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There are a lot of questions on this site asking how to justify space battles at different ranges (including one by me). However, I have not been able to find much information on what range a realistic space battle would actually happen at.

The technological level in question is roughly similar to that in the Honor Harrington / and Prince Roger series. In other words:

  • FTL exists, but you aren't going to use it during a space battle.
  • Energy generation is done by fusion reactors / small antimatter reactors (small meaning "only produce slightly more energy than fusion"; the reactors themeselves are pretty big, and thus only on big ships).
  • There are energy weapons, but they aren't useful at long ranges.
  • Missiles have a long range and don't suffer as much from accuracy issues. However, there are various kinds of active and passive countermeasures.
  • Shields are a globular field around ships which capture 99% of energy that passes through them, storing it in capacitors.
  • Since shield capacitors aren't infinite in size (and it's rather hard to get rid of energy in space), they have to be lowered when the capacitors reach full capacity.
  • The shields also have to be lowered to fire weapons out. As a result, weapons fire is generally grouped in large salvos in order to minimize the amount of time the ship is exposed.
  • Shields do not block matter very efficiently. As a result, ships are rather weak to railguns.
  • Railguns are only effective at very close range, for the same reason as energy weapons.
  • Most missile warheads are Casaba Howitzers or radiation-pumped lasers.
  • While shrapnel warheads exist, ships generally have enough armor for them to be ineffective.
  • In case I missed anything, more information is available upon request.

EDIT: Here's some more details I missed.

  • Ship drives produce output in the centigees.
  • Battles primarily occur within solar systems, but aren't usually in orbit of planets (at least, not planets you want to remain habitable). Generally the battle is already decided once it reaches orbit.
  • Yes, you can use energy weapons, railguns (although that's wasting ammo) and engines to dispel energy from shields. While this is effective over time, ships can't dump energy more quickly than it's coming in due to [insert handwavium here].
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    $\begingroup$ That really depends on the composition of the combatants and the ship design philosophy of each side (a missile heavy fleet will want to stay at long range and bombard whereas a gun heavy fleet will want to get close fast for a knife-fight), which in turn depends on the relative effectiveness of guns vs missiles. For example, in wet navies today, missiles are so effective that warships only carry token cannon armaments. $\endgroup$ Mar 20 at 20:51
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    $\begingroup$ There is no generally, it all is tech dependant. Based on tech specs one builds tactics and strategy. $\endgroup$
    – MolbOrg
    Mar 20 at 20:56
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    $\begingroup$ This is just a matter of opinion as there are too many variables and insufficient information has been provided. For example energy weapons and missiles aren't useful at "long" ranges. How long is that? Ships are rather weak to rail guns. How weak? But as a generalisation I suggest it would be a giant missile fest with the side that has the most / best destroying the opposition at "long range". $\endgroup$
    – Slarty
    Mar 20 at 21:48
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    $\begingroup$ ‘Centigee’ is an amazing unit. $\endgroup$
    – Joe Bloggs
    Mar 21 at 8:35
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    $\begingroup$ I suggest removing the word "realistic" since this question involves FTL, antimatter reactors and globular energy shields. $\endgroup$
    – Innovine
    Mar 21 at 10:18
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DISCLAIMER: The only reason I didn't VTC this question as 100% opinion based is because I believe so fervently that my answer is the only realistic answer. I'm sure others will disagree with me... but you'll need to prove me wrong with more than casual logic.

Realistically there will be no space battles

Space [...] is big. Really big. You just won't believe how vastly hugely mind-mindbogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space. Douglas Adams, The Hitchhiker's Guide to the Galaxy

Honestly, even the people who understand that statement tend to take it for granted. The space surrounding a planet is big... really big... mind-mindbogglingly big....

And for this first reason alone, there will be no space battles. Why waste time and energy banging away at your enemy's ships when you can circumvent them entirely in complete safety and bomb the snot out of their planets? Space stations don't matter because for any design and/or placement you give me I can basically ignore them by sending a flotilla of FTL slugs (say big balls about a metric ton each) to slam through and/or past the stations and into the planet.

