The Douglas D-558-1 Skystreak and D-558-2 Skyrocket were, with the Bell XS-1, the earliest transonic research aircraft built in this country to gather data so the aviation community could understand what was happening when aircraft approached the speed of sound (roughly 741 miles per hour at sea level in dry air at 32 degrees Fahrenheit). In the early 1940s, fighter (actually, in the terms of the time, pursuit) aircraft like the P-38 Lightning were approaching these speeds in dives and either could not get out of the dives before hitting the ground or were breaking apart from the effects of compressibility-increased density and disturbed airflow as the speed approached that of sound and created shock waves.