Space Combat Missile Design

As for all New Diasporia technical game rules missiles are designed using GURPS 3rd Edition rules from VE 2nd Edition, modified for the New Diasporia Universe. If you use another system either just import the operating statistics into your ruleset or design your own, based upon what ever design rules that ruleset uses.
Standard missiles (250mm & 500mm) were designed using the vehicle rules from VE2 and using GURPS Vehicle Builder, a program which I believe is still available from SJGames. I created a custom Barnes-Gutierrez Engine Module for GVB. Basically it is a Gravity Drive with a low power deflector representing the Barnes Manifold Interface. It was created at TL12 and improves over the next two TLs per standard GURPS Tech Level rules.
Viper anti-missiles were created using the space missile design rules on VE2 p122. The P factor has a progression which goes from TL4 to TL11+. This progression discounts the fact that according to GURPS Reactionless Thruster (and Gravity Drive) rules a drive becomes 1000 times more efficient (that is its weight to thrust ratio is a thousand times greater at TL13+ than it is at TL9.) By increasing setting P=P*1000 to reflect this progression, I got a very high acceleration, but short endurance, anti-missile missile.
X-Ray Laser warheads use damage from GURPS Traveller rather than Gurps Space Third Edition, that is they do 9dx200(2) damage. Note divide by 2 only against Armor, not force fields. As can be seen at that level of damage a flight of 10 missiles could damage even a dreadnought, even with an operating force field. 10 flights of them could easily do in even such a first rater as a super dreadnought.
Missiles with X-Ray warheads are not susceptible to typical point-defense weapons (lasers and rail guns) nor to nuclear dampers at the ranges they deploy at. Anti-Missile Missiles are the best defense against them. Typical ranges of Vipers is 1 million miles (or 100 hexes if using the Traveller 10,000 mile hex based combat system.)
Point-defense weapons typically are only effective when missiles get within 3,000 miles of the target, or within the same hex. X-Ray Lasers detonate at ~10,000 miles from the target or just within the same hex.
Nuclear dampers are only effective in the < 100 mile range. Since nuclear tipped missiles are effectively contact weapons this is more than adequate. Even an active force field will not protect against the 12dx20,000 damage from a 250mm warhead or the 12x2,000,000 damage of a 500mm warhead.
I'll cover more about New Diasporia space combat philosophy in another post.