to a box with one side the size of a football field and a height extending to over 830 feet tall. The helium must provide enough lift to offset the weight of the balloon and its suspended load, and still lift the balloon to the desired altitude. As the balloon rises, the ambient pressure goes down and the gas expands to fill the balloon. The sunlight heats the balloon film during the day, which in turn heats the helium. (Helium itself is essentially transparent to--and hence unheated by--all frequencies of solar radiation.) As the helium warms, it seeks a new equilibrium of pressure, temperature, and volume, constrained primarily by the ambient air pressure and the enclosing balloon film. From the interplay of these factors, the net result as the helium warms is that the balloon expands and hence rises. If such a process were unregulated, it could easily lead to a rupture of the balloon film. To prevent this, zero-pressure balloons include in their lower portion open vents allowing excess helium to escape.
As night approaches, cooling replaces warming. The helium, no longer heated by its enclosing film, releases radiative heat energy through the film to the dark cold space above and the cool dark earth below. The balloon's internal pressure and size decrease, which means that its lift decreases and hence it loses altitude. However, because it has lost helium, the balloon now has less intrinsic lift at any temperature than it had when it was launched. To maintain altitude, therefore, the balloon flight system must reduce its weight, which it does by dropping ballast in the form of a fine powder.
The process of losing helium during the day and dumping ballast at night is a limiting cycle that prevents most zero-pressure balloons from flying for more than a few days. One way around this is to fly from Antarctica during its summer, so the balloon will be in constant daylight. NASA annually launches these types of flights from McMurdo Station, Antarctica, with flights typically lasting 10 days to two weeks. Of course, not all research can be accomplished from the Antarctic region within a few weeks' time. Hence the motivation for developing balloons that achieve longer flights at midlatitudes.
Since the 1960s and '70s, an alternative to zero-pressure balloons has offered the intriguing promise of extending the length of scientific balloon flights. In this design, the balloon maintains a constant volume and amount of gas while the gas pressure varies according to its heating or cooling through the full cycle from bright day to dark night. These types of balloons, called superpressure balloons, originally were Mylar spheres inflated with helium and then sealed so the gas could not escape. Although heating during the day increases the internal pressure and the nighttime cooling decreases it, so long as the internal pressure is greater than the outside pressure, the balloon remains at a constant volume and will remain at a stable altitude.
Spherical Mylar superpressure balloons have flown with small lightweight payloads for very long periods at low altitudes. As a balloon material, however, Mylar is less than ideal. Not only is it hard to work with in fabricating a balloon but also its performance characteristics are marginal. Mylar is susceptible to pinholes and cracks. Although it is strong, it has very poor resistance to tearing. Beyond a certain size, Mylar balloons were highly likely to contain flaws that would lead to catastrophic failures. As a result, the Mylar spherical superpressure balloons could not fly high enough or carry enough weight to meet the growing needs of research.
New thinking about an old idea
Although spherical superpressure balloons that flew for several hundred days were tested, there was no apparent way to upgrade them to carry the heavy payloads required by scientists. In a spherical superpressure balloon, the strength requirement of the film is directly related to both the balloon's radius and its differential pressure (internal pressure minus external pressure) at its designed flight altitude. The bigger the balloon, the larger the radius and thus the stronger the material had to be. Stronger materials generally mean heavier materials, which require an even larger balloon. Spherical balloon design, thus, is a vicious circle with diminishing gains as the volume increases. Introducing the payload weight into the balloon has also proved to be a difficult design challenge.
Enter a design that had been around for decades, but whose potential had not been actualized with then-existing materials. The superpressure pumpkin balloon is a moderately flattened spheroid in shape with tendons running from apex to base. Because the tendons, which are braided ropes, are shorter in length than the shell upon which they sit, they cinch in the flattened spheroid, producing a bulge or lobe between neighboring tendons--hence the "pumpkin" name.
The pumpkin-shaped superpressure balloon holds considerable advantage over the spherical superpressure balloon. Hidden in that advantage is an important concept about the design of very large superpressure balloons. A major stress factor for superpressure balloons is their differential pressure at their design altitude. This pressure will rise and fall with temperature, but the balloon volume must remain the same, and the material must be strong enough not to burst or expand as pressure rises. Putting our alternate balloon designs in perspective:
* the zero-pressure balloon is vented to the outside and has minimal differential pressure;
* the spherical superpressure balloon has a differential pressure, and the entire spherical film must collectively bear the stress (or pressure load) of that differential pressure;
* the superpressure pumpkin balloon has a differential pressure, but the many cinched-in tendons carry the main pressure load, leaving the film to carry only the local load between tendons.
The pumpkin design permits the film to remain thin even as the volume increases. This design also places the payload weight on the tendons. Given these considerations, then, the pumpkin design opens the way for integrating better films and fibers for making tendons into a new class of scientific balloon. The ULDB is NASA's entrant in this new class.
The lobed design and the advances in film and fiber technology (see sidebar) have made it possible for the superpressure pumpkin balloon to emerge at this time. This design also allows the balloon to be scaled up in size without requiring the materials to be proportionally stronger.
Who wants a ride?
Scientists who use balloons as platforms for collecting data have diverse needs. They fly some payloads that "look" out at the universe and others that look in toward Earth. They exploit the advantages of balloon flights for such diverse disciplines as infrared astronomy, cosmic and heliospheric physics, plasma physics, high-energy astrophysics, solar physics, and upper atmosphere research. Scientific balloons have demonstrated that they can flexibly and inexpensively help scientists attain great heights from which to divine the origin of the universe, calibrate solar cells for use on spacecraft, or perform a multitude of other tasks.
