Bear with me.
Premise – we discover a material that’s 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 it’s 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 wouldn’t work for bombs, I mean.
...Anyhoo, getting to the point. Make a rocket with a reaction chamber that’s 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 – you’ll 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, I’m sure we’d 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 didn’t want to write the above from Jim’s perspective. I think Jim is a weirdo.
[words to the effect that I don’t think this is even a very original idea. I just can’t find it fleshed out anywhere convenient. It’s got parallels with other nuclear propulsion concepts, at the least]