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Old 09-07-2016, 10:38 AM   #2
Join Date: Jul 2013
Default Re: Building a Fun Spacecraft Combat Paradigm

Defense against Missiles

There are five possible defenses against missiles: Vector change, counter-missiles, point defense, dodge, and armour.

Vector Changes
Compared to the spacecraft's reactionless engine, the missile looks pretty short-legged. What the spacecraft can try to do is accelerate laterally, trying to cause the missile to miss them. For this, I'm going to assume the missile to match the spacecraft's lateral acceleration using half its dV budget; the other half is used to (instantly; we're simplifying here) accelerate initially. Both spacecraft are at rest relative to each other; the spacecraft has a total acceleration of 1G.
The missile, closing in at 5mps, can be evaded by the spacecraft every time after 800 seconds (13 and change minutes), during which it covered a distance of only 4,000 miles. When not starting from rest, the every five mps of the launching spacecraft adds another 4,000 miles to that range. A major UV laser on an SM+10 spacecraft has a longer range than a missile launched from a 20mps velocity difference!
A larger missile gives us twice that dV; from rest, it reaches 12,800 miles. This is a longer range, and each 8mps from the launching craft gives us another 12,800 miles. We can work with that, but the result is that small missiles are actually short-range weapons (at least, using these assumptions)!

Counter missiles are interesting in that they perform better the more missiles are incoming. A nuclear-tipped counter-missile is simply going to destroy every missile in a ten-mile hex, whether there are ten or a thousand. As such, counter-missiles are useful as an oh-crap-button, and should form a complement against missile numbers that can't be stemmed by point defense. We can assume it hits every time, since the incoming missile can't expend much dV - if it does, and therefore doesn't hit the spacecraft, we've won anyway.

Point Defense
Point defense is mostly going to use the smallest VRF guns that fit, since each hit is guaranteed to destroy a missile. We're therefore looking at 30KJ UV lasers, with a range of 300/1,000 miles and a fire rate of 100 per 20-second turn. Skill modifiers are +12 for range, -1 for missile SM and up to +7 for RoF, for a total of up to +18. Obviously, that's really not needed, so we're going to split RoF between multiple missiles. Assuming skill-12 for the computer-gunners, we can fire at five missiles with each gun, for an effective skill modifier of +12 (range) +4 (RoF) - 1 (SM) -10 (multi-tasking) = +3. This means each gun has an expected 5.7 missiles killed. This, of course, assumes that we're allowing arbitrarily-small guns on there; the Spaceship Design Spreadsheet has those as Peripheral Batteries.

Now, how about those X-ray warheads? Assuming they'll close to 30 10-mile hexes, point defense suffers from a -3 in range. This changes total modifier from +18 to up to +3. Splitting RoF is still possible, but a two-missile split results in a modifier of +0, for an expected interception of only 1.5 missiles. Amusingly, not using UV lasers means that X-ray warheads can attack (with 1/2 damage) from outside of their range!

There's another way to kill missiles: Your point defense batteries can fire in non-point defense. The whole 1,000 mile range of the 30kJ lasers can be used. How many interceptions are possible depends on the range; if we assume 5mps closing speed a total of ten shots can be fired. Those have a lower hit chance; but the furthest out is still only -5, for a skill modifier of +1, i.e. 83% hit chance. This means you're expected to intercept more than 8.3 missiles, or about another six of X-ray warheads.

In summary, point defense should kill the missile fired (at once) by fourteen equally-sized missile-launcher systems, or by about five x-ray warheads-firing ones.

Dodge & Hoping it won't hit

What's the typical to-hit roll of a missile? It has an sAcc of 2 (3 for larger ones). Let's assume a large-ish spacecraft of SM+10, and a closing velocity of less than 100 mps. The roll modifier is at least +2 (sAcc) + 10 (SM) - 12 (speed) = 0, for a hit probability of 75%. This increases to 98% when using proximity detonation. Hoping it won't hit is, clearly, not a good approach.

Dodge itself rolls against pilot skill/2 + handling + dodge modifiers, at most for 6 (skill) + 0 (handling for SM+5; larger spacecraft are worse) + 1 (evasive manoeuvre) = 7, plus up to three from ECM. Best case, therefore, is half of the attacking missiles missing; without ECM it's only 16%.

Summary: You can rely on not getting hit by missiles, hoping you can dodge and hoping they won't hit. You can also detonate your reactor; that should be quicker and roughly as deadly.

As we've seen before, armour does not help against contact detonation, but does help against x-ray warheads.

Missile Conclusion
All in all, this leads us to the following results:
- A spacecraft with a single peripheral battery will be able to kill many more missiles than an equivalently-sized spacecraft can put out. Including a few missile launchers (for counter-missiles) increases protection against huge strikes.
- The only way to effectively damage a spacecraft is using x-ray warheads from a stand-off range.

This means all spacecraft will mount at least one system of peripheral batteries, for a thousand weapons half-ton weapons at SM+10. Attack spacecraft, on the other hand, mount the same number of missile launchers, launching 66kg missiles (roughly a Sidewinder or IRIS-T) at each other.
All in all, this seems like an unsatisfactory situation.
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