There have been a few articles about the Navy\'s new Mach 7 33 Megajoule railgun

Question

There have been a few articles about the Navy's new Mach 7 33 Megajoule railgun. As a physics teacher, I have a couple of questions about this, and was hoping for some help.

Is the kinetic energy of the projectile 33 MJ, or is that the energy that was delivered from the capacitor bank that was used to fire the projectile, so that some of it would have been lost in resistive heating, etc.

The photos of the projectile show a cloud of hot gas behind the projectile? What is this? superheated air? Plasma from the electrical breakdown inside the railgun? What causes this?

Explanation / Answer

33 MJ is probably the projectile kinetic energy. The stored electrical energy required is this value, divided by the efficiency. Since this is a military project, the actual efficiency value is probably sensitive, and therefore not released.

The fireball behind the projectile may be burning vaporized metal from the projectile and rails. In a railgun, the current flows from one rail, through the projectile, and back into the other rail. The projectile makes a sliding electrical contact with the rails as it gets launched, and this contact carries an electrical current of tens of thousands of amps. This kind of current through any sort of moving or sliding high-current electrical contact is probably going to burn off a little metal from both the projectile and the rails.

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