It gets rather frustrating when designing a spaceplane as you have to stick all your reactors right on top of the engines (unless you're using nuclerar-electrical engines) usually causing issues with balance, or making you use three tiny reactors when one big reactor would have done the same job and you'd have a more aerodynamic design. Certainly reactors like molten salt reactors can pump their coolant (molten salt) to where it's needed if a pump is provided. More exotic reactors such as fusion or antimatter reactors would naturally need a heat exchanger and pump to get the heat to where it's needed. No idea if this has been said before, but I find it so limiting when there are things like bicouplers (on planes) and placing a reactor in a cargo bay right before said bicoupler will cause the engines to fail completely. I mean... if you can place radiators anywhere shouldn't the same be true for reactors; provided you've got a way (hopefully not too heavy) to move the heat to the thrust source.