I Have been experimenting with using a derivative of a rocket I used for landing on the mun that was made only from the base parts to escape the solar system and actually was able to reach a very high hyperbolic excess velocity and leave the system at a very high rate of speed. I left the system at top time compression all night and woke up to see that the whole system was barely visible in the map view and I was still travelling at over 10,000 m/s. The way I did this was essentially a dual gravity assist (sort of) You start on a launch pad on Kerbin travelling around the center of mass of kerbin which has a the mun travelling around it and which travels around Kerbol. I started the mission and waited 4 night cycles by speeding up time right away and then launched eastwards into a minimum orbit just a hair over 70,000 m. the waiting had put the mun in the right position to perform the gravity assist as my intercept point would be the point in the orbit of the mun where it is traveling around kerbol faster then kerbin (outside of the orbit) Which also happens to be my launch direction. I came in behind the mun and lowered my hyperbolic trajectory to just 800m over the surface on the far side. When I reached close to periapsis, I fired the engine full throttle, shed my 'descent stage' and fired the ascent stage until cut out. this left me traveling faster then the mun by about 6000 m/s which was traveling faster then kerbin which in turn is traveling around kerbol...... all in all this left me in a trajectory about twice the velocity of kerbin (at periapsis) in the Kerbol system and I went sailing off with far more then escape velocity.