The cheapest way to the airport. Nothing listed is quicker.
S11 from under the terminal
S11 · DB Regio NRW for the VRRPickup, hours & luggage
Boards at. Düsseldorf Hauptbahnhof, the S-Bahn platforms — the S11 towards "Flughafen Terminal", not a train to "Düsseldorf Flughafen"
A Preisstufe A single, valid 90 minutes on any bus or train inside Düsseldorf, the SkyTrain included; a child aged 6 to 14 is EUR 2.10, under-sixes free. A Deutschlandticket is valid, and no group ticket makes a single ride cheaper.
Luggage. An S-Bahn with open floor space by the doors; no published luggage rule.
Hours. Weekdays every 20 minutes from 05:03 to 19:23, then every 30 minutes to 00:53; at weekends every 30 minutes from 05:23 to 00:53.