Author: Jason Croft
Date: 02-23-09 19:09
Today's spacecraft carry their source of power. The cost of space travel could be drastically reduced by leaving the fuel and massive components behind and beaming high-intensity laser light or microwave energy to the vehicles. Experiments sponsored over the past year by the National Aeronautics and Space Administration and the U.S. Air Force have demonstrated what I call a lightcraft, which rides along a pulsed infrared laser beam from the ground. Reflective surfaces in the craft focus the beam into a ring, where it heats air to a temperature nearly five times hotter than the surface of the sun, causing the air to expand explosively for thrust.
Using an army 10-kilowatt carbon dioxide laser pulsing 28 times per second, Franklin B. Mead of the U.S. Air Force Research Laboratory and I have successfully propelled spin-stabilized miniature lightcraft measuring 10 to 15 centimeters (four to six inches) in diameter to altitudes of up to 30 meters (99 feet) in roughly three seconds. We have funding to increase the laser power to 100 kilowatts, which will enable flights up to a 30-kilometer altitude. Although today's models weigh less than 50 grams (two ounces), our five-year goal is to accelerate a one-kilogram microsatellite into low-Earth orbit with a custom-built, one-megawatt ground- based laser--using just a few hundred dollars' worth of electricity.
Current lightcraft demonstration vehicles are made of ordinary aircraft-grade aluminum and consist of a forward aeroshell, or covering, an annular (ring-shaped) cowl and an aft part consisting of an optic and expansion nozzle. During atmospheric flight, the forward section compresses the air and directs it to the engine inlet. The annular cowl takes the brunt of the thrust. The aft section serves as a parabolic collection mirror that concentrates the infrared laser light into an annular focus, while providing another surface against which the hot-air exhaust can press. The design offers automatic steering: if the craft starts to move outside the beam, the thrust inclines and pushes the vehicle back.
A one-kilogram lightcraft will accelerate this way to about Mach 5 and reach 30 kilometers' altitude, then switch to onboard liquid hydrogen for propellant as air becomes scarce. One kilogram of hydrogen should suffice to take the craft to orbit. A version 1.4 meters in diameter should be able to orbit microsatellites of up to 100 kilograms by riding a 100-megawatt laser beam. Because the beams we use are pulsed, this power might be achieved fairly easily by combining the output from a group of lasers. Such lasers could launch communications satellites and de-orbit them when their electronics become obsolete.
Lightcraft with different geometries can move toward their energy source rather than away from it--or even sideways. These variant vehicles have potential for moving cargo economically around the planet. Lightcraft could also be powered by microwaves. Microwaves cannot achieve such high power densities as lasers, so the vehicles would have to be larger. But microwave sources are considerably less expensive and easier to scale to very high powers.
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