A stirling engine is a type of engine which converts thermal energy into mechanical energy. An example of a rather large one is shown here
I know this is pretty much the same principle that an expander cycle engine uses, but could a stirling engine possibly make a rocket engine more efficient, and more reliable? The nozzle of the engine could be attached to the heating part of the stirling engine and the rotational energy could be used to drive the pumps for the engine. Also would a stirling driven engine be confined to the same limited amount of thrust an expander engine has?