Wikipedia on mass drivers

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http://en.wikipedia.org/wiki/Mass_driver

Brief: "

A mass driver or electromagnetic catapult is a proposed method of non-
rocket spacelaunch that would use a linear motor to accelerate and
catapult payloads up to high speeds. All existing and contemplated
mass drivers use coils of wire energized by electricity to make
electromagnets. Sequential firing of a row of electromagnets
accelerates the payload along a path. After leaving the path, the
payload continues to move due to inertia.

A mass driver is essentially a coilgun that magnetically accelerates a
package consisting of a magnetisable holder containing a payload. Once
the payload has been accelerated, the two separate, and the holder is
slowed and recycled for another payload.

Mass drivers can be used to propel spacecraft in two different ways: A
large, ground-based mass driver could be used to launch spacecraft
away from the Earth or another planet. A spacecraft could have a mass
driver on board, flinging large pieces of material into space to
propel itself. A hybrid design is also possible (see coilgun, railgun,
or helical railgun).

Miniaturized mass drivers can also be used as weapons in a similar
manner as classic firearms or cannon using chemical combustion."

Full detail: http://en.wikipedia.org/wiki/Mass_driver