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So in my questions about the intergalactic highway and simulated ship quarters I established a universe where the navy fights a cult of alien worshipers over star gates. 500 to 700 years in the future. I want the navy to have a “super weapon” that they can use to attack the cult’s heavily defended world. My ideas are:

Planet cracking railgun
This gun is mounted on the brand new UHFS Cayuga. The gun will be able to accelerate a massive shielded projectile to near c while it is going down the barrel. I want the gun to shear continents into pieces and destroy the cult’s bunkers and cities.

10,000 megaton bombardment nuke
This is also mounted on the new ship. It would be used to destroy the cult cities.

Which is more realistic and effective? A question will follow regarding the details of the battle and the tactical situation.

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  • $\begingroup$ UHFS Cayuga is a space ship? And what is "plant cracking"? $\endgroup$
    – Trioxidane
    Commented Dec 14, 2020 at 14:57
  • $\begingroup$ Yes for United Human Federation Ship $\endgroup$
    – 11Bravo
    Commented Dec 14, 2020 at 14:58
  • $\begingroup$ it is recommended for several reasons to wait with selecting an answer. Better alternatives might arise and it gives other people a chance to give their answer or opinion on the answers. $\endgroup$
    – Trioxidane
    Commented Dec 14, 2020 at 15:12
  • $\begingroup$ Although user Trioxidane's answer is great, I believe you should wait on awarding it the green check mark. You only asked the question half an hour ago, and most of the people who will see your question have not seen it yet. You might still get useful answers from other users $\endgroup$
    – BMF
    Commented Dec 14, 2020 at 15:14
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    $\begingroup$ @11Bravo How hard sci-fi? What tech does your ships use to travel? $\endgroup$
    – Alpha Delta
    Commented Dec 15, 2020 at 14:23

7 Answers 7

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You should check out Wichell Chung's Atomic Rockets and 'Matter Beam's Tough SF Websites for realistic details on warfare and weapons in space.

The key takeaway message from both however is that at high C velocities your ship is the weapon!

Any vessel accelerated to high velocities accumulates kinetic energy, as do ALL the objects on that vessel.

Mr Chung uses the amusing example of a space ship traveling at a significant % of light speed who counts among its crew the ships cat - and, as a bi-product of the aforementioned feline, a supply of used kitty litter.

The ship is on an intercept course with a planet occupied by an evil genius or similar (well evil from the cats perspective anyway). Solution? Just dump a couple of kilos of used kitty litter out the airlock and then maneuver the ship off at a tangent to its original course to avoid a collision between the ship itself and the planet. Separate to the ship the kitty litter just keeps on keeping on!

Result? Multi-megaton death by kitty litter!

EDIT; So if the ship is traveling fast enough the choice of a weapon is pretty much irrelevant. Any mass that can be detached from a ship traveling at a significant % of the speed of light will do the trick. Applying the KISS principal one or more metallic 'darts' perhaps equipped with a passive targeting system and simple gas maneuvering retros to maintain target alignment will do the trick. As would a 'shotgun' spread of multiple small 'dumb' weights depending on the size/nature of the target.

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  • $\begingroup$ Shoot, you got this answer before I did. +1 $\endgroup$
    – DWKraus
    Commented Dec 15, 2020 at 4:41
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Railgun

Neither are particularly realistic. You're more likely to fire multiple nukes instead of a 10.000 megaton one. Firing things close to C is also particularly difficult.

That being said, something often conflated with railguns is your best bet. Coil or gauss cannons. It can be done without shielding, as you can operate in space and thus a vacuum. The shell can be suspended and launched by magnetic forces, reducing damage to the gun part of the ship. Any energy imparted on the shell will increase velocity, allowing you to put a lot of energy in the projectile.

Although it will be difficult to accelerate to C, it's not needed. There are idea's to just drop tungsten rods from satellites. Simply their speed from falling to the Earth is enough for incredible devastation, while being near impossible to stop. Extrapolating this to your space combat, you can just put any (partly)magnetic material in a gauss cannon, fire it and watch the devastation.

Potentially you just fire them from outside the solar system and can even get gravity assisted speed boosts from planets and the star before landing on the target. Having enough payloads on board is basically the only requirement for senseless devastation. It is all potential energy waiting to be released from there towards the planet. Finally, it is also less complex than atom bombs, depending on how far the railgun is developed. Fewer chances things to break.

