I was watching this show about all of the leading ideas going into a mission to mars.

The part that got me was about fuel. Specifically Nuclear Power.

Years ago, I somehow got it into my head that a nuclear power rocket wouldn't need fuel. It would just keep reacting and such going on. I admit I'm wrong as the show pointed out. Still I'm just a bit confused. Just hoping for a little bit more of an explanation.

Does the reaction make fuel, or is the fuel the reaction itself? If it's the reaction then what are the short comings aside from having a nuclear reactor doing a challenger in low orbit over cape canaveral?

The fuel gets used up in the process. What happens is one type of material gets converted into another one, which isn't suitable for fission or fusion, depending on the type. (fusion reactions aren't yet controllable) After all the fuel gets used up, the reaction ends. Now, in the case of a star, the sheer amount of pressure inside the star is enough to sustain several different types of fusion. First the hydrogen fuses. Then after all the hydrogen is gone, the helium starts fusing, and the star becomes a red dwarf. All elements up to iron can be fused by a star. But fission reactions require that the material be able to be split by the free neutrons caused by former splits. Eventually no fissionable material remains, and the free neutrons don't have anything left to split. Many atoms can be split multiple times. Any atom will split if bombarded with a high enough speed neutron, but sustainable chain reactions are only feasible with certain high atomic weight elemental material.

BTW Napster, I forgot to say thanks. Can't say I totally understand, but I am at least beginning too. :)

and thus children, is the lesson of , you can't get something from nothing.