The first Starship Congress conference session was devoted to solar sails, leading off with Jim Benford’s keynote, followed by Les Johnson, who described current and near-term work. Right now the only propulsion method that will get us to interstellar velocities is the sail, and even then we’re talking no more than a couple of hundred kilometers per second.
Benford described a series of experiments he and his brother Greg made on beamed propulsion back in the year 2000. The researchers were working with a carbon fiber mat shaped into a small sail with a thickness of less than 1 millimeter.
Jim Benford is organizing Project Forward, named after the legendary Robert Forward, as an Icarus Interstellar effort to further refine the interstellar beamed sail concept. Asked to name the biggest problem areas for sails, the group came up with several, but at the top of the list was deceleration. How do you slow a beamed sail down when it arrives at its target? Forward proposed staged solar sails and others believe in magnetic sail braking.
Carbon fiber is ideal for sail work because when you put a microwave beam on the sail the material absorbs energy and begins to heat. A sail made of aluminum would begin to melt as you reach about 900 K, limiting possible accelerations, but carbon fiber has a low areal density (about 8 grams per square meter in the material the Benfords used) and a microwave reflectivity approaching 90 percent. The material is actually a carbon-carbon microtruss, meaning a core of carbon fibers is fused to a textured outer surface. With carbon nanotubes woven into the material, this microtruss is capable of temperatures up to 3000 K, at which point it doesn’t melt but sublimes, going from solid to gas with no intervening liquid state.
Working with a sail in an Earth-bound laboratory means you have to achieve an acceleration of one g just to lift off, but the Benfords were able to get up to 10 gs in these experiments, using a wavelength of about 3 centimeters and a pulse duration of 0.2 seconds.
Having learned that a sail can indeed be pushed to high accelerations by using a microwave beam, the Benfords were also able to show that a sail of the right shape — concave and something like a parachute — will be stable and stay centered in the beam. In fact, the beam induces a sideways restoring force so that even assuming a certain amount of ‘jitter’ in the beam itself, the sail is capable of riding the beam.
Sunjammer next year
The Sunjammer solar sail folds up into something the size of a dishwasher, but when deployed in space it will have spread to 1200 square meters, seven times the area of the IKAROS sail, while weighing a scant 32 kilograms (ten times less than IKAROS).
Sunjammer is a NASA-funded Technology Demonstration Mission, scheduled for launch in Q4, 2014, that will demonstrate NASA’s newest propulsion technology, solar sails. Sunjammer will be made of Kapton.
Sunjammer has a square sail, 124 feet (38 meters) wide on each side (total area 13,000 sq ft or 1,208 sq m). It will travel from the Sun-Earth L1 Lagrangian point 900,000 miles from Earth (1.5 million km) to a distance of 1,864,114 miles (3 million kilometers)
Tethers Unlimited was awarded a $100,000 NASA Innovative advanced concepts grant to develop the SpiderFab project. They will develop a process for automated on-orbit construction of very large structures and multifunctional components. The foundation of this process is a novel additive manufacturing technique called ‘SpiderFab’, which combines the techniques of fused deposition modeling (FDM) with methods derived from automated composite layup to enable rapid construction of very large, very high-strength-per-mass, lattice-like structures combining both compressive and tensile elements. This technique can integrate both high-strength structural materials and conducting materials to enable construction of multifunctional space system components such as antennas.