Railgun recoil: Newton won’t be denied!, an article by Tim C. Taylor

In his last post for the blog, NSFWG member Tim C. Taylor wrote about the creep of technobabble into TV and movie Sci-Fi, and why he wanted more believability in my written fiction (which you can read here).  This time, he looks at more technology.

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.

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 it has exactly the same recoil.

Take a projectile of the same mass and push it out of a barrel at the same muzzle velocity and it will push back on the breech with the same recoil force. Whether that projectile is propelled by a chemical explosion or by electrical repulsion makes absolutely no difference. (Actually, I am simplifying slightly — recoil isn’t all felt in one go. See http://www.positiveshooting.com/RecoilMain.html.)

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.

The Universe is a Conservative Place

There are many conservation laws of the Universe. These say that if can draw a ring a bunch of stuff (the technical term for this is a ‘system’) and promise not to influence that system from the outside, then a host of basic properties of that system must stay the same.

The law we’re interested in here is conservation of (linear) momentum.

What’s a system? Well, a railgun is one, the balls on a pool table are nearly another, but we’re going to start with a bomb.  I’m talking about the kind of bomb beloved of terrorists and revolutionaries of the late 19th century. The kind that somehow has become a thing of amusement today.  That bomb is a system.  We’ve lit the fuse. It’s going to blow, so you’d better take cover.

BANG!

Bits of bomb casing fly out in all directions.  Each fragment has mass and velocity. Momentum is mass multiplied by velocity, so each fragment has momentum.  There are dozens of fragments, each with its own momentum. There’s a whole lot of momentum going on here. Except…

Hold on a moment!

Conservation of momentum says that the momentum of our bomb system must not change unless we act on that system from the outside. And to start with, the bomb is at rest. Its momentum is zero.

So the momentum must still be zero after the explosion.

Yet the fragments are not at rest.

The answer to our conundrum is that momentum is mass times velocity. And velocity is not the same as speed: velocity always has a direction.

Back to our bomb. If we add up the momentum of all the exploding fragments, we will see them start to cancel out. For example, if we have a fragment flying out to the left side of the page and a fragment of equal mass and speed going right, then the combined momentum of those two parts is zero.

And that’s what we get with our bomb system. The momentum starts off as zero, and after the explosion the net momentum of the system remains zero at all times, even though individual parts of our system do acquire momentum.

Now let’s move to railguns…

BTW: if the bomb starts off on the floor, then the ground will push back on the bomb fragments, interacting with our system. So just imagine the bomb is actually in outer space and then it really is isolated from any external force.

Consider all the parts of our railgun to be one system, just as we did earlier with the bomb. That’s the breech, barrel, projectile, capacitors to store electrical charge, sabot, the firing button and anything else we need.

Now assume the gun is at rest. That means the momentum of our system is zero.

We press the firing button.

The result is something like this:


We know what’s happening here because we’ve just seen that with our bomb.

The projectile flies out with huge momentum.

Now, if the railgun system was as truly isolated as our bomb (let’s assume floating in deep space) then the gun would move backward with sufficient speed to cancel out the momentum of the projectile. It will leave net momentum as zero (which is what rocket engines do).

The cue ball hits, knocking balls everywhere. But the net momentum stays (very nearly) the same

A practical railgun fired on a planet’s surface isn’t an isolated system, though. It doesn’t fly backward. Something pushes back very hard to cancel out the momentum of the projectile.

That’s what recoil really is. It’s the practical implication of the gun trying to fly backward with an equal and opposite momentum to the projectile.

Or, if you like, it’s our way of defining our gun system so that it is not in isolation. We add a gun carriage, tripod, or a rifleman, Navy ship, or some other mechanism that is ultimately braced against the Earth’s surface.

Recoil is complicated and we can engineer tricks to manage recoil, but we can’t change the law of conservation of momentum. Which is why my teenage idea of spaceship railguns was wrong, and why the recoil force acting on the breech of a railgun is exactly the same as for a conventional (chemical propellant) gun firing a projectile of the same mass and velocity.

BTW: Imagine our railgun is mounted on a spaceship or space station. That’s a very different proposition. There’s no planet to push back against to resist the recoil. The spacecraft’s momentum will change to cancel out the momentum of the projectile.

In my Human Legion books, I’m after believability rather than science lessons. So I wanted to learn more about the practical experience of recoil, and design some of that into weapons such as the standard Marine weapon, the SA-71 carbine.

Designers of real-life rifles, for example, can lessen and use the recoil by diverting expanding gases to expel the spent round and chamber the next one. But if the gun system is not to fly backward something must still push hard against the breech.

With a rifle, that would traditionally be the firer’s shoulder.

And it’s important we get this bracing right, because it is accurate bullets that kill the enemy, not volume of fire. And if the firer can’t control the recoil, they can’t fire accurately. (The very earliest siege cannon teams knew this better than anyone: the recoil from each fire would damage the gun carriage, meaning they had to rebuild it each time. Two shots a day was a respectable rate of fire)

One of the weapons experts I consulted was my father. During the 50s, he faced off against the Russians in the Cold War. The rifle he used was the First World War issue .303 Lee-Enfield. You quickly learned how to hold the Lee-Enfield properly, he told me, because even when held correctly, every time you fired, it hurt!

He much preferred the Bren gun, a light machine gun design from the 1930s that he qualified for as a marksman (I think light might be better called ‘portable’ — they weren’t light if you carried one any distance). He said that when braced on its bipod, you couldn’t feel the recoil, and this helped to make it an extremely accurate weapon. He could put a bullet through a mug at 100 yards every time. (I think his eyesight was better back in the 50s).

The Bren used gas venting, springs, and other clever tricks to reduce the experience of recoil to near zero. The law of conservation of momentum still holds for the Bren, but if they could reduce the recoil experience so much in the 1930s, they can certainly do much better centuries later in my Human Legion books.

So there you have my journey through both railgun physics and a little practical understanding of recoil.

The SA-71 carbine is the standard personal weapon of the Human Legion, and their predecessor/ rivals, the Human Marine Corps, My starting point for its design was the Bren Gun. But the Bren Gun wasn’t designed for space combat, compatibility with stealth suits, and carrying an immense electrical charge. So, having paid homage to the law of conservation of momentum, when working out these other features, I allowed myself a little more flexibility.

And that is what I like other writers to be doing. By limiting the techno-babble, and with at least an acknowledgement of real-life physics, as a reader I’m much more ready to go with the flow of all the more fanciful future technology.

I’m happy to report that having read plenty of military sci-fi this past year, other authors are doing us proud. In fact, believability in future weapons and, to some degree, combat tactics, has been greatly improved by the introduction of so many self-published books.

To say amateurs are beating the pros seems strange, but I think it is because so many new self-published writers have military backgrounds, or go do their research first.

Mind you, there is still a tendency for spaceship lasers to have range limitations that make no sense (the effective range of lasers in vacuum is determined by diffraction). And Marines of the far future still tend to carry bolt-action rifles.

But that’s just minor niggles.

Whether you’re reading, writing or both: military science fiction is a great place to be right now.


This article originally appeared, in two parts, at Tim's website which can be found here