How can I drill a hole 8 ft deep through hard sandstone with a rotary hammer?
The roto hammer is not designed to drill any where near that deep.  You can try to drive the ground rod with the roto hammer. They rent an attachment.  If it is indeed sand stone and not too hard it may drive it through.  Other wise be content with the trenched ground rods.  This method is widely and successfully used. You could contact a highway construction company and find out the going rate for a rock boring drill.