TL1: TransNewtonian Starting tech.
TL2: Any tech 2.5K or less with no prerequisite techs costing more than 1.25K. Also includes Pebble Bed reactors, Nuclear Pulse propulsion, and Jump point Theory.
TL3: Any tech 5K or less with no prerequisite techs costing more than 2.5K. Also includes Fast Reactors, Ion propulsion, and any tech 2.5K or less that required Jump Point Theory.