Direct Impulse
A beam could also be used to provide impulse by directly "pushing" on the sail.
One example of this would be using a solar sail to reflect a laser beam. This concept, called a laser-pushed lightsail, was analyzed by physicist Robert L. Forward in 1989 as a method of Interstellar travel that would avoid extremely high mass ratios by not carrying fuel. His work elaborated on a proposal initially made by Marx. Further analysis of the concept was done by Landis, Mallove and Matloff, Andrews and others.
In a later paper, Forward proposed pushing a sail with a microwave beam. This has the advantage that the sail need not be a continuous surface. Forward tagged his proposal for an ultralight sail "Starwisp". A later analysis by Landis suggested that the Starwisp concept as originally proposed by Forward would not work, but variations on the proposal continue to be proposed.
The beam has to have a large diameter so that only a small portion of the beam misses the sail due to diffraction and the laser or microwave antenna has to have a good pointing stability so that the craft can tilt its sails fast enough to follow the center of the beam. This gets more important when going from interplanetary travel to interstellar travel, and when going from a fly-by mission, to a landing mission, to a return mission. The laser or the microwave sender would probably be a large phased array of small devices, which get their energy directly from solar radiation. The size of the array obsoletes any lens or mirror.
Another beam-pushed concept would be to use a magnetic sail or MMPP sail to divert a beam of charged particles from a particle accelerator or plasma jet. Jordin Kare has proposed a variant to this whereby a "beam" of small laser accelerated light sails would transfer momentum to a magsail vehicle.
Another beam-pushed concept uses ordinary matter and works in vacuum. The matter from a stationary mass-driver is "reflected" by the spacecraft, cf. mass driver. The spacecraft neither needs energy nor reaction mass for propulsion of its own.
Read more about this topic: Beam-powered Propulsion
Famous quotes containing the words direct and/or impulse:
“Parliament must not be told a direct untruth, but its quite possible to allow them to mislead themselves.”
—Norman Tebbit (b. 1931)
“Sometimes we sailed as gently and steadily as the clouds overhead, watching the receding shores and the motions of our sail; the play of its pulse so like our own lives, so thin and yet so full of life, so noiseless when it labored hardest, so noisy and impatient when least effective; now bending to some generous impulse of the breeze, and then fluttering and flapping with a kind of human suspense.”
—Henry David Thoreau (18171862)