As I said, maybe it's because of the first problem I described, and it just stopped calculating while adding the fuel lines (and it stopped on me often yesterday). I always wanted to give KSP plugin coding a shot, so I'm fiddling around with a build system (on OSX) right now, and if it works, I'll try the new code Padishar pushed to GitHub. Don't want to install VS in a VM, but XamarinStudio/MonoDevelop should work, shouldn't it? Padishar, if you like, you can PM me a binary of the code you pushed, so I can see if it fixes the problems I encountered. @GavinZac: I'm playing KSP since .18, and have used older versions of KER which worked just fine. It's not because of funky staging, I'm pretty sure.