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    $\begingroup$ A railgun projectile need not be magnetic, or even conductive -- the force is applied by a plasma created from a conductive coating or wire, vaporized by the driving current. You're probably conflating with a gauss-gun, in which the projectile must be (or be encase in) a conductor that can carry the eddy currents that produce the counter-field. $\endgroup$
    – Zeiss Ikon
    Commented Dec 14, 2020 at 15:13
  • $\begingroup$ @ZeissIkon for ease of use I didn't change it to coil/gauss cannon. I'll edit it. However, I thought a railgun was electric by definition, requiring an electronic loop to be finished by what thrusts the projectile, which is generally shot as well for ease of use? Can you point me to more information, as it arouses my curiosity. $\endgroup$
    – Trioxidane
    Commented Dec 14, 2020 at 16:01
  • $\begingroup$ The propelling force in a rail gun is (IIRC, been forty years since my electricity course) the Lorentz force that attempts to expand the enclosed area of a current loop. You've got the general idea, and these can be made with a conductive shoe, but high powered ones use an expendable vaporizing bridge that forms a conductive plasma, which pushes the projectile. Or so the declassified information said several years ago. Can't give a link; I read about this in Analog magazine fact articles in the 1980s (took a long time to make the rails reusable). $\endgroup$
    – Zeiss Ikon
    Commented Dec 14, 2020 at 16:46
  • $\begingroup$ @ZeissIkon interesting! I'll try to find more. From what you say, it seems the expanding plasma will just replace the normal contact points for the closed loop, which would reduce friction (and thus damage to the gun). As plasma can be moved by magnetic fields and thus Lorentz, it can close the loop just behind the projectile, allowing for maximum energy into propelling the projectile. The only problem would be closing the loop before the projectile, but the plasma might move well enough along the Lorentz force to not be present earlier. $\endgroup$
    – Trioxidane
    Commented Dec 14, 2020 at 17:41
  • $\begingroup$ Garbage... you fire the ship's garbage. Those trash compactors on the detention level could be set up to make some nice, tight cylinders. :) $\endgroup$
    – T.J.L.
    Commented Dec 14, 2020 at 21:28
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Have you ever heard of a Nichols dyson beam? Fascinating concept. Basically a Dyson sphere reconfigured into a Death Star.enter image description here

If you want hard Sci fi planet killers, you really can’t do any better. Combine it with a star gate, like the ones you have in your universe you can bypass taking centuries to hit the target.

But if you are on a budget and can’t afford to dome over a star, RKVs or relativity bombs are probably the way to go, and the concept is rather simple. Take a single 100 ton missile like the one seen here.enter image description here

Then all you have to do is get it up to light speed, aim for a planet and it will hit with enough force to make the meteor that wiped out the dinosaurs look like a fire cracker.

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  • $\begingroup$ Moving a 100 metric ton object to 95% the speed of light requires about 4.0x10^21 joules of energy. That's about the equivalent of 970,000 1 megaton nukes. Why not just drop half of the nukes on the planet instead? $\endgroup$
    – stix
    Commented Feb 25, 2021 at 20:57
  • $\begingroup$ All the destructive power and none of the radioactive fall out to clean up so you can move your colonists in as soon as the bedrock cools. $\endgroup$
    – Jacob Badger
    Commented Feb 25, 2021 at 22:32
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    $\begingroup$ With that much energy dumped into the planet, the bedrock cooling will take decades, maybe centuries. $\endgroup$
    – stix
    Commented Feb 26, 2021 at 17:26
  • $\begingroup$ @stix But you can simply reduce the speed to the 67% of the spped of light and get the same effects that half the nukes without the radioactive decay. $\endgroup$
    – Erdel von Mises
    Commented Oct 27, 2021 at 6:29
  • $\begingroup$ @ErdelvonMises At the amount of energy you're dumping into the planet, you will see radioactive decay as you will be forcing fusion of all kinds of atoms. You'll also see X-ray spalling, forced fission, pair production, etc... $\endgroup$
    – stix
    Commented Nov 22, 2021 at 20:57
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Rock falls. Everybody dies.

You don't need relativistic speed for an asteroid to be deadly. The gravity well of the planet will give a good-sized rock enough speed to be a devastating kinetic impactor. That idea combined with tech is sometimes called a "rod of god" - a massive, dense object like a tungsten rod dropped from space. Add a few thrusters if your targeting isn't precise enough. Add rocket propulsion if you're not satisfied with just gravity - falling from outside the solar system can take a while.

If you need more sci-fi, launch the thing with a rail gun to give it a better starting velocity so it'll arrive just a bit faster.

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  • $\begingroup$ Nice first post Extrarius, welcome to wbse. $\endgroup$
    – Escaped dental patient.
    Commented Feb 25, 2021 at 15:38
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The most realistic of your two choices is the 10-gigatonne nuke.

Accelerating a projectile to the speed of light is an exceptionally difficult task, and is best done over scales of millions or billions of kilometres, or even more. Obviously building a gun with a barrel this long is impractical, regardless of technology. It requires absolutely mindboggling amounts of power, to the point where if you're present in the same solar system as your enemy you'd probably be better off using that power in a large number of much slower weapons.

Thermonuclear warheads on the other hand, even with the original Teller-Ulam design, can be scaled up to more-or-less arbitrary sizes. It might even be possible for us to build a gigatonne nuke today, not that it would be a very useful thing to do.

Such a nuke is probably excessive, though. Let me propose an alternative plan:

Probably should should start by softening up the planetary defenses.

