score:7
I arrive in Frankfurt Hahn airport (a good 90mins drive from the city) at 9:15PM.
No worries! The HHN airport is indeed far from Frankfurt, but the connections are not that bad. Just take the 10.30 pm bus. Ticket for an adult is 15€ (14€ if you buy it online).
You find the bus schdules via the airport's official site: Getting there - by bus. Click Frankfurt on the map and this timetable page opens (English translation via Google Translate).
September 29 is Tuesday, and the last departures from the airport are:
Dienstag, Abfahrt Hahn - Ankunft Frankfurt
Hahn Terminal B Frankfurt Airport   Frankfurt Hbf
[...]
20:00           21:30               21:45
22:30           00:15               00:30
23:30           01:15               01:30
00:00           01:45               02:00
00:30           02:15               02:30
(Note that departures at/after midnight are shown in the next day's—in this case Wednesday's—timetable.)