Comment by db48x

Production ion drives have ISPs between 1000s and 4000s (https://en.wikipedia.org/wiki/Ion_thruster#Comparisons). A fission fragment drive would have an estimated ISP of over 100,000s (https://www.nasa.gov/general/aerogel-core-fission-fragment-r...). Of course you can just buy an ion drive off the shelf, which is definitely an advantage. And making the fuel for a fission–fragment drive is probably going to be fun.