A balloon flight over Antarctica in January 1999, for example, demonstrated the quality of science that can be performed from a balloon. Over a 10-day period, the Boomerang payload traveled 5,000 miles (8,000 kilometers) in a great circle around the South Pole while capturing the clearest images yet produced of the early universe. Boomerang's data helped show that the universe is flat and that it will continue to expand forever. This discovery has been considered one of the ten great discoveries of the past decade. Consistent with the pattern of scientific ballooning, the Boomerang payload had flown before this history-making flight and will fly again to provide even greater understanding of the origins of our universe.
Balloons are not useful for every science discipline, but they offer some significant advantages. Low flight costs and high weight-carrying capabilities are the most obvious. A typical balloon mission can be tens of millions of dollars cheaper than a rocket mission. Scientific ballooning can accommodate very large payloads both in terms of weight and volume (payloads up to 8,000 pounds have been flown). The large available payload volume can carry instruments freed from the rocket missions' constraint of being compacted into a canister and then unfolded in space.
The gentleness of a balloon launch is a big plus in comparison to the strong vibrations and G forces of a space launch. Thus balloon payloads do not need to be so rugged and they can be fabricated more quickly and at lower costs than comparable space-based versions. One of the most attractive features of ballooning is that it allows payloads to be recovered, refurbished, improved, and reflown. Science instruments that fly over and over again with improvements made for each flight allow the cutting edge of technology to be tested in very short time frames.
A balloon launch is even more dependent on nature's caprices than a space launch, especially because of its need for very low velocity surface winds. Furthermore, the balloon's flight path is a function of the winds at float altitude. One can never foresee the exact flight path, although weather forecasting permits scientists to estimate it. Decisions about terminating a flight and landing the payload are made by a pilot and observer in an airplane watching the balloon. The payloads are landed in remote areas with sparse population, which does limit the locations of balloon flights.
With ULDB promising much longer flights at stable altitudes, scientists are beginning to raise their expectations for what they can achieve with a scientific balloon. Scientists using instruments that previously could have been flown only on space-based missions will have a new intermediate alternative that, to all the benefits of zero-pressure balloons, adds flight options up to 100 days. Potential new payloads include instruments offering far better resolution than present space- based instruments, solar telescopes, and planet finders.
Advances made as part of the ULDB project have already been used within and beyond the ballooning community. Spin-off technology is already being used in the clothing industry. The breakthroughs in materials and designs are being applied to more advanced balloon systems to explore Mars and other planets. The integrated balloon technology has direct applications for large space-based inflatable solar arrays, signal detectors, and habitats, while the fibers alone will find uses in space such as in stronger, lighter-weight tethers and structures.
Both commercial and military scientists have high hopes for the superpressure pumpkin balloon in the emerging markets for high-altitude airships and position-holding balloons. Commercial companies are vying to place balloon-borne telecommunication systems over expanding markets. Several groups are working on concepts for relaying cell, voice, data, and other content from a single station-keeping balloon over a metropolitan area. Likewise, military users are bounded only by the imagination. Battlefield surveillance, communications, target identification, and signal jamming are just a hint of the potential.
In all of these areas and more, the design and materials advances issuing from the ULDB program alter the functional ground rules. In the station-keeping market, the only real competition is the promise of solar-powered airplanes [see "Soaring with the Sun," The World & I, August 1999, p. 166]. They offer access to similar regions of the upper atmosphere, but with significantly lower maximum payload weight capacity. In comparison, balloons can carry payloads well over 10 times the solar-powered airplane payload to the same altitudes.
Building on success
The successful test flight in June 2000 involved a balloon of 2.4 million cubic feet. A larger balloon is required to meet the ULDB mission goal to fly up to 100 days with 4,500 pounds of suspended load. The first balloon capable of achieving such lofty goals is scheduled for launch in January 2001 from Alice Springs, Australia. If all goes as planned, that 18.4-million-cubic-foot balloon will take off from the Australian Outback, circle the globe in approximately 15 days, and then return to Australian soil. If the January flight is successful, a full 100 days' duration flight will be launched in December 2001. After that, NASA hopes to be ready to offer a new low-cost tool to the scientific community. In addition, the technology will be available to commercial and military interests.
Looking beyond Earth's up[per atmosphere and orbits around Earth, planetary scientists are so impressed with the potential of superpressure pumpkin balloons to explore other planets that initial tests are now being conducted. It is only a matter of time. NASA's superpressure pumpkin balloons are living up to the motto "Giving Science a Lift to the Outer Limits."n
Henry M. Cathey Jr. works for New Mexico State University's Physical Science Laboratory. He is also balloon vehicle manager for NASA's Ultra Long Duration Balloon project, which is headquartered at the Goddard Space Flight Center's Wallops Flight Facility in Virginia.
On the Internet
Ultra-Long Duration Balloon (ULDB) Project
National Scientific Balloon Facility
Raven Industries, Inc. makes the balloon itself
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|Title Annotation:||Ultra-Long Duration Balloon Project|
|Author:||Cathey Jr., Henry M.|
|Publication:||World and I|
|Article Type:||Product Announcement|
|Date:||Jan 1, 2001|
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