English: The SOFIA aircraft, a Boeing 747SP, is pictured flying above the clouds over Waco, Texas during a test flight in 1998. In this photo, the aircraft bears the SOFIA acronym and the acronyms of the project sponsors, the United States' National Aeronautics and Space Administration (NASA) and Germany's Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR) but is otherwise still mostly in the livery of the aircraft's previous owner, United Airlines. A black square has been painted on the aft port fuselage to indicate the location of the 18 feet (5.5 m) by 13.5 feet (4.1 m) door that will be opened in flight to allow the telescope access to the sky.