I want to build a weapon targeting Interstellar distances. How viable are railguns?

Question

I am in a timeline which would be tens of thousands of years advanced from yours, and humans have colonized a significant portion of the stellar neighborhood, multiple thousand light-years across (Last I checked - a century maybe?). Sadly, the laws of physics you devised, on the grand scale, seem to mostly (with a few exceptions) have held up (importantly - relativity). Many of you can probably extrapolate our technology (Ignore the timeline communicator - It was given to me by the Great God of Plot and will be taken away by Him after I have received my answer). Probably the greatest invention is fusion energy and torchships - capable of supporting constant 1g (or more upto Plot) acceleration for an extended period of time, without being a weapon of mass destruction from the heat (due to Plot. One of the few deviations from your world physics. Thermodynamics turned out to be somewhat forgiving! It is still destructive due to its kinetic energy.)

I have reached my destination, System X. I was sent here to build a semi-colony semi-military-base, and am the grand authority here. This system is particularly close to routes fleets of opposing factions would need to take to into allied space. I have a few million colonists, and a drone fleet that was sent at greater accelerations and arrived a few years before. It has mined up a lot of the resources we would need to begin colonization in luxury. They even have a Dyson swarm set up that can extract as much energy as I will need. This system is ours for all intents and purposes.

The test target is an enemy fleet that will - at its closest point - pass within a few light-years of this system. This will happen in a bit more than a century. We know the ship design and know they have accelerated to maximum velocity - They don't have enough fuel for significant deviations from this course. Thus, I have a few decades to build my weapon.

What I have in mind is a huge battery of railguns - as many as Plot demands! They will be capable of accelerating a projectile of mass Plot to around 0.2c max (if that is insufficient, it will be accelerated to Plot). Using my massive fleet of drones, and some calculations about the rate of material gathering, I believe this can be done in the required timespan.

But is it viable? Can using thousands of railguns to attack this fleet at interstellar distances work? I must send a report of my plans to my superiors in a few hours (technically, years), so I must be certain my plan will work. It will be really bad for me if they calculate it will not - and will thus have wasted more then a decade by the time their new orders get here. So I want to be sure this will be able to significantly damage any fleet which passes within a reasonable distance of this star system.

Additionally, will they or a nearby star system be able to tell exactly what I am constructing? Not that they could do much about it - this fleet is doomed either way (even if it changes course, it won't have the fuel to stop). But having the element of surprise for the century or so before this fleet is destroyed and their information reaches the enemies would be very useful, especially if our enemies decide to launch other fleets during that time.

As a bonus, there is a rather pesky system a few parsecs away whose continuous existence and defiance is a thorn in the Empire's side. Can I use these railguns to attack them? Perhaps I will be able to destroy their off planet installations. Maybe I'll even be able to bombard their planets.

Answer 1

They even have a Dyson swarm set up that can extract as much energy as I will need

You should be building a Nicoll-Dyson beam. If you weren't aware of these engineering marvels, it involves constructing a phased laser array on the outside of each element of your Dyson swarm. In the limit, this results in a laser weapon that can turn the output of your star into a beam of destruction that can reach out to stupendous distances with astonishing power... a full swarm with a 1AU diameter with a diffraction limited phased array emitting 500nm light can render planets uninhabitable 100s of lightyears away, and should be able to project a huge zone of deadly power at something mere lightyears away.

This isn't to say that building your railguns is pointless as such, but their power and effectiveness is unlikely to match that of even a partial star-sized laser array. If you've got stuff to spare that isn't useful for expanding the Dyson swarm, knock yourself out.

Really though, the laser can reach further in the same period of time, striking the fleet further away, inflicting early damage and casualties. It can also apply damage for a longer duration, maximising the chances of you destroying everything flying towards you. As the beam propagates at lightspeed, there's no practical way for the fleet to realise they're under attack until things start exploding, unlike railgun projectiles which might be detectable by various means that I won't go into here (but feel free to ask a separate question on that).

The biggest problem you may face is if the fleet is a) watching you and b) has larger fuel and propellant reserves than you thought, and is able to tweak their course slightly. It'll take years for you to notice and respond, but as they have to keep playing this game for at least a century they'll eventually run out of juice and you can waste them at your leisure. They might even send you a surrender message, and dealing with that is definitely a job for a separate question.

You could, of course, construct vast zone plates and blow them towards the enemy fleet like solar sails, and put enough intelligence in them to appropriately direct and focus your death beams if you happen to be a bit off. So many different possibilities!

