The starter gear section of a typical direct cranking starter used for large reciprocating aircraft engines:

The starter gear section of a typical direct cranking starter used for large reciprocating aircraft engines employs a sun and planetary gear reduction system because this design effectively reduces the high-speed rotation of the starter motor to a lower, more manageable speed necessary for cranking the engine. This system increases torque while decreasing the speed of the output shaft, allowing the starter to engage the engine smoothly and efficiently under loads.

The configuration of the sun and planetary gears provides a compact solution that is well-suited to the space limitations and design requirements of aircraft engines. It also distributes the load more evenly, reduces wear on components, and enhances the reliability of the starting system when dealing with the high torque requirements of larger engines.

In contrast, a simple gear mechanism might not provide the necessary torque multiplication or speed reduction. A friction drive typically relies on slipping surfaces and can be less reliable under the heavy demands of starting an engine. A belt system is not practical for starter applications in this context, as it cannot provide the direct torque and engagement required for starting powerful engines.