space@mit-mc (03/05/85)
From: Ted Anderson <OTA@S1-A.ARPA> Several people have noted that the energy gained by a solar sail happens in the form of red shifting the reflected light. As most people probably know a rocket is most efficient when it is operating speed is close to its exhaust speed. You can get a feel for this by noting that when the rocket's speed is the same as its exhaust the exhaust particles aren't moving in the initial frame of reference. This means they have no kinetic energy in the rest frame either. In other words, the efficiency argument can be stated by saying that any kinetic energy left in the exhaust in the rest frame is wasted. The implication of this for light sails is that at low velocities they are abysmally inefficient (though of course they have certain clear advantages, such as low fuel consumption), but at substantial fractions of the speed of light they are wonderful. Actually they lose some attraction a very close to the speed of light since if the light comes from a stationary source it is seriously red shifted by the time it reaches the sail in the first place. This brings to mind another idea I saw a few days ago. I was reading the new Pournelle collection called "Far Frontiers" which has an article by Robert Forward which reviews interstellar propulsion technologies. In there he suggests making a interstellar probe out of a microwave sail. Similar to the idea of poking 1000 Ang. holes in an aluminum visible light sail to save weight, with a microwave sail you only need a 1 cm wire mesh. Forward's suggested design called for a 10GW microwave beam, a 16 gram sail, a 4 gram distributed processing element, 115 g acceleration, and a terminal velocity of 20% c. This gives an acceleration time of 3 days and a distance during acceleration of only 50 AU which greatly simplifies the beam divergence problem. Since 10GW is about the power level commonly suggested for Solar Power Satellites Forward suggested that launching these might be a good use for an SPS during initial checkout. A wonderfully cheap and easy way the explore our local neighborhood.