Ideally I would use a large diameter brushless motor directly, but doing the sums the brushless motors in my price range just couldn’t provide enough torque, and again, I want to put together an affordable project.

The ghost Minitaur is great, https://www.youtube.com/watch?v=_YrWX9ez3jM, but the motors are just too big and expensive relative to the body.

So a compact gearbox was needed, and a planetary gearbox seemed the ideal solution.

The design was inspired by some of the planetary gearboxes on thingiverse, such as the example at https://www.thingiverse.com/thing:8460

I ran a variety of experiments with planetary gearboxes for the servo.  The original gearbox had a ratio of about 40:1, though this was later dropped to  21:1.

First I printed some experimental planetary gear to check they would fit and mesh correctly.

The two halves of the prototype gear assembly

Planetary gears

The assembled unit.