I'm making a world where sub-FTL interstellar travel is a thing, and so is interstellar war, but for the purpose of this question we'll assume the scale to be of a single system.

Spaceships have evolved quite a bit since our current era, and they range in size from several meters (something like 2 times larger than your average air superiority fighter, having a space-worthy manned vessel can't get any smaller than this in my universe) to up to 20 kilometers in the shape of a scaled-up space submarine.

Weapons comes in varying shape and size (proportional to the ship that bear them) and can be split in 3 categories:

• Missiles : Just your everyday payload carrying self propelled firestick, you could also call them torpedoes at this point, it doesn't really makes a difference as far a I know. They won't get bigger in size than an ICBM and the tech didn't evolve much aside from targeting and space-worthiness.

• Lasers: They are mainly used as a countermeasure and/or to mess with enemy targeting systems at close range (and occasionally to blind the enemy commander trough the window as a prank).

• Railguns: This is where R&D was the most successful, since those guns can get pretty big (remember the 20km ships?) and the biggest projectile to date is a whopping 100m long and can travel at 30Km/s. Picture throwing Saturn V at the enemy vessel). Please note that for smaller projectiles the speed can get up to 150km/s, but that is a very specialized gun and the average is less that 100km/s for a decent gun.

Concerning energy requirements, capacitors went a long way and are now extremely efficient, as well as cooling, even in space (firing a railgun more than once won't melt it for the first couple consecutive shots) and fusion is the go-to mean of energy production across the civilization. Dyson sphere project started in some systems, and the question of the access to the host star sparked the wars in the systems.

Now for the question: Is it of any use to strap a nuclear payload to the railgun projectile or does the impact of a plain old solid steel slug at several kilometers per second outperform such a projectile?

Bonus point: If it ever gets efficient, at what scale? (from the smallest tactical nuke to a Saturn V sized monstrosity of sheer destruction)

EDIT: My question differs from space-born ship-to-ship combat 150 years from now w fission/fusion tech as it is about the efficiency of a specific type of weapon regardless of the fact that it may not be the best kind of weapon. Although the two questions are very much linked, I do not think this is a duplicate.

• @L.Dutch I edited the question to adress that, feel free to ask for precisions or edits if it can improve my question – Alexcommil May 22 '18 at 9:42
• To add to LDutch's answer. Simplicity. Assuming it were possible to detonate the nuke at the exact right moment. Such a shell would be highly difficult to manufacture and handle. Rather, it is easier to hurl a hunk of lead at thy foe. Same reason explosive bullets aren't a thing. – Aron May 22 '18 at 14:53
• When using this kind of railgun, don't forget the bit they explained in Mass Effect: When you fire one of these, sometime, someplace, you ruin someone's day, even if you miss. – Perkins May 22 '18 at 21:44
• "(something like 2 times larger than your average air superiority fighter, having a space-worthy manned vessel can't get any smaller than this in my universe)" we can already make today unmanned space vessels which are considerably smaller, and we could potentially have vessels whose size is determined by the weapon they carry. Not saying you can't have manned space craft, but if you've got sub ftl space travel, interstellar war isn't going to be done with manned space pilots. With the lack of ability to dissipate heat with out radiation, IR sensors become 1000x more powerful in space. – opa May 23 '18 at 15:14
• @LukeMcGregor can you prove it? that sounds really cool – amara May 23 '18 at 19:45

The kinetic energy of a bullet (for non relativistic speeds) is given by the known formula $E_K = 1/2 m v^2$.

For a velocity of $150 \ \text{km}/\text{s}$ you need about $100 \ \text{kg}$ to deliver 1 Kton ($4 \cdot 10^{12} \ \text J$) of kinetic energy.

Apparently then you can double the delivered energy by strapping a nuke to the bullet.

But...

As in vacuum you will have no shockwave to help you deliver damage but only radiation damage, you have to time the nuke to explode once in the target, which leaves you with a margin of error of about 1 millisecond.

One millisecond is way more than the time it takes for the explosion to happen, by then, the deploying explosion would have exited the target. The image below depicts an atomic explosion 1 millisecond after it started.

And on top of that, your bomb has to withstand the impact, else it would just fizzle.

