Thoughts on Space Combat (Heavily Inspired by CoaDE)

Engagement ranges are on the order of tens/hundreds of kilometers, not more, and they are mostly linear

Weapons are largely ineffective farther away (we'll talk about this in a bit), but since range is a tremendous advantage, no one will survive a prolonged closer encounter. Unless your intercept is retrograde, one side will die before anyone can maneuver appreciably closer or farther.

The battle space is also mostly linear (two sides facing off across a no-one's-land). A battle "line" is really a 2D plane in space, but aside from this, it's not much different. 2D-thinking (or even 1D thinking!) is quite sufficient.

Why should this be? While ostensibly space is 3D, when you're flying a real ship, you have delta-v constraints. The space of engagement is large relative to that, and your acceleration is slow to boot (you do have a low-thrust, high-ISP engine, right?). Additionally, since you're probably rendezvousing from a different orbit, you'll have a single dominant direction of approach. You spread out when you attack, sure, but if you're at the distance where you can completely outflank your opponent, you're at a distance where both sides have long since torn gaping holes through each other with k-slugs.

Maneuvering is almost worthless

As a direct result of the above, the only purpose for thrusting at all is to dodge k-slugs. You can't do it very well, though, since unpredictably dodging requires rotation—but rotation is slow, and costs lots of delta-v.

Moreover, to move laterally probably means turning, which means exposing your flank to the enemy. That makes it a bigger target. In CoaDE, this is balanced by the fact that weapons shoot sideways, so to attack, you must make yourself vulnerable. In reality, all weapons just shoot forward.

On that note, K-slugs are actually great

A k-slug is essentially a high-velocity bullet. The conventional wisdom on k-slugs is that they don't work in space because your target moves literal kilometers just in the time it takes your bullet to move down the barrel. This is a bit of an exaggeration. If you're rendezvousing with the enemy anyway, the relative velocity is low—probably less than several hundreds of m/s. An ordinary bullet moves a bit faster than this, and a k-slug probably 3-10x as fast again. However, the problem is real.

The solution is to lead your target. As above, an opponent can't really dodge effectively. But, the inaccuracy of projectile weapons means you're pelting an entire volume with k-slugs anyway, so a bit of jitter from enemy maneuvering is essentially meaningless.

Note: CoaDE models k-slugs with railguns and coilguns, which I think are probably optimistic/unrealistic (some versions fire >100 / sec, including cooldown). They also make an argument for tracer rounds on every shot, since stealth is meaningless in space. I disagree; tracer pyros are extra mass you need to accelerate, and ballistics computers have essentially no use for visual confirmation of a hit.

K-slugs are effective at any range, though obviously accuracy decreases as range increases. It's mainly a question of how much mass, in the form of k-slugs, you can afford to have miss.

Example: a base in a hollowed-out asteroid will be willing to fire k-slugs at any distance. This opens the door to interplanetary-scale bombardment.

Lasers are basically worthless

Because of divergence, effective laser power decreases brutally with distance (constant divergence angle => inverse square falloff). With higher frequencies, you get lower divergence, but unfortunately, higher frequencies are hard to generate and in many ways are less damaging (though that's way beyond scope). Since the engagement envelope is measured in tens/hundreds kilometers, your laser basically needs to be a thousand, a million, or a billion times as powerful, just to do the same amount of damage at range.

Example: A diffraction-limited 532nm green laser with a 2mm aperture has a minimum beam divergence of 0.085 milliradians. This corresponds to a factor of 23 million billion reduction in flux density over the mere 1.3 light-second distance from Earth to the Moon. So the whole thing about light-speed lag playing a role in laser targeting is garbage, because your city-sized 22-terawatt death-star-laser literally looks like a laser pointer at a distance of 1 light-minute.

Oh sure, you can do a lot better by increasing the aperture (at inverse square again, but thankfully not scaling with distance). And, in fact, any even remotely practical laser weapons system operates with huge apertures and a lens or mirror to move the beam waist towards the target (all of which are vulnerable themselves)—but you're still going to play a losing battle with diffraction, and CoaDE correctly shows a depressingly abrupt asymptotic drop to zero with distance.

But the even larger problem is the heat generated. A laser outputs only a tiny portion of its power as coherent light. The rest is dumped as heat, which goes into radiators. To radiate a literal power-plant's worth of thermal energy into space requires several square kilometers of radiator. That makes you a huge, immobile, sitting duck that still can't defend itself because lasers are worthless.

Example: A space station with an enormous 1 GW ultraviolet laser was disarmed easily, at range, by a lone gun skiff with a 3mm railgun, firing in the general direction of the radiators.

The point is it's not worth it. Enemies can't dodge anyway, so you might as well use something that actually retains all its destructive power at range and doesn't produce an obscene amount of waste-heat. The only case I've found for lasers is blinding (but again, not really damaging) drones and missiles.

