The UltraFlex 175 solar array resembles something between a fan and an umbrella. In its stowed configuration it is folded somewhat like a fan. When it deploys, its umbrella-like gores unfold, pivoting around a central hub. The end panels remain a few degrees short of closing to complete the circle at which point the gore tensioning method developed under the ST8 program brings the UltraFlex 175 to its final deployed shape. The gore tensioning mechanism consists of flat steel flexure-type spring elements that attach the two end panels to their adjacent gore material. During the final few degrees of closure, the flexure springs deflect and introduce a known, in-plane tension into the deployed structure’s triangular gores. This provides a more consistent gore loading and uniform tension distribution that is particularly beneficial for scaling to large sizes.
The UltraFlex 175 solar array has ten gores between the spars—interconnected triangular shaped panels made of an open weave fabric called Vectran. The solar cells are attached to this open weave fabric, which is super tough, lightweight, and remains flexible and strong even in a very cold environment. The airbags used for the landings of the Mars Pathfinder Rover and the Mars Exploration Rovers Spirit and Opportunity were made of Vectran fibers. On the UltraFlex 175 solar array, the rectangular shaped solar cells are arranged on each Vectran gore to maximize the area covered, while leaving an uncovered strip down the center of each gore to allow the flexible Vectran fabric to fold in half when stowed.
Arrangement of solar cells on a single, triangular gore of Vectran.
Detail of solar cells attached to Vectran substrate.
The solar cells themselves are triple junction, Gallium Arsenide (GaAs) cells. Multijunction solar cells integrate several layers of semi-conductor materials into a single structure that converts more of the light spectrum to electrical power. The solar cells used in the UltraFlex 175 solar array can attain 28% efficiency, meaning 28% of the energy that strikes them is converted to electricity that can do useful work on the spacecraft. Just a few years ago, this level of solar cell efficiency would have been considered impossible to achieve. Although manufacturing costs are higher for multijunction solar cells, they are especially advantageous for space applications. Fewer cells are required if they are of higher efficiency, thus reducing the size and mass of the solar array.
From its stowed position, the solar array structure begins deploying when the tie-down mechanism releases. The still-folded package rotates away from the spacecraft around the base mechanism and latches in place. Then a motor-driven lanyard that is wrapped around the outside of both rigid end panels (one is stationary and the other pivots) begins to be retracted by a pulley system, pulling the pivot panel and unfurling the solar array blanket nearly 360 degrees. When the blanket is fully unfurled, the lanyard continues to retract, putting tension on the blanket in a controlled manner via the spring tensioning method described previously. The array then becomes a preloaded membrane with exceptionally high stiffness for its considerable size.
