Last time in my Aurora tutorial series, we looked at the basics of beam weapons and warship design. But beams are not the only threat out there, and the hardest thing for new players to get is usually missile defense. I once ran a multi-player game where several of my friends (unfamiliar with Aurora) were each given a race and turned loose. I also ran my own power, which I attempted to nerf to keep things relatively even, and use as a wildcard. As it turned out, I significantly underestimated how much nerfing I needed. When one of them tried to attack me, I managed to destroy his fleet with essentially zero difficulty despite a significant technological disadvantage. I knew missile combat and he didn't, so his defenses were ineffective.1
Despite this, building an effective missile defense isn't particularly difficult. It just needs to be taken seriously, because the AI opponents can throw a lot of missiles. For beam defenses, this basically means that you need to optimize for the maximum number of hits (of any damage, because missiles are killed by 1 damage) at point-blank range against fast-moving targets. The math on all of this is made easier by the fact that pretty much everything scales linearly. In particular, if a target is moving faster than the attack's tracking speed,2 the chance to hit is equal to the chance for that range times (tracking speed/target speed). At any given tech level, it's easy to build effective missiles that are faster than 4x racial tracking tech,3 so it's best to just treat the turreted weapons as 4X more likely to hit for analytical purposes. There's a box on the right side of the ship-design window that lets you set the speed and range the FC to-hit values will display for.
OK, so enough theory. What are our actual options? In the early game, it comes down to 10cm railguns and gauss cannons. Gauss cannons do only 1 damage per shot, but can fire more than once per 5-second increment and can be mounted in turrets. The problem is that they take up 6 HS4 and the early tech only lets them fire relatively slowly (2 shots/5 seconds). 10cm railguns, on the other hand, take up 3 HS and can fire 4 shots/5 seconds, but can't be mounted in turrets. So which is best? That depends on your tech level. Railguns also require power, but this is balanced out by the fact that a turret requires 30-40% extra space to allow it to track at higher speeds, and the fire control for the turret will be larger and more expensive. On the whole, at the default tech level you have now, railguns are probably a better option. Note that if the ship is faster than the racial tracking speed, then you can build a fire control that matches ship speed and the railguns gain relative to gauss weapons. As the game goes on and you research higher levels of gauss ROF, those weapons gain a clear advantage. To a very rough approximation, a single shot from a turreted gauss cannon is equivalent to 4 from a railgun, and with ROF 4 and above, the gauss turret is the clear winner for effective shots per HS.5
It's also worth taking a brief digression into fire control. Besides making sure the tracking rate is matched to the rate of the weapon, you should also pay attention to the range, balancing hit rate with the size and cost of the fire control. In the early game, this can be quite important. Point defense takes place at 10,000 km, so going from a 32,000 km FC to a 16,000 km FC could cut effectiveness by almost 50%. On the other hand, bigger FCs do cost more and take up more space. This is probably something to play around with using prototypes before you commit, at least in the early game. Later on, as the base FC range rises, the benefits of overranging FC become less important. Another note is that back in the days of VB6 Aurora, it was my standard practice to fit every gauss turret with its own FC. This was because each FC could only shoot at one target per turn, and there was considerable risk that an FC with multiple turrets would kill a salvo with the first turret, leaving the others inactive.6 This changed in C#, and while I'd still emphasize the importance of multiple FCs on any warship, there's not really any need for 1:1, which also lets you build bigger and better FCs.
The one other thing a missile defense ship needs is a good sensor, capable of picking up the missiles while they are well out. This is because tracking the missile for an extended period can offset the penalty from a missile being faster than the fire control's tracking speed. The tracking bonus offsets 1% of the missile's to-hit speed penalty for every 5 seconds the missile was on an active sensor before impact, with a cap based on the racial Tracking Time tech. For instance, take a missile going at a 133% of the weapon's tracking speed (25% penalty). If sufficient tracking tech was available and the missile had been tracked for 125 seconds before it was engaged, then there would be no penalty. The obvious need, then, is to pick the missile up as far out as possible, and it's probably best to make sure the sensor is at least size 4 or size 5. Making it bigger than this usually faces sharply diminishing returns.
