Cartridge starter

A type of starter used on some large aircraft reciprocating engines. A cartridge, looking much like a shotgun cartridge, is fired in the breech of the starter. The gas produced by the burning powder pushes down on a piston inside a cylinder. The piston is attached to a helical spline, a spline cut in the shape of a spiral, in such a way that as the piston moves down, the spline twists. The twisting spline turns a gear that meshes with a gear on the engine crankshaft. When the cartridge is fired and the piston is pushed down, the helical spline turns the gear and cranks the engine fast enough for it to start.