When and why are spinally-mounted railguns superior to turreted railguns for space combat? Is it a power issues(longer barrels could probably be mounted spinally, and the longer barrels might allow for longer acceleration) or a barrel wear issue(maybe turreted would be better if the rails run out too often, and the railguns need constant rail replacement)? Any ideas?

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    $\begingroup$ That depends on what is the meaning of the phrase space combat. For shotgun range, turreted autocannon may be of some use. For fighting at reasonable "space" distances, any kind of gun would be utterly useless; try guided missiles. $\endgroup$
    – AlexP
    May 12, 2022 at 19:22
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    $\begingroup$ How big is the ship? It's a lot easier to move a turret than aim a 2,000,000 ton lump of spacecraft. $\endgroup$ May 12, 2022 at 19:23
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    $\begingroup$ "Maybe 10 kilometers to maximum hundreds of kilometers": and why doesn't the enemy kill you from tens of thousands of kilometers with ordinary guided missiles? $\endgroup$
    – AlexP
    May 12, 2022 at 19:25
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    $\begingroup$ easy one , when you engage more than one enemy. $\endgroup$
    – John
    May 12, 2022 at 20:10
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    $\begingroup$ @AlexP: A guided missile can be defeated by many means: counter missile, antimissile railgun fire (which would fill the same roll as AA fire and rockets fill in modern sea-ship defenses), hacking/jamming, etc. Rail gun rounds are faster (and potentially do more damage), are harder to "see" and defend against, and are much cheaper (at the moment, by two factors of magnitude). $\endgroup$
    – sharur
    May 13, 2022 at 9:08

7 Answers 7


This question asks a trade-off between traversement speed and projectile energy. In a turret, your ability to point the weapon is limited by the speed at which you can turn the turret. This gives the turret a field of fire that can be as large as an entire hemisphere. With a spinal mount, you have to turn the entire ship in order to point it, so you have to rotate everything (engines, crew, etc) in order to tell the shot where to go.

The advantage of a spinal mount is the amount of acceleration you can pack into your projectile. Overall, for a rail gun or coil gun, as long as it's in "the barrel" you can continue to add more energy to it. With a barrel the entire length of the ship, you can build up enough energy to punch through a small asteroid. A turret mounted railgun could still get through any imaginable amount of armor, but couldn't destroy a whole ship at once.

With greater power comes slower fire rate. Although this is entirely story-dependent, a huge spinal mounted rail gun probably requires a much longer charging time than the smaller turret equivalent. It would also have more expensive shots. With current technology, you have to replace the rail every few shots, so the cost-per-shot goes up immensely. With a spinal mount, you probably have a maximum number of shots before you have to go back to dry dock and refit new rails.

Addendum: @Gillgamesh commented "with a spinal mount you have the potential to deliver projectiles measured in tones, and at devastating speeds"

At first my thought was that "energy is energy, and mass doesn't matter," but that's not quite true. If you put the energy of a 16-inch round into a 45apc, it would hit just as hard, but more of that energy would be spent penetrating. It would just go right through most targets because it doesn't have a large enough cross section, and you don't want that.

Additionally, you can pack more energy into a larger projectile for any length of rail. If you take the amount of energy to throw a smaller projectile off the rails, the larger projectile will still be on the rails, and you can pack more energy into it.

Rail guns are a fun idea because they don't require the containment of an initial explosion to get the object moving. Instead, you have to brace the rails to prevent the force from yanking down the entire structure. Have you ever looked at The Orion Project? We're talking about dropping a nuke out your back end and riding the explosion through shock absorbers. Given the energy levels you could generate with an interplanetary ship, that's what it would be like to fire a spinal mount railgun, but without the radiation.

Addendum 2: @Willk suggests that rotating a turret might cause the rails to bend. Initially I thought so, too, but then I realized that the configuration is wrong. A railgun isn't like a chemical propellant gun, where all the stress is near the center of rotation. It is basically an electrical slingshot, where all the stress is on the end of the barrel. With that configuration, the turret wouldn't be a little bubble with a barrel sticking out if it, it would have to be a large globe that rotated the entire mechanism.

