I believe the problem would be “knocking”. In an engine with a spark plug to ignite the fuel in the chamber there is a condition where the engine could run too hot and the fuel ignites from the heat before the spark plug fires, and that leads to a rough running engine and lost power. A similar condition is likely in a fission powered piston engine where a stray neutron from a spontaneous fission sets off the fission too soon and you lose power.
I can imagine how such an engine might work in theory but in practice I can imagine a spontaneous fission causing timing problems on the “detonation”. I doubt a fission explosion is required to make it work, it just needs to get hot after compression and then have a cooling off cycle somewhere. The “spark plug” could be something like a proton gun or a small fusion device, whatever would be a reliable and controllable neutron source, but the uranium fuel and fission products could trigger the critical condition too early with a spontaneous neutron.
If put in a continuous critical fission state for heat, like some kind of rocket or turbine, then the issue of early (or late) fission is not an issue. With enough fission events going on the statistical timing of these events becomes a smooth curve that can be calculated out and put into the design. There’s no need to know when each critical event happens, there is just one continuous critical state. 479
Just keep the hot ball of gas in the right spot and the engine keeps running smoothly.
Knocking, that’s the problem I see in a piston nuclear fission engine. Maybe with the right neutron absorbers the knocking goes away but then you are throwing away a lot of valuable neutrons you could have used for fission somehow. With neutrons having no charge they can’t be funneled into an adjacent piston easily, or stored in a cyclotron (or whatever they are called).
I like the theory. I’m just thinking knocking will kill the idea in practice.
Edit to add:
I was talking about this hypothetical engine with my brother and he mentioned something to the effect of combustion engines being able to use valves to release pressure, potentially a simple means to prevent knocking in an engine. So the solution to the problem is likely much simpler than I considered initially. This still leaves the issue of fuel being burned and not turned into useful heat, instead the pressure from the heat is vented out the exhaust. Perhaps the lost heat is trivial but over time that would likely count against the idea with other engines that aren’t “burping” excess waste heat out of the exhaust with every premature detonation. Delay things somehow to prevent premature detonation and that’s also lost efficiency because the power stroke was shortened.
I believe this can be made to work, it’s just going to have efficiency problems. 479