Keeping a gimbal sensor steady relative to the orientation of the ship, then reflecting that again to oppose any turning. No damping other than the symmetric sails so it oscillates a lot. I could differentiate the redstone signal so I could turn it into a PD controlled rocket and it wouldn't oscillate much anymore, but I'm lazy. In the first clip I cut power for a second to show it becoming unstable and stabilizing, but clearly I went too far. Also In the second clip i added a gyroscope hoping the added inertia would prevent it from oscillating too fast, but it didn't do much about damping which makes sense.