Once you've reduced your foes to a thin cloud of expanding ionised gas, you can still use your ND beams for peaceful purposes including terraforming, transportation (both local and interstellar) and reminding your neighbours that if they like the planets they live on or the stars they live around that they'd better not try any funny business.

Answer 2

The problem with an unguided projectile at interstellar scale isn't accelerating it, it's hitting anything at the other end.

Here's the scale of the problem. Assume that your railgun has perfect bench accuracy: that is, if you fire it twice at the same point, it will hit the exact same point in space, every time. Assume that it's 1km long, but despite that length, its muzzle end can be oriented with nanometer precision (and its back end will remain perfectly stationary as you orient it).

Even with this ludicrously well-calibrated gun, when you're aiming at a target 1 LY away, the effect of each nanometer of traverse on the end of the barrel is to move the potential point of impact about 10km. If your opponent's ships are significantly smaller than this, your gun still isn't accurate enough to address them.

And this is assuming that pointing the gun is the only aspect of accuracy, which it isn't. If there's anything at all even slightly off about the gun - mass concentrations in the projectile, imbalances in the magnetic field caused by minor variations in conductivity along the barrel - or targeting parameters - minute gravitational pull from other planets in your system, the impact of solar radiation - anything that could throw its trajectory off by so much as a nanometer as it leaves the barrel, you're doing no better than shooting blind.

(This is assuming your projectile's width is negligible, which it probably would be if it's a railgun, unless someone has plans for a hyper-accurate railgun with kilometer-wide slugs. A gas cloud, plasma, or beam of light would expand as it goes, making hits more likely... but spreading its force of impact across the whole area. It would still need to be extremely accurate to stand a chance of hitting, though.)

Answer 3

Railguns have to be useless if there is relativistic travel

Unfortunately, absolutely base table-stakes for being to travel at any significant fraction of C is some kind of detection, deflection, or shielding technology that can shrug off incoming projectiles, because by traveling at that speed, you have turned everything you run into into a projectile traveling at a similar fraction of C. As per the very-well-worth-reading paper, The interaction of relativistic spacecrafts with the interstellar medium, mono-atomic collisions can cause non-negligible wear and tear over time. A single 10-micron speck of dust can crater the surface of the ship. And while collision with something larger is unlikely, over large interstellar distances, with sufficient ships, it becomes a real problem that needs to be confronted. “How do we not die if we collide with a fist-sized meteorite” is a question that will have to have been long-since resolved for the setup of your universe.

Therefore, we have to presume that if ships are traveling at that speed, they have technology that makes those collisions pain-free. As a result, tossing projectiles at them will have no effect, even if you could hit an elephant at that range.

Answer 4

Fast moving space ships will be watching for incoming impactors.

This fleet is moving fast and can't dodge. Space junk and rocks are a threat to them. They will be watching for such and they will have countermeasures as have frequently been bandied about on this stack. Space is not hazy and you can see a long way using various EMR frequencies. 0.2c is fast but radar and lasers are faster. As regards the fastest of those two they argue but races always wind up a tie.

In any case these ships will have countermeasures to divert or disable incoming trash which includes your railgun rounds.

Answer 5

You could never hit, at that range, with a rail gun - but not for the reasons listed. You're firing projectiles with mass on fight paths that last years. Every single object in area will interact with them, in terms of gravity.

They'll be attracted in very slightly different directions, depending on the flight path. Unless you have perfect knowledge of all mass concentrations in a volume measuring cubic light years and the capability to predict the motions and interactions of all these objects... you're projectiles are going to drift this way and that... Pulled by that rogue planet, slowly moving off course...

Answer 6

Railguns are unlikely to produce velocities higher than a few km/s. The physics involved in electromagnetically propelling the projectile by an electrical current passed through the rails and projectile mean they will be slow in comparison to a simple chemical rocket. I will assume some translation error and take "rail gun" as referring to some other mass driver/beam launch technology capable of reaching the desired velocity.

Apart from that, any mechanism for accelerating a projectile to 0.2c is likely to be somewhat noisy. Even a tiny fraction of the total power being emitted as electromagnetic radiation will be noticeable from a great distance. If they have the slightest reason to be on guard, they are likely to notice you shooting decades before your projectiles arrive. However limited their propulsive abilities might be, they're probably sufficient to dodge given this much warning.

Your projectiles likely need some tracking ability just to enable them to hit their targets at such extreme range, but this can be countered by firing projectiles to intercept them, damaging or destroying their tracking and propulsion systems. The advantage is on the side of the defenders: they can launch smaller projectiles at slower speeds to cripple your projectiles. It comes down to an arms race between weapons, countermeasures, and counter-countermeasures.

