Bear with me.
Premise we discover a material thats a good reflector of neutrons. In the mirrors are a good reflector of light sense, not low-efficiency, slow scattering reflection you get from existing reflectors.
Two reasons. First - it would mean that critical mass wasn't so important
anymore. In theory, if you could make a sphere that was a perfect internal neutron reflector, ANY suitable fissile (gives off as many or more neutrons under decay, as it takes to trigger the decay - this I know isn't the textbook definition of fissile, but what I need for this concept) mass placed inside (that underwent even slow, natural neutron-emitting decay) would fairly rapidly undergo complete fission, once its neutrons started coming back in force. The closer it was to the original critical mass, the quicker it'd go off. Of course in application I'm certain there'd be limitations with structural integrity, etc it probably wouldnt work for bombs, I mean.
...Anyhoo, getting to the point. Make a rocket with a reaction chamber thats internally reflective for neutrons, ported at the exhaust. Work out all your reaction curves beforehand
Now throttle the rocket by putting some suitably fissile material into the reaction chamber, and based on the ratios of exhaust port size, fissile mass, reactivity, and chamber size youll get a controllable, throttle-able nuclear rocket. Yes, with the fissile mass as the reaction mass
Obvious improvements would be shaping the reaction chamber for progressive reaction, boosting with fusion-able fuel, and perhaps not building it in the first place. If we did find a suitable neutron reflector, Im sure wed come up with better uses, like for more efficient reactors, or probably just better bombs the rocket idea was just the first to come to me.
Originally intended as an anno to [madness]s atomic rocket idea. I just didnt want to write the above from Jims perspective. I think Jim is a weirdo.
[words to the effect that I dont think this is even a very original idea. I just cant find it fleshed out anywhere convenient. Its got parallels with other nuclear propulsion concepts, at the least]