Using the posted information about missiles I did some small thought experiments about booster stages and their efficiency. Using the missile design postet under Rules, and the Resolution-class destroyer as orientation, I redesigned the missile a little bit.
here's the original:
Nuclear Anti-ship Missile
Missile Mass: 5 Tons Warhead Strength: 250 Kilotons Guidance: Fire Control
Engine Power: 0.24 MN Fuel Use: 134.40 litres per hour
Launch Acceleration: 48 m/s (4.89G) Per Hour: 172.8 km/s Per Day: 4147.2 km/s
Final Acceleration: 68.6 m/s (6.99G)
Fuel Mass: 1500 litres Delta-V Budget: 2,285 km/s
Full Burn Duration: 11.15 hours Distance Required for Max Velocity: 47m km
Fireball Radius: 1000 MJ/m: 289m 300 MJ/m: 527m 100 MJ/m: 912m
Proximity Detonation Range (PDR): 250m Damage MJ/m: 1331
Cost Per Missile: 2.75
Materials Required: 1.55x Tritanium 0.25x Uridium 0.9x Gallicite Fuel x1500
Development Cost for Project: 275RP
Mostly defining two stages:
1. Booster Stage, transporting a 1. 75 coasting stage, totally 5tons.
Using basically the same engine, only cut to 204kN by reducing the size to 1. 8 and therefore the consumption of fuel to 120. 96l/h. The fuel has been decreased slightly to 1. 45tons, and therefore 1450liter.
The statistics for this stage should be (compared to the original missile):
Delta-V: 2200km/s from 2285km/s
MaxBurnDuration: 11. 98hrs from 11. 15hrs
DistanceToMaxV: 44,7mkm from 47mkm
Therefore it is (slightly) slower, and needs more time to reach final velocity. One reached, however, the second, coasting or steering stage emerges:
2. Coasting Stage, 1. 75 tons
Carrying the warhead of 1. 5 tons and the firecontrol guidance module, this mounts a very, very small engine and only 150liters of fuel.
The used engine is a smaller Fuel Efficient 160 KN Missile Engine, providing a measly 8kN power, and needing only 7l/h fuel. This allows the quite good burn time of 21. 4 hours at an acceleration of something between 4. 57 and 5 m/s², depending on the fuel left. Here, too, the statistics:
Delta-V: 368km/s
MaxBurnDuration: 21. 4h
DistanceToMaxV: 154mkm, assuming the velocity of 2200km/s has been reached, and the missile only makes course corrections.
This gives the missile a theoretical reach of nearly 2000 mkm against an evading Resolution, while still having an end velocity of about 2200km/s, which the previous design couldn't, as it would tap into the acceleration-delta-v-budget for course correction.
This wouldn't be the only improvement, as for everything except the boost-phase, the missile wouldn't be 5tons - it would be 2, pushing it under the minimum detection size, and maybe making interception more difficult.
The statistics, copied together, of the missile would be:
Two-Stage Nuclear Anti-ship Missile
Missile Mass: 5 Tons Warhead Strength: 250 Kilotons Guidance: Fire Control
Engine Power: 0.24 MN/0.008MN Fuel Use: 134.40/7 litres per hour
Launch Acceleration: 43.2 m/s (4.4G)/ 4.57m/s (0.47G) Per Hour: 155.5 km/s / 16.452 km/s Per Day: 3732.4 km/s / 394.8km/s
Final Acceleration: 60.85 m/s (6.2G) / 5m/s (0.51G)
Fuel Mass: 1600 litres Delta-V Budget: 2200 km/s / 368km/s --> 2568km/s
Full Burn Duration: 11.15 hours/21.4hours --> 32.55 Distance Required for Max Velocity: 44.7m km
Fireball Radius: 1000 MJ/m: 289m 300 MJ/m: 527m 100 MJ/m: 912m
Proximity Detonation Range (PDR): 250m Damage MJ/m: 1331