This thing is insane as a final stage! Just got it up to a few hundred K before lighting it and it ended up at 4400 K and 8458 m/s before the final decouple.(with 1 fuel tank) I'll try to get it up there with more tanks, or maybe two stages or double engines. EDIT: Just made 12863 m/s and 10800K before the final decouple with two fuel tanks. Also, I'm now at 12861.7 m/s and not decelerating anymore.