• Comments are not for extended discussion; this conversation has been moved to chat. – Monica Cellio May 25 '18 at 16:39
• <strike>1 megaton =~ $4 \times 10^{15}$ Joules. Seeing as the OP states the highest energy rail gun is the one that produces $4 \times 10^{12}$ (~100kg at 150km/s) wouldnt it be more accurate to say that a thermonuclear warhead adds 1000 times the energy? </strike> Since modern thermonuclear warheads have yields in the megaton range, wouldn't it be more accurate to state the nuke adds x1000 the energy? – cms May 25 '18 at 17:05
• @cms, I referred to 1 kton – L.Dutch May 25 '18 at 17:52
• What's the scale on that picture? – Ray Jun 18 '18 at 19:38
• You do still have a shockwave in a vacuum, but it's composed only of the expanding cloud of ionized gas that was originally the explosive payload. The solution is for your warhead to be a "casaba howitzer". That is: a nuclear shaped charge that transforms your megaton warhead from a spherical burst into a hellish lance of highly charged particles travelling at a measurable fraction of the speed of light. Correctly built they will concentrate approximately 90% of the energy into a beam with only a 5 degree spread. – Perkins Dec 21 '18 at 20:35

I won't expand on the math done by L.Dutch as it's correct and speaks for itself, however I would advise against the use of nukes in space for a few reasons:

Cost

Not only would you have to figure out the design aspects of the weapon, but you'd also be throwing expensive material away with each shot, if you had to choose in a war zone between throwing an iron projectile at a ship, which you'd need to reproduce or resupply, or send a nuke. One costs very little as it's basically just refined metal, the other consists of many more expensive refined metals and a lot more engineering, to do comparatively little extra damage. The only time the nuke would be worthwhile is if it detonated exactly where it was supposed to, in the middle of the ship. Anywhere else and it becomes exponentially less effective, all that for double your damage and 10x the cost - why not just throw another slug at it!

This is already a factor in combat zones at the moment, why throw that million-dollar missile at that guy when the comparatively primitive auto-cannon or conventional weapons on a chopper would do it.

Kessler Syndrome

If you shot a hole through a ship, it would pretty effectively cripple it unless it somehow missed everything of worth; it would cause a ship to have an exit wound similar to a human, and leave debris flying out the back of the ship. But, it is heading away from you very fast, so you're still fairly safe in theory, but if you detonated a nuke inside the ship, then you'd send debris in all directions: unless the ship was able to contain the blast somehow, this debris would make maneuvering very hazardous at best, but at worst could damage your own ship. Imagine the damage your massive 100m slug would do on impact... now imagine a 1km chunk of previously destroyed ship impacting yours, nowhere near as fast, but still with huge kinetic energy due to its size and mass.

This is the smallest of the factors but the radiation and EMP caused by nukes, would effect navigation and other ship board systems, I know if a ship can survive inside the Van Allen Belt then this is a fairly moot point, but what about in conflict defenses and protection can be damaged, if your ship survives the battle but took a few hits as well, you'd think twice before entering the Van Allen Belt of a planet, you'd at least wait until you were certain it was safe to do so.

Spoils of War

Why destroy a ship if you can capture it? The Allies tried again and again to capture a German U-Boat just to capture the Enigma machine on board it; in a war zone where both sides are technologically advanced it would be worthwhile allowing a ship to surrender and capturing it or just going straight in, boarding it and capturing it the old fashioned way, just to see what new tech the other side may have been working on.

Basically ship-to-ship battles in space would lead to chaos for navigation and crew survival at the most basic level, let alone if you started throwing nukes around.

• I think you mean the Kessler Syndrome, not the Kepler Syndrome. – YSelf May 22 '18 at 12:01
• Oops... edited, cheers – Blade Wraith May 22 '18 at 12:33
• The cost of your answer ignores the cost of accelerating the slug. Sure, fusion is plentiful and easy, but that might make uranium dirt cheap as well. Is steel slug + "propellant" worth more or less than nuclear bomb + less "propellant" (* Propellant is short for fusion fuel, capacitors, wiring whatever here.) – Odalrick May 22 '18 at 13:20
• @Odalrick pretty sure the OP stated that energy capacitors and Fusion Energy were the thing, so energy requirements go out the window, i admit that includes for the Energy for nuke manufacturing process as well, but that doesn't change the infrastructure behind it all to build them being far more complex and therefore more expensive. but purely in reference to your statement, yes, it would be cheaper to throw a 20kg iron slug, mainly because 20kg is not very heavy, in fact i would wager its not that much heavier then the nuke you'd want to throw, possibly its even lighter – Blade Wraith May 22 '18 at 14:54
• @Odalrick: The nuke would need to be built not only to survive the impact on the target, and penetration of its armour, but also just the launching of it, Nukes are somewhat complex devices, and as such cna break when to exposed extreme G forces such as going from 0m/s to upwards of 100km/s in 100m, really not wanting to keep this back and forth going, so disagree if you want, i'll leave it here – Blade Wraith May 22 '18 at 14:59

Space shotgun.

