[Hunpecked]

El_Phil writes: "...where did you get that railgun length from..."

A railgun that accelerates a projectile from velocity 0 to velocity "v" in "t" time has to be at least as long as the distance the projectile travels in that time. That distance "d" is given by

d = (1/2)*a*tE2

where "a" is the acceleration. We get "a" from

a = v/t

Approximating "c", the speed of light, as 3E8 m/s and using 1s or 10s for "t", we get the numbers in my earlier post.

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Edit: Actually, I was off 10-fold on the 1-second rail gun (damn decimal point!). The 1-second rail gun has to be 15,000 km long, not 1,500.

Mea culpa
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I was frankly surprised when I saw the results, until I remembered the post calculating Newtonian acceleration to "c" at 1 G: hundreds of days consumed and trillions of kilometers traveled.

If we settle for projectile speeds of a few km/s, as illustrated at the "Atomic Rockets" web site, rail gun lengths become more practical.