Editing Beam-powered propulsion (section)

==Background==
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Rockets are [[momentum]] machines; they use mass ejected from the rocket to provide momentum to the rocket. Momentum is the product of mass and velocity, so rockets generally attempt to put as much velocity into their [[working mass]] as possible, thereby minimizing the needed working mass. To accelerate the working mass, [[energy]] is required. In a conventional rocket, the fuel is chemically combined to provide the energy, and the resulting fuel products, the ash or exhaust, are used as the working mass.

There is no particular reason why the same fuel has to be used for both energy and momentum. In the [[jet engine]], for instance, the fuel is used only to produce energy, and the air provides the working mass the jet aircraft flies through. In modern jet engines, the amount of air propelled is much more significant than the amount used for energy. However, this is not a solution for the rockets as they quickly climb to altitudes where the air is too thin to be useful as a source of working mass.

Rockets can carry their working mass and use other energy sources. The problem is finding an energy source with a [[power-to-weight ratio]] that competes with chemical fuels. Small [[nuclear reactor]]s can compete in this regard, and considerable work on [[nuclear thermal propulsion]] was carried out in the 1960s, but environmental concerns and rising costs led to the ending of most of these programs.

Further improvement can be made by removing the energy created by the spacecraft. If the nuclear reactor is left on the ground and its energy is transmitted to the spacecraft, its weight is also removed. The issue then is getting the energy into the spacecraft. This is the idea behind beamed power.

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 and greatly improving performance.