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Whenever rockets leave Earth, the satellites and other cargo they carry account for just a fraction of their weight. Most--up to 90 percent--of a rocket's weight comes from its fuel. This presents a problem for rocket designers: the more a rocket weighs, the more fuel you need to boost it into space. But how do you reduce a rocket's weight without reducing the fuel needed to launch it? Leik Myrabo, an aerospace engineer at Rensselaer Polytechnic Institute in Troy, New York, says the solution is to take the power source off the rocket and leave it on the ground, reducing launch costs to at least a hundredth of what they are now. Working at the Air Force Research Laboratory in California with aerospace engineer Franklin Mead, Myrabo has built a number of six-inch-wide prototypes that in early tests have reached a height of more than 70 feet without so much as a drop of onboard propellant...However, above about 18 miles, there's not enough air to form the plasma, so the craft would have to switch to a reserve of liquid hydrogen fuel, which the laser would ignite to boost it into space.