Stop feeding the troll ;) If a person can't handle an argument without name calling, they're not worth your time.
For what anyone not trolling :) There is nothing magical about existing on Earth that allows a nuclear reactor to run. Earth does provide a few conveniences, mind you - your mass budgets are unlimited, and cooling is easier. But nothing about either bulk nor mass prevents nuclear reactors from operating in space, by any stretch, and the two main things limiting their use have been a lack of need and NIMBY (the former being little applicable in the former USSR, they used them quite a bit, although they still lacked a need for high powers and so generally kept them fairly small; in the US, NIMBY limited the US to just one launch, although the US developed a number of other systems, some to flight-ready status, on the ground).
The typical mass balance for a in-solar system fission fragment rocket (measured simply by MWt, not MWe, since thrust is direct) is about 20% payload, 20% structural, 35% reactor, and most of the rest toward various aspects of cooling. The nuclear fuel makes up only about 2% of the total mass (figures from the Callisto baseline). For an interstellar mission, however, the fuel would make up the a large minority or the majority of the mass, trading significantly reduced acceleration for significantly longer acceleration times. On an in-solar-system version, power density is about 6kWt per kilogram of reactor mass (that 35% figure above). This is actually quite low by large-space-reactor standards; many modern multi-megawatt reactor research projects for NEP and defense purposes (example) often deal with density figures of 50-100 kWe per kilogram, including cooling. But a fission fragment reactor has a sparse core and has to rely extensively on moderation / reflection to keep up a sufficient neutron flux; higher core density is prohibited because then the fragments would thermalize.
One thing that's neat about a fission fragment reactor is that, like systems like VASIMR, it can operate in various output modes, trading ISP for thrust as needed. In pure fission fragment mode it's ISP is is ridiculously high, nearly 1m sec; your thrust is purely the relativistic fission fragments from each reaction, carrying the majority of the reaction's energy away. However, you can inject gas into the stream as reaction mass, limited only by the density to which your magnetic nozzle can keep the stream confined. So where higher thrust maneuvers are needed, you can use the same engine (up to the aforementioned extent, of course; you're not going to take off from a planet with a FFRE!)