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Comment by jug6ernaut | Hacker News Reader

The amperage is not the issue(main issue at least).

The way a rail gun works the projectile(or its casing) must make physical and solid contact with the barrel. This is because the electric current actually flows through the projectile from one rail to the other rail[1]. This produces huge amounts of wear on the barrel, making it extremely difficult to maintain the barrel. Both in form and in being able have the projectiles make solid contact.

[1] http://upload.wikimedia.org/wikipedia/commons/9/9a/Railgun-1...

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Ah, that makes a lot of sense. Thanks for that image. Until now I had assumed it was a coil that hurled a solid metal core (which always had me wondering where the rail part of the name came from). No idea how I failed to catch that for years.

Why couldn't there be small metallic wheels in the barrel making contact with the projectile? Wouldn't that prevent wear?

I imagine there's still a tremendous amount of unstable vibration being introduced by sudden acceleration to mach 7. There's just lots of force involved in these things.

I imagine you could do something like this.

Off the top of my head this would pose a few issues.

1) While the magnetic force must go from one rail to the other, the amount of electric force is effected by how much of the body is doing the conducting(if my memory servers me). So while you could use wheels, you would still want the current to flow through the entire(or large portion) of the projectile housing(for lack of a better term). This leads to the second issue.

2) Melting. With these huge amperage's there is also huge heat levels. If you were to conduct this much current through a small point(as would be the case with a wheel) the heat generated would be even worse. Most likely causing the wheels/barrings to melt.

Neither of which cant be accounted for, just issues that would have to be considered.

Many wheels, possibly in the form of many small ball bearings on a rail should work. Many small cylinders instead of spherical bearings would be better.

Even better would be a highly conductive grease non-flammable grease.

The bearing idea isn't a solution for a few reasons:

1) Rotating bearings still have internal friction, which will still generate tremendous heat at these velocities.

2) A lot of the heat being generated here is caused by the flow of electrical current, which isn't negated by bearings. In fact, it would probably be made worse, because any spherical contact patch is going to be tiny compared to a flat surface. The suggestion of using many small bearings would seem to alleviate this problem, but the smaller you make the bearing, the more fragile it becomes.

At its heart, this is a materials science problem. You need a material that can conduct the electricity, withstand the heat, and not break down under high friction loads while doing both of these. Simply dropping in a bearing doesn't solve any of these problems

The highly conductive, non-flammable grease is just another materials challenge. At the kinds of temperatures present in a rail gun, "non-flammable" becomes a very high bar for a lubricating fluid.

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