This is a good site for options: http://www.projectrho.com/public_htm...enginelist.php
Smoother than the nuclear pulse orion concept is the Nuclear Salt Water Rocket
Its fringe-worthy--but look how well it does with a 90% solution on this graph:
Best yet: The advantage of NSWR is that this is the only known propulsion system that combines high exhaust velocity with high thrust.
Thrust 13,000,000 newtons with 4,700,000 m/s exhaust velocity--a one two punch!
NTR DUMBO looks more do-able
This would provide 3,500,000 newtons thrust
Now imagine you use that in place of a Centaur upper stage, and have AIMSTAR as a payload:
This would provide 598,000 m/s exhaust velocity. This alone would allow a trip to 10,000 AU in 50 years. Couple this with DUMBO NTR to get most of its useful life extra-solar--NOW you have a souped up New Horizons.