SciFi Weapons: Ballistic
The science behind ballistic weapons. What will do X damage to Y’s butt? What are the real life implications of using something like an antimatter torpedo on your ex-wife’s star cruiser?
The following article was written by Raymond Frazee.
If you talk science fiction long enough (and who doesn’t?), there will come a time when the discussion turns to every fanboy’s fancy: weapons. As in, “What can I use to do X damage to Y’s butt?” weapons. Let’s face it, often times sci-fi drama means things getting killed, and to do that you need weapons.
But what kind of weapons can you use? What are their advantages and disadvantages? What looks cool and what’s boring as hell? And, if you are the sort of person who likes “science” in their “science fiction”, what are the real life implications of using something like an antimatter torpedo on your ex-wife’s star cruiser?
There are many different types of weapon used in science fiction. Lets start with something simple: the ballistic weapon, also known as the kinetic kill weapon, or honking big space gun.
At this point I should probably say that I’m talking about ship-mounted weapons here, not handheld weapons. These are the kind of large-scale weapons you would mount on your battle fleet, not your trendy gun belt.
This is a pretty simple arrangement, and one that nearly everyone understands. Get a barrel—or something like it—a projectile, and something to send our projectile on its way, preferably at as high a speed as possible. Locate, aim, fire, wait for the projectile to do its thing, and repeat as often as possible. Aside from beating your opponent over the head with big stick, this is about as simple a form of combat as you can get.
There are basically two ways in which you can fire a projectile, both of which work well in space. First, you can use a self-propellant round, like a conventional bullet. This weapon fires a shell with a (usually) solid warhead on one end and an explosive propellant on the other end. You ignite the propellant, which then begins turning into a rapidly expanding gas, and this process launches the warhead down the barrel towards your enemy. Alternatively, you could turn your gun barrel into the means of propulsion. This is called a “railgun”. It hurls your warhead at the bad guys by means of powerful electromagnets (see diagram).
Guns Vs. Railguns
Each configuration has it own advantage. The standard gun configuration is simple and, needless to say, cheap. You load the round, you ignite the propellant (which has its own oxidizer, and so definitely can be fired in a vacuum), and off goes your round. If you desire really high rates of fire, you would probably make your round “caseless”, with the propellant shaped so that it holds the warhead nice and secure until the moment you stick it in your gun and fire. That way you don’t have to worry about getting rid of that casing (which was excess mass anyway, and why do you want that on your ship?), you only have to worry about venting excess gases.
Meanwhile, a railgun configuration has the advantage of being able to propel a warhead to very high velocities. A normal rail gun configuration—two charged rails with a projectile placed between them—can accelerate a warhead to maximum velocity of 6 kilometers (3.7 miles) per second, while a “coil gun” configuration—a series of donut-shaped magnets that use repulsion and attraction to launch the warhead—can achieve accelerations high enough that it could even be used as a propulsion system (this is known as a “mass driver”). This tremendous acceleration means that your projectile doesn’t need to be as large, as the sheer speed of the thing is likely to do a lot of damage. Therefore, you can carry more projectiles and do a lot more damage, and people will tremble in fear when your ship enters their system.
What are the disadvantages?
With your ship’s gun, you have to store both warhead and explosive propellant aboard your vessel, with special emphases on “explosive”. In order to get those kick-butt higher velocities for your warhead that will punch really nice holes in your enemy, you’ll be required to use larger amounts of propellant, which means larger “shells”. Your gun will also have to be scaled accordingly.
With railguns it’s a whole different set of problems. Rail guns are under tremendous forces when they fire: the electromagnetic fields are constantly trying to pull the different section of the gun apart—this can also be read as “tear apart” and “explode”. This means that even though the guns might be small, they require a tremendous amount of bracing. And if we want to get into “real” science and engineering, rail guns are tremendous power hogs—or to put it another way, if your railgun has a 40 percent efficiency, and you want one warhead to do 4.184 million Joules of kinetic damage (which is the equivalent of 1 kilogram of TNT), your gun is going to use 10.46 million Joules of power (or 2.5 times as much energy) to fire each single round.
Of course none of this will matter on your ships because you’ve built your guns using ‘unobtanium’ and you’ve got them hooked up to your ‘handwavium’ generators, so we’re good, right?
The Science of Ballistics
Let’s take a look at what makes puts the “kinetic” into kinetic kill weapons. That means we gotta look at some math. But it’s fun—really. Trust me.
KKWs work off the simple concept of something with mass hitting another object at speed. The formula to figure out what sort of energy you’ll get from an impact is:
Ke = 0.5 * M * V²
Where Ke is kinetic energy in Joules, M is mass of the warhead in kilograms, and V is the velocity in meters per second squared (and this is important) relative to your target.
It takes a lot of Joules to give Jamie the big boom he likes. As already pointed out 1 kilogram (2.2 pounds) of TNT produces 4.184 million (or 4.184e6) Joules, and if you play with your numbers you’ll see that if you keep kicking your velocity upwards, your kinetic damage potential goes up considerably.
