I'm also researching fission-based engines. I'll probably handwave things a little, and abstract a lot. Zozer's excellent Orbital 2100 has some of the rules I need, though I'll probably want somewhat better (more optimistic?) drives than his NTRs.
Potentially Nuclear Lightbulbs?*
The kind of rockets I am looking for are:
1. Work with a fission power-plant ala MGT1/CE.
2. Relatively high delta-V and acceleration (and I'll probably add some "optimistic handwaving" to performance)
3. Work with hydrogen (or water?) propellant/reaction-mass
If possible - without too much radioactive exhaust.
* How often do you have to refuel the "bulb" itself, i.e. the uranium gas core? If this is not too often, this seems like an excellent engine for interface craft, as it lacks radioactive exhaust and there are already launch vehicle designs for this
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