Mass driver

A mass driver is a method of spacecraft propulsion that would use an electromagnetic accelerator to move a payload up to high speeds. All existing and contemplated mass drivers use coils of wire energized by electricity to make electromagnets.

Prototype mass drivers have existed since 1975. Most were constructed by the Space Studies Institute in order to prove their properties and practicality.

Refined designs use semiconductor switches to control the coil. The coils can be constructed of aluminum to save mass, and to permit them to be constructed from lunar materials. The best known performance occurs with an aluminum coil as the payload. The coils of the mass-driver induce eddy-currents (paramagnetism) in the payload's coil, and then act on the resulting magnetic field. There are two sections of a mass-driver. The maximum acceleration part spaces the coils at constant distances, and synchronize the coil currents to the bucket. In this section, the acceleration increases as the velocity increases, up to the maximum that the bucket can take. After that, the constant acceleration region begins. This region spaces the coils at increasing distances to give a fixed amount of velocity increase per unit of time.

In the prototypes, the payload would be held in a bucket and then released, so that the bucket can be decelerated and reused. If mounted on a planetary surface a mass driver could accelerate payloads to escape velocities (or provide a boost to orbital velocity, using a small onboard rocket to circularize the orbit once in space).

In this mode, the major proposal for use of mass-drivers was to throw lunar dirt at space habitats so that they could process it using solar energy. The Space Studies Institute showed that this application was reasonably practical.

Another variation is to have a mass-driver on a spacecraft, and use it to "reflect" masses from a stationary mass-driver. Each deceleration and acceleration of the mass contributes to the momentum of the spacecraft. The spacecraft need not carry reaction mass, and doesn't even need much electricity, beyond the amount needed to replace losses in the electronics.

If mounted on a spacecraft, a mass driver could use any type of mass for reaction mass to move the spacecraft. This, or some variation, seems ideal for deep-space vehicles that scavenge reaction mass from found resources.

One possible drawback of the mass driver is that it has the potential to send solid reaction mass travelling at dangerously high relative speeds into useful orbits and traffic lanes. To overcome this problem, most schemes plan to throw finely-divided dust, or liquids. Propelling the reaction mass to solar escape velocity is another way to ensure that it will not remain a hazard.