Consider the payload from a railgun. It packs a wallop because of its velocity. The more massive it is the harder the wallop. A high mass projectile the size of a rocket would pack a very big wallop. It would also be energetically very costly to get it up to speed.

But compared to space, even a rocket is small. And a very fast moving rocket sized projectile would shine like a star on radar, which would work better in space than on Earth. You would see it coming thousands of kilometers away. And although the size of a rocket, it is not a rocket - it is a dumb projectile following a totally calculable trajectory. All your ships have the power to move. They will see this thing coming and get out of the way and the enormous energy to accelerate it will have been wasted. Miyagis best defense: no be there.

Re the nuke: as has been pointed out, these make more sense in an atmosphere where the atmosphere itself is part of the weapon. In space the only thing the nuke has to throw is pieces of rocket, and its own gamma rays. A spacecraft will be able to handle some radiation; there is a lot out there anyway.

The fix: if you want to hurl projectiles at speed towards small distant fast moving objects, you want a shotgun. The best approach for your giant railgun projectiles would be to have them break up shortly after launch into individual fragments. You could use explosives for this to give the fragments lateral momentum relative to each other. Each piece retains the momentum given to it by the railgun, but now you have a spreading cone of destruction. You could make the pieces difficult to see with radar and so if your target missed the initial launch the incoming 1 kg fragments might be hard to detect. Once the cone was close there would be no getting away from it. You cannot outrun it. Unless you are close to the edge you cannot dodge it. There are too many pieces to block individually. You are toast. Or swiss cheese.

This would be similar to a nuclear strike, because it is indiscriminately destructive. Actually, even more destructive - an atomic blast has a circumscribed area and time of effect, but not this cone. If anything you like is downstream from this cone (which will continue to spread and spread, kinetic energy unabated) those things are also cheese toast.

Thinking about such a weapon, it would be better for planetary defense. You can brace for the recoil against the planet. You have more energy available. The cone of destruction comes out from your planet and so your friends are all on the good side of that cone. The first stage rocketsized projectile would be great for traversing the atmosphere and it would break up into components once through.

I'm voting for the nukes, and not just because I'm a natural pyromaniac

I think that many people spend their lives watching Star Trek and Star Wars and think that space battles are a lot like watching pre-1900 wars where ships of the line stood broadside and unloaded short-range light-damage cannon in bulk.

In reality space battles will be much more like post-1900 naval warfare where the ships themselves will be ever further apart as technology improves.

Why?

Because railguns hurt

Railguns pack enough punch that they can rip through pretty much any ship of any size — if you give them the opportunity. As anyone who'se tried rabbit hunting with a .22LR will tell you, the closer the bunny the deader the bunny.

And this assumes the bunny doesn't have nasty things like electronic counter measures, false heat signatures (can you imagine the size of flares left in space to confuse heat-seeking missles?), or magnetic deflectors to push aside incoming ordnance.

The natural behavior of ships will be to attack at such long distances that the effectiveness of their defensive abilities is maximized and it becomes impractical to retaliate with point-solution projectiles. It's simply too much of a pain to hit the target.

Thus, nukes

The issue isn't that nukes have more or less punch. The issue is that they don't require the pinpoint accuracy of railguns. You need to think of them more in terms of flak guns. All you need is to either set a "distance" marker for detonation or have a means of detecting the shift of mass from in front of the warhead to behind the warhead to indicate the need for detonation (or, if everyone's using magnetic deflectors, a simple magnetrometer and a timer to arm the warhead after it's outside your own magnetic deflector).

However, the railgun isn't useless...

