From the hotel to site you can walk, the problem is getting from aiport to Bitburg, its a fair drive and AFAIK there's no easy way to do it by public transport, especially on a holiday weekend. So yeah, you do need a car (or s****y silver minibus with tinted windows from hertz at Frankfurt Hahn )
makes it cheaper to rent a car in Frankfurt and drop it in Bitburg, rent another one to go back to Frankfurt when it is all over, than to hire one for the full 5 days...