Ballooning To Mars

FAYETTEVILLE, Ark. - Every day Larry Roe grapples with questions like: How do you inflate what does not exist, under unknown conditions, using unknown materials, for an unknown application? Roe, a mechanical engineering professor at the University of Arkansas, faces these problems in his work with NASA and the Jet Propulsion Laboratory to invent inflation devices for space missions.

"Some people see these questions as insurmountable obstacles," Roe laughs. "Those kinds of questions can drive them crazy. But to me they are creative challenges."

Roe’s current challenge is designing systems to inflate structures for near-term space missions that are in various stages of planning. For near-term missions, Roe’s designs are grounded in known engineering principles and existing hardware. He is looking at structures that range in size from a baseball to 50 meters (161 feet) across. Subsequent missions may call for inflatables that are 3 to 5 kilometers (2 to 3 miles) across.

For missions to Mars or other planets with sufficient atmosphere, Roe envisions reconnaissance balloons. These small, lightweight devices could collect and transmit data or carry cameras to provide additional views of the planet.

A number of inflatable other devices have also been identified for these missions, according to Roe. These structures include data collection and communication antennas, solar shields, thermal radiators, solar concentrators, light sails, landing systems, rover tires and habitats. Although vastly different in design and purpose, they all have a common need for some type of inflation gas.

"Currently, devices such as antennas and reflectors are expensive, heavy and prone to failure," noted Roe. "Inflatable devices can overcome these obstacles, making space explorations more reliable and affordable."

Inflatable structures are classified according to the type of inflation they require, regardless of their size. The three main categories are continuously inflated (CI) structures, rigidized inflatable (RI) structures, and single-inflation (SI) devices. The gas used to inflate all three types must be light, non-contaminating to the spacecraft instruments, non-condensing under given pressure and temperatures, non-reactive with structural elements and able to be delivered reliably and controllably.

SI and RI devices are only inflated once. SI devices, such as landing bags are then discarded. RI structures, such as communication antennas, light sails or solar shields, are made of materials that can be folded and packaged, but harden once inflated and exposed to sunlight.

CI structures, such as data collection antennas, rover tires or habitats present a different set of problems. They must be continuously inflated throughout the life of the mission, and they invariably leak. In addition, some CI devices can be very large. A reflecting membrane currently envisioned for the ARISE (Advanced Radio Interferometry between Space and Earth) mission may be 25 meters (around 83 feet) in diameter.

"Preliminary characterizations studies show that either a tanked-gas or systems that generate inflation gas via chemical reaction are indicated," Roe explained. "Our prime candidate is the catalytic decomposition of hydrazine, which produces a mixture primarily of hydrogen and nitrogen."

Roe is doing conceptual designs for projects that are about 10 years in the future. He is focusing both on determining good ways to inflate devices and eliminating things that don’t work. For example, subliming solid systems used in the Echo series of balloons have contamination and control issues that must be addressed before they could be used reliably.

Optimization is another key issue, Roe indicated. Once a viable process is determined, it must be made more efficient. Inflation systems have already been reduced in size by 50 percent, but Roe believes that further efficiencies and size reductions are possible.

"For the near-term the primary development focus is on RI and CI structures," said Roe. "Inflation technology can be incorporated into deployable solar arrays for power, solar shields for spacecraft thermal management and antennas for data collection and communication."

Roe is presenting his findings in Atlanta on April 4, 2000, at the American Institute of Aeronautics an Astronautics Joint Conference on Structures, Structural Dynamics, and Materials.