You do still need the railgun (or, at least, you can justify it). The problem with those distances is that they're distant. The time needed to get a missle up to speed can mean the difference between a sucessful strike and being the dead bunny yourself. Railguns become the rocket for the "rocket assisted artillery" of the future. It gets the nuke moving really fast, really fast.

Conclusion

I gladly predict that the future of space combat will demand the ships be so far apart that today's over-the-horizon naval engagements will seem like a Marx Brothers skit. In fact, the distances could be so great that even Battlestar Galactica-esque carriers with swarms of fighters will be useless. They're simply too slow over to great a distance. Delivering area-of-effect weapons quickly over long distances will likely become the only method of ship-to-ship combat.

Man, I love making sweeping difficult-to-prove-right-or-wrong statements. It just perks up the morning, you know? :-)

• "the closer the bunny the deader the bunny." +1 Aside from that, I like the aproach of using the railgun for the nuke and not the other way around – Alexcommil May 22 '18 at 14:17
• As L.Dutch Explained below Nukes in space don't have an atmosphere to move and form the pressure wave, which is the area of effect part of the Nuke going off on earth, therefore the AOE would be significantly reduced compared to on earth. I will agree that space battles will occur over incredibly great distances, but this will most likely lead to Rail guns that fire big old balls of bird shot, scattering a large area with difficult to detect and counter balls of steal, space battles in space will be very rare, targeting a ship orbiting a planet while it lands troops on the other hand... – Blade Wraith May 22 '18 at 14:29
• @BladeWraith, I stand by my answer and point you to my answer to "Are nuclear-propelled shotgun slugs viable?". The spread pattern from the launching ship would be so wide over any sensible distance that it would be worthless to fire birdshot from the ship. – JBH May 22 '18 at 15:15
• @JBH, A good well rounded point on the other post, however it's effectiveness depends on what it was orbiting, orbital mechanics make this matter very complex. but as you said, lets have some fun with this, I'm going to "gather additional data" when i get home, i'll play Kerbal Space Program and put my best guess info into the buckshot idea and see if i can hit a target and see what the spread would be. i'll let you know tomorrow – Blade Wraith May 22 '18 at 15:31
• @JBH in Star trek: The Original Series starships fired their weapons with devastating effect at distances of tens of thousands or hundreds of thousands of kilometers. It was only in Star Trek II: The Wrath of Khan and later movies and TV shows that bigger effects budgets and the influence of Star Wars led to starships fighting at close distances. – M. A. Golding May 22 '18 at 20:43

While a nuclear warhead by itself seems somewhat useless (as per L Dutch's answer) there is a similar effect that can be achieved that is simpler* than Nuking the opponent, which would be to have a small amount of anti-matter (assuming you know what the opposing hull is made of) in your projectile. When the projectile hits the hull, it will fracture and let the antimatter particles hit the hull; these will annihilate causing small nuclear explosions (?) near where it struck.

*Simpler to get working. Harvesting Anti-matter is a pain; but you don't need much of it.

These explosions don't have the issue that L Dutch points out because the explosions will only happen when they contact the hull. This is to say that much of the forces that come out of annihilation will be send as shockwaves throughout the hull itself causing the hull to shift in various ways and hopefully tear itself apart.

An aside: People talk about Lasers and Railguns all the time, but forget about Particle Cannons. Particle cannons travel at near light speed, can pepper someone with extreme amounts of kinetic energy to tear their hull apart, and if unsuccessful will instead impart large amounts of heat. What's more? They've already been invented and proven to work (so is well within the realm of possibility.)

While I'm not sure, you can combine these ideas and have an anti-matter-particle-cannon, firing anti-matter particles at the opponent to not only deliver massive kinetic energy, but annihilate a (very very) small chunk of their ship, and hope the shockwaves sent throughout the ship's hull are enough to rend it apart.

• While this is a decent discourse on effective space weaponry choices... it doesn't answer the question, which was "would making our railgun projectiles nukes have any benefit?" – Delioth May 22 '18 at 19:46
• @Delioth I've changed the first paragraph in an attempt to address your point. If that isn't enough, I wouldn't mind a suggestion on how to reform my answer in a more helpful way. I do think that if he wanted a nuclear warhead, the anti-matter projectile is a reasonable approximation of the same effect, but should actually work (I think..) – blurry May 22 '18 at 19:52
• This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review – Mołot May 22 '18 at 20:09
• @Mołot Yes it does, in the first sentence: "While a nuclear warhead by itself seems somewhat useless" is a direct answer to the question "Would it be of any benefit[...]?" I then provide a solution to his conundrum and address the point that L Dutch brings up: How would you make a similar weapon work; which no one else has touched on, thusly providing a helpful direct answer to the question and a helpful alternative to the question. – blurry May 22 '18 at 20:22

I suggest the following:

Since a classical nuclear weapon is just big enough heaps of radioactive material to pass criticality and explode..