Whipple shields are stupid; slanted armor is OP

Slanted armor vastly increases your survivability; this has been known since antiquity. And, when you're going against hypervelocity k-slugs, it's basically your only option. Make it thick enough and slanted enough, and you can shrug off a continuous hailstorm more-or-less indefinitely (at least, if CoaDE is any guide). If the opponent is, stupidly, using lasers, their beam spreads out with the secant of the armor slant angle, to say nothing of the greater Fresnel reflection at angles. Every piece of armor on your ship should be slanted.

Example: Against an incoming 532nm laser, Aluminum armor has a refractive index of 0.90175. This means that you can actually get total internal reflection. Armor slanted at more than ~64.389 degrees will experience no effect whatsoever from the laser, no matter how powerful!
(EDIT: this was a miscalculation; I am not aware of a material where TIR losses are practical from vacuum. Secant and Fresnel losses still apply, and so slanting armor is still effective versus lasers.)

Conversely, whipple shields are useless (a whipple shield is a sacrificial layer of thin armor that shocks k-slugs into plasma, which can then diffuse). One problem, however, is that this theory only works if the projectile is orthogonal to the armor (which would mean your armor isn't slanted). In fact, if your whipple shield is slanted, k-slugs tear huge gashes that quickly render it worthless.

This is a special case of whipple shields being helpful only once. A whipple shield will block one bullet, but not two. If you have a battle where millions of k-slugs being fired, that's basically no protection at all.

Missiles ruin everything

In CoaDE, missiles lock onto the greatest heat source. This makes radiators a vulnerability (although I don't know if occlusion is considered by the game). In CoaDE, this is basically completely countered with flares.

In real life, missiles won't be anywhere near so dumb. First, countermeasures are not 100% effective. Vietnam-era "Sidewinder" missiles had a kill probability of 18%, which is already terrifying. Modern missiles are around 90%. Pure-infrared systems are imager-based these days, making them basically immune to countermeasures. But these are being phased out—today, we have multispectral guidance systems that are essentially unstoppable, operating on radio, visual, and thermal frequencies. And that's not factoring in literal centuries of technological development before the first space battle of the future. Ships are also much bigger and (as before) less-agile targets than fighter jets.

But the real difference between a k-slug and a missile isn't payload, but maneuverability. Unlike a ship, missiles have an enormous delta-v budget, and they are cheap and small enough to be nigh-vulnerable to weapons fire. This means they can outflank enemies, shooting them from essentially any direction. In addition to striking the more vulnerable sides of a ship, it makes slewing a point-defense cannon around more difficult. And even if you can disable a missile at distance, its debris is still going to slam into you at several km/s relative velocity. Missiles are massive enough that this is probably a mission-kill anyway.

Indeed, the ideal tactic is to shoot many small missiles and have them converge from different directions. There is no realistic defense against this. Missiles are even less dodge-able than k-slugs and they're much heavier. Shoot it down with point defense or blind it with lasers, and you still have a gaping hole through your hull. Fail to disable even a single one, and you have a nuclear warhead going off point blank.

In case you don't believe me, think about reality. Fact: a modern warship (the boat kind) has trouble shooting down a single missile with point defense. If you have dozens of missiles with sci-fi armor, all traveling at quadruple the speed (no air resistance), approaching in three dimensions, and you have maybe 1/10th the armor (mass, delta-v limits) . . . well, it's just not going to work out very well for you.

So how should one design a battleship?

You don't. It's an obvious consequence of missiles: if your battleship can be obliterated by a tiny missile, and there's no real defense against such a thing, you don't build battleships—you build missiles and send them against enemy infrastructure. Obvious secondary effect: War between such factions is attritional, and at most only one major space-based faction survives.

[Oh, fine. Let's handwave the missile-defense problem for now.]

The uselessness of maneuverability suggests exposing the smallest possible cross-section to your enemy. For a given mass, this means making your craft long. The slanted armor means making a sharply pointed nosecone, which will also contain all of your armor budget. This will ameliorate the unreasonable effectiveness of k-slugs. Maybe, your entire ship can just be a highly tapered cone.

All your heat is dumped via a single retractable radiator extending out the ship's rear, and therefore hopefully hidden from enemy weapons' fire.

Weapon systems have narrow gimbaling, if any, and poke through tiny holes in the forward cone. These are almost-entirely rapid-fire k-slug launchers, of whichever SF-inal technology you please. IIRC IRL railguns have trouble with repeatability, but a few spinal-mount linear accelerators seems plausible. You probably want a few kilowatt-scale lasers to engage incoming drones and missiles, but nothing too fancy. You can place these on the sides, behind your cone-shield, shooting sideways.

Since missiles can turn, put your missile launchers behind your ship. Each missile splits into hundreds of individually targeted warheads that spread out and then converge on the target at an angle, as described before. Missiles are optimized for delta-v, and consume all of it before impacting. Rocket-powered guided (non-explosive) k-slugs are also an interesting possibility.

All weapons are optimized for range, since the aggressor who strikes first and longest is the victor.
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