Thanks for the information. For the balancing thing, if the main goal is to make them more powerful and fuel efficient, then there's really no need to 'balance' it, just set the thrust and Isp values ridiculously high and make sure it doesn't collapse. On the other hand, if the main goal is to make the engines work like realworld engines do, there're some approaches. First, max thrust should be set reasonably considering the size of nozzles and the physical/chemical limit of it. It means, smaller engines are normally not as powerful as those bigger ones are due to smaller nozzle size (which means that it can't redirect too much flame thus can't achieve a high fuel flow rate because of the underexpansion of nozzles). Also, engines that burn woods should have less power than those burn LH2/LO2/Hydrocarbon (xD). And when it comes to the Isp, well, things are getting complex now. Various categories of engines have different characteristics, but any kind of engines do have a natural limit of Isp. See below. The limit varies with engine type and mach number. Also, design and production process play much more important roles. That's just said, what if we use new type of engines, like those driven by electricity and have way higher Isp. The main idea is to first work out the certain principle (i.e. how your engines are gonna work), then set the numbers while referring to useful information for given design. FYI, O2/H2 burners do have higher Isp over HC burners, but no higher than 9000 sec (ASL). There's an exception, the GE CF6 turbofan which powers the Boeing 747s/767s can deliver an Isp of 11700 sec at sea level using hydrocarbon fuel (Jet-A1). PS: I'm not a programmer, so I don't have an idea on the feasibility of having the Isp gradually reduced as mach number increases in this game. If it is practical, then it's gonna be the best engine mod ever (already is though, lol, nice work bro). Also, you need to set the mass/cost properly to make it more realistic. Normally (but not necessarily), powerful engines weigh more. But this only applys when same type of engines are compared. Rocket engines are the lightest (TWR, thrust-weight ratio) due to the lack of fans, compressors and shafts which weigh the most in a jet engine, whereas jet engines (turbofan/turboprop/turbojet) weigh much more. Ramjets and scramjets are between that two. Unit Cost:...um, whatever. When you put more technology (read: metal) into it, it costs more. More discussion is welcomed and appreciated. Will be happy if I can help.