Why not just fire many small chunks of radioactive material, to impact the enemy hull in quick succession.

• clunk
• clunk
• clunk
• "What's that?" ...
• clunk
• nuclear explosion

The downsides would be:

• Possible waste of nuclear materials if you miss.
• Having to calculate and adjust for the motion of the enemy.
• Design and management of your weapon.. don't want it to jam and build up chunks of material.. in your own gun.

The upsides would be:

• The chunks would not need to penetrate the hull, their mere proximity would be dangerous to the crew.
• Fairly cheap compared to refined metals, especially if you're using fusion to produce power, no real demand for fissiles.
• With good aim and relying on pure trajectory, there's no electronic countermeasures, detection itself could be difficult.
• That bullet list is hilarious :D – Alexcommil May 24 '18 at 13:38

Yes there is a reason for railgun rounds to have nukes on them, but it is not to deal more damage

Any relatively hard sci-fi universe (which yours seems to be) is going to have ships engaging each other at mind boggling distances so even if you are launching slugs at several hundred km/s it could give the opposing ship seconds or minutes to dodge the round. so each rail gun slug would have a guidance package on it that is capable of targeting enemy ships and turning the slug in flight. Then once your slug has crunched a lot of math it figures out which way it needs to aim and sets off the nuke for quick and violent course correction onto the course that it thinks has the highest probability of hitting wherever the enemy dodged to. Though since the nuke only has one speed setting which is boom, if the rail gun slug doesn't need to change course it would just use the nuke to hit the enemy at higher speeds or maybe not even blow the nuke so it doesn't decrease its mass for impact and can irradiate the enemy ship.

Nukes, or at least nuclear material, would be useful for at least two things:

1. radioactives are very dense so the rounds can be heavier, and thus have more kinetic energy, at a given size if they're made of say depleted uranium. This gives the ability to vary the impact of rounds while using standardised rails and capacitors, which is handy to have even if you don't often use the versatility.

2. radioactive materials are radioactive so you can use radioactive rounds, whether these are nuclear weapons or shells full of hot Caesium doesn't really matter and will probably change depending on mission profile, to devastate planetary targets and write off extraction and manufacturing facilities in such a way as to completely deny any future use to your opponent.

It is useful if it is cheaper/easier to put a nuke inside a railgun projectile than to make the projectile and railgun larger in order to get the same damaging power. I would imagine that it is as nuclear tech is today and tomorrow it will already regarded as simple as a light bulb compared to giant railguns. Shooting smaller bullets with the same damage as a larger one could probably be shot with a higher frequency or requiring less energy.

Assuming that shooting projectiles is the norm of the space war vehicles would also evolve to dodge enemy projectiles, perhaps by shooting projectiles tangentially from the enemies in order to gain momentum almost instantly. Therefore nukes could be timed so that if the projectile misses the ship it will detonate right after passing its original location inflicting some damage anyways.

Since there is radiation in space anyways all the space ships would be protected from it fairly well. The damaging part of the nukes would then be the electromagnetic pulses that could disable enemy tech in addition to the actual exploding of the projectile.

Not directly answering the question but supposing you were to use some inert material as slugs, why would it be simple steel? Even right now on Earth we know steel is far from being the best metal to make shells with, and we use depleted uranium. Ideally you want the densest material that you can mass-produce, which could be osmium in a sci-fi universe.

Secondly, a single big slug might not be the ideal projectile shape to inflict maximum damage on a huge spaceship far away. Supposing the fights happen at a distance where the railgun projectiles take maybe a minute to reach their target (the chances of hitting would depend on targeting systems, size and agility of the target). If the material used as projectile and the speed provide sufficient kinetic energy at the point of impact to puncture the enemy ship then it actually doesn't help to increase the size of the projectile, as the damage is the existence of a puncture. Instead, you should try to shoot more projectiles and make as many punctures as possible to cause more damage.

Yes, if you're simply using the railguns as a launch mechanism to get nuclear bomb-pumped gamma ray/X-ray lasers or Casaba howitzers (two different methods that allow you to focus a significant portion of the energy of a nuclear bomb into a single direction, as either a laser or particle beam, respectively) to a minimum safe distance from the ship launching them, rather than using them as a method to deliver a high-velocity projectile towards your enemy to damage them through a kinetic impact.

I wouldn't feel completely secure in a spaceship where massive amounts of explosive atomic material were applied intense magnetic forces + extreme heat + extremely strong acceleration by a railgun :-D

As said above, the weak point of the railgun is its "fire and forget" nature : accuracy is paramount, and depending on the target distance, it leaves time to detect whatever has been thrown and to move. Nukes may aso not be a weapon of choice due to the lack of atmosphere to propagate the shockwave.

However, why not considering a mix of both : a missile with no explosive material aboard but an heavy arrowhead (steel, depleted uranium), and engines to let it reach very high speeds + correct its trajectory so that it reaches (goes through) the target even if it moves.

It is true that in the vacuum of space there is no atmosphere to transmit the blast effects of a nearby atomic explosion. But what if a nuclear missile brought its own atmosphere with it for blast damage on enemy ships?

Says that a ten meter diameter object with the density of ice travelling at Mach 172 or 57 kilometers per second would start to break up in Earth's atmosphere at a height of 98,800 meters (98.8 kilometers) or 324,000 feet (61.3636 miles). The Earth's atmosphere is dense enough even that high to damage objects, if they are moving that fast.

At a height of 100 kilometers the atmosphere has a density of 5.25 times ten to the minus seven kilograms per cubic meter. The surface density is 1.225 kilograms per cubic meter, about 2,371,441 times as dense.

Suppose that an atomic war head is surrounded by a hollow shell containing air at the density of Earth surface pressure. When the warhead explodes it will vaporize the hollow shell and heat up the air to plasma temperatures. The plasma will expand in all directions. The shell of plasma will have a density as great as Earth's atmosphere at 100 kilometers when it has expanded to about 133.35 times the radius of the hollow shell it was originally contained in.

If the shell was travelling at 57 kilometers per second relative to the target when it exploded, the cloud of plasma will be damaging to the enemy ship at distances up to 133.35 times the diameter of the shell. Normal railguns in this setting are specified to give speeds of less than 100 kilometers per second, plus or minus the relative speeds of the two space ships and also plus the speed the plasma will be travelling at.

What if the atomic warhead is surrounded by a hollow pressurized shell containing air at 1,000 times the sea level atmospheric density? Then the sphere of expanding plasma would be at least as dense as Earth's atmosphere at 100 kilometers out to a distance of 1,333.5 times the radius of the hollow pressurized shell.

Iron has a density of 7.874 grams per cubic centimeter, or 7,874,000 grams per cubic meter, or 7,874 kilograms per cubic meter, or 6,427.7551 times the density of Earth's atmosphere at sea level. Thus if the atomic bomb vaporized a shell of iron around it the expanding plasma would be as dense as Earth's atmosphere at 100 kilometers at a distance of about 2,480.31 times the radius of the iron shell.

Osmium, the densest stable element, has a density of 22.58 grams per cubic center, or 22,580,000 grams per cubic meter, or 22,580 kilograms per cubic meter, or 18,432.653 times Earth's atmosphere at sea level. Thus if the atomic bomb vaporized a shell of iron around it the expanding plasma would be as dense as Earth's atmosphere at 100 kilometers at a distance of about 3,521.7735 times the radius of the osmium shell.

Thus for each meter of the radius of the shell of matter around the atomic warhead, the expanding shell of plasma will be dense enough to be damaging to an enemy ship at distances of 133.5 to 3,521.7735 meters, depending on the density of the matter in the shell. Assuming that the shells of matter around nuclear warheads are one to ten meters in diameter, their expanding plasma shells could be dangerous to space ships at distances of 133.5 to 35,217.735 meters.

I have read that atomic bombs can be designed to concentrate most of their energy in one direction, which could permit designs for a space nuclear warhead to eject plasma in only one direction that would be dense enough to be dangerous to enemy spacecraft at much greater distances.

Then there are nuclear pumped lasers, which get their power from atomic reactors or atomic explosions. Project Excaliber in the Strategic Defense Initiative was designed to use orbiting stations to destroy incoming ICMBs with X-ray lasers pumped by exploding atomic bombs.

Thus a future space armada might use railguns to shoot missiles at enemy space ships, missiles that aim at the enemy ships when close and then use atomic bombs to power lasers to blast the enemy ships.

• Your answer suffers from way too many numbers given with way too many significant digits. – Arsenal May 23 '18 at 14:08
• In particular, specifying "damaging ... at distances of 133.5 to 3,521.7735 meters" is ridiculous. You mean "damaging ... at distances of 0.1 to 3km". (There is no way that moving half a millimeter further away at a range of over two miles is going to turn "full damage" into "no damage".) – Martin Bonner May 23 '18 at 15:41
• Picky,Picky. Why should I bother to decide how to round the numbers? The effectivve distance to damage a ship varies a lot and is very small compared to the distances of the ships. My point is if such warheads can be designed to detonate with the damaging distance, whatever it is, of their targets they are effective weapons, but if they cannot get with that distance, whatever it is, and detonate within that distance, whatever it is, they are useless. So the writer needs to consider whether he thinks it is plausible for the missiles to get close enough to damage the enemy when detonating. – M. A. Golding May 23 '18 at 19:53

1 Kt of TNT is 4.184e+12 J.

30 km/s is a Newtonian velocity, so 1/2 mv^2 is accurate. If we solve for 1/2 mv^2 = 1 Kt of TNT we get 10 tonnes of metal. At 150 km/s we get 25x that energy.

A 4 m diameter 300 m long projectile of iron has a weight of about 30,000 tonnes. So it will deliver 3000 kT, or 3 MT, of energy.

If left as a column much of this energy will "blast through" a relatively small object (like a space ship, or Ceres). We can use Newtonian projectile impact depth, which basically says we'd need at least 300 m of armor to significantly slow down this projectile (if it is made out of iron). Spalling in that case will be fatal to whatever is behind it.

Generally, a projectile whose length times density is much larger than the depth times density of our target is going to blast through the target and waste kinetic energy.

We can improve our ability to deposit energy on a "thin" target by shattering our projectile. You'd want to shatter the projectile at a distance measured by the rate of spread of the shattering and the density-depth ratio of your projectile to the target. This "spread" also enhances accuracy, as near misses become hits.

A nuclear charge may be viable here to cause rapid spread, and also boost the velocity of some of the components. It would go off before you reach the target.

Now we can get even fancier. Anti-tank weapons today are rockets that carry a charge that generates a shaped-charge of high density plasma, which is used as a higher velocity impactor to penetrate the high density hull of the target.

We can use the 30 thousand tonnes of the projectile and fill it full of hydrogen bombs. A tsar bomba has a weight of about 30 tonnes and a yield (when not reduced on purpose) of 100 MT. Suppose we manage to replace 10% of the mass of this projectile with tsar bomba; this weapon now has a yield of 100 GT, 30,000x greater than the metal projectile.

Each kg of steel requires 500 J to heat it by a degree kelvin; and it melts below 2000 Kelvin (so < 1E6 J to heat it to melting). Another 272 kJ to turn it into liquid, for a total of 1.272E6 J to melt a kg of steel.

Each m^2 of steel has a weight of 8 tonnes / m of depth. If we assume 10% energy deposit, melting a meter-thick steel plate requires 8*10*1000*1.27E6 J =~ E11 J.

A 100 GT nuclear explosion releases 4E20 energy; a mere 0.25E-9 fraction of it melts a meter thick steel plate. This is roughly when a circle of radius r has an area of 4E9 m^2, or a range of about 17 km.

So a weapon that explodes with the power of 1000 Tsar Bombas would melt somewhere between 1 to 10 meter-thick steel armor at a range of 17 km.

A modest improvement to 100000 Tsar bombas gives us a range of 170-500 km to melt (1 to 10) meter-thick armor, as the limiting factor of making a bigger H bomb was "there is no point" not "there is a fundamental problem here".

Such weapons would be insanely lethal against smaller craft (as you cannot put meter-thick steel on a fighter), but probably a direct hit from a 300m long solid slug would be harder to armor against for a 20 km long ship. If you can figure out how to make shaped nuclear bombs you could possibly do even better.