>> What's It Made of and How Does It Work?

Basic deployable mast technology has been previously used successfully on numerous space missions.

The shuttle Radar Topography Mission (SRTM), an 11-day mission flown in February 2000, mapped Earth's topography using two radar antennas, one in the Shuttle's cargo bay and the other at the end of a 60-meter-long deployed mast.

The fiberglass-based coilable mast or boom is a well-understood, flight-proven and mass-efficient structure capable of reliable self-deployment. Two key structural elements, longerons and battens, for these previously flown coilable masts were made of fiberglass, which gave them a deployed mass of around 93 grams/meter.

Now, the use of advanced, highly flexible graphite fibers instead of fiberglass gives still better performance and even lower mass. The fiber strain limitations of these composites does require that the individual structural elements be more slender than were the fiberglass elements. Luckily, such very slender elements are exactly what is needed for gossamer applications like solar sails. This very slenderness, though, makes the elements more sensitive to any imperfections in the manufacture and assembly of the structure. These imperfections can significantly reduce the stiffness and strength of the mast.

New, carbon-fiber masts use the same proven structural design as the previous masts—continuous, coilable, longerons running the complete length of the mast. These longerons can be coiled and stowed in less than 1% of their deployed length, making a compact package for launch. The mast is self-deployed, using the strain energy stored in the coiled and stowed longerons. A lanyard controls the rate of deployment. The mass of the carbon-fiber structure is less than 35 grams / meter when deployed.

The coilable ultra-lightweight mast uncoils.

The mast fully deployed in the laboratory, supporting one quadrant of a solar sail.

See a video (3 Mb) of the mast deploying in the laboratory.

See a video (7 Mb) of the deployment of a solar sail quadrant in the laboratory.

Structure of mast uses carbon fiber longerons and battens, and strong, flexible thread diagonals to balance tension and provide stiffness.

The mast design consists of three continuous, carbon-fiber longerons forming an equilateral triangular section and kept apart by small structural elements called “battens," which retain a preload when deployed. “Diagonals” are small structural elements that connect diagonally opposite batten connections to provide shear stiffness for maintaining the straightness of the mast. The combination of battens and diagonals is a design element repeated regularly along the length of the mast, each element forming a “bay." The diagonals are of a length that buckles the battens, thus maintaining tension in the diagonals and providing stiffness.

For more information on the design and technical specifications of the SAILMAST, please see the technical paper "The ST8 SAILMAST Validation Experiment," prepared by Michael A. McEachen et al. for the American Institute of Aeronautics and Astronautics.

Diameter of carbon fiber longeron used for SAILMAST compared with diameters of longerons used on the Shuttle Radar Topography Mission (SRTM); the Solar Array Flight Experiment (SAFE), which was a 1984 Space Shuttle experiment deploying a "large" solar array; and the Cassini mission to Saturn. In the table, L/D is the ratio of length to diameter.

This ultra lightweight graphite coilable mast is of a slenderness far beyond that of any other structure previously flown, and providing an advance beyond the current technology of deployable structures in addition to its low mass per unit length for a given stiffness. This structure is being developed for near-term use as a solar sail mast, but it is also applicable to many lighter booms that stow compactly that will be needed in the future. This new technology will be broadly enabling to future gossamer spacecraft missions, just as the coilable fiberglass boom structure has been employed in many past and ongoing missions.