The second reason is attributed to a Franciscan monk, William of Ockham: the Law of Parsimony, lovingly known as Occam's Razor. My favorite version of it is, "all things being equal, the simplest answer is usually correct."

Unless there is a MASSIVE technological difference between combatants, there is simply no reason to engage in any form of space battle. In fact, it might not even be possible to. FTL travel hints at FTL communications, which means you can always counter your enemy's movements.

And this is an honest flaw in almost every scifi story: the ability to detect an oncoming fleet such that you can do anything about it in an FTL universe is equal to your own speed. Which means unless you have the worst possible luck (as either attacker or defender), you'll never know another ship is there for a battle to take place anyway. Heck, to have a battle you'd almost have to agree beforehand with your enemy to meet at a specific place at a specific time.

The supreme art of war is to subdue the enemy without fighting. Sun Tzu, The Art of War

And just to add icing to this bizarre little cake, the economic cost of space battles would be debilitating on a planetary (if not a multi-planetary) scale. You don't get to recover the debris and recycle it to before the next battle. It's lost.

And this suggests the nature of space-faring beings, if we ever meet any. They'll either be master diplomats, because the reality of battle in space is so incredibly ugly that everyone wants to avoid it like the very worst plague, or they'll be the most vicious and hateful people you ever meet (or, realistically, don't meet) because they'll bomb us out of existence to guarantee we never become a threat. But, that's must my opinion.

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  • $\begingroup$ The only reasonable place for space battles is is orbit (because that’s the only place interesting things are), but even there it’s more like asteroid defence with added steps. $\endgroup$
    – Joe Bloggs
    Mar 21 at 8:39
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    $\begingroup$ Does not seem like a good answer to me. Space-to-ground attacks are extremely likely. You just put a bunch of mass in orbit (say, bringing it back from asteroids), and it requires little to no maintenance until the day you decide to drop it on a city. To prevent this, some space-to-space capability is required. Space also has huge observational intel benefits, which you will want to deny your opponent in a conflict. $\endgroup$
    – Innovine
    Mar 21 at 10:26
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    $\begingroup$ @Innovine I, personally, would not be letting Damocles' decorators hang swords all over my home. Similarly, you can park all your big dumb dangerous mass over there, well away from this nice place where I live. $\endgroup$ Mar 21 at 11:15
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    $\begingroup$ "Heck, to have a battle you'd almost have to agree beforehand with your enemy to meet at a specific place at a specific time." Isn't that pretty much how ALL major battles (except invasions) have been fought? That in almost all of history, both sides knew the night before that the next day they would be in battle, and where that battle would take place? Really, isn't 'war' just another type of 'organized team sport' between two well-trained, well-prepared 'sports teams' with pre-scheduled 'competitions'? $\endgroup$ Mar 21 at 14:29
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    $\begingroup$ In most sci-fi space opera war stories, the author has to go to great lengths to develop something even close to a scenario that would preclude these space battles as a necessity. The two waring sides almost ALWAYS decide to have a battle, somewhere, some place, just for the sake of having a battle to see who is the better team. $\endgroup$ Mar 21 at 14:35
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There are a bunch of problems here.

  • Firstly, you're not having realistic space battles, on account of energy shields.
  • Secondly, you haven't said anything about where battles might be fought, or why. Low planetary orbit will be dramatically different from interplanetary space, for example.
  • Thirdly, you haven't constrained your drive technology. Do your ships have sustained thrusts measured in centigees or multiple gees?
  • Lastly, and perhaps most importantly, you haven't said what sort of range you want.

That last one is kinda important. Do you want starwars-style WW1-dogfighting? Would you prefer the enemy be so far away that you can only just see the nuke flashes?

The easiest variable to tweak to make things fit your dramatic and aesthetic requirements is the effectiveness of shields. Tougher shields require getting up-close and personal with your enemies.