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    $\begingroup$ I would add that with a spinal mout you have the potential to deliver projectiles measured in tones, and at devastating speeds. Depending on your world 1 tone projectile 50,000-100,000 mph ~.5KT - ~200kt TNT on impact. Thats a smallish projectile, about the volume of an oil drum if you cut 1/3 off the top. or a 4 ft cube. $\endgroup$
    – Gillgamesh
    May 12, 2022 at 20:18
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    $\begingroup$ With greater power comes slower fire rate. Not necessarily. In opposition to conventional guns, it could be feasible for a railgun to fire a second (third, fourth) projectile while the first one still has not exited the gun, specially if we are thinking about a railgun mounted in a spaceship which most likely won't have any air inside the gun. And it could be very well that the limit to the gun rate of fire would not be energy consumption, but the fact that you only can impart so much force into the projectile without ripping your ship apart. $\endgroup$
    – SJuan76
    May 12, 2022 at 21:02
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    $\begingroup$ @DWKraus No contact? How do you expect the current to conduct through the projectile with no contact? $\endgroup$
    – DKNguyen
    May 13, 2022 at 0:45
  • $\begingroup$ @DWKraus I thought that maybe you meant to say coil gun which would make more sense. Since with a coil gun you could magnetically suspend the projectile. Or just mass drivers which are basically all forms of coil guns which is important if you are using one to launch cargo payloads. $\endgroup$
    – DKNguyen
    May 13, 2022 at 1:01
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    $\begingroup$ @DWKraus With a rail gun, you pass an electric current down the rail, through the projectile, and up the other rail. The current actually pulls the projectile down the rail. You couldn't put a second projectile on the rails until the first one is clear. Well, you could, but it wouldn't do anything because all of the current would be short-circuited through the first projectile. Because the projectile is literally scraping its way down the rail, there is significant wear. Our scientists haven't figured out a way around that. $\endgroup$ May 13, 2022 at 3:30

Rail torque.

Rotating the turret with long protruding railgun puts torque on the rails because forces are applied only to one end of a long structure. They wind up bending out of true. At best this messes with aim; at worst asymmetric acceleration causes catastrophic failure.

You can turn the turret very slowly and avoid this. You might as well turn the whole ship which supports a spine mounted railgun along its entire length.


Size of ship

My small, lightweight, fast-moving ship can only support a spinal mounted railgun because the kickback of any non-spinally-centered shot would induce a spin. I also need the potential kickback to be offset by my main-thruster — my port thrusters are for moving the ship, not offsetting railgun recoil!

My large, heavy, slow-moving ship is so big the turreted railgun's recoil doesn't matter — it's huge! Additionally, my large ship's turreted railgun is much larger and sophisticated that it includes recoil-compensation hardware — try fitting that on your fast movers!


Since it’s a railgun, I would go with a mass driver gauss rifle design. It essentially speeds up an object pushing a payload (which is your ammunition) and stops the object pushing the payload, launching your shot. This is done with electromagnets and has wear and tear on the stopping mechanism.

To understand the difference between the two options, think of it like this. Your spinally mounted one is like the machine guns on fighter craft, while your turrets are like the ones defending bombers.

For space combat, most spacefighters (Eta-2 Actis-class light interceptor from Star Wars and such) have fixed guns and fly the ship around the enemy, trying to outmaneuver them. This lets them have the largest caliber guns they can fire. With the turrets, you don’t have to move you craft, just the turret and gun. Odds are you’re flying something that resembles a star destroyer (Star Wars) or a Hapan Battle Dragon (also Star Wars). With the star destroyer, its appears to have and 8x2 or 8x1 set up of big guns, with some smaller guns elsewhere. The turrets will be called A,B,C,D,a,b,c, and d, with the lowercase denoting the left side. Guns B,C,b, and c all have the most limited firing angles, maybe 45 degrees at most. Guns D and d have a slightly larger firing angle, and Guns A, and a have the largest firing arcs. Now in space, there’s no air resistance and so the movement of the turret won’t be fast. Turrets of such large guns never are, not in Star Wars, not in real life. On the Hapan Battle Dragon, all the guns are fairly short barreled, so I assume the torque is much less. Thus the Hapans are able to fire the front gun, rotate the lot of them so a fresh gun is in place, fire it, repeat.


Essentially, use spinal mounted guns for your fighters and use turrets on larger ships that can take hits and don’t need the guns to move as fast.


