I like the sound of the new 'Decoy Missile' mechanics, mostly.
I agree with @Bremen, that the name needs work. Missile Decoy would work for me if not for the fact that it's already taken by missile launched decoy penaids. Maybe 'Defensive Decoy', or something similar?
Really, the only issue I have with it, is the prohibitive mass cost. 2.5% of the ships mass for a single decoy that gives 50% reduction in hits? That's a _lot_, for a decoy that only lasts for a single attack. If not for the cost, I could envisage missile combat where multiple waves are survived, by using multiple decoys per wave, or one huge wave is survived by using several decoys, but not at this cost in terms of ship mass. 10 decoys with equal signature (50% hit reduction), would be a quarter of the entire ship's mass!
Perhaps a tech line that starts at 200t/MSP, and increases sharply from there? I disagree with Bremen's take on a signature/MSP tech line, as effectively greater signature just means you can carry more decoys, as there's diminishing returns on decoys with greater signatures.
Obviously playtesting will inform this, but on the face of it, the mass cost seems really high to me.
Is there any reason for the 5 MSP minimum size? What if I want my fighters to have decoys to defend against AMM size missiles?
Finally, a request for clarification - how would these interact with laser heads? I've gotten a little fuzzy on how they're going to work, but they don't go through the CIWS phase right? Will these decoys affect laser warheads? It sounds like they won't but I feel like they should.
I don't mean to sound so critical. Overall I really like the core mechanic here.