Frankfurt has 2 airports. Hahn is the small one and a long way from Frankfurt. Fortunately bus services run at times roughly synchronised with arrivals to major cities. The bus to Heidelberg takes 2h40m and costs €16 single.
You will almost certainly have to walk across the tarmac on arrival and departure, no idea what happens if it is raining.
Hahn appears to have been an airforce base in the past, judqing by the reinforced hangars and attractively beige buildings.