A mass driver is not the same as a railgun, with which it is sometimes confused. A railgun uses the payload itself (or the bucket) to close a circuit gap and drive a continuously variable magnetic field. The mass driver is a more complex device that is designed to be more robust, scalable and controllable. In a mass driver, the guide rails for the payload are not powered. The bucket contains an electromagnet (or regular magnet depending on the application). The rails run through the center openings of a series of electromagnetic coils of fine tolerance. The bucket is initially propelled down the rails either by mechanism or thrusters. When the magnetic field of the bucket has passed the centerpoint of the first coil a capacitor is discharged through the coil, creating a magnetic field of identical polarity. This pushes the bucket down the rails faster. When it passes the next coil, that one fires, and so on. The advantages of this approach over railguns are manyfold if you intend to use the system repeatedly:
  • No ionizing arc across the bucket/rail interface
  • No extremely high current need pass through the payload, allowing more sensitive items to be launched
  • It's controllable; depending on capacitor charge power and timing changes, you can vary the boost
  • This means that if you're using it in space you can vary the course (or trajectory) of the payload

Of course, the railgun can be made to stuff more acceleration into a shorter space, which makes it ideal for weapon uses. For maximum efficiency, you could even package each round of a railgun as an 'all-up round,' with a complete set of rails and projectile in each one. This would obviate the problem of rail degradation. The mass driver is a better bet for cargo use and anything of a large scale. The railgun also avoids the need for complex control systems for the rings.

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