I talked in my last post about the creep of technobabble into TV and movie Sci-Fi, and why I wanted more believability in my written fiction.
Back in the early 80s, I used to design ships as part of the game, Traveller. And then I designed how the elements of a multi-ship force would work together for the game expansion called Trillion Credit Squadron (anyone still play that?) Typical ship armaments included meson cannons, pulse lasers and railguns.
Science fiction is fast becoming science fact. It’s a cliché, I know, but it’s happening in my lifetime. One of those exotic theoretical space weapons, railguns, looks likely become the main medium-range ship weapon for the US Navy over the next 20 years. They are already planning how to retrofit existing ships with enough electrical power in readiness.
Armed with a whole load of real technical data on railguns, they were a pick for my weapons in the Human Legion universe. (A special thanks goes out to all those dads who filmed their children’s railgun science projects).
So, what is a railgun?
To make one, you take two parallel rails made from an electrical conductor (such as copper) and wire them up to a direct current supply. Place another conductor to touch both rails. This armature, as it’s called, must be able to move along the rails. When you supply the electrical power, a magnetic field is induced in the rails which pushes the armature along until it falls off the end of the rails, which breaks the electrical circuit.
In a school science project, the rails might be strips of aluminum foil taped to a board and the armature a steel bar that gently rolls along the board.
Real ones look might use a conducting sabot for the armature. A sabot is a jacket that wraps around a shell or bullet, enabling it to be fired more effectively. Search the ground after one of the fight scenes in my Human Legion books and you will be wading through spent sabot casings.
ASIDE: Coilguns and gauss guns. I’m talking here about railguns, because that’s what I chose as the equivalent of a modern rifle in the Human Legion books. But in another series I write, the Four Horsemen Universe from Seventh Seal Press, I’ve also written about coilguns and MAC cannons (Magnetic Accelerator Cannons).
Magnetic accelerator rifles, Gauss guns, and coilguns are all names for the same thing: a weapon where a sequence of coils around a barrel are switched on and off in a precisely timed sequence, accelerating a projectile. The recoil properties are essentially the same as railguns.
Anyway, back to Traveller and railguns. When I was designing heavy cruisers after school back in the 80s I used to imagine that when a railgun fired you would hear a hum of power build up followed by a whoosh. No bang. I expected it was recoilless too. Maybe that was because there was no explosive charge. With no exploding gases required to push the projectile along a barrel, there would be no recoil pushing back on the gun breech. Right?
Turns out I was wrong on all counts. It seems obvious to me now, but I knew a lot less physics when I was 13, and my misconceptions have stuck with me,
If you watch videos of real railgun test firings [such as this US Navy video below] there is a big bang when it fires. Lots of sound and lots of light. That’s what happens when you suddenly discharge a huge amount of power in an enclosed space, but it is not a chemical explosion as with conventional munitions. You don’t have all those hot, expanding gases pushing back against the breech. So does a railgun have less recoil than a conventional gun?
The answer is that the projectile has exactly the same recoil. In a conventional modern day rifle, this is equivalent to the recoil from the bullet leaving the barrel, and it’s called primary recoil.
But that’s not the whole story, because in a conventional firearm, there’s also a secondary recoil, which overwhelmingly consists of the hot gasses from the propellant escaping from the end of the barrel. In a rifle, the secondary recoil is usually greater than the primary.
You only have to see the muzzle fireball in the video above to see that the railguns also have secondary recoil, even though there’s no chemical propellant. However, the hot gasses in the railgun are, I presume, largely a result of the projectile compressing the air in front of it as it passes along the barrel, like a space capsule on re-entry. It’s a side effect rather than a cause of the shot and so although I don’t have the numbers, I make an assumption that the secondary recoil of a railgun is smaller than the primary recoil, possibly much smaller.
And as commenter, Matt, rightly points out, if the railgun is a spaceborne weapon and the inside of the barrel is a vacuum, then there are no hot, compressed gasses to expel out the tube.(*1) Secondary recoil would be significantly reduced. You might have some secondary recoil from expelled armature sabot sleeves (but only if the armature isn’t integrated with the projectile).
So certainly you have recoil with a rail gun, but if my assumption that the secondary recoil is much lower than primary recoil (relative to a conventional rifle) then the total recoil will be less than with conventional weapons for a projectile with the same mass and same muzzle velocity.
I emphasized that last point because the recoil of the weapon ultimately depends on choices made by its designer. In the Human Legion Universe, soldiers are usually heavily armored. To penetrate, you might fire heavier projectiles, or fire projectiles similar to modern bullets but at a higher velocity. Or you might prefer to take the railgun’s ‘saving’ in recoil and use it for a higher or more accurate fire rate with some kind of armored piercing shaped charge rounds.
One of the features of the SA-71 personal railgun often seen in the Human Legion books is that it uses a great number of fire mode options and a flexible variety of rounds in their cavernous ammo bulbs. It’s the primary weapon for all theaters of combat, including shipboard and deep space operations, and it needs to be effective against pretty much anything it can come up against.
Having said all that, it still feels intuitively odd that there’s not explosive propellant in a railgun and yet it still has significant recoil kicking back against the breech.
It’s all to do with the law of conservation of momentum. It’s the same law that makes rockets fly into space. Also… It’s a basic law of the universe that you can’t get around by waving a technobabble phrase.
So in going for believability, conservation of momentum is something I can’t ignore.
I’ll cover more about railgun recoil in part2.
*1 The issue of spaceborne railguns is an interesting one. Current railgun testing involves three most common designs for armature. The simplest is where the projectile is made from a conducting material and acts as its own armature. Others use plasma (ionized gas), which allows for a non-conducting projectile, or a hybrid which uses plasma but not exclusively. Using high pressure plasma inside a depressurized gun tube will present additional challenges. Perhaps spaceborne and land-based railguns would develop along different paths for this reason.