In fact, there is Rick Robinson’s First Law of Space Combat, which states that, “An object impacting 3 km/sec delivers kinetic energy equal to its mass in TNT”. Put it another way: put one kilogram of anything in your gun, fire it at a target, have it impact at 3 kilometers a second, viola! You’ve got yourself the equivalent of 1 kilo of TNT going off. (If you need a visual of how much TNT this is, one stick is about 200 grams, so 5 sticks of TNT.) This gives you a nice little rule of thumb to work with in terms of killing off someone quickly.
But what did we mean by “relative to your target” in that equation? Keep in mind that if you have KKWs on your ship that means you’re probably shooting at a target that’s moving relative to you, which means it’s also moving relative to your warhead. What really matters is not that your warhead is leaving your gun at 3 km/sec, but that it’s hitting your target at 3km/sec, and that means to get the same kind of big boom you love, you’ll either have to ramp up the velocity at which your warheads leaves you gun, or you’ll need to put your ship in a position where you maximize the impact.
So if your target is pulling away from you, and you fire your warhead at 3 km/sec, the impact is probably going to be much less, meaning your exit velocity might need to be adjusted. On the other hand, if you’re shooting at a ship that is moving really fast towards you—say you’re standing still and it’s flying along at 8 km/sec—you don’t even have to get a lot of velocity behind your warhead; just chuck it out an airlock and let that fool hit it, because impact relative to your target is still 8 km/sec, and that’s gonna do a lot of damage. (In case you’re wondering how much damage. . . 3.2e7, or 32 million, Joules, which works out to about 8 kilograms [17.6 pounds], or 40 sticks, of TNT. Yeah . . . big boom.) So, with that in mind, should you decide to launch a round straight into the nose of this ship, maybe at 4 km/sec, and get a combined impact velocity of 12 km/sec, hilarity ensues!
Examples in Science Fiction
So what sort of science fiction ships carry these types of weapon? Two come instantly to mind: the Galactica and the BC-304 Battlecruiser, otherwise known as the Daedalus-Class from Stargate SG-1.
The BS-75 Galactica is a huge ship: 1438 meters long, 536 meters wide, 183 meters tall. It was designed to be a command and control ship that would fly into your space, launch fighters, and then begin blasting the hell out of you with its main guns. The specs for the ship indicate it had 24 primary “duel Kinetic Energy Weapons”. While there was an argument as to whether these were railguns or conventional guns (even the production staff were confused, with some saying they were railguns but many writers saying they were conventional), because there were many allusions to the need for propellant for the rounds, we’ll go with conventional guns.
Most of these weapons were deployed along the dorsal (top) of the ship, located in turrets, but there were a few mounted in the bow with a forward arc of fire. When it came time to bring the pain to the Cylons, these were the guns employed. They had a great rate of fire, and could pretty much take out a Base Star, should the odds fall in Galactica’s favor.
And while we’re talking about KKWs, the Galactica employed another, very important kind of KKW: the Point Defense Weapons. If you watched the show these were the itty-bitty guns that started filling the vacuum with lead so there was a defensive flak screen around the ship that would prevent missiles and enemy fighters (and, if you weren’t too careful, your friendly fighters) from getting way too close. The Galactica had a lot of these little guns: 512 total, positioned all over the vessel.
This made Galactica a very formidable ship, but it also required it to carry a lot of supplies—propellant for their guns and fuel for those pretty Vipers and Raptors. This explosive cargo also meant that the entire ship could go “BOOM!” pretty easily. I mean, when the Columbia exploded it looked very pretty—something the 3500 crewmembers aboard couldn’t appreciate because they were too busy dying.
The need for storing all this material—propellant, warheads, fuel—was one of the reasons the Galactica was so big. And the size also helped offset a problem you would encounter firing all those weapons: recoil. Fire them long enough and it might have had an effect on the Old Girl, but given the mass of Galactica, it would have taken a lot of shooting to impart any reasonable spin.
Now let’s look at the Daedalus-Class. I have a special affinity for the Daedalus, simply because a long time ago I worked up the stats for the ship for a Stargate SG-1 game I was going to run. That game never got started, but someone else saw the work I’d done (I’d posted it on a forum for the game) and asked if they could use the information. I said sure, and imagine my surprise when, years later, I found a home-made Stargate Atlantis RPG and there, under the ship listings, was my Daedalus. Made me sort of proud, you know?
The Daedalus was another projectile boat like the Galactica, but it was lean and mean. When the Daedalus first showed up, it sported hangar space for 16 F-302s, 16 missile tubes in a configuration that made it look a little like a Typhoon–Class submarine, and 32 railguns. 225 meters long, 95 meters wide, 75 meters tall, it was the quick and deadly alternative to Galactica, using its hyperdrive to do hit and run attacks on the enemy, and its energy shields to keep it from harm (unless the story said it was supposed to come to harm, in which case, those shields weren’t worth that much).
The railguns on the Daedalus had the sort of incredible rate of firepower you’d expect, but it seemed like every time a Daedalus-Class went up against another alien vessel it would get it’s butt handed to it—at least until it got beam weapons, and then look out! But that’s another story.
Though they might seem weak—and not nearly as cools as beams of light coursing through space—ballistic weapons play a big part in science fiction. Just wait until we talk about the sort of overkill you can do with mass drivers flinging rocks into a planet at 40 km/sec!
A note of thanks to the site Atomic Rockets by Winchel Chung, which was used extensively for background on much of this article.
This article was written by Raymond Frazee.
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