Basic Rocketry
From Kerbal Space Program Wiki
Basic Rocketry is the second node on the Tech tree, and the only stage 2 node on the tree. This is the first node the player researches. It costs 5 Science, and includes four parts, including the first decoupler.
Required Research
As this is the first node the player researches, it only requires the one node the player starts with:
Available Research
Three nodes branch off of this one on the tree: