Beam-powered propulsion
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Beam-powered propulsion is a class of spacecraft propulsion mechanisms that use energy beamed to the spacecraft from a remote power plant.
Thermal propulsion
With beamed propulsion one can leave the power-source stationary on the ground, and directly (or via a heat exchanger) heat propellant on the spacecraft with a maser or a laser beam from a fixed installation. This permits the spacecraft to leave its power-source at home, saving significant amounts of mass, greatly improving performance.
Since a laser can heat propellant to extremely high temperatures, this potentially greatly improves the efficiency of a rocket, as exhaust velocity is proportional to the square root of the temperature. Normal chemical rockets have an exhaust speed limited by the fixed amount of energy in the propellants, but beamed propulsion systems have no particular theoretical limit (although in practice there are temperature limits).
Ablative Laser Propulsion is a form of laser propulsion that uses a laser to create a plasma plume from a metal propellant, thus producing thrust.
In addition, microwaves can be used to heat a suitable heat exchanger, which in turn heats a propellant (very typically hydrogen). This can give a combination of high specific impulse (700-900 seconds) as well as good thrust/weight ratio (50-150). http://etd.caltech.edu/etd/available/etd-06022006-160023/ Kevin Parkin's PhD thesis on microwave/thermal propulsion.
Electric propulsion
Some proposed spacecraft propulsion mechanisms use power in the form of electricity. Usually these schemes assume either solar panels, or an on-board reactor. However, both power sources are heavy.Beamed propulsion in the form of laser can increase the power delivered to photoelectric panels, however careful design of the panels is necessary as the extra power tends to cause a fall-off of the conversion efficiency due to heating effects.
Alternatively, Microwave broadcast power has been practically demonstrated several times (e.g. Goldstone, California in 1974), rectennas are lightweight and can handle high power loads.
Other propulsion techniques
A beam could also be used to provide impulse directly, for example using a solar sail to reflect a laser beam or using a magnetic sail or MMPP sail to divert a beam of charged particles from a particle accelerator.
Testing
Early in the morning of 2 October 2000 at the High Energy Laser Systems Test Facility (HELSTF), Lightcraft Technologies, Inc. (LTI) with the help of Franklin B. Mead of the U.S. Air Force Research Laboratory and Leik Myrabo set a new world's altitude record of 233 feet (71 meters) for its 4.8 inch (12.2 cm) diameter, 1.8 ounce, laser-boosted rocket in a flight lasting 12.7 seconds. Although much of the 8:35 am flight was spent hovering at 230+ feet, the Lightcraft earned a world record for the longest ever laser-powered free flight and the greatest "air time" (i.e., launch-to-landing/recovery)from a light-propelled object. This is comparable to Robert Goddard's first test flight of his rocket design. Increasing the laser power to 100 kilowatts will enable flights up to a 30-kilometer altitude. Their goal is to accelerate a one-kilogram microsatellite into low Earth orbit using a custom-built, one megawatt ground-based laser. Such a system would use just a few hundred dollars' worth of electricity, placing launch costs per kilogram to many times less than current launch costs (which are measured in thousands of dollars).
Myrabo's "lightcraft" design is a reflective funnel-shaped craft that channels heat from the laser, towards the center, using a reflective parabolic surface causing the laser to literally explode the air underneath it, generating lift. 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.
Non-spacecraft applications
In 1964 Brown flew a microwave powered helicopter for an extended period.
In 2002 a Japanese group propelled a tiny aluminium airplane by using a laser to vaporize a water droplet clinging to it, and in 2003 NASA researchers flew an 11 ounce (312 g) model airplane with a propeller powered with solar panels illuminated by an infrared laser. It is possible that such beam-powered propulsion could be useful for long-duration high altitude unmanned aircraft or balloons, perhaps designed to serve as communication relays or surveillance platforms.
A "laser broom" would be intended to sweep space debris from Earth orbit. This is another proposed use of beam-powered propulsion, used on objects that were not designed to be propelled by it, for example small pieces of scrap knocked off of ("spalled") satellites. The technique works since the laser power ablates one side of the object, giving an impulse that changes the eccentricity of the objects orbit. The orbit would then intersect the atmosphere and burn up.
See also: spacecraft propulsion
References
External links
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