A prototype railgun has been designed for a science fiction RPG I am running. It fires a 0.3kg bullet at 9 kilometers per second. The bullet itself generates a magnetic shield to keep it from vaporizing.
I’ve been told that the resulting Kinetic Energy will be 12,150,000 Joules, or aproximately the effect of 9 lbs. of TNT.
I have several questions though:
How much noise will there be from the bullet passing through the air?
Will there be a sonic boom?
Will there be a visible trail?
What kind of damage is this likely to do to a person? (I imagine simply pass through them.)
What about asphalt? What would it take to stop the bullet?
Answers to these and any other relevant information would be greatly appreciated
Since this is an RPG, the bullet itself will be piercing damage, while the shockwave would probably be blunt.
The speed of sound is about 331 m/s, or .331 km/s, so there will definitely be a sonic boom. Remember that the bullet is faster than the sound it makes though, so you won’t hear the sound until it’s through you.
As for trail, I’m thinking that a relative example would be the ending of the dragon battle in Terry Pratchett’s ‘Guards! Guards!’
Also, remember that the 9km/sec is going to be the ejection velocity. It will then immediately start slowing down due to air friction, and that’s how you determine the weapon’s range…
The problem with describing what your railgun would do is that you’ve got a piece of magic tech (the ‘magnetic shield’) that strongly affects how it works. Presuming the magnetic shield pushes aside air without offering much friction (an actual magnetic field wouldn’t do much to air, since air is electrically neutral, but I’m presuming it’s a typical SF forcefield), there wouldn’t be much of a shockwave since it’s not going to interact much. There’d be a sonic boom something like a bullet since it is displacing air (my estimation would be a loud crack, a lot louder than a bullet but not as loud as firing a gun). Any visible trail depends pretty much entirely on how the shield works. I’d expect it to punch through a person without much (in relative terms) incidental effect, I’d give it a highly random damage (like two dice multiplied together) and have it ignore normalish armor.
As far as stopping it, I’d expect it to punch right through a person, normal wall, etc, though again the shield really determines how much it can go through. It would have problems with being effective because it’s so penetrating; it would be hard to actually put all of that energy into the target and so would probably work better carefully aimed to hit something vital in a vehicle than just firing and relying on the enrgy to break the target.
Also, unless you’ve got some other magic tech, I’ll note that this isn’t going to be a shoulder-fired weapon - the recoil from it would likely kill the firer. You need some kind of really good capactiors/battery/generator (likely capactiors) to power the shots from it (the army experiment mentioned earlier required a shed of capacitors using modern tech), so it should be possible for people to have other high-energy weapons in the campaign world - if this is a SF game with lots of lasers, then no big deal since that is already around, but if it’s near-future then whatever is powering the device is more valuable than the railgun itself.
Could you adjust the perameters of the gun? Say it only fires a .03kg bullet at 20km/s. That way you could, in theory, make the gun smaller while keeping the power.
Of course I’m no expert so take this with a grain of salt…
I’m not familiar with Terry Pratchett’s ‘Guards! Guards!’ Care to explain further?
As for the ‘magnetic shield’, I’m considering getting rid of it. I wasn’t really comfortable with that bit of technobabble anyway. The purpose behind it was to have the bullet remain intact. Is there simply a material that can be used that would be able to survive until it reached it’s target.
I think your KE value is off by a factor of 2. m=.3kg, v=9000m/s, mv[sup]2[/sup]=.390009000=24,300,000 joules. That’s muzzle energy, of course.
I’m not sure how that translates into dynamite, but let’s see…
The kiloton is defined as 4.186x10[sup]12[/sup] joules. You railgun’s muzzle energy is 5.81x10[sup]-6[/sup] (about 6/1,000,000) of that.
Since a kiloton is the energy roughly equivalent to 1000 tons of TNT, we can say your gun is equivalent to about 11.6 lb of TNT. A quick search on the web tells me that dynamite is about 65% as powerful as the same amount of TNT, so you’re looking at almost 18lb of dynamite per shot.
As for sonic boom, heck yeah. Since the muzzle velocity is about mach 45 (not 4.5), you’ll have a hell of a sonic boom.
The actual effect of shooting a human with this monstrosity would be similar to shooting a squirrel with a hunting rifle (only more so): you’d be left with a red splotch on the ground and not much else. It wouldn’t just punch a neat hole, because of hydrostatic shock. The primary wound cavity would likely be significantly larger than the actual body, thus blowing the target to smithereenees (to quote Yosemite Sam). And that’s the projectile impact, not any sort of shockwave effect (which would likely be insignificant by comparison).
My question is where you get the power to accelerate a 300g projectile to 9000m/s! I mean, is this thing carried by a soldier? Mounted to a vehicle? Mounted to a battleship? Those 24 million joules have to come from somewhere, right? If you want reasonable accuracy, the round needs to leave the rifle within 1/100 of a second after trigger pull (I’m guessing here). That would give a pretty hefty jolt when it’s fired – think > 2 billion watts. That 24 million joules will get transfered into the gun’s mounting over that 1/100 of a second, and needs to be generated for each shot.
Well, consider this: Your bullet is being accelerated from zero to mach 45 within the length of your gun barrel. Just to survive that it’s going to have to be made of something remarkably strong (although it does depend on how it’s being accelerated - if the acceleration method works on the entire bullet evenly, it’s not so bad). Secondly, at the speed it’s going, you probably don’t need it to survive for more than a few seconds anyway - by that time it will have hit something or flown into space.
Also, the bullet’s going to be deccelerating something fierce over it’s path, shedding a lot of energy into creating one hell of a sonic boom. The main damage to soft targets (like people) will be from this secondary sonic shock effect. Hitting someone directly will poke a small neat hole through them, then blow them apart from the sonic boom. Missing someone by a few feet will have the same effect without the neat hole - even if the bullet doesn’t hit them directly, the overpressure from the sonic wave will still injure and possible kill them.
Hitting something hard (such as a material made from something comparable in strength to the bullet itself) will produce a spectacular explosion, as the bullet shatters into still-supersonic bits.