Your shields, I think, need more thought. The most obvious problem is this statement:

Since shield capacitors aren't infinite in size (and it's rather hard to get rid of energy in space), they have to be lowered when the capacitors reach full capacity.

You're getting free energy from your enemies attacks. Why not give it back to them in the form of laser beams and railgun rounds? This presents something of a "game balance" issue here; attacks that fail to cause significant damage boost the point defense capabilities of the target!

What happens when a capacitor fills up? Does it go bang? How forcefully? This totally sounds like something that could be weaponised, you know.


Anyway, as I see it, missiles are going to be the long range weapon of choice, because they're (potentially) long ranged and fast and relatively clever (compared to a railgun round) and perhaps most importantly: they can be shielded. Energy absorbed by the shields can be used to fire defensive weaponry against incoming interceptor missiles, or to boost the engine if you're using a drive that can be boosted just by turning the power up.

A missile, therefore, is in fact a missile bus whose job it is to penetrate the point defense boundary of a target deeply enough that it can despatch a bunch of (possibly shielded) warheads and submunitions and decoy devices to best effect. "Deeply enough" depends on what the submunitions are... nuke-pumped lasers, casaba howitzers, one-shot railguns, other modified nuclear warheads such as explosively-forged projectiles seem like sensible choices, and you might reasonably expect to have them work at ranges from 100s to 1000s of kilometres depending on your tech level.

At a 10G acceleration (which is pretty high, by the way), it'll take a little over 2 minutes for a missile to travel 1000km from a standstill, at which point they'll be travelling 14km/s and will take a little over a minute to travel the next 1000km even if they don't accelerate anymore. If your ships stand off a few thousand kilometres away from each other and have kill ranges between 100km and 1000km, it seems like you've ended up with something that isn't entirely implausible, and does have a certain amount of dramatic tension but lacks serious action and the opportunities for your characters to engage in derring-do, tactical awesomeness and heroics are sharply limited. Shorter ranges will be a slaughter, longer ranges might give too many opportunities for missile interception or even outright escape, depending on how much fuel and reaction mass the missiles have.

Decide on the feel you want your combat to have, then constrain the setting to suit. Doing things the other way round risks disappointment and frustration.

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  • $\begingroup$ In response to your questions: (1) Why? (2) Battles are within solar systems, but not in orbit. (3) Sustained thrusts in the centigees. (4) I don't know; that's what this question is for. $\endgroup$
    – The Daleks
    Mar 21 at 0:15
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    $\begingroup$ "What happens when a capacitor fills up? Does it go bang? How forcefully?" Electroboom with your answer! $\endgroup$
    – Cort Ammon
    Mar 21 at 1:29
  • $\begingroup$ +1 for @CortAmmon's comment. 30+ years ago my classmates and I had a ton of fun blowing up caps during our High School electronics classes. If you want realism, it won't involve energy weapons because the capacitors (or any other energy storage device) used to build up the energy before release is nothing more than a giant bomb inside your own hull. $\endgroup$ Mar 21 at 3:30
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    $\begingroup$ Traditional battlegrounds have almost always been located at a site of strategic value. This would likely mean Lagrange points, very high orbit in the solar systems plane, and low equatorial and polar orbit. The only interplanetary strategy advantage would be one of surprise attack on traveling ships. Battles are likely to include space-to-ground attack. I agree that the question should mention this aspect. $\endgroup$
    – Innovine
    Mar 21 at 10:21
  • $\begingroup$ @TheDaleks (1) they're magic, hence not realistic. Your magic system can be coherent, but it can't be realistic, cos magic. (2) there's nothing in deep space to fight over, so how do you arrange a battle out there? (3) that's a fairly critical piece of information to add to the question (4) because space warfare doesn't exist, and you have too many variables. The only constraint is what you want from your fictional world into which you have already added magic in order to change how space battles work to better fit your needs which you aren't sharing with us! $\endgroup$ Mar 21 at 11:12
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You have FTL, continuous-centigee sub-light drives, and antimatter technology, but your civilization still managed to get into an armed conflict. Kudos.

