In the last post I talked about railguns and recoil, but didn’t put the two together. That’s what we’ll look at here in part2.

The Universe is a Conservative Place

There are many conservation laws of the Universe. These say that if can draw a ring around a bunch of stuff (the technical term for this is ‘defining 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.



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.

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!

Bren Gun

Bren Gun

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.


  1. […] Railgun recoil: Newton won’t be denied! Pt2 […]

  2. […] at the Human Legion website today, I’ve finished off my personal journey through the world of railgun recoil, using an old-school bomb, and a game of billiards, and a little help from my […]

  3. SGT Mike says:

    Nothing wrong with a bolt action rifle, some thing’s are kept for purely iconic reasons. Look at the use of horse cavalry into the 1930’s!!

    • timctaylor says:

      Agreed, SGT Mike. And I’d go further and say robustness, ease of maintenance, and ease of production of ammo/ spares is important too, especially if your supply chain is lengthy and so is your campaign.

  4. SGT Mike says:

    Don’t forget the old adage of “if it ain’t broke, don’t fix it” being applied as a guiding principle too!!!

  5. Jas says:

    So how would you negate the recoil of a Railgun; please don’t go into too much detail.

    • tctaylor says:

      Hi Jas,
      Depends on the size. In my Human Legion books, when we’re talking warships in space, I’m using essentially the kind of recoil slide you would get with the main armament of a modern-day battleship. These evenly distribute the recoil shock through turret and hull. For the handheld carbines, some of the energy is compensated for because they guns are designed to be used in conjunction with their powered armor, which is linked into the weapon and knows how to brace and push against the recoil. Additionally, there’s a recoil absorption system that transforms some recoil shock to heat and then dumps that energy out to another dimension.

  6. bobby bob says:

    I’m curious, when you throw a ball is there recoil? The problem with your distinction between gas driven projectiles and magnetically driven projectiles is that you’re comparing an explosion with kinetic energy and pressure moving in all directions to a throw with kinetic energy moving in one direction. I know there’s a big fireball coming out of that, but you have to understand that’s not an explosion. Also important to take into account that we’re talking about using the weapon in a vacuum, the weapon would only need to really shoot a 20mm projectile at standard velocity (a 97% reduction in weight and 59% reduction in velocity from the navy’s example, with proportionate reductions in all other variables), and of course the advancements that will be made by the time we actually are moving weapons into space. In essence you can’t compare a chemical explosion to a magnetic throw, they are not the same thing.

    • tctaylor says:

      Hi Bobby Bob. Yes, there is recoil when you throw a ball. I struggled with this for a long time because , as you say, an explosion throws a pressure wave out in all directions (channeled in a weapon) and intuitively I can see that if there’s a controlled explosion just in front of me, then I’ll feel some of it will hit back at me as recoil. Without an explosion, the law of conservation of momentum feels cold and less intuitive to me, but it is still a universal law that applies in every situation. So, if you throw a ball in space, or fire a dart out of a railgun, then there will be a recoil force. You make a very important point that without all the complexity of a round needing a case and carrying around all that chemical explosive, a round fired from a railgun will be more mass efficient. In the Human Legion books, I take advantage of that in a variety of ways, but the most common is to have the fire rate of a contemporary mini gun and place it in a handheld carbine without needing to carry enormous drums of ammunition on your back.

      Back to the ball; I found a video of an astronaut throwing things on the ISS here: 🙂

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