Inflatable structure forms space antenna

Tustin, CA--Space-based antennae can present design engineers with seemingly contradictory requirements. Due to the tremendous transmission distances involved, they must be very large; yet, limited space within the launch vehicle requires them to store compactly. They should be light, but also rugged enough to withstand launch forces and a lifetime of cyclical thermal loading. And though precision devices, they must be inexpensive enough to be feasible, given current budgets.

Engineers at L'Garde, Inc. believe they have the perfect product to meet these requirements: an inflatable antenna. Scheduled to launch aboard shuttle STS 78 in May 1996, the Inflatable Antenna Experiment (IAE) is intended to demonstrate that a pneumatic structure can be made precisely enough to function as an antenna, and that it can be successfully deployed in space. The company claims their design is one to two orders of magnitude cheaper, and more compact when stored, than existing rigid designs. They say it's also two to eight times less massive.

L'Garde's antenna consists of three inflated components: a parabolic reflector, a circular torus, and three equal-length struts. In space, a single nitrogen bottle fills all three components via a simple solenoid valve. Separate valves between the components maintain the pressure in the torus and struts at 3 psi and in the reflector at a wispy 3 x 10 ⁴ psi. "By terrestrial standards, the pressure in the reflector is an excellent vacuum," says Costa Cassapakis, vice president of advanced programs.

To form a concave parabolic reflector--a seeming impossibility with inflatable components--engineers joined two parabolic-shaped sheets of Mylar(TM) 1/3-mil thick at the edges. One sheet is optically transparent, the other is uniformly coated with a few hundred microns of aluminum. Inflating the structure creates a lenticular balloon with a circular aperture 14m in diameter.

The reflector is supported by a tubular torus 15.2m in diameter with a 20-inch minor diameter. Made of rubberized DuPont Kevlar(TM) fabric, the relatively high-pressure torus serves as a frame for the lenticular reflector. It maintains the reflector's parabolic shape, a shape that would become spheroid if left unsupported.

Also made of rubberized Kevlar, the struts are positioned equidistant around the torus and extend 28m to the reflector's center of curvature. They attach to the Spartan spacecraft, which carries three CCD TV cameras and a large light panel. Used to test the reflector's accuracy, the panel is dotted with 3,700 LEDs grouped in 530 clusters.

In theory, "the mean accuracy from a perfect paraboloid is plus or minus 1 mm," says Cassapakis. "That means you can use this as an antenna for frequencies all the way up to 10 GHz, maybe more." Engineers will use the light panel and cameras to check this accuracy (and measure the thermal-distortion effects of the sun) by illuminating patterns of LED clusters and recording the reflected image.

As expected, building an accurate reflector 46 ft in diameter from strips of plastic sheet proved quite a challenge. Engineers found Mylar to be anisotropic, possessing a different modulus of elasticity along the warp than the fill. In addition, its modulus changes with stress, particularly at low stress levels.

"In order to know how to design the gore shapes you must take these variabilities into account," Cassapakis explains. To do so, L'Garde developed proprietary computer codes in house that determine the shape of the gores while accounting for known material and manufacturing variabilities. The system works so well that error from the seams between gores remains within about plus or minus 0.1 mm.

Punctures and leakage might pose the greatest threat to an inflatable space structure (though not for the several-hour-long IAE test). For permanent satellites, engineers claim that a reservoir of nitrogen should accommodate any tiny holes that appear in the extremely low-pressure, reflector canopy. A puncture in a strut or torus, however, could prove catastrophic.

To address this issue, engineers are experimenting with Kevlar, graphite fiber, and fiberglass cloths impregnated with UV-curable or water-based epoxies. Both types harden to a rigid structure in minutes. "You could lose pressure and not worry about a thing," says Cassapakis.

Additional details...Contact Dr. Costa Cassapakis, V.P. Advanced Programs, L'Garde, Inc., 15181 Woodlawn Ave., Tustin, CA 92680, (714) 259-0771.