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Based on the “blue balloon” pre-prototype studies, it was concluded that it would be very difficult to achieve desired pointing accuracies with a design which rolls on the ground surface, because of issues of base balloon deflections, distortions, and hysteresis effects associated with the roll-to-point paradigm, and also because of an inability to reach high tilt angles. Several variants of the “blue balloon” configuration were tried, including a version with in which the tethers were attached to an equatorial band, and a version in which the toroidal mirror support element was installed with a tilt angle bias. These variants were also unsuccessful in achieving the kind of pointing control that is necessary.

Trade studies were conducted to examine alternate design concepts utilizing inflatable-structure support for the reflective surface, but wherein the base balloon does not roll over the ground to

accomplish the two degree-of-freedom pointing function.

An initial




a base balloon, which


on a ring base with

prototype design was a ring of ball bearings

supporting the base balloon in a spatially fixed in elevation and azimuth. The base balloon in above it, which in turn supported a reflective

location while permitting it to change orientation turn supported a toroidal (donut-shaped) balloon membrane mounted on a lightweight perimeter

frame. Pointing control was achieved by motorized winches accomplishing differential control of several control tethers located around the perimeter of the heliostat. This prototype design was constructed, and is illustrated in Figures 4a through 4e.

length initial

Unfortunately, the initial prototype design was also found to have significant problems in precision pointing control and stability for high tilt (low elevation) angles. Upon study, the principal investigators concluded that such control problems were due to the overturning moment associated with the weight of the toroidal balloon and the reflective surface it supported, at the high tilt angles. The high overturning moments in turn caused substantially imbalanced tether loads for control tethers on opposite sides of the heliostat, and also resulted in the base spherical balloon lifting off the support bearing ring on one side.

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November 24, 2002

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