Machine Design

Superpressure Balloons Reach New Heights

Balloons originally designed to drift in the Earth's upper atmosphere for months may visit Martian skies in the next few years.

Cameras on balloons can provide a better view of surface objects than cameras on orbital spacecraft.
No, it's not a floating jack-o'-lantern. The pumpkin-shaped ultralong-duration balloon should be able to fly above the Earth's atmosphere for up to 100 days.

Though they were first designed in the late 1700s, balloons may turn out to be a key technology for extraterrestrial exploration. Researchers at NASA's Goddard Space Flight Center, Greenbelt, Md., are developing unmanned balloons that withstand the harsh environment of earth's uppermost atmosphere and whose next mission may be on Mars. Their current task is to investigate gamma particles that could give clues to the origin of the universe. These "superpressure" balloons are sealed and made from material that can hold up under dramatic pressure and volume changes caused by the sun heating the gas.

This technology is being applied to aerobots, self-deploying and inflating balloons that could float through the Martian atmosphere, taking pictures and dropping probes. The first mission is scheduled for December with a payload that tracks cosmic rays.

Balloons as a means of transport compare favorably to rockets in a couple of important ways. For instance, balloons can lift payloads without the vibration and g-forces of rocket launches, and often their payload is recoverable. A conventional balloon will stay afloat for one to two days, but the pressurized, pumpkin-shaped balloon on which NASA is working should be able to fly longer.

The design goal, says NASA, is to support a 2,200-lb scientific payload and deliver 800 W of continuous power to it for 100 twelve-hour days. The balloon payload collects solar energy with three solar panels and stores power in nickel-metal hydride batteries. Scientists will be able to command instruments and read scientific data back on Earth via the Internet. The telemetry design may use a commercial satellite network as well as TDRSS, the satellite on which the space shuttle relies for communications.

The pumpkin-shaped design differs from the typical spherical version. Spherical balloons are prone to stresses along meridian and circumference areas because the balloon envelope carries most of the load. The pumpkin shape relies on meridian tendons binding sections of material together, also called gores. The balloon will have 20 miles of seams, making sealing procedures critical. To maintain tight fabric tolerances, a laser cuts the gores. There are 134 gores on the ultralong-duration balloon (ULDB). It measures 104.5-ft tall and has a 174.4-ft diameter.

Development of material for the balloon has been the effort's biggest challenge, according to Steve Smith, Goddard Space Flight Center manager for the ULDB project. "We tried to come up with a single layer of material that would do all the wondrous things we want it to do and unfortunately there is no such thing."

One of the demands is that the fabric be tough, able to resist tearing if a puncture occurs so the hole doesn't enlarge. Mylar is strong, but has zero tear toughness when there's a hole. Polyethylene resists further tearing, but lacks high strength.

"We had to try to combine the materials, keeping the good properties of one and canceling out the bad properties of the other," says Smith. The candidate material also must resist UV degradation at altitude and stay pliable at nighttime temperatures as low as –90°C. And it must be lightweight.

The superpressure balloon itself is made of layers containing polyester fabric, Mylar film, and polyethylene film bonded with a proprietary formula adhesive from Shell. The balloon is super-pressurized with a cap analogous to a cap on a radiator. Durable composite plastic and fabric materials are being developed to let the ULDB maintain lift, size, and shape, and not react to atmospheric influences.

NASA's Jet Propulsion Laboratory in Pasadena, Calif., is designing smaller ULDB balloons to navigate the Martian atmosphere. Technology is in the works to visit Mars via superpressure balloons in 2003. These aerobots will deploy in the upper atmosphere. The Mars Aerobot Technology Experiment (Mabtex) entails a low-cost test of technology for Mars balloon exploration. It will involve a superpressure balloon deploying and inflating midair in a thin, Martian atmosphere 2 to 3 km above the planet's surface.

Navigation of the aerobot and altitude control will be tested. A miniature wide-angle camera will gather stereo images of Mars to bridge the resolution gap between the orbital and rover data. The projected date launch of the Mabtex is April 2003.

NASA balloons may be headed toward Venus and its moon, Titan. Venus has a dense carbon dioxide atmosphere and surface temperature around 450°C. No ordinary balloon design could handle such conditions. One possible approach under investigation uses a reversible fluid. Condensation of a heavier-than-air fluid at higher altitudes makes the balloon descend. Vaporization makes it rise. Using argon as a reversible fluid permits numerous visits to the surface for hours or even days before rising to altitude and drifting to another location.

Balloon technology already has a pretty good track record on Venus. In 1985, the Soviet Union and France deployed two balloons which lasted for at least two days in the Venusian atmosphere. Future missions would most likely focus on surface and surface/atmosphere interaction. An aerobot 50-km high could drop small probes which, for example, might measure atmospheric conditions and magnetic fields. A reversible-fluid balloon could descend to the surface and then rise before its electronics package overheats.

"I would say in the next five to ten years you could actually see a mission to one of the other planets with a balloon system," says Steve Smith, of Goddard Space Flight Center. NASA is currently demonstrating and proving out the basic technology.

TAGS: Defense
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