Rail gun is a projectile weapon - meaning there is a physical object being thrown at a target. How it will be used depends on what your world (and universe) looks like and, specifically, what tech level you're envisioning. This is important, because railgun has a few characteristics that makes it a somewhat unique weapons system. In other words: how well it will do depends on the role you want it to fulfill.

First some things to consider:

  1. Energy delivered to the target is dependent on two factors: mass and velocity,
  2. ...but not on the distance - so range is limited only by precision of the targeting systems;
  3. Depending on the mass of the projectile, there is limited storage;
  4. Depending on the available tech you can go either for smaller but faster or bigger but slower rounds, which will dictate your rate of fire,
  5. ...and your energy budget.

There are also limitations:

  1. Projectile is a physical object, so it can be detected, using various methods. But in essence the problem is: if you know it's coming, you can evade it.
  2. and this effect is bigger the greater the distance to the target - more time to detect and the farther you can "see", the easier evasion will be
  3. and how well it will perform if it hits the target depends on shape and composition.

To answer your question, then.

Spinal mount allows for much more powerful weapon system compared to turreted one. Which for railgun means both bigger and faster projectiles (as mentioned elsewhere, longer "barrel" can impart greater velocity), but limited firing cone (few degrees in each direction, at most). Turreted mount, conversely, is smaller - so less powerful - but has much wider firing arcs.

What to go for depends on the protection it needs to defeat. If the space combat is between vessels carrying armor and/or additional em-style shielding, then projectile's velocity or mass must be significant to defeat either or both). In case of the railgun the main difficulty is speed, because it is dependent on length of the barrel. However, increasing projectile mass to compensate, necessarily reduces it's velocity.

Space combat by the very nature of space involves both higher speed and greater distance, . So, if the anti-ship railgun weapon system needs to defeat significant protection at significant distance (several hundred of kilometres, if not more), it needs to be very, very fast, which means either huge amount of energy to propel it or very long barrel (as in kilometres, dozens). Otherwise it can be detected early and the slower it moves, the easier it is to avoid.

So, to bottom line it all: railgun is not feasible as a an anti-space-ship-combat-system. It could be of some use as a close-in-defence system (anti-missile and/or anti-fighter), especially if it will employ scattering payload (sand, ball bearings - whatever, just needs to be small and lots of it), but it's limited (depending on factors) fire-rate makes it mediocre at best.

Best possible use would be planetary bombarding and/or anti-orbital-defence weapon - so everywhere where target can't or has very limited movement capability.

Unless you make big projectiles capable of some manoeuvring...


Railguns are heavy

Railguns need to be very strong. Whenever one is fired, the two rails create enormous magnetic forces which try to push them apart. To avoid bending the rails need to be strongly built and well supported

Railguns are powered by giant capacitors. You cannot just run a wire from the capacitor to the rail - it would melt instantly. Instead solid copper bars are used and the capacitors are mounted as close to the rail as possible. The bars need to be strong, or they will bend due to the magnetic forces.

Railguns create a strong EMF spike. A lot of shielding needs to be added to the railgun to stop the spaceship from frying itself after every shot.

All in all, this makes the gun far heavier than the ship in most cases. Adding a turret wouldn't rotate the gun, but the ship.


Why build any long barrel, why any big gun?

Your question sounds as if it were irrelevant how long the barrel or how big the gun is, but it absolutely is not. I don't even need to go into any reasons: if it were irrelevant they'd use 1cm short "guns" with the diameter of a hair. But they don't, they build "proper" guns.

So going bigger has it's reason. That implies that going even bigger also likely has advantages. With guns on turrets, you need a large ship relative to the gun. To account for the turret torque, recoil (which has to be caught in a joint large enough to sustain multiple shots) = even more size/weight. Turning a longer barrel also incurs shear forces which could bend delicate magnet alignments over their use without sufficient (HEAVY) structural support.

Spinal mounts have these counterpoints as their advantage. Recoil can be passed directly into the structure of the ship, and the entire ship is structural support for swiveling. Also, you get the maximum size of your gun for your buck. Anything on that ship goes directly into supporting the one enormous gun.

But why one massive gun to begin with? Well, that could be a lot of reasons. Maybe shielding technology is quite advanced, so that a barrage of smaller shots (over some spread) are virtually useless. Or physical armor made those strides. Either way, in your world, shots are all or nothing. Either they pierce, or they barely result in camera shake on the bridge.


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