Minecraft wiki entry for Flying_machines, associated text: A two-way engine can be made with as few as 6 blocks – 2 Observers, 2 Slime Blocks, and 2 Sticky Pistons. Two-way engines A and B (see gallery above) show two different ways to do this. In both cases, the direction of flight depends on which observer is updated first. Note that in the diagram shown, each observer directly powers a slime block. The dock shown is placed so that the trapdoor will cover the face of the incoming machine's near observer, which lets it send the machine back the way it came.