[Spaceships] getting into orbit without superscience?

[Rupert]

Originally Posted by Varyon

I had always assumed the radioactive exhaust was a form of secondary radiation - bits of your fission core flaking off seems like a really bad design, so it makes more sense if you're actually looking at the exhaust being made radioactive thanks to neutron bombardment. The way to avoid that would be more shielding for the reactor.

Actually, some designs would leak radiation - in the form of fuel. You see, having a barrier between the energy source (fuel) and the propellant limits how hot you can run the rocket to what that barrier can handle. So some designs relied on careful flow design to keep the fuel from (mostly) leaving. Better performance at the cost of radioactive exhaust and consuming expensive radioactives at a much higher rate.