Hessen Troops in America:
Search database for those participating in the Revolutionary War in America in Hetrina. For example, a search for the surname "Hess" will bring up references to mercenaries and their enlistment dates, service, desertion, etc.
See also German article: https://publishup.uni-potsdam.de/opus4-ubp/frontdoor/deliver/index/docId/6136/file/giersch_grAf.pdf