Day use of airpot lounge in Frankfurt

Day passes do indeed exist for lounges. Here is a breakdown of what is available at Frankfurt: Day use of airpot lounge in Frankfurt: http://www.frankfurt-airport.com/content/frankfurt_airport/en/services_at_the_airport/lounges.html

Thank you Michael for a very speedy reply. My next question is: how do you get an invitiation from your airlines? I don't meet any of the criteria, except for having $45! I have a letter of inquiry out to Condor, but have not yet gotten a reply.

I called Lufthansa and they said no, they don't sell single-day access to their lounges.

I think you would at least need to be a part of their frequent flier program, or possibly of one of their partner airlines. From there, if you have one of their credit cards, a day pass may be a perk, or you inquire. I know with delta, I can get a $25 pass at any time, but only due to certain conditions.

The invitation is like a free coupon airlines give to passenger so they get free access. The Sky and Luxx lounges don't require invitations from everyone, if you don't have one you can pay cash. While regulations vary in different countries, at least in the US, the airlines are required to offer some sort of day pass arrangement to the general public because airports are public, tax payer subsidized facilitates. But the airlines can limit sales to only passengers flying on their flights, and capacity restrictions.

I have frequent flyer programs with both airlines, but don't want another credit card.
Thank you all for your ideas, I'll try my best to get an invitation at the airport.

It looks like you can get into the SKY Lounge just by paying but it seems pretty pricey:
"Passengers of other airlines are also welcome. The charge is 30 € for 3 hours, 60 € for 6 hours etc., payable also with credit card. US-Dollars are not accepted anymore. Diners Club cardholders are welcome."

With just 5 hours as a layover, not really sure if a lounge is worth it. You only have about 2 hours to spend relaxing or eating, etc. If you want to shower, there are showers in Arrival Hall A, upper level, that cost 6 euro. Have a German lunch over at the Squaire, perhaps at Paulaner, or for fresher veggie/fruit options, the Marche' Cafe' at Arrival Hall B-1. There is a food court under Arrival Hall A, on the same level as the DB counters, and the ticket machines for the S-bahns. Lots of various options. If you just want to lay down for a while, there are lounge type chairs in various locations.