In 2017-18, I was the captain of my school's Team America Rocketry Challenge (TARC) team. We designed a system of two 3D-printed servo-actuated drag flaps embedded in the tail of the rocket to provide active control of the rocket in our effort to hit the target altitude for the contest.
This video shows our first successful test flight, where the drag flaps were programmed to open and close repeatedly. Subsequent flights used a rudimentary control algorithm based on data collected from identical flights, one with the flaps completely stowed, and one with the flaps completely deployed. We then took the velocity/altitude plots made from altimeter data collected on these flights and shifted the trajectories so that the x-intercepts(velocity of zero, i.e. apogee) both fell at the target altitude. This created a "funnel" between the two trajectories where the rocket would be considered to have acceptable velocity for its current altitude, and if the velocity was outside of that region, the flaps would deploy or retract to create the desired corrective effect. This whole system was designed long before any of the team had any formal education in control theory, and I'm proud of how well it turned out. In a qualifying flight for the contest, we were able to achieve a flight within 5 feet of the target altitude.