Add a Orion Nuclear Pulse Engine, that is fast, but heavy, not as manuverable as regular liquid engines, can't use liquid fuel (add new tanks for nuclear bombs, but they should be heavier and bigger than a normal liquid fuel tank) and you can't land with it ( because you can't land with an engine using nukesas propellants ,unless you want to land in an pile of dust, that was a planet before your arrival ) and if you try, the ship will explode