I don't think the lightbulb is that OP at all, considering how heavy and expensive it is.As well as being deep enough in the community tech tree to be late game. It doesn't _fundamentally_ change the purpose of an interplanetary nuclear stage, it's just more efficient than using a bunch of NERVs to accomplish the same thing. In my opinion, the most game balance and style affecting engine is the RTG one. It allows for nuclear level ISPs on probes and upper stages. I like it, but If there's any one thing about it I would change, I'd say the gimballing should be scaled down a bit.