Cobble13

Asteroid belt

Each rocket engine shouldn't have a fixed ISP, but should instead become more efficient over time, after researching certain technologies. Non-nuclear engines shouldn't change much but should at least have one or two upgrades that improve the specific impulse of them by a fairly small amount. Nuclear engines, however should change a lot. The only current nuclear engine should begin with something like 100 ISP in atmosphere and 400 in vacuum. With more techs this should increase to 800 then 1000 and finally 1200 or something like that. There should also be a bigger, more powerful nuclear engine unlocked later on that starts with a very low ISP but can be upgraded a lot like the other. Ion engines should also be upgradable but not for efficiency but to give them more power. Some parts should also be upgradable giving them more strength to withstand higher velocity collisions and struts should become stronger with upgrades. Batteries could be upgraded to store more electricity. These are just some possibilities that could enhance science much more without adding very many new parts. It also would mean that the tech tree wouldn't just end when you've unlocked all the parts.