Yep, if there was ever a game that deserved a custom game engine, it would be Kerbal Space Program. The version of Unity that they used constrained them to 3.5 gb, and 32 bit floating point coordinates, in a game that deserves a true scale solar system.

I keep hearing that double precision floats for coordinates are unrealistic because of the optimisations that are available only for 32 bit floats, but I am willing to make serious sacrifices to free myself from the Spheres of Influence and the Orbital Railroad of Destiny; the real solar system mod forces a full scale environment, and even though the visual geometry is off by roughly a metre from the physical geometry, I still consider it to be an improvement.