Rocket League’s physics are based on the Bullet physics engine.
Psyonix used Bullet rather than relying solely on the physics system built into Unreal Engine 3. Bullet tracks the movement and collisions of cars, the ball, and other actors, helping the game deliver the fast, responsive interactions that define its car-soccer gameplay.
The choice was closely connected to online play. Rocket League needed networked physics that could remain responsive for players while periodically synchronizing the game state across clients. Psyonix’s developers valued Bullet because it was open source, which made it possible to inspect, debug, and modify the engine.
A common mix-up is to answer PhysX or Unreal Engine. Unreal Engine 3 provided the broader game framework, while Bullet supplied the physics implementation discussed in the game’s development history. Bullet is also a general-purpose engine used for collision detection and rigid-body dynamics beyond Rocket League.