If you've got big lasers, zap everything that looks military from a long way away, if possible. If they've got big lasers in hardened platforms (say, a small moon or asteroid) you should swat them with a long train of big rocks sufficient to overpower any defenses. Use some suitable nuclear engines to boost the rocks into a fast intercept trajectory; you've presumably got decent nuke engineering skills, so orion drives may be entirely practical here.

Finish off everything softer by throwing a load of debris into orbit on inconvenient trajectories, smashing up everything in near-planet space and triggering an ablation cascade. This can be done with missiles. If you've got big lasers, they might be a cleaner way of doing the same thing, but really you want space around the enemy world to be extremely hostile to anything coming off the surface... it is easier for you to drop heavily armoured things into a gravity well than it is for them to fly them out, after all.

Once that's done, drop a cloud of nuclear warheads of moderate size (megatonnes, not gigatonnes), possibly armoured in nice big lumps of rock, and detonate them at the edge of the atmosphere. This causes massive high-altitude electromagnetic pulses. Note that antimatter warheads would be more effective than fission/fusion here, if you have them, because they produce a higher proportion of gamma rays in their blast even at the same overall yield. This will fry all sorts of electronic and electrical things on the surface, including power grids, and toast any electronics still functioning in lower orbits.

Finish off with a sustained asteroid bombardment, focussing on areas of known deep fortification. Keep this up for as long as you can be bothered; the more rocks the better. Spice things up a little by including some salted bombs to produce deadly and long-lasting nuclear fallout, to make sure that cracks in bunkers become lethal and to discourage anyone venturing out once the dust has settled.

When you go home, leave a bunch of observation platforms in high orbit to see if anything stirs on the surface. You should probably park a load of spare rocks and warheads with re-entry engines up there too for a swift automated response, in case it takes too long for human verification. Better safe than sorry!

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Firing a projectile near lightspeed from a ship is an impossible task.

First, you'd need stellar levels of energy to get to high fractions of c; second, the accelerator would dump the reaction momentum into the ship's structure, producing potentially lethal or hull-fracturing levels of acceleration; third, as with very large nukes, the damage would be too localized to have the effect you seem to be after.

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    $\begingroup$ So the railgun is better but near c is unrealistic. $\endgroup$
    – 11Bravo
    Commented Dec 14, 2020 at 15:27
  • $\begingroup$ Look up what firing nine 16-inch naval rifles in a broadside does to a battleship. Now multiply by roundly 200,000 to get from muzzle velocity to lightspeed (ignoring mass dilation). Get the projectile anywhere near c and you'd scrap the firing vessel, or take minutes (and millions of miles) to accelerate the shot. $\endgroup$
    – Zeiss Ikon
    Commented Dec 14, 2020 at 19:30
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    $\begingroup$ @ZeissIkon Not that I disagree with your post but regarding your example, firing a broadside does not cause a battleship to move appreciably: see navweaps.com/index_tech/tech-022.php (To preempt a common misunderstanding of the linked text, "recoil slide" refers to the recoil slide mechanism that is integrated with the gun to buffer recoil, not movement of the ship.) $\endgroup$
    – GrumpyYoungMan
    Commented Dec 14, 2020 at 20:48
  • $\begingroup$ So you haven't seen the famous photo of the New Jersey with a sideways wake while firing the second broadside? Aerial shot from the Korean war. $\endgroup$
    – Zeiss Ikon
    Commented Dec 15, 2020 at 12:03
  • $\begingroup$ @ZeissIkon "what firing nine 16-inch naval rifles in a broadside does to a battleship" Nothing. It slightly rocks the ship. "New Jersey with a sideways wake" it's not wake, those are muzzle blasts. You can clearly see they're overlapping "spheres" that project from the muzzles and blast the water. navweaps.com/index_tech/tech-022.php $\endgroup$
    – Agent_L
    Commented Dec 15, 2020 at 13:30
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If you'd like scalpels for your super weapons, might I suggest the Ultra-Relativistic Electron Beam, or the Macron accelerator? The former even works rather well in atmospheres!

For UREBs, you avoid the issues of electrons spreading out due to having like charges due to the intensity of the relativistic effect of their speed. At TeV energies there are so many 9s after the decimal that time passes very slowly for them - and if you try to slow them down or push them to the side, you create intense x-ray radiation. Your options are "don't be hit", "have a very, very thick block of tungsten between you and the enemy", and "be deep underground."

And since it is an electron beam, aiming with magnets lets you quickly track different targets and even saturate areas with rapid sweeps not unlike a CRT in an old TV.

For Macrons, while you're not shooting these ultrahypervelocity dust grains nearly as fast, or in atmospheres, you can easily push them into the regime of impact fission, and 10,000 km/s is still very fast. This means you can use good old kinetics (albeit in an extreme case) and have more energy on the target than your accelerator had to use to accelerate the projectile.

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