This DIY Model Rocket Computer Has Raspberry Pi Pico-Controlled Steering
SandwichRising was able to build a robust flight control computer based on a Raspberry Pi Pico to achieve active steering on a budget.
When launching a model rocket, the typical goal is for it to shoot straight up and gain as much altitude as possible. That sounds simple, but it is a surprisingly difficult thing to achieve. If the center of gravity isn’t perfect or a gust of wind hits, the rocket is going to naturally veer off at an angle. To prevent that, the rocket needs some way to detect deviation and then correct its trajectory. SandwichRising was able to build a robust flight control computer based on a Raspberry Pi Pico to achieve that on a budget.
This system can be divided into three primary sections: the steering unit, the sensor puck, and the control computer. The two modules in the sensor puck are a GPS unit and an IMU (inertial measurement unit). The latter is necessary for monitoring the rocket’s orientation, in order to implement corrective steering. There is also a provision for adding a barometric pressure sensor later and data from all of the sensors can be logged on an SD card for post-flight review.
The steering module is where all the magic happens. Modeled in Autodesk Fusion 360 as a kind of extension tube for an Estes Green Eggs model rocket kit fuselage, this 3D-printed module contains four outward-facing servo motors mounted 90 degrees from their neighbors. Fins attach to those servos, so the rocket can adjust its own trajectory. Because each servo operates independently, there is a lot of potential for complex maneuvers—though only small angle changes should be necessary for typical launches.
Finally, the controller itself consists of a Raspberry Pi Pico development board on a breakout board, connected to an SD card adapter and with power coming from a 2000mAh lithium battery. The choice to use the Pico wasn’t arbitrary. SandwichRising selected it because the RP2040 microcontroller has unique PIO (Programmable Input/Output) capability — essentially, pins that can act independently of the main processor cores. The servos operate through PIO control, so they’re always smooth and responsive. Once a new servo angle is calculated, the PIO pins can control the servos while the main processor cores do other work.
SandwichRising used OpenRocket software simulation to get a basic idea of the rocket’s center of gravity and center of pressure. But it can’t account for active steering, so SandwichRising had to develop their own algorithms that link the servo positions to IMU readings. That is not trivial and the work is impressive.