But wait. Do we have to do all this messing around with point-defense weapons? Couldn't we use a longer-range beam weapon to pick the missiles off earlier? The answer is that we can, in theory, and there's a mode in the game to let us do so, but it's not particularly practical because of how fast missiles often are. Your best odds of hitting are at point-blank range, so if you want to use area defenses, you'll need to have multiple shots from each weapon while the missile crosses its engagement zone,7 and this means that you need good-sized weapons with reasonable range and enough capacitor tech to recharge in 5 seconds. This usually means 15cm C6 lasers, and big fire controls. Even if we take a best-case scenario where the missile is only moving at 4x racial tracking speed, you have to be reasonably far up the tech tree before this starts to make sense, and faster missiles cut both accuracy and number of chances to hit.8
One last feature of beam missile defense deserves a brief note, namely CIWS. This is a system much like the Phalanx CIWS that equips so many warships today, a self-contained missile defense system that can be bolted onto any ship to provide a last line of protection. These are often a good deal, particularly with higher gauss techs, and can be fitted to civilian ships to give them minimal missile defense capabilities. Most of my high-value warships often have one or two as well, just in case.
But beam weapons aren't the only way to deal with missiles. There's also countermissiles, which have their own benefits and drawbacks, which I will discuss at length next time.
1 I've spent a lot of time thinking about missile defense after being badly burned in some early games, so this post is going to have more general theory than the previous installments. ⇑
2 As a reminder, the tracking speed for an attack is set by the lower of the weapon's tracking speed and the fire control tracking speed. FC tracking speed is built in, while weapon tracking speed is listed in the ship design window, and is the higher of the ship's speed and the base racial tracking speed for weapons not in turrets. For weapons in turrets, it's set in turret design. ⇑
3 In fact, I'd argue that any general-purpose missile which isn't faster than that is probably by definition a bad missile. Obviously, there are lots of special cases which are exceptions to this, but it's a good rule of thumb. ⇑
4 It's possible to build smaller gauss cannons, but you trade off accuracy. A 3 HS gauss cannon has 50% of the accuracy of a full-size gauss cannon, for instance. To a first approximation, this drops out and can be ignored, although it can be important in more detailed analysis. ⇑
5 An argument could be made on the basis of cost that railguns are always the winners, because of how cheap basic railguns are. Personally, this strikes me as an exploit, and I avoid it. ⇑
6 This behavior could be exploited if you were trying to get missiles through someone's defenses, by fitting lots of fire controls on your missile ships to keep salvo size down, and with certain other techniques that I'll talk about at some point. ⇑
7 Weirdly, the math for this works out so that on average, an area-defense beam weapon will score 0.5 hits for each 5-second increment the missile is in its engagement zone, because the hit chance rises as the missile gets closer. Note that this is an average, and the rate for a given salvo could be higher or lower depending on the exact geometry. ⇑
8 There's one other problem, which is that weapons shooting in area defense suffer the 1% failure rate that offensive beam weapons do, and that beams shooting in point defense appear to be immune from. This looks to be a recent update. ⇑
Comments
A few points I want to be sure I understand correctly:
Either you've gotten a number wrong, or I'm understanding this wrong. If it's one percentage point of penalty per increment, i.e. 0s tracking -> 25% penalty, 5s tracking -> 24% penalty, 10s -> 23% and so on, it should only take 125s to get 0% penalty. There are a few other ways to interpret the percentage, but the math doesn't work for them either.
If I have this right, this assumes the beam fires every increment, and '0.5 hits' means '0.5 hits for each hit it would score at point blank'. Which makes sense--if you fire at the missile the whole time it's approaching, the average salvo occurs while it's at half of maximum range, and thus half the accuracy of point-blank fire. Wouldn't over-ranged fire control change this in favor of area defense, though?
The first one was a math error on my part. No clue what was going on there.
As for over-ranging your FC, you could in theory do that. But 15cm lasers usually seem to fall at around 3.5x base beam FC range, so you’re going to need to go past that, and a max-size beam FC is big and very expensive. On the whole, it’s just not worth it. For instance, in my current game, I’m facing a foe so far down the tech tree that my total casualties so far are a single Marine. Their missiles travel at 12500 km/s, while my turrets track at 32,000 km/s. My standard destroyers (5x2 15cm lasers) equipped with my latest FC (which they aren’t yet, because the refits would have been too expensive) would be able to take out an average of 28.8 missiles each on the way in. My gauss destroyers are 5x2 R5 gauss cannons, and one of them will kill 90+ missiles at about 70% of the cost of the laser destroyer. (It’s a lot less effective in a beam action, which is why I have laser destroyers.)
But this is a best-case scenario for the area defenses. If I doubled the speed of the enemy missiles, (which is still well below my tracking speed) the laser destroyer drops to only 14.4 kills, while the gauss destroyer stays the same. Yes, the laser destroyer is still better than it would be using PD fire (~10 missiles killed) but not by very much. And there are other advantages to PD fire, to the point that I don’t bother with area beam defense. It's not that I would never use it if I happened to end up with a destroyer division facing down a bunch of my current enemies who are throwing big salvoes at it, but I'm not going to build my fleet around it, either.