Answer 7

While it has been mentioned in passing in a couple of the answers, I don't feel like the gravity problem hasn't been given enough airtime here.

Even if you solve all of the aiming problems (pointing it in the right direction, getting exactly the right energy output, etc.) and even if you have total knowledge of the distribution of gasses, radiation, and so on in the path, you still need to solve the N-body problem to accurately predict the path of the projectile. Not only do you have to solve a problem we can't even consider an answer to (except in certain very limited example cases), you have to solve it for every gravitational source in the vicinity, for the entire flight time of the projectile.

Which is another way of saying: nope, not going to happen.

For the sake of argument, let's assume that you have solved the N-body problem. You have a Matrioska Brain capable of the calculation, and some way of actually measuring all of the mass in the system with sufficient accuracy to get the input data the calculation needs. You calculate the trajectory and make the shot.

Once the shot is made any disturbance that wasn't already part of the calculation introduces errors. A ship changes course unexpectedly, an asteroid collision breaks up a couple of rocks in an unexpected way, a slight change in solar emission alters the net force by a tiny amount... and the projectile misses it's appointment.

Railguns are fantastic because of their kinetic potential, not their accuracy. Getting hit with a mass travelling at an appreciable fraction of light speed is going to ruin your day. But over long distances? Either it's going to miss or you'll have enough warning to get out of the way. Even if you don't have detectors sensitive enough to see them coming in all you need to do is make tiny adjustments to your engine output at random and you're effectively impossible to hit at relativistic ranges, even with SOL weapons like lasers.

If you really want to hit them that far out, send some AI-controlled torpedoes with exciting sub-munitions like nuclear pumped lasers, or drones with railguns that will do the damage but from close in instead of impossibly far away.

Answer 8

Relativistic shotgun (only works on approaching ships).

You don't need to get anything up to speed, the incoming ships are already moving plenty fast. What you need is to overwhelm their automatic impact defense systems and also keep them from avoiding your weapon. Bring in the shotgun approach...

Figure out how good their defense systems are, how accurate your guess is as to where they'll be, and how much maneuvering they can do to avoid you. This gives you the mass you need and the volume you need to put it in.

Then find at least enough mass to equal that. Something that's not very solid works well: one of the asteroids that's basically a pile of rubble would be a good bet (note that you'll need some way to contain it during transportation).

Drag it out to the area you anticipate them passing through.

Then shake it. Pieces everywhere, big huge cloud of crud filling up enough volume that they won't be able to avoid it with enough density that it will overwhelm their defenses.

Sit back and await results.

Answer 9

If there have been Questions more loose, someone might be able to point them out.

In building weapons targeting any great distance, the viability of railguns seems like a reasonable consideration… until you spoil it with all that other stuff.

To stay on topic, a weapon targeting interstellar distances has three basic requirements:

Launch power; accuracy; warhead power… who sees another, please chip in.

May we ignore the warhead? It gets there, or it doesn’t…

As posed, your Question seems to ignore accuracy, leaving the launch power of rail-guns.

Are you asking whether a rail-gun could do the job at all or how efficient it might be, as compared to other technologies? If this Worldbuilding technology, why not simply state that your railguns have that capability?

If this is about real-world technology, can you explain how SE Worldbuilding permits that?

“Ordinary” guns have put shells 111 miles high and whether at that height they came close to escape velocity, you might like to research. Apparently, explosive-powered guns can’t readily give their projectiles a muzzle velocity of more than ≈2 km/s, while railguns might exceed 3 km/s.

If you’re in a timeline tens of thousands of years advanced from ours, why are you asking about such trivia?

How could it matter that humans have colonized a significant portion of the stellar neighborhood, umpty light-years across?

Why is it “sadly” that the laws of physics seem to mostly have held up? More usefully, how have those laws failed? Can you explain, or would you rather drop that?

How could even you, let alone any ordinary person, “extrapolate” technology tens of thousands of years advanced?

What is the timeline communicator you’d like readers to ignore?

What shows that your Great God of Plot is more than a dream you had?

How in thousands of years is the greatest invention fusion energy and torch-ships? Is that limitation your own, or what?

When your ships are capable of supporting a constant 1g or more “upto Plot” what does that mean? 3G? 15G? 315G? If you’re dealing with a mere 1G, how could any period of time matter?

What kind of interstellar object could not also be a weapon of mass destruction due to its mass and velocity or as you yourself said, kinetic energy?

Did anyone else retain the will to live even up to this point?