Ooh, I can help with this! The Saturn V, in the 50s/60s, had a planned larger version that (I believe) was to be called the Saturn S-N. It used NERVA nuclear thermal engines on either the first and second stages or just on the second (or some other variation - not quite sure). There were other variants planned as well, but the NERVA was an extremely good, very efficient engine, and this rocket would have been extremely capable. However, the NERVA program was cancelled due to lowered NASA funding, just like pretty much every other new, amazing form of propulsion. Whenever I read about these things, I get really hopeful for the future, and then I see that the program was cancelled and get sad. It really is a shame. Luckily, though, we don't have to worry about KSP programs being cancelled. Looking forwards to getting the Vega stage in KSP
