Be sure you see the AI icon in the top right during flight. It doesn't do anything, and it's only there during flight, just to indicate that the AI is watching you.
You said "rocket". The AI does not see this as a SSTO (even if it can get to orbit with one stage). The conditions for the SSTO include having lifting surfaces (wings), having turbine engines (jets), being landed on the runway, and having more than 7000 deltaV (which is just a rough guess by me).
The AI is unpredictable. There's something like a 50/50 chance that, if all the conditions match, the SSTO flightplan will take over within a minute. This is evaluated many times per second, but the math comes out to somewhere like 50% per minute for that flight plan.
If you want, you can review the flight plans yourself in the GameData/RogueAI/FlightPlans folder. I tried to make the conditions and maneuvers as self explanatory as I could.