score:11
5 hours is borderline. The trains work very well. The S9 goes every 15 minutes and it takes about 15 minutes to get to/from the city. You don't have to get off at Hauptbahnhof (which isn't necessarily the nicest spot in Frankfurt) but you also can go to, say, Hauptwache.
However the airport itself is huge and it can take quite a while to get from the gate to the train station (you want "Regionalbahnhof") see http://www.frankfurt-airport.com/content/frankfurt_airport/en/directions/arriving_by_bus_ortrain/airport_railway_stations.html. Actual time will depend on where you arrive and whether you have to clear immigration and custom.
On the way back you have the same rigmarole plus you have to go through security which can a lot of time as well. Especially for an international departure. US flights leave typically from Terminal Z with an extra layer of immigration and doc check.
So I'd recommend to be at the airport two hours before an international departure. If you budget an hour to get from the arrival gate to the train station and have the tickets sorted out plus another 45 minutes travel (including waiting for the next train), that leaves a you little more than an hour in town.
EDIT: The Lufthansa bus makes this much more feasible. You will get 2-3 hours in town. Luggage storage is available and it's not particularly expensive. http://www.frankfurt-airport.com/content/frankfurt_airport/en/checkin_luggage/luggage_neu/storage.html
Upvote:4
Will I have any transport problems considering it will be on a Sunday morning in August?
Five hour is plenty to get to the S-Bahn go to the main train station (Frankfurt am main Hauptbahnhof) get a grab and return to Frankfurt airport. The subway (S-bahn) goes every 15 mins between the city and the airport. But going towards the central attractions is possible but you might have to hurry a little.
But do note that on Sundays (except for verkaufsoffene-sonntag), the shops will be mostly closed in Frankfurt city and there's little you can do there.
Also, do note that it's advisable to reach the airport 2-3 hours before (especially when you're traveling out of the schengen states) so effectively you have around 1-2 hours for travel + food around the Hbf. There's quite a variety of food around the Hbf.
If you're taking the Lufhansa bus, it's not exactly at the airport but at the Squaire building near the conference hall. So do take into account another 10-15 walk to and from the Squaire to the S-bahn station.
Will I be able to leave my luggage somewhere in the airport? (Or quite possibly in Frankfurt Hbf?)
Luggage storage is available http://www.frankfurt-airport.com/content/frankfurt_airport/en/checkin_luggage/luggage_neu/storage.html
And there are also quite a lot of coin-operated lockers at the Hbf too.