I'm no master of stealth missiles this but I finally managed to get a working design, so I'll give a reliable and tested method to all my much confused bretheren.
The idea of a submarine launched "stealth" missile, is about killing without using active sensors. You passively detect their signature, then launch missiles using a waypoint where they're heading to, and it homes in, all without revealing your location. (you cannot target passive signatures directly sadly...)
To do this, you must basically launch a rocket transported minefield. Nothing I did seemed to be able to get ordinary Missiles with active sensors on them to target lock without firecontrols. mines however do work for this task, and if you think about it, a mine is more versatile anyways. In a nutshell: You're using the first stage to transport and deploy a mine to the location you see the enemy is heading, then when it reaches the waypoint, it stops and becomes a normal mine (2nd stage), they fly into its sensors, and it targets it with 3rd stage payload missiles.
1st stage is for traveling there nothing else. sensors do nothing on this stage - it seems missiles do not start become guided by their active sensors until they're started with targeting a ship via a fire control and then lost the target.
2nd stage you need to make a "no engine" stage with nothing in it but a sensor, holding some stage 3 minelets for payload - this stage is the mother mine. The separation range for entering this stage should be 0 - if it is set say 150k distance, it will switch to stage 2 150k before it hits the waypoint. I have not tested if that breaks it, so to be safe just do 0 km, which I've confirmed works. You NEED to have an active sensor on this stage for it to function - it only runs when detected from this sensor, external sensors do nothing!
3rd stage is the payload stage. You can put whatever missile you want here - conveniently it does NOT need an active sensor on this stage! The 2nd stage's active sensor WILL stick around until the 3rd stage explodes! You may have noticed this when using 2 stage missiles, the previous stage's name weirdly stays behind. Don't forget to make the separation range for your payload similar to your active sensor's detection distance, it launches when the enemy reaches the separation range, not the active sensor range.
Note: Launching at a waypoint can be done at any distance, regardless of fire-control size, so you can put the cheapest fire control. It will fire towards the waypoint even if the missile lacks the range to actually reach it too.
And thats it basically. Now for unnecessary fun talk: Realistically speaking, if someone saw a mine field entering in front of their position, they'd move away - not to mention with them watching it come in, they now know your firing angle. Its stealthy, but it's not totally stealthy. Also, trying to calculate the narrow zone you can deploy the mine so that it it activates the moment it enters perfect range is cumbersome in practice too. Instead, its probably (?) best to launch your mines far ahead so they simply run into them.
Both the payload and detection range needs to be long enough that they cannot detect the mother mine so as to avoid its zone / shoot it down from afar, and to instill confusion as to the true source of the missiles. However it should probably be designed to defeat ordinary units with average sized sensors, and simply accept that good sensors counter it, avoiding those targets. Of course, you can always try launch into those targets.
Note Note: Mines with high resolution active sensors are potentially visible to passive em detection if they build a huge one (which they probably won't tbh) - if you use a thermal sensor mine as an extra stage before the regular mine, your mine will be invisible to em until after its triggered. And because mines are so small, they are effectively invisible to active sensors too. I have NOT tested a 4 stage transport -> thermal detection -> active detection -> payload works cuz i'm sleepy, but I assume it works. I did confirm thermally detecting a ship does sets off the next stage, though it fizzled out because in my test the next stage had no active sensors. I did not confirm if a 3 stage transport -> thermal detection -> active sensor payload works, it probably would? I recommend not taking chances and trying the safe version.
Note i'm talking out my ass a little philosophizing here, because I didn't take the time to design a proper design yet - i really just wanted to get the basic functionality down. For all my tests, i sent them straight in range rather than using them like a mine field like i suggested, and was only 30 million km away.
