I' There's another situation where it doesn't when the two coils are in the same plane. i>f'IVI 9 C..I\. .. G>" 6UtMl-T ~~ ~M~"-'t/E CO U.. , , F•lle~ "110 · , ACC.• LallA,,_, --· • It - • - • • • • ! • 3 pay to turn on the drive current: In that situation, the magnetic force would act to try to expand or compress the bucket coil, but would not provide any forward force, as you can see by the sketch on the left. So the purpose of the electrical circuitry of the mass-driver is to turn on the drive current just at the times, and in the directions, that will give the maximum amount of forward push for the least total expenditure of power. The relationships of the drive current and the bucket position are shown in the sketch below: MA,X·. + ae.-0...-----..... t----......... c~a..n- '"' l>ft. I Y£ GOIL - The currents have to change very quickly. For a mass-driver used to launch material from the Moon, for example (a machine with a drive-coil diameter of about 12 inches) the drive current at the high speed end, where the bucket has reached nearly full speed, has to rise to its maximum value of several thousand amperes, and then come back to zero again, in less than fifty millionths of a second. As you can see, each drive coil provides first a pull, as the bucket coil approaches, and then a push, as it recedes.
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