Navy's new Mach 7 Rail Gun Photo

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?

Nice question! I took a look at some of the news stories about this, but they don't seem to give the answer to your questions. Hopefully someone who's more familiar with military technology can come along and help you out. — David Z, Commented Dec 12, 2010 at 22:34
I agree. This is a nice question, which could fall under pop sci, but it's asked as a proper physics question, so I'm happy. :) — Noldorin, Commented Dec 13, 2010 at 20:53
Here is a short video of the railgun in action. At the 55-second mark there is a slo-mo of the projectile leaving the barrel followed by a huge plume of hot gas. You can also see some material flying off the projectile. — Brian Rogers, Commented Oct 4, 2014 at 16:15

Jeremy · Accepted Answer · 2010-12-13 02:21:02Z

3

33 MJ is the electrical energy. I think the projectile is about 1kg, so the efficiency is about 10%, not so bad.
Plasma from electrical breakdown, which then gets accelerated the same way the projectile does, with ExB force

answered Dec 13, 2010 at 2:21

Jeremy

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$\begingroup$ After <a href="dailymail.co.uk/news/article-1338112/… searching</a>, it seems the mass of the projectile is 9 kg, which, at mach 7 (2382 m/s) would be 77% efficient, which seems unrealistic.... $\endgroup$
– occam98
Commented Dec 13, 2010 at 4:05
$\begingroup$ Good point. if the projectile is in fact that massive, it would be very hard to make anything so efficient. But in that case, I think they should measure the energy in tons of TNT! (0.008 kT TNT) $\endgroup$
– Jeremy
Commented Dec 13, 2010 at 14:12
$\begingroup$ 10% "not so bad" for the military.. LOL $\endgroup$
– Kyle Hotchkiss
Commented Jan 11, 2012 at 0:35

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mkClark · Accepted Answer · 2010-12-16 00:12:25Z

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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