You want something, the other party is most dastardly not giving it (up) for reasonable-ish amounts of love or money. Freedom, Oil, Smoking - you name it.

Your society does love its warriors, but they may not have a warrior-death, Ever. Thus, it is drone-warfare and complete superiority all the way. The opposition(s) you had until now was crushed, obliterated, broken. (Actually parts of the opposition were thusly dealt with. The rest withdrew, melted away, took their resentment and some longish knives with them. You never put boots on the ground in any appreciable numbers, because of the threat posed by theses disgruntled loosers, and the rest of civilization seems to think you did not win the encounter because of that. Fools.

An appreciable percentage of your domestic product is sunk into gargantuan pieces of technology that fractally defend and are defended by slightly smaller units, down to the tip of the spear, your warriors.

You now are opposed by a society that is on your level. They are moving battlestars towards your main population centers (they could, of course, at the flick of a switch simply destroy those population centers, but they seem to want to control them.) They will stand off in empty space many FTL-minutes away, of course, because if any of those battlestars got killed, it would ruin their economy. Instead, they will be the base of operations for vessel that will approach to FTL-seconds, releasing, in turn, drones that will close with the enemy (you) to breath-on-your-neck light-seconds, releasing munitions, which release submunitions, you get it. they are you.

Of course you will never let them close that much. You meet them, FTL-days away from your population centers and theirs, and the drone-fractal unfolds: sentient sub³munitions gang up on submunition, C-beams glitter in the dark near the Tannhäuser Gate (some drones will be quite impressed by this), carrier groups flank, the works.

Now the distances: Sentients vs Sentients : Close Quarter Combat; Humans vs Humans: FTL-days or weeks; Sentients vs Humans (later, but arguably part of the same conflict): Close Quarter Combat (in the rain, possibly)

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Rule of Cool Says You Get Space Battles

JBH has probably the most accurate answer. There is not really a chance of realistic space battles.

That said, the rule of cool says that there are space battles, even if that doesn't make sense. Given that there are space battles, how far apart are they?

What are the weapons?

If you have a few hours, take a look at this document which details a lot of the technology that might be used in space warfare.

From that, I would say that we're looking at mostly solid munitions fired at speeds in the 10s of km/s with limited guidance potential.

Modern technology what it already is, and considering your rules about energy shields and energy weapons, I would then estimate that you're looking at distances of less than 10km. Any farther than that, and your target has more than a full second for their autocannons to lock onto your easy to notice munitions and vaporize them (or a part of them) with a short range energy weapon, presuming they can't just dodge.

In terms of the scale of space, a distance of 10km might as well be 0.

If we were talking ships at sea, this would be the same as fighting a battle after the two ships had already run into each other.

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You mentioned the Honor Harrington series--yet that is a basically perfect example of why there can't be answer to this. The universe comes reasonably close to your specifications.

At the start of the series missiles aren't worth much against heavy ships, fleet battles are settled with lasers. Then the standoff warhead (for those who haven't read the books, using the warhead to drive bomb-pumped lasers) was developed--nowhere near the boom but it made missile defense much harder--now the balance shifts more towards missiles, but you still see energy weapons in fleet engagements. Then the missile pod comes along, salvo size goes up and it favors missiles even more. Then we see the multi-drive missile (for those who haven't read the books--3x the burn time and thus something like 9x the range. Yes, missiles don't really have a max range, but once the drive burns out it's ballistic and easy to dodge. Later versions allow ballistic phases between the 3 stages.) Now energy weapons are basically meaningless. (To the point that with all the new tech Manticore took on and old-style fleet of 100x it's tonnage--and it was over long before the old fleet could engage.)

This is all a matter of tweaking the exact details and yet